NORTHRANGE CONCRETE INC.
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NORTHRANGECONCRETE, INC.


Concrete Color

It's unrealistic to expect your contractor to precisely match the color of your concrete driveway to a showroom sample, a neighbor's decorative concrete driveway, or a photo from a brochure. Even plain concrete can exhibit color variations, especially if the project requires more than one load of concrete or if the concrete placements are made on different days. Most of these variations are minor and will fade over time.


An Overview of Concrete Driveway Costs and Basic Price Ranges

Longer-lasting, less maintenance, and cost-effective are the top three attributes of driveways installed with decorative concrete. Driveway costs can range from $5 per square foot to more than $15 per square foot depending upon the design and coloring effects desired. They are an ideal choice for getting the look of natural stone, patterns, or pavers without the cost associated with those authentic materials. Plus, with concrete driveways, you have the added luxury of having a long-lasting surface that is durable and wear-resistant in the harshest environments, along with little required maintenance. In addition, the design options and customization choices are nearly endless providing you with a palette of opportunity to create a one-of-a-kind driveway that can greatly enhance your home's curb appeal. *Note: Costs shown vary by market depending upon the size of the project and the prices for materials and labor.


Basic: $5 to $12 per square foot
An economical upgrade compared to plain, gray concrete, you can enhance your concrete driveway for a reasonable cost. Basic installations usually include:

1.     Cement

2.     Water
3.     Aggregates (rock and sand)


 Cement - The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.
 

Water- Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio, the stronger the concrete. (higher strength, less permeability)
 

Aggregates- Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.
 

Desired Properties of Concrete
1. The concrete mix is workable. It can be placed and consolidated properly by yourself or your workmen.


2. Desired qualities of the hardened concrete are met: for example, resistance to freezing and thawing and deicing chemicals, water tightness (low permeability), wear resistance, and strength. Know what you are trying to achieve with the concrete.

3. Economy. Since the quality depends mainly on the water to cement ratio, the water requirement should be minimized to reduce the cement requirement (and thus reduce the cost).
Take these steps to reduce the water and cement requirements:
·         Stiffest mix possible
·         Largest size aggregate practical for the job.
·         Optimum ratio of fine to coarse aggregate.



Concrete Admixtures: Most Common Types and What They Do
Admixtures are additions to the mix used to achieve certain goals.
Here are the main admixtures and what they aim to achieve.


Accelerating admixture-accelerators are added to concrete to reduce setting time of the concrete and to accelerate early strength. The amount of reduction in setting time varies depending on the amount of accelerator used (see your ready mix supplier and describe your application). Calcium chloride is a low cost accelerator, but specifications often call for a no chloride accelerator to prevent corrosion of reinforcing steel.


Retarding admixtures-Are often used in hot weather conditions to delay setting time. They are also used to delay set of more difficult jobs or for special finishing operations like exposing aggregate. Many retarders also act as a water reducer.


Fly Ash- Is a byproduct of coal burning plants. Fly ash can replace 15%-30% of the cement in the mix. Cement and fly ash together in the same mix make up the total cementious material.

  • Fly ash improves workability

  • Fly ash is easier to finish

  • Fly ash reduces the heat generated by the concrete

  • Fly ash costs to the amount of the cement it replaces

Air Entraining Admixtures- must be used whenever concrete is exposed to freezing and thawing, and to deicing salts. Air entraining agents entrains microscopic air bubbles in the concrete: when the hardened concrete freezes, the frozen water inside the concrete expands into these air bubbles instead of damaging the concrete.

  • Air entrainment improves concrete workability

  • Air entrainment improves durability

  • Air entrainment produces a more workable mix

Water reducing admixtures-reduces the amount of water needed in the concrete mix. 
The water cement ratiowill be lower and the strength will be greater. Most low range water reducers reduce the water needed in the mix by 5%-10%. High range water reducers reduce the mix water needed by 12% to 30% but are very expensive and rarely used in residential work.


Concrete Reinforcement: Fibers vs. Welded Wire Mesh
Fibers can be added to the concrete mix in lieu of welded wire mesh.


The problem with welded wire mesh is that it often ends up on the ground from being stepped on as the concrete is being placed. (Particularly if no support blocks are used). Another problem is that mesh does not prevent or minimize cracking-it simply holds cracks that have already occurred together.
If you could look into a section of concrete poured with fibers you would see millions of fibers distributed in all directions throughout the concrete mix. As micro cracks begin to appear due to shrinkage as water evaporates form the concrete (plastic shrinkage), the cracks intersect with the fibers which block their growth and provide higher tensile strength capacity at this crucial time.


ADJUSTING CONCRETE MIXES TO CORRECT PLACING PROBLEMS
When the concrete sticks to the trowel, when it is lifted off the concrete or concrete sticks to the finisher’s knee boards then too much sand in the mix or higher than necessary air entrainment are most likely the causes.


Excessive bleed water will delay the finishing operation and can cause serious problems with the surface of the concrete. Adding more sand to the mix, adding more entrained air, using less mix water, or adding cement or fly ash are possible cures.
Make sure your ready mix supplier knows if you will be pumping concrete. Pumping mixes require a sufficient amount of fines and there are limits to the size of the aggregate in order for the mix to be pumpable. Fly ash and air entrainment improve workability and pump ability.


Setting time of the mix can be slowed with retarders.
The mix may be cooled in hot weather by replacing part of the mixing water with ice, sprinkling water on the aggregate pile at the ready mix plant, or injecting liquid nitrogen into the batch.


Setting time of the mix can be sped up with accelerators.
The mix can be heated at the ready mix plant by heating the mix water and aggregates.


Installing Concrete
Placing Concrete
Normal concrete weighs approximately 150 pounds per cubic foot and should be placed as near as possible to its final position. Excess handling can cause segregation of the course and fine aggregates. Wetting up the concrete so it can be raked or pushed into a location far from where it is discharged is not acceptable.
 Concrete is poured directly from the chute of the ready mix truck, wheeled into place with a buggy, or pumped into place with a concrete boom pump (see concrete pumping). Concrete is normally specified at a 4-5" slump. Industrial, commercial, and some residential projects require an inspector on concrete pours who monitors the concrete slump and takes slump measurements at the required intervals.


 Spreading Concrete
The purpose of spreading fresh concrete is to place concrete as close as possible to finish level to facilitate straight edging/screeding the concrete.
Short handled, square ended shovels are recommended for spreading concrete. A come-along (a tool that looks like a hoe and has a long straight edged blade) can also be used. Do not use a round edge shovel for spreading concrete since it does not spread the concrete evenly.
Any spreader used should be rigid enough to push and pull wet concrete without bending: Normal concrete weighs approximately 150 pounds per cubic foot
 .
Why is Hot Weather a Problem?
Concrete sets as the cement hydrates. Hydration is an exothermic reaction, meaning it generates heat, and that reaction goes faster when the concrete is hot. So the main concern with the concrete's strength and set time isn't really the air temperature but the concrete temperature. When cement hydrates it sucks up water and grows crystals around the aggregate particles. When it's hot and that reaction is rapid, the crystals grow quickly but don't have time to grow strong. Early strength will be higher but 28-day strength suffers. When the concrete is hotter than normal  such as 88 degrees instead of 70 degrees, the ultimate compressive strength will be about 10% lower.
In hot weather, as the cement sets up, slump decreases rapidly and more mixing water is needed. This can also contribute to lower strengths (as much as another 10% lower), and in integrally colored concrete, can lead to variations in water content which can result in significant differences in concrete color between adjacent pours.


Surface Drying Another potential problem in hot weather is surface drying although this one comes with a caveat. If the concrete is warm and the sun is shining and there is a hot dry wind across the concrete, yes, you are likely to get more drying and surface shrinkage. But let's go back to something every concrete contractor should know about beer.
In 1987, G.E. Munro who was manager of technical services for Lafarge Cement wrote an interesting article for the Lafarge newsletter. He started by posing a question: Does warm weather increase the probability of plastic shrinkage cracking? Most people would say, yes. But he advises us to think about a glass of beer on a hot summer day. If the beer is colder than the air, what happens? Water condenses on the outside of the glass. Concrete works the same way. If it is cooler than the air, Munro says by about 18F or more, the chances are that water will be condensing on the surface rather than the surface drying out. The real problem with drying is when the air is cooler than the concrete. So if we can get cool concrete, we should be OK. This is NOT to say that hot weather can't cause increased cracking and plastic shrinkage cracking, because it can if the concrete temperature is high and the humidity is low.


To determine if evaporation will be a problem, use the monograph from ACI 305 (see below). Knowing air temperature, relative humidity, concrete temperature, and wind velocity, you arrive at a rate of evaporation. If the rate is greater than 0.1 pounds per square foot per hour, shrinkage cracking is possible.

Start at air temperature; draw a vertical line up to relative humidity, then a horizontal line to the right to concrete temperature, and then go vertically down to wind velocity, and horizontally to the left to determine the evaporation rate. If the evaporation rate is greater than 0.1, plastic shrinkage cracking is likely.
Hot dry subgrades and formwork can also result in cracks by absorbing water from the mix. Another problem with heat and concrete is thermal differentials. All that means is that one part of the concrete mass is warmer than another part. If the differential is greater than about 20F then you are likely to get cracks. This tends to be a particular problem in mass concrete (members thicker than about 18 inches).
Finally, maintaining air content can be a problem in warm concrete. Mixing is more likely to drive air out of the concrete making the level difficult to control.


****Hot Weather Concreting Note
As a general rule, each 10o F increase in ambient temperature reduces slump about 1". A 30o F increase in mix temperature can cut setting time in half, increase water requirements, and reduce the 28-day compressive strength as much as 25%.
A switch from ASTM C494 Type A to Type D water reducing and set retarding mix may be part of an effective plan for hot-weather concreting.


Delamination in Concrete Slabs
What causes them and how to avoid ?
Delamination in concrete slabs can be a serious problem, but often they are "heard" before they are seen. Here's an example. A concrete floor slab placed days ago has hardened and cured. As work from other trades continues in the area, someone drops a hammer from a ladder onto the floor, and an unexpected sound is heard. Instead of the clear ringing sound you would expect when metal hits solid concrete, a drumming sound or loud clack draws your attention. Using the hammer to tap around this spot, you identify more hollow-sounding areas. What's been discovered is a surface delamination in the concrete floor.


How do delaminations happen?
When fresh concrete is placed and compacted, the solids (cement and aggregate) settle. This natural settlement causes excess mix water and entrapped air to be displaced (called bleeding), and the lighter materials migrate toward the surface. If finishing operations start prematurely and close or seal the surface before bleeding is completed, air and/or water are trapped under the densified surface mortar. As concrete hardens, subsurface voids develop where the water or air is trapped. These voids create weakened zones right below the surface that can eventually detach during slab use. Very thin mortar layers over delamination may even detach when struck with a hammer, as you try to sound out the extent of the delaminated area.


When delamination can be a problem
If the delamination is isolated to just one spot, it might not affect the performance of a concrete slab. But if it's widespread, then you have a more serious problem that should be addressed.


Delamination comes in various forms and degrees of severity. Blisters are small, isolated delamination usually 1 to 3 inches in diameter Blisters frequently happen when relatively sticky mixes with higher percentages of fines are placed. They occur in areas subject to moderate to high rates of evaporation and that are finished too soon.
When finishing operations occur too early, uniformly over the slab, a delamination problem can be quite widespread and affect larger zones of a slab surface


How to avoid delamination
The simplest way to prevent delamination is to start final finishing of the slab after the bleeding process has run its course. But this may not be as easy as it sounds. Following consolidation, and once bleeding is complete, there is nothing left to be trapped to cause voids. However, a finisher's job to properly time final finishing can be an art because job-specific influences impact this timing on each project. Stickier mixes, thicker slabs, different types of subgrades, initial concrete temperatures, different cementitious contents, and changes in ambient temperatures all affect the bleeding process and impact when finishing should begin.
Here are a few dos and don'ts to follow to help you avoid delamination:


Don't seal or close the slab surface too early.
 Bleeding must be completed before starting finishing operations that close the surface (Photo 4). Be aware that     sticky mixes with higher cementitious or sand contents tend to bleed more slowly.


Do take precautions when finishing if the concrete is air entrained.
At air contents greater than 3%, a dense, hard-troweled surface isn't necessary. There is less potential for delamination if you give the surface a light steel-troweled finish or a broom finish in exterior areas where slip 


Don't finish slabs placed on impervious surfaces too early.
When concrete is placed on an impervious sub base, bleed water must rise to the top because it can't exit out of the slab bottom. Compared to slabs cast on a porous sub base, the bleed water quantity will be greater and bleed time may be longer, especially in cold weather.


Don't place concrete on cold subgrades when ground temperatures are below 40F.
The cold ground slows setting of the bottom part of the slab, making the timing of finishing even more difficult to judge. To reduce this differential set retardation, keep the subgrade covered until the time of concrete placement.


Do warm the concrete or use small doses of set accelerator to promote more uniform setting of the mix.
This will make it easier to judge the timing of final finishing.


Do take special precautions when ambient evaporative conditions are high and exceed concrete bleeding rates.
Under these conditions, it's tougher to judge when bleeding is complete. Here's a trick learned on the jobsite: Place a domed garbage can lid on the surface for a few minutes, and if you see bleed water after lifting the lid, bleeding is still in progress even if it's not easily visible over the entire slab.

 Do take measures to counteract rapid evaporative conditions on the jobsite,

such as wind, low humidity, and hot temperatures. Otherwise, crews are often forced to start final finishing of a surface before it gets away from them. The best ways to stop the surface from crusting and prevent high evaporation rates is to erect wind breaks, fog the air 5 feet above a slab (Photo 5), or spray on an evaporative retardant (such as E-con from Euclid Chemical or Confilm from BASF).


Remedies for delamination
If drumming-sounding areas on a slab are easily detectable by hammer or chain dragging, the subsurface probably contains voids. Depending on their severity and slab use (such as exposure to wheel loads and heavy traffic), these zones are likely to detach sooner than from a sound surface.
To achieve a high-quality surface, remove the defective concrete to a depth where only sound concrete remains. Proper removal of unsound concrete by suitable methods such as shot blasting grinding, or hydro demolition, is critical if subsequent steps are to be successful.
After proper removal of unsound concrete, you can then apply a new surface. In some cases, the slab can be ground smooth and sealed, resulting in an attractive final surface with an exposed-aggregate appearance. Alternately, you can bond a topping or concrete overlay to the prepared roughened surface. You can find resurfacing products for both interior and exterior use that are formulated to bond to a sound substrate and provide an attractive, sound new working surface. Conduct a small mock-up with the resurfacing material in a noncritical area of the slab to verify that the final appearance and surface texture will be acceptable to the owner for the use intended.


Why is cold weather a problem?
 There are two main problems with concrete in cold weather:

  • Concrete can freeze before it gains strength which breaks up the matrix

  • Concrete sets more slowly when it is cold very slow below 50F; below 40F the hydration reaction basically stops and the concrete doesn't gain strength

But these are concrete temperatures not air temperatures. So when it's cold, we need to protect the concrete until it can handle the cold on its own. The general rule is that once the concrete has gained strength to about 500 psi then it's OK. The magical thing that happens is that at almost the same time that the concrete achieves 500 psi compressive strength, hydration of the cement has consumed enough of the water in the original mix so that even if it does freeze, there's not enough water left in the pores to damage the concrete. With most concrete, even at 50, this happens during the second day.
To help it reach that 500 psi strength, then, there are two things we can do in cold weather: Change the mix to get it to set more quickly or protect the concrete from the cold or more likely, both.
When to Protect Fresh Concrete From Freezing
Q. The current project that I am bidding has a cold-weather clause in the specifications, which requires heating materials and protecting concrete from freezing during freezing or near-freezing weather. This specification is vague to me. What is near-freezing?
A. Cold-weather clauses can be found in many specifications. For example ACI 318, Building Code Requirements for Reinforced Concrete, says: Adequate equipment shall be provided for heating concrete materials and protecting concrete during freezing or near-freezing weather.
And ACI 301, Specifications for Structural Concrete says: When the mean daily outdoor temperature is less than 40F, the temperature of the concrete shall be maintained between 50F and 70F for the curing period.
ACI 306, Cold Weather Concreting, defines cold weather as a period of more than three consecutive days in which the following conditions exist:
1) The average daily air temperature is less than 40F and
2) The air temperature is not greater than 50F for more than one-half of any 24-hour period.
This specification may not always protect fresh concrete from freezing. For example, temperatures on a job located in a part o the country that has dramatic weather changes, like the Rocky Mountain states, may be 30F one day and above 50F the next. These weather conditions are not classified as cold weather because the temperature did not remain under 40F for three consecutive days. Use good judgment and protect concrete whenever there is a possibility of freezing. During periods not defined as cold weather but when freezing temperatures can occur, protect concrete surfaces from freezing for the first 24 hours after placement.
ACI 306 recommendations to protect fresh concrete from freezing can be broken down into two categories:
1) Modify the mix design so the concrete will gain the necessary strength before freezing, or
2) Protect the concrete from freezing by external means (i.e., blankets, enclosures, or heaters).


Protection and Curing
So you're prepared and you've got the right mix, now what? For flatwork, the traditional, and still the best way, to protect concrete from the cold is to cover it with blankets after it's been finished. Since the ground is a bit warmer and the concrete generates its own heat, blankets will keep it warm even if the temperature goes below 20F. A few things to think about are:

  • Remember the definition: If the air is below or expected to go below 40F, then use cold weather techniques.
  • When finishing concrete in cold weather, you still need to wait for all the bleed water to evaporate. Bleed water is basically the concrete particles settling (like mud in a stirred up pond) and squeezing out all the extra water. If you finish that water into the surface, you increase the water-cement ratio and get weak surface concrete. Since the concrete is setting more slowly in the cold, bleeding starts later, lasts longer, and you can get more bleed water. You can try getting it off with squeegees or vacuums--or you can wait.·  Typically, you only need to keep the blankets on for a couple of days, if the concrete is warmer than 50F.
  • If you want to make sure of that, check the concrete temperature using an infrared temperature gun, or use maturity methods. Maturity is a way to determine if the concrete has gained enough strength to be on its own and it relies on the combination of time and temperature. Learn more about the maturity method (PDF).
  • To determine how much insulating value you need to keep the concrete at 50F, check out the tables in Chapter 7 of ACI 306. The insulation needed is based on concrete thickness, cement content, and the lowest air temperature anticipated for the protection period.
  • ·  Place triple layers of insulating blankets at corners and edges that could freeze. Wrap any protruding rebar. Make sure the blankets won't blow off during the night.
  • ·  If blankets alone aren't enough to keep the slab warm (or the walls for formed concrete) then you can use hydronic heating pipes or electric heating blankets laid on top of the slab and insulated.
  • ·  If that's still not enough, or if it's too cold to even place the concrete, then you would need to enclose the work and heat the air. Temporary enclosures are expensive, but if the work must go forward, sometimes that's the only option.
  • In an enclosure or even in a building heated by temporary heat, you need to consider the potential problem of carbonation. With unvented heaters (salamanders), or even with gas-powered equipment, the carbon dioxide levels can increase. This carbon reacts with the concrete, creating a chalky carbonated layer at the surface. This layer will be soft and generally unacceptable.
  • Heaters are available that exhaust to the outside of an enclosure or building and just blow in warm air. That eliminates the carbonation problem. Assign someone to make sure the heaters are fueled and will stay on all night.
  • When using hot, dry air in an enclosure, the concrete surface can dry out quickly, leading to crusting or plastic shrinkage cracking. Also, be careful about fire with propane heaters.
  • If the concrete is kept at around 50F, protection can typically be removed after two days. If the concrete remains at 50F, depending on what kind of cement is used and how much accelerator, you should wait a couple of weeks better to wait 4 weeks before actually putting it into service. You can always test to determine the strength if it's essential.
  • Removing the blankets suddenly in cold weather can cause a temperature differential to build up between the outside of the concrete and its middle. This can cause cracking from the thermal differential, but typically only in thicker members.


 Cure concrete in cold weather without additional water; adding water will keep the concrete saturated so that freezing will damage it even after it reaches 500 psi compressive strength.
Concrete in cold weather absolutely does need to be cured the surface can dry out even faster than in warm weather, if the concrete is warmer than the air.


Concrete Curing
Curing concrete will make your concrete stronger and better, but only if you do it right timing is key
When concrete is born when you place fresh concrete where you want it to live out its life it's like a baby: very sensitive and easily ruined. If you take good care of it, when it's young it will grow up to be a strong and reliable adult; neglect it, and you'll be sorry!
Curing is all of the things that we do to keep our concrete baby happy during the first week or so of its life: maintain the proper temperature (neither too hot nor too cold) and dampness (I know, most babies prefer to be dry concrete likes being difficult). Curing is easy to skip in the instant but that will have a major impact on the quality of your finished work. While curing is important for all concrete, the problems that arise from not curing are most obvious with horizontal surfaces. An uncured slab, whether decorative or plain gray, is likely to develop a pattern of fine cracks (called crazing) and once it's in use the surface will have low strength that can result in a dusting surface that has little resistance to abrasion.


Curing Concrete:
What is curing and what does it do to the concrete
When most people think of curing, they think only of maintaining moisture on the surface of the concrete. But curing is more than thatit is giving the concrete what it needs to gain strength properly. Concrete strength depends on the growth of crystals within the matrix of the concrete. These crystals grow from a reaction between Portland cement and water a reaction known as hydration. If there isn't enough water, the crystals can't grow and the concrete doesn't develop the strength it should. If there is enough water, the crystals grow out like tiny rock-hard fingers wrapping around the sand and gravel in the mix and intertwining with one another. Almost sounds like a horror movie our concrete baby has turned into a monster!
The other important aspect of curing is temperature the concrete can't be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is what's important here, not necessarily the air temperature. Below about 50 F, hydration slows down a lot; below about 40 F, it virtually stops.
Hot concrete has the opposite problem: the reaction goes too fast, and since the reaction is exothermic (produces heat), it can quickly cause temperature differentials within the concrete that can lead to cracking. And cement that reacts too quickly doesn't have time for the crystals to grow properly so it doesn't develop as much strength as it should.


Curing Concrete:
How do we cure
Now let's narrow this conversation down a bit. Let's talk only about horizontal concrete and only about the moisture part of curing. To learn more about working in temperature extremes get a copy of ACI 305, Hot Weather Concreting or ACI 306, Cold Weather Concreting.
Let's also narrow things down to curing of colored concrete. We'll define that as any concrete with color, whether integral or dry-shake, whether it is going to be stamped or not. First, and most importantly, colored concrete is not really different than any other concrete; it needs exactly the same treatment to end up with quality concrete. Some of the methods, though, need to be a bit different since appearance is so much more important than it is for an industrial slab.
There are three ways to cure concrete: either we add water to the surface to replace the water that is evaporating or we seal the concrete to prevent the water from evaporating in the first place or we do both. Note that adding water to the surface is NOT adding water that will be worked into the concrete mix--that would increase the water-cement ratio of the surface concrete and weaken it, ruining all our curing efforts.
You need to think about initial curing when the bleed water is evaporating too rapidly to keep the surface wet prior to initial set. Traditionally that has been specified at greater than 0.2 pounds per square foot per hour. Many mixes today bleed at much lower rates than this, so if there is less bleed water then the evaporation limit needs to be set lower more like 0.05 to 0.1 pounds per square foot per hour. The best approach for decorative concrete is to try to alter conditions so you don't need to do initial curing: block the wind, keep the sun off the concrete, get cooler concrete. If that's not possible, fogging just enough to keep the surface damp is possible, but the simplest approach is to use evaporation retardant. This chemical can be sprayed on to form a thin membrane on the surface that prevents the water from evaporating. It completely dissipates during finishing operations. Keep some of this around for dry windy conditions.


Curing Concrete:
When do we cure

So the objective is to keep our young and impressionable concrete damp and at the right temperature (ideally between 50 and 85 F). The most frequently overlooked curing aspect is keeping exposed concrete surfaces moist while they are hydrating. Most concrete, especially most decorative concrete, will have plenty of water initially in the mix to completely hydrate the cement. The problem is that if the exposed surfaces dry out then the concrete can't hydrate and our young concrete ends up with very sensitive skin easily scratched and sometimes actually dusty.
There are three phases of curing and the length of time each lasts depends on the concrete and the environmental conditions. When concrete is first placed for a slab, bleed water rises as the concrete mixture settles. During this period (initial set), if the bleed water is evaporating from the surface faster than it is rising out of the concrete then you need to do some initial curing or else you are likely to end up with plastic shrinkage cracks. To know if that's necessary, you need to know the evaporation rate (see below).

  • Between initial set and final set, intermediate curing would be needed if the finishing (or stamping) is complete prior to final set.

  • After final set, you need to do final curing.

During initial set, the rate at which the bleed water evaporates depends on a combination of factors: air temperature and humidity, concrete temperature, and wind velocity. The classic, and still best way to estimate the rate of evaporation is the Menzel/NRMCA nomograph an easy-to-use chart that combines all of these factors. You can get this nomongraph out of ACI 308 or it's also available in an excellent piece in the March 2007 Concrete International, "Estimating Evaporation Rates to Prevent Plastic Shrinkage Cracking." You can also estimate evaporation rates using a free online program developed by Luke Snell and Amir Munir.
So you use these methods to figure out how fast the bleed water is evaporating--if it's greater than 0.2 pounds per square foot per hour, then initial curing is necessary because the concrete will be drying out. In the next section we'll discuss how to do initial curing.
After initial set, the concrete surface still needs moisture and now there's no bleed water. This is when you really need to cure the concrete. You need to assume that your concrete needs to be cured it does! You don't want your perfect baby concrete to turn into a juvenile delinquent, do you?


Properly Curing Concrete Slabs
·          Build a High Quality Slab on Grade
·         Make Sure the Subgrade is Compact
·         Use a Low Water to Cement Ratio How to Calculate Water to Cement Ratios
·         Properly Curing Concrete Slabs
·         The Three Types of Foundations
·         A case for allowing the time in the schedule to water cure
·         Special considerations for driveways, walkways, and patios
·         Be active in deciding where control joints will be placed
·         The Foundation Construction Process
·         Why Concrete Cracks


Why cure concrete. Curing serves two main purposes.

  • It retains moisture in the slab so that the concrete continues to gain strength

  • It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking.

Properly curing concrete improves strength, durability, water tightness, and wear resistance.


How to cure concrete.

  • Water cure:
    The concrete is flooded, ponded, or mist sprayed. This is the most effective curing method for preventing mix water evaporation.

  • Water retaining methods:
    Use coverings such as sand, canvas, burlap, or straw that are kept continuously wet. The material used must be kept damp during the curing period.

  • Waterproof paper or plastic film seal:
    Are applied as soon as the concrete is hard enough to resist surface damage. Plastic films may cause discoloration of the concrete-do not apply to concrete where appearance is important.

  • Chemical Membranes:
    The chemical application should be made as soon as the concrete is finished. Note that curing compounds can effect adherence of resilient flooring, your flooring contractor and/or chemical membrane manufacturer should be consulted.

All the desirable properties of concrete are improved by proper curing!
Proper Curing Techniques for a Concrete Driveway


Cure the concrete as soon as finishing is completed
Curing of the concrete is the final step of the process, and one of the most important. Unfortunately, it's also one of the most neglected. In extreme cases, failure to cure the concrete immediately after final finishing can result in strength reductions of up to 50% by reducing the concrete's resistance to the effects of weather and increasing the possibility of surface defects.
Methods of curing include covering the concrete with plastic sheets or wet curing blankets, continuous sprinkling, and application of a liquid membrane-forming curing compound. For slabs that are to be acid stained, wet curing is the best approach, since a curing compound would have to be completely removed to allow the acid stain to penetrate. The most common way to cure plain or integrally colored concrete, though, is to use a liquid curing compound. Read more about why curing concrete is important and how it's done
Why build a high quality slab on grade is best answered with, "what happens when you don't!"


If the concrete is out of level (greater than " in 10') it causes expensive shimming or cutting of framing. If corrections are not made during framing lid lines (where the wall meets the ceiling) may be noticeably out of level.


If the water cement ratio is above .50 the concrete can be overly permeable causing adhesives for vinyl flooring to loosen, mold or mildew to form under vinyl, vinyl to yellow, and grout in tile to become wet. Excessive cracking can cause further problems with flooring materials and water permeation through the slab.


If air entrainment is not used in the mix in cold weather climates, water inside the concrete can expand when freezing temperatures hit and fracture the concrete.


If control joints are not used with the proper spacing and placement unsightly cracks can develop, telegraphing through vinyl or breaking tile grout.


If wire mesh is used in the slab but blocks to support the mesh are not used the mesh can end up on the ground-not in the concrete, thus cracks that develop may widen causing problems with flooring or water permeability. Consider fibers in the concrete mix instead.


If the subgrade is not compacted and the ground becomes saturated after a good rain, the plumbing trenches under the house may collapse or the utility trenches connecting to the street may collapse under the driveway or other concrete flatwork-leaving no support for the concrete.


If the concrete is not properly cured the concrete may develop cracks that would not otherwise have occurred. Proper curing delays the drying shrinkage until the concrete is strong enough to resist shrinkage cracking. Concrete which is moist cured for 7 days is about 50% stronger than concrete exposed to dry air for the same period.
One of the most common questions received on ConcreteNetwork.Com is about cracks that are developing in newly poured concrete. The homeowner will question why it is cracking and did they receive a shoddy job.
When installed properly, concrete is one of the most durable and long lasting products you can use around your home. But it is important that concrete contractors follow well-established guidelines with respect to concrete placement. Durable, high strength, and crack resistant concrete does not happen by accident.


Why Concrete Cracks
Reason #1 - Excess water in the mix
Concrete does not require much water to achieve maximum strength. But a wide majority of concrete used in residential work has too much water added to the concrete on the job site. This water is added to make the concrete easier to install. This excess water also greatly reduces the strength of the concrete.
Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.


The bottom line is a low water to cement ratio is the number one issue effecting concrete quality- and excess water reduces this ratio.
What you can do about it:
Know the allowable water for the mix the contractor is pouring- or be very sure you have chosen a reputable contractor who will make sure the proper mix is poured. It is more expensive to do it right- it simply takes more manpower to pour stiffer mixes.


Reason #2 - Rapid Drying of the concrete
Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.
You can make sure that the necessary water is available for this reaction by adequately curing the slab.


What you can do about it:
Read here about the methods to cure concrete and understand how your contractor will cure the concrete.


Reason #3- Improper strength  of concrete poured on the job
Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.


Reason #4 - Lack of control joints.
Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). So 4"concrete should have joints 8-12' apart.
Read more about control joints here.


Other reasons:
Never pour concrete on frozen ground.
The ground upon which the concrete will be placed must be compacted.
The sub grade must be prepared according to your soil conditions. Some flatwork can be poured right on native grade. In other areas 6"of base fill is required along with steel rebar installed in the slab.
Understand what you contractor is doing about each of the above listed items and you will get a good concrete job.

Why is there Water Vapor in Concrete?
Most people, even many people in the concrete business, think concrete is water tight. After all, we make water tanks and dams out of concrete. But the truth is that although concrete does a good job of containing liquid water at least when there are no cracks water vapor moves readily through concrete at a rate that depends on the concrete's porosity and permeability. All concrete starts out wet. If there wasn't water in the mix, you couldn't place it and it would never gain strength. At a water-cement ratio of 0.50, there is about 300 pounds of water and 600 pounds of cement in a cubic yard. As the concrete begins to set, some of that water (about half) combines with the cement (through hydration) and some rises to the surface as bleed water where it evaporates. The rest is in the pores of the concrete.
After the curing period, the slab begins to dry. At this point there is a lot of liquid water in the concrete pores in fact, the slab is saturated. This liquid water begins to evaporate from the surface and if no additional water gets into the concrete, within about 90 days for normal-weight, 0.5 w/c concrete, the slab will be dry enough so that most floor coatings won't delaminate.
Water vapor leaves the surface of a concrete slab at a rate that is called the Moisture Vapor Emission Rate (MVER). When you read in a sealer data sheet that the MVER needs to be 3 pounds or 5 pounds, what that means is the number of pounds of water vapor per 1000 square feet per 24 hours. Envision a 31.6 x 31.6 foot section of concrete (1000 square feet) and imagine 3 pounds of water evaporating from the surface each day. Three pounds of water is about three pints ("a pint's a pound the world around"), so that's not much.
But what if the slab is placed on the ground without a vapor barrier? Think about what happens when you dig a hole in moist ground. Long before you get to the water table (liquid water), you will encounter damp soil. That's how the soil beneath your slabs looks damp. The ground beneath nearly all concrete slabs is damp in fact; it nearly always has a relative humidity of 100%. That means it is a continuous source of water vapor into the slab and the slab will never dry out especially if you put a coating on the surface that restricts the movement of water vapor. ACI 302.2R-06, Guide for Concrete Floors that Receive Moisture-Sensitive Flooring Materials, states that "A concrete slab-on-ground without a vapor retarder/barrier directly beneath it may have a final relative humidity profile that does not benefit from any initial drying."


What are Vapor Barriers? All of the problems associated with moisture vapor movement in a concrete slab will go away in time as the slab dries, as long as there is no source of additional water into the slab. Since the most common source is moisture in the ground beneath the slab, the solution is to completely take the ground out of the equation, by sealing the bottom of the slab. The best way to accomplish that is with a vapor barrier under the slab. Vapor retarders have been used since the 1950s. Recently, though, research has shown that the old traditional layer of 6-mil Visqueen (polyethylene plastic) under the slab is seldom effective for two main reasons: Although it may seem water-tight, this grade of material allows a lot of water vapor to pass through. 6-mil plastic often gets damaged during placement of reinforcement and concrete, creating holes that can let a considerable amount of water vapor into the slab. Thin plastic like this is often called a vapor retarder meaning it slows the vapor down but doesn't stop it. A much better approach is a true vapor barrier, with characteristics that conform to the requirements of ASTM E-1745, "Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs." This specification has three classes of vapor retarders (or barriers--the terms are still often used interchangeably), Class A, B, and C. For all three classes of vapor retarder, the permeance (a measure of how much vapor can pass through) must be less than 0.3 perms.

Most experts today don't think that's low enough, and a few materials have recently become available that have permeance values less than 0.03 perms some as low as 0.01. These low permeability materials completely eliminate any moisture migration from the ground, allowing the slab to dry out much more quickly and to stay dry. ACI 302.2R-06, Guide for Concrete Floors that Receive Moisture-Sensitive Flooring Materials, estimates that concrete with a w/c of 0.5 will dry to an MVER of 3 pounds/1000 sq. ft./24 hours in 82 days with a vapor barrier, compared to 144 days when exposed to vapor from below.  The other characteristic of a good vapor barrier that makes it effective is resistance to punctures and tears. ACI 302.1,

Guide for Concrete Floor and Slab Construction, states that the minimum thickness of an effective vapor barrier is 10 mils. This was verified by some field studies conducted by Concrete Construction magazine. Thinner plastic can't stand up to the abuse of construction. ASTM E-1745 specifies minimum values for tensile strength and puncture resistance that increase from Class C to Class A.  A 10-mil vapor barrier may be sufficient for residential construction in terms of puncture resistance, although the 10-mil barriers can't completely isolate the slab from ground moisture. The newer very low permeability barriers, such as those from Stego, W.R. Meadows, Fortifiber, Interwrap, Raven, Reef, Polyguard, Grace Construction Products, Strata Systems, and Layfield are 15 mils (15 thousandths of an inch) or greater. This thicker material is much less susceptible to tears or punctures and has a lower permeance. To be effective, a vapor barrier must be installed in a way that prevents any water vapor from getting to the slab. ASTM E 1643, "Standard Practice for Installation of Vapor Retarders Used in Contact with Earth or Granular Fill under Concrete Slabs," provides details, but here are a few tips: Most exterior slabs do not need a vapor barrier. If you are going to seal an exterior slab, find a sealer that transmits water vapor (breathes). For some tips on this, see Sullivan's Corner. . In general, place the concrete slab directly on top of the vapor barrier, with the sub base below. If the sub base is sharp angular gravel, a thin layer of sand can be placed on top of the gravel sub base next to the vapor barrier. See below for more discussion on this. The vapor barrier can be placed under the sub base, if the building is closed in when the dry sub base material is placed. Never punch holes in the vapor barrier to allow bleed water to escape. Do not pound form stakes through the vapor barrier. Some supports come with wide bearing pads. Another solution is the new VaporStake that seals to the vapor barrier and is then cut off and left in the slab. Seal all vapor barrier seams with the tape provided by the barrier manufacturer. Seams should overlap 6 inches. Seal around all pipe penetrations and blackouts; Run the vapor barrier up onto the footing or seal it to the foundation wall or both. Protect the vapor barrier as much as possible during construction and repair any damaged spots with the manufacturer's tape.


Where to place the vapor barrier

has been argued about for years. For a time in the 1980s and 1990s, ACI recommended using a "blotter" layer, on top of the vapor barrier. In 2001, the recommendation changed to typically placing the concrete directly on the vapor barrier. The argument is that putting the concrete directly on top of the vapor barrier prevents water from leaving through the bottom, which leads to longer bleed times and to slab curling. While this is true, we've shown in other parts of this article that the sub base will end up damp and a continuous source of water vapor into the slab. Today, most slabs experts recommend placing the slab directly on the vapor barrier unless the sub base is protected and can be assured of staying dry. Curling can be controlled with well-graded aggregate and a little more reinforcement in the lower part of the Joints in Concrete Slabs  Properly creating and locating joints keeps concrete looking its best Concrete is not a ductile material it doesn't stretch or bend without breaking. That's both its greatest strength and greatest weakness. Its hardness and high compressive strength is why we use so much of it in construction. But concrete does move it shrinks, it expands, and different parts of a building move in different ways. This is where joints come into play. Although many building elements are designed and built with joints, including walls and foundations, we'll limit this discussion to joints in concrete slabs. Here's an overview of the types of joints, their function, and tips for locating and installing joints. The Purpose of Joints in Concrete Slabs As concrete moves, if it is tied to another structure or even to itself, we get what's called restraint, which causes tensile forces and invariably leads to cracking. Restraint simply means that the concrete element (whether it's a slab or a wall or a foundation) is not being allowed to freely shrink as it dries or to expand and contract with temperature changes or to settle a bit into the subgrade (see Subgrades and Subbases Concrete). Joints allow one concrete element to move independently of other parts of the building or structure. Joints also let concrete shrink as it dries preventing what's called internal restraint. Internal restraint is created when one part of a slab shrinks more than another, or shrinks in a different direction. Think how bad you feel when part of you wants to do one thing and another part wants to do something else! Concrete feels the same way. In slabs, there are three types of joints: Isolation joints (also sometimes functioning as expansion joints) Construction joints (which can also function as contraction joints)  Contraction joints (also sometimes called control joints)
Different joints in concrete slabs all have the same bottom-line purpose of preventing cracks.


Causes of Craze Cracking
Hotline Problem
What causes the fine random cracks that show up on a flatwork surface, especially after if gets wet?


Troubleshooting Response
They're called craze cracks or crazing. Usually the cracks develop at an early age and are no more than 1/8-inch deep. They don't affect strength and rarely affect durability or wear resistance but they are unsightly. There are many possible causes:

  • Using concrete that's too wet

  • Excessive use of a jitterbug

  • Over floating the Concrete

  • Finishing while there's bleed water on the surface

  • Sprinkling cement or a cement-sand mixture on the surface to blot up bleed water

  • Intermittent wetting and drying during the curing period

  • Early drying of the surface before curing begins

Circular Cracking Pattern in Floor
 Early Random Cracking in Floors


Hotline Problem
A supplier in the sun belt reported that a floor built by a contractor customer had developed many random cracks within 2 or 3 weeks after construction. The floor was built in June, and during the subsequent weeks the ambient temperatures were between 105 and 110 degrees F.
The supplier thinks the slab might have been about 130 degrees F during this time. The 4-inch slab was made with a 5- to 6-inch slump concrete containing 15-20 percent fly ash. A hand tool was used to make the score marks - to -inch deep, dividing the floor into 12-foot squares. A reddish colored curing compound was used, but it was not applied uniformly. The supplier wanted to know what had caused the cracking.


Troubleshooting Response
The main trouble appears to be inadequate joints. The depth of the joints should be thickness of the concrete so that they will perform their function of controlling cracking. In this case they should be a full 1-inch deep.
An auxiliary contributor to the random cracking described is not enough care taken in application of the curing compound. The slab undoubtedly dried nonuniformly because of the uneven application, and rapidly because of the high temperature, causing random cracks to form.
A joint spacing of 12 feet would be satisfactory for many slabs, but it is too large for this one. A good rule of thumb is that the joint spacing in feet should be 2 to 3 times the slab thickness in inches. According to this rule, the joints for a 4-inch slab should be spaced 8 to 12 feet apart. However, it is also suggested that the intervals between joints should be shorter whenever there is reason to expect shrinkage to be high or the range of temperature to be great. It would have been wise to space the joints 8 feet apart of less.


Foundation Waterproofing and Drainage
Other than burning down, probably the worst thing that can happen to a residential structure is a foundation problem. The foundation is literally what the house is built on, what keeps the building where it was built, transferring the dead loads and the live loads into the ground.
The source of the vast majority of foundation problems is water. Wet soil beneath a foundation can swell or lose strength.
And that's only the first reason to keep the foundation dry. Then there's the little problem of wet damp concrete basements and crawl spaces that can breed mold and make below-ground interior spaces generally unpleasant. The problem is that typical concrete is not waterproof. Although uncracked (and what concrete is uncracked?) it will typically keep out liquid water, water vapor can still penetrate quite easily. Keeping water drained away from concrete foundations and preventing it from moving through the concrete are essential to a successful structure.

 

 

Foundation drains
Drainage is a key element of any waterproofing or damp proofing system. Surface drainage is the first opportunity to mitigate water infiltration, but it is very difficult to keep all water away from foundation walls both surface water and ground water can lead to problems. So it is very important not to use partial measures in draining water that ends up at the base of the foundation wall.


A typical sub-grade drainage system consists of:
·          A layer of 6-mil plastic forming a trough in a trench at the bottom of the footing to capture as much water as possible
·         A rigid or flexible 4-inch perforated drain pipe laid over the plastic and covered with clean well-graded gravel; some pipe is available with a sleeve to filter finesthis pipe should outlet to daylight, adequately drained soil, or a mechanically drained chamber (a sump)
·         A covering of geotechnical filter fabric, as a barrier between the gravel and the backfill
·         Backfill compacted in lifts to rough grade
·         Finish grade sloped away from the wall


Here are a few things to keep in mind:

  • Rigid drainage pipe, although more difficult to handle, may be preferable to flexible drain tile since it is easier to establish and maintain grade and avoid low spots where sediment can accumulate.
  • Lay rigid drain pipe with the slots to the sides, which creates a continuous channel for water to run in but still allows rising ground water or water from above to easily get into the drain.
  • Maintaining slope on a pipe may require it to be higher or lower than the footer in certain spots.
  • Even a level pipe will move water if the outlet is lower than the bottom of the pipe.
  • There are strip drains that lay against the outside edge of the footing. Some double as stay-in-place formwork for the footing. One example of this is Form-A-Drain which incorporates a double channel, one to use as the foundation drain and the other for radon mitigation or other ventilation, if needed. Brittleness in cold weather and deforming of the product in direct sunlight are potential problems.
  • Not all subsurface water trickles from above. A rainstorm or melting snow miles away may end up sending a surge of water underground into a basement or crawl space.
  • In very wet areas, an interior drainage collection pipe and trench and a sump pump can be employed to keep water under control.

Waterproofing versus dampproofing


 There is a distinct difference between damp proofing and waterproofing. Damp proofing is intended to keep out soil moisture while waterproofing keeps out both moisture and liquid water. Buildings have been damp proofed for years, a practice that used to be mistakenly referred to as waterproofing. The International Residential Code (IRC), in Section R406, specifies the conditions that require either damp proofing or waterproofing. Any concrete or masonry foundation walls "that retain earth and enclose interior spaces and floors below grade shall be damp proofed from the top of the footing to the finished grade." The IRC then provides a list of the permissible materials, which include bituminous coating and acrylic-modified cement. Waterproofing is only required by the IRC "in areas where a high water table or other severe soil-water conditions are known to exist."


 Damp proofing is a coating, usually asphalt-based, that is either sprayed on or hand applied to the outside of the wall. Though less frequently recommended in modern residential construction, it is still an acceptable form of treatment in many situations. The drawbacks include an inability to seal larger cracks or holes left by form ties and the potential for damage by coarse or careless backfill. But with proper surface drainage, correctly installed foundation drains at the footing, and the absence of hydrostatic pressure to drive water infiltration, damp proofing can supply adequate and long-lasting protection for many crawl spaces and basements.


 Waterproofing a foundation requires the same care as damp proofing in regard to surface treatment and drain pipe, but is much more exacting in the treatment of the wall itself. Obviously, if there is any doubt about whether or not damp proofing will do the job, it's best to spend the extra time and money to waterproof, particularly for habitable space.
   Local conditions vary widely and help determine the right choice for preventing water infiltration. Some areas with drier climates and lower water tables regularly build basements with foundation walls that may be 10 feet tall. Other areas, with high annual rainfall, high water tables, and no frost in the soil, more commonly use slab-on-grade foundations. But even these foundations need protection to keep moisture from migrating up from the ground through the slab (see Vapor Barriers for Concrete Slabs). And consider topography walkout basement built into the side of a hill has a good opportunity to use gravity to move sub-surface water away, whereas a full basement on the flats in a wet area may have to consider mechanical means in the absence of nearby well-drained soil.
 Of course the depth of the foundation wall and the use of the interior space will also determine the choice of methods. A 36-inch frost wall for an unheated crawlspace built on well-drained soil and employing a gravity drain is a good candidate for damp-proofing. The house next door with a 10-foot-high foundation wall and a finished basement might opt for a fully waterproofed foundation wall system.


What are Concrete Footings
Concrete Footing Fundamentals
Under every house is a foundation, and under most foundations are footings. Most of the time we take footings for granted, and usually we can: For typical soils, a common 16- or 20-inch-wide footing can more than handle the relatively light weight of an ordinary house.
·         On the other hand, if you build on soft clay soil or if there's a soft zone under part of your foundation, there can be trouble. A footing that performs well in good soil may not do so well in weak bearing conditions. We don't often see outright failure, but it's not uncommon to see excessive settlement when soil bearing capacity is low.


If the whole house settles slowly and evenly, some additional settlement is no big deal; but if settlement is uneven (differential settlement), there could be damage. A frame house with wood siding and drywall interiors can probably handle up to 1/2 an inch of differential foundation movement, but even 1/4 of an inch of uneven settling is enough to cause cracks in masonry, tile, or plaster.

  •   It's the unusual situations that cause the most trouble. When the footing is laid out off-center so the wall misses its bearing, when you encounter a soft zone on site, or when the footing is undersized, the builder faces a judgment call. If you think there's a problem ahead, you know you should stop and call an engineer. But if the risk is
  • In these tough cases, it's helpful to understand the bearing strength of soil and the reasons behind footing design rules. In very strong soils, minor mistakes probably aren't a big deal. In weak or marginal soils, however, it's best to be very cautious some of the solutions contractors think up may not really work.
  • I get called in to a lot of problem situations. I find that people understand the problems better if they have some background knowledge. For the benefit of builders in the field and at the risk of oversimplifying, I'm going to use non-technical language in this article to briefly explain a little about how footings work and to present some ideas for dealing with special situations. As you look at the solutions I recommend, however, keep in mind that high-bearing-capacity soil is assumed. Any time you're in doubt about the soil under your foundation, you'd be wise to get professional help.
  • In addition to providing a level platform for forms or masonry, footings spread out the weight of the house so the soil can carry the load. The load spreads out within the footing itself at about a 45-degree angle, and then spreads out in the soil at a steeper angle, more like 60-degrees from the horizontal.
  • As the load under a footing spreads out, pressure on the soil diminishes. Soil directly under the footing takes the greatest load, and therefore should be thoroughly compacted.
  •   Because the load spreads out, the pressure on the soil is greatest right beneath the footing. By the time we get down below the footing a distance equal to the footings width, the unit soil pressure has dropped by about half. Go down the same distance again, and the pressure has dropped by two-thirds. So it's the soil right under the footing that is the most critical and also, typically, the most abused.


 When we excavate for the footings, the teeth on the bucket stir up the soil and mix air into it, decreasing its density. Also, soil from the embankment may fall into the trench. Soil that loose has much less bearing capacity than the original soil. That's why it is so important to compact the trench bottom (use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay). If you don't compact that soil, you could get 1/2 inch of settlement in just the first 6 inches of soil.

  • If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it won't settle to any significant degree.
  • Soil Bearing Capacities

Class of Materials

Load-Bearing Pressure
(pounds per square foot)

Crystalline bedrock

12,000

Sedimentary rock

6,000

Sandy gravel or gravel

5,000

Sand, silty sand, clayey sand, silty gravel, and clayey gravel

3,000

Clay, sandy clay, silty clay, and clayey silt

2,000


Soil types and bearing.
The type and density of the native soil is also important. The International Building Code, like the CABO code before it, lists presumed bearing strengths for different types of soils. Very fine soils (clays and silts) typically have lower capacities than coarse granular soils (sands and gravels).
However, some clays or silts have higher bearing capacity than the values in the code tables. If you have a soil test done, you could discover that you have denser clay with a much higher bearing strength. Mechanically compacting the soil can also raise its bearing capacity.
You can get a pretty good idea of the soil bearing capacity in the trench bottom using a hand penetrometer. This pocket-sized device is a spring-loaded probe that estimates you the pressure the soil can resist and is calibrated to give readings in tons per square foot. In my opinion, every contractor and building inspector should have one of these. It can help you avoid a lot of trouble.
Footings Introduction
So, how does soil bearing capacity relate to the size of footings? The footing transmits the load into the soil. The lower the bearing capacity of the soil, the wider the footing needs to be. If the soil is very strong, the footing isn't even strictly necessary just the soil under the wall would be enough to hold the building up.

Minimum Width of Concrete or Masonry Footings (inches)
 
Load-Bearing Value of Soil (psf)
 
1,500
2,000
2,500
3,000
3,500
4,000

Conventional Wood Frame Construction
1-story
16
12
10
8
7
6
2-story
19
15
12
10
8
7
3-story
22
17
14
11
10
9

4-Inch Brick Veneer Over Wood Frame or 8-Inch Hollow Concrete Masonry
1-story
19
15
12
10
8
7
2-story
25
19
15
13
11
10
3-story
31
23
19
16
13
12

8-Inch Solid or Fully Grouted Masonry
1-story
22
17
13
11
10
9
2-story
31
23
19
16
13
12
3-story
40
30
24
20
17
15

Source: Table 403.1; CABO One- and Two- Family Dwelling Code; 1995.
You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. As you can see, heavy houses on weak soil need footings 2 feet wide or more. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. Under an 8-inch-thick wall, that's the same as saying you have no footing.
These numbers come from assumptions about the weights of building materials and the live and dead loads on roofs and floors. The allowable bearing capacity of the soil under the footing has to equal the load imposed by the structure. Reading down the table, you see that the code calls for a 12-inch-wide footing under a two-story wood-frame house in 2,500-psf-bearing soil. A 12-inch footing is 1 square foot of area per lineal foot, so the code is saying that the portion of a two-story wood house that bears on the outside walls weighs about 2,500 pounds maybe a little conservative, but reasonable. The same size footing is called for under a one-story house if it has brick veneer the brick is assumed to weigh as much as a whole second story.
If you had an engineer design the footing based on soil testing numbers and your prints, he'd add up the actual weights of the concrete, wood, and brick you'd be using in your building, factor in the required live loads, and come up with an estimate of the weight your actual house puts on the footing. It might be a little less or a little more than the code assumes. Then he would take the known bearing strength of the soil what a square foot of the soil can be trusted to support and design the footing so that the area under the footing multiplied by the bearing strength of the soil would equal or exceed the actual load.
In practice, you don't have to do this engineering on most houses. The amount you'd differ from a standard code-compliant footing isn't worth worrying about. Unless you have retaining walls or some other special situation, an engineer’s fee probably isn't justified.
In any case, I wouldn't recommend that builders cut back on their standard footing size even if they know they're building on strong soil. Regardless of bearing requirements, masons and poured-wall contractors want footings for their block or their forms to sit on. But the lesson to take is that when soils are very strong, (4,000-psf capacity or better), the footings may not be strictly necessary from the standpoint of bearing. This means it is less important, for example, whether the wall is correctly placed in the center of the footing.


What Retaining Walls Do
 Retaining walls provide lateral support to vertical slopes of soil. They retain soil which would otherwise collapse into a more natural shape. The retained soil is sometimes referred to as backfill.
Retaining walls can be constructed of many different materials and with a variety of building techniques. This discussion will focus on rigid, monolithic, poured concrete walls as the structural material, but steel, timber, and reinforced soil are often used too.
This retaining wall discussion will focus on walls that are constructed from the bottom up and where a stable back slope exists (at least temporarily) prior to wall construction. With poured concrete retaining walls, backfill is placed between the wall and the slope after the wall is constructed.
This discussion will talk about the walls themselves, their design and some important construction considerations. It will not talk about groundwork or compaction, since these are entire topics on their own right. Designers and builders of any kind of retaining wall should be familiar with and follow the procedures and methods for soil preparation and backfill compaction methods dictated by the appropriate local building codes.
 
Choosing a Concrete Cleaner or Degreaser
Learn how the various types of concrete cleaners work and how to choose the best one for your needs
For concrete-related applications, cleaners generally fall into five categories:

A wide variety of cleaners are available that are designed to clean or remove contamination from concrete surfaces. Understanding what these different cleaners do and how they work can save you time and money on your next cleaning project.
First, without going into complex chemistry, let's discuss how a basic soap works. We all know that oil and water do not mix. Soap consists of fatty acids that emulsify oil, grease, and dirt, allowing the particles to be removed with a water-based solution. Without soap, just plain water doesn't have much cleaning ability. By surrounding and entrapping stubborn oil or organic-based dirt, a soap or cleaner allows the grime to be rinsed away more easily.
The origin of basic soap goes back hundreds of years, and today there are a multitude of modern detergents and cleaners to choose from. Some utilize complex chemistry to target specific types of contamination and dirt in a wide variety of environmental conditions.
Even with the best cleaner, good old-fashioned elbow grease and patience really make the difference when removing a stubborn stain from concrete. Concrete is porous, holds dirt well, and can be a tough surface to clean. Doing some research and trying a few different systems can really pay off. Once you find the cleaner that really works, you will cut your time for concrete surface preparation or maintenance in half.


pH-Neutral Cleaners
When to use them:

These mild cleaners are primarily designed for cleaning interior sealed concrete surfaces that do not have imbedded dirt. They can also be used on exterior or interior unsealed concrete that only needs a mild cleaning, with no major dirt contamination present.


How they work:
These cleaners are typically concentrates that you dilute with different amounts of water, depending on the level of cleaning required. Saturate the concrete surface with the cleaner, followed by scrubbing or light agitation. After a few minutes, vacuum or mop up the residue, followed by rinsing with clean water. If dirt remains, try repeating the process or using a stronger solution.


Where to get them:
pH-neutral cleaners are readily available at most specialty concrete distribution outlets, hardware stores, and janitorial supply outlets.


Acidic cleaners
When to use them:
These types of cleaners are primarily designed to remove stains, dirt, and contamination that are soluble in an acidic solution. They are especially effective for removing
efflorescence on concrete, an insoluble metallic salt that appears as a white powder or crystalline residue on the concrete surface and will not wash away in plain water. Exposure to hard water, a high salt content in the concrete, and high soil alkalinity are common causes of these types of stains.


How they work:
As you might expect, acidic cleaners contain acid as the active ingredient. They come in both concentrated and ready-to-use formulations and are applied directly to the contaminated area. Sometimes scrubbing or agitation is needed, and stubborn stains may need additional applications. It is critical to neutralize the concrete after cleaning with an acid-based cleaner, followed by a clean-water rinse. Consider using a sealer to protect the area from future alkaline or salt contamination.


Where to get them:
Acid based concrete cleaners are available at most specialty concrete distribution outlets. Diluting standard muriatic acid is a widely used and accepted type of acidic cleaner. It is important to note that over-the-counter cleaners like Lime Away and soap-scum removers are designed to clean similar types of contamination in a bathroom environment, but do not contain acid, so they may not be as effective on concrete



cleaners
When to use them:
Also known as "concrete degreasers," alkaline cleaners are most often used to eradicate oil, grease, or other hydrocarbon-based stains in concrete. The high alkalinity of these cleaners emulsifies, or breaks down, the oily contamination. The other application for alkaline cleaners is to neutralize concrete surfaces after acid staining or acid cleaning. Alkaline cleaners are the best way to bring the pH of concrete from acidic to alkaline, which is concrete's natural state. Modern alkaline soaps and cleaners far exceed the neutralizing ability of the old-school method of baking soda and water. They are economical too, since 1 gallon of cleaner fully diluted will treat approximately 4,000 square feet of concrete.


How they work:
Alkaline cleaners typically come as concentrates and are diluted with water, depending on how aggressive the stain is. Apply the cleaner full strength for deep or older stains, and dilute it for newer stains that haven't penetrated far. Agitation or scrubbing to work the cleaner into the oil stain is critical for good results. A common mistake when using alkaline cleaners is to not allow enough time for the cleaner to work. Depending on the type and depth of the oil stain, multiple applications may be necessary, with each being allowed to work for a few hours to get acceptable results. Another important step when using alkaline cleaners is to "lift" the oil stain out of the concrete once the stain has been emulsified. You can use an industrial wet vac, poultice, or rags. Remove the residue with clean water, and reapply more cleaner if needed until the stain is gone.
Enzymatic/Bacterial Cleaners


When to use them:
The newest types of concrete cleaners are those that use organic chemistry and active enzymes to attack, break down, and in some cases digest stains and contamination. They are also known as "oxidation cleaners," with the most popular brand being OxiClean. Generally, they are formulated to break down very specific soils, such as starch-based stains, hydrocarbon-based stains, and protein based stains. For example, eliminating pet urine stains on concrete is a common problem. Using an enzyme cleaner that targets protein (the odor and stain in urine is comprised mostly of protein) is the best way to remove these types of stains. The other type of active cleaner is bacterial. These cleaners use genetically engineered bacteria that actually feed on oil, grease, or hydrocarbons.


How they work:
A major benefit to using these types of cleaners is that most do not require water for activation, and there is little or no residue to wash away. The enzyme or bacteria simply consumes the contamination until it's gone. The biggest drawback is that they require time to work. Depending on the size of the contamination or stain, the enzyme or bacteria may require days or weeks to do its job. Most of these types of cleaners are applied directly to the stain with little or no agitation required. For large or stubborn stains, additional applications may be required.


Where to get them:
Active cleaners can be a bit harder to find than the other types. Some hardware and home-improvement stores and specialty janitorial supply outlets carry them. I have found that buying a cleaner targeted for the specific stain you're removing yields the best results.
Specialty cleaners
Specialty cleaners remove specific types of contamination and may be a blend of two or more of the four aforementioned cleaner types. Examples of these would include rust removers and non-acid-based efflorescence removers. These products can be hard to find, and I have found that their success varies depending on environmental conditions.


What causes damage to concrete?
So we've done our evaluation and now we are ready to determine what caused the damage this is often called troubleshooting. Start by thinking about the basic characteristics of concrete strong in compression, weak in tension. Therefore, a crack implies that the concrete was in tension. Recognize that the crack is always perpendicular to the direction of the tension always!
Think of a typical shrinkage crack running diagonally from a re-entrant corner in a concrete slab. The concrete was shrinking back in each direction from the corner and that diagonal crack is perpendicular to the direction of shrinkage. Look at a diagonal crack across the corner of a slab panel where it was run over by heavy traffic or the sub base was poorly compacted the bending force created tension across the top of the slab. Take a saltine cracker and bend down the corner it breaks in a diagonal line exactly the same as a concrete slab. I guess you could think of a concrete slab as nothing more than a big cracker!
Here are a few typical concrete problems and their causes:



  • Corrosion of reinforcing steel: Steel rebar is protected inside concrete because the concrete is very alkaline which prevents rust. But if there are chloride ions present, such as from deicing salts, the chloride destroys the "passivating layer" of alkalinity around the steel, allowing it to rust. Rust has greater volume than steel and the expansion presses against the concrete putting it in tension and causing it to crack and pop off. Chlorides get to the concrete through cracks or by simply penetrating through the concrete's pore structure.

  • Freeze-thaw disintegration: Concrete is porous, so if water gets in and freezes it breaks off small flakes from the surface. Deicing salts make it worse. This is typically called scaling and it can occur during the first winter and get worse over time. When severe, it can lead to complete destruction of the concrete. Proper air entrainment completely prevents scaling.

  • Alkali-aggregate reaction AAR is a result of reactive aggregate in the concrete that forms a gel around the aggregate particle. When that gel gets wet it expands and can destroy the concrete. There are now some lithium products that can mitigate AAR.

  • Drying shrinkage cracks Most concrete has more moisture in it when placed than is consumed by the hydration reaction. As that water evaporates, the concrete shrinks about 0.15 inches over 20 feet, depending on how wet the concrete starts out. If you could hold a slab up in the air while it shrank, it probably wouldn't crack, but since it is on the ground it drags on the subgrade and the shrinkage is restrained and cracks form. Joints in concrete control the drying shrinkage cracks at least joints let us decide where the crack will form. Learn more about concrete joints.

  • Plastic shrinkage cracks When concrete is placed, if the surface is allowed to dry out before the concrete has gained strength, a pattern of cracks will form on the surface. This is also called crazing These cracks are very shallow and narrow and are seldom a serviceability problem although they can be objectionable to some owners, especially on decorative surfaces. The solution is proper curing, although synthetic fiber reinforcement can reduce this phenomenon.

  • Blisters/delamination Bubbled concrete surfaces may be blisters caused by prematurely finishing and sealing the concrete surface and sealing in air and bleed water. This can especially be a problem with air entrained concrete and heavy finishing equipment.

  • Cracks from structural loads here’s how reinforced concrete works: the steel reinforcement is completely useless until the concrete cracks. That crack may be very narrow, even invisible, but until the concrete cracks the steel doesn't start getting pulled to hold the concrete together. If there is no steel, inadequate steel, or the loads are too heavy (either during construction or in service), then the cracks can become wider. For a slab on ground that may not be significant, but for a wall or beam or column, cracks may signal structural problems.

Concrete Foundation Repair Methods
Past techniques for repair of sunken concrete has varied. Wood, concrete, cement and steel have been poured, pushed, turned or somehow forced into the ground trying to salvage these foundations and slabs, while early on, anyone and everyone, trained or untrained, became "experts" at this type of repair. Often as not, the repairs proved to be futile.
Other, more successful, methods of remediation involve extensive disruption of the family or business using the building. Usually, it is desirable that settlement of building slabs and monolithic foundations in residential areas be corrected without having to move all furniture, appliances, and possibly the whole family, or in commercial areas, without disrupting business.
However, with today’s technology and trained experts, there are a number of very successful solutions to the problem of sunken concrete that involve little or no disruption to normal living or business routine.


The two most common methods of this type of repair are slab jacking and hydraulic jacking (also known as piering).
In a slabjacking operation, grout is pumped beneath a slab or beam to produce a lifting force that restores the member to its original elevation.
In piering, steel posts are driven through unstable soil and hydraulic jacks are used to raise or stabilize concrete slabs affected by changes in the underlying soil. The repair method used depends on the type of distress being treated.
Find a Foundation Repair Contractor


What Causes Foundations
and Slabs to Sink?
Foundation settlement and movement can be caused by building on expansive clay, compressible or improperly compacted fill soils, or improper maintenance around foundations.
Whatever the cause, settlement can destroy the value of structures and even render them unsafe.
In any case, water is the basic culprit in the vast majority of expansive soil problems. Specific constituents of certain soils tend to swell or shrink with variations in moisture. The extent of this movement varies from soil to soil.
Those soils highest in clay content are generally more susceptible while that lowest in clay content is the least affected. In some areas the movement in insignificant; in others, it is quite pronounced.
When unstable soils are used as a base for a foundation, the tendency for movement is transmitted to the foundation. Since soil movement is rarely uniform, the foundation is subjected to a differential or upheaval. The problem shows up in both slab and pier and beam type foundations.
If all the soil beneath a foundation or slab swells uniformly, there usually is no problem. Problems occur, however, when only part of the slab settles. Then, the differential movement causes cracks or other damages.
In residential properties, slab settlement problems can result in potential damage to the structure, potential accidents, and loss of real estate value. Poor drainage, tripping hazards, rough floors, unsightly cracks, and equipment malfunctions may also result from concrete slab settlement.


Warning Signs of Foundation/Slab Failure
Bulging floors, cracked walls, and doors that won't close are all signs of foundation distress. Sixty percent of all homes built on expansive soils suffer from foundation distress. The problem occurs when only part of the foundation heaves or settles, causing cracks and other damage.
This differential movement is largely caused by differences in soil moisture. Loss or gain of soil moisture can cause serious shrinkage or swelling.
If the frame of a house does not begin to distort until after three or more years of satisfactory performance, it is doubtful that the distortion is caused by full-depth foundation settlement, which is always evidenced by matching cracks. Cracks occur at each side of a portion of the foundation wall that is undergoing downward movement caused by soil bearing failure.
Settlement cracks are nearly always vertical, and they should not be confused with cracks that occur when a wall is subjected to lateral movement from soil pressure.


Exterior Warning Signs

  • Wall Rotation

  • Separation around garage door, windows and/or walls

  • Cracked bricks

  • Broken and/or cracked foundation

  • Displaced Moldings

Interior Warning Signs

  • Misaligned Doors and Windows

  • Cracked sheetrock

  • Cracks in Floor

·         Piling or piering is the technique of driving steel pipe pilings to remedy failing building foundations and to correct foundation settlement.


Push piers consist of sections of galvanized or epoxy-coated steel pipe that are driven into the soil with a hydraulic ram.


  Helical piers use screw piles with steel shafts. The lead section, with one or more helixes attached, provides the needed bearing capacity. The piers are screwed into the ground with a hydraulic torque motor.
·         With either system, one or more steel piers are driven to rock or a suitable soil bearing layer and are connected to the foundation through a metal head assembly. Once a suitable bearing stratum is reached, each pile is tested to a force greater then required to support the structure.
·         Hydraulic jacks attach to the embedded steel piers and are used to raise the foundation back to its original elevation. Once the structure is restored to the desired elevation the piles are affixed (bolted or welded) to wall brackets, locking the new elevation of the structure.
·         Piers also offer an affordable solution for decks, porches, patios, hot tubs as well as pre-fab buildings.


Advantages of Piers on Concrete Foundations
No disruption or loss of use of the dwelling. The repair is performed with the building being used as normal.
The equipment is portable and can be easily used in tight spaces or carried by hand where access is a problem.
Remedies  are both the cause of the settlement (unstable soil) and the consequences (dwelling out of level) in one step.

  • No yard destruction

  • No heavy equipment

  • 3'x4' excavation is made adjacent to foundation and approximately 10" below the grade beam.

  • Soil is scrapped from the footing bottom and the foundation is chipped smooth to ensure proper fit of support bracket.

  • Brackets and hydraulics are installed and a guide sleeve is advanced through the support bracket.

  • Starter and pier sections are advanced to refusal at an average penetrating power of 50,000 pounds of total driving force.

  • The last pier section is cut approximately 5" above the support bracket and a fastening plate is installed on top of the pier column. A

  • The hydraulics are reconnected and are operated sequentially to raise the structure.

  • When the structure has been raised to the desired height the fastening plates and support bracket are permanently attached to the pier column.

  • When the structure is secure, depth, pressure and elevation readings are recorded for each pier.

  • The excavated soil is replaced and compacted.

  • Shrubbery and concrete removed for the pier installation is replaced.

How to Hire a Contractor for Interior Concrete Work


Find a Local Contractor to Do Concrete Work

 

 

 

8 Tips for Hiring a Concrete Contractor
Use this guide to make sure you're prepared when it comes to hiring a contractor for concrete work. These 8 simple steps are your key to knowing what information you should look for, what questions you should ask, and to understanding some vital do's and don'ts during the process. Your preparedness can make a difference in how smoothly your project gets installed. Print a copy of this diagram and refer to it through each step of your project. Before you know it, you'll be enjoying your new concrete too!


How to find out if a contractor is licensed?
All states that license contractors and tradesmen keep lists of license holders. Click on the abbreviation for your state in the map to see the phone number (and usually a Web address) for your state licensing board. A phone call or several mouse clicks will identify exactly who is licensed and who isnt.
Except for plumbers and electricians, tradesmen paid by the hour usually don't need a license. Contractors bidding on any significant work (more than a few hundred dollars) need a license in most states. If you have a problem with a licensed contractor or tradesman, the state licensing board can be a powerful ally in resolving issues. Get into a dispute with an unlicensed contractor or tradesman and youre on your own.
All construction contractors need liability and workers compensation coverage. Request a certificate of insurance from the contractor you select. Insurance carriers provide certificates showing policy limits and coverage dates. There's usually no charge for these certificates.
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