PREVENTING CONCRETE CRACKS

A young engineer who wants to learn to make good quality concrete will always search and look for the good books, journals, articles and the guideance from the experienced seniors. Through this post,I hope some of the points will somehow help in the enlightenment process concerning the anatomy of concrete cracks.

Every engineer knows that concrete is composed of the following ingredients:

1.CEMENT.
2.AGGREGATE-CA+FA.
3.WATER.
4.MINERAL ADMIXTURES-FLY ASH. GGBS, etc.
5.CHEMICAL ADMIXTURES-SUPERPLASTICISERS, RETARDERS, etc.

Even after good selection of the above ingredients and producing a good concrete mix will be of no use if in the construction site the following important procedure is not followed.

a. Controlling water quantity.
b. Good compaction.
c. Careful and continuous curing.

It is well established that adding more water is as bad as consuming more food to a human body. More water is the culprit who makes many problems with the properties of concrete. The layman-Masons like to add more water for their easy workability . It should be controlled by the engineer at site.

It is hoped also that every one should appreciate the importance of Compaction.

It is known fact the concrete should be nourished by continuous curing. The importance of curing can be read in many good books on concrete and jounal, article.

To help the young engineer some information below were posted below on various websites . More detailed information can be read by browsing the web site.

Preventing Concrete Cracks
http://www.concretenetwork.com/concrete/preventing_concrete_cracks.htm

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 concrete poured on the job

Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.
Talk to the ready mix supplier
Consult with the Ready Mix Concrete Association in your area.

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.

http://www.cement.org/tech/faq_cracking.asp

Frequently Asked Questions

Cement & Concrete Technology Home > FAQs > Cracking.

Q: What causes concrete to crack?

A: Unexpected cracking of concrete is a frequent cause of complaints. Cracking can be the result of one or a combination of factors, such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. Cracking can be significantly reduced when the causes are taken into account and preventative steps are utilized.

Crazing is a pattern of fine cracks that do not penetrate much below the surface and are usually a cosmetic problem only. They are barely visible, except when the concrete is drying after the surface has been wet.

Plastic Shrinkage Cracking: When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth. These cracks are often fairly wide at the
surface.

Drying Shrinkage: Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. Restraint to shrinkage, provided by the subgrade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete. Restraint to drying shrinkage is the most common cause of concrete cracking. In many applications, drying shrinkage cracking is inevitable. Therefore, contraction (control) joints are placed in concrete to predetermine the location of drying shrinkage cracks.

D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more years of service. Due to the natural accumulation of water in the base and subbase of pavements, the aggregate may eventually become saturated. Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate at the bottom of the slab and progresses upward until it reaches the wearing surface. D-cracking usually starts near pavement joints.

Alkali-aggregate reaction: Alkali-aggregate reactivity is a type of concrete deterioration that occurs when the active mineral constituents of some aggregates react with the alkali hydroxides in the concrete. Alkali-aggregate reactivity occurs in two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).
Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or displacement of different portions of a structure.

Thermal cracks: Temperature rise (especially significant in mass concrete) results from the heat of hydration of cementitious materials. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and center is too great. The width and depth of cracks depends upon the temperature differential, physical properties of the concrete, and the reinforcing steel.

Loss of support beneath concrete structures, usually caused by settling or washout of soils and subbase materials, can cause a variety of problems in concrete structures, from cracking and performance problems to structural failure. Loss of support can also occur during construction due to inadequate formwork support or premature removal of forms.

Corrosion: Corrosion of reinforcing steel and other embedded metals is one of the leading causes of deterioration of concrete. When steel corrodes, the resulting rust occupies a greater volume than steel. The expansion creates tensile stresses in the concrete, which can eventually cause cracking and spalling.
Cracking in concrete can be reduced significantly or eliminated by observing the following practices:

1. Use proper subgrade preparation, including uniform support and proper subbase material at adequate moisture content.

2. Minimize the mix water content by maximizing the size and amount of coarse aggregate and use low-shrinkage aggregate.

3. Use the lowest amount of mix water required for workability; do not permit overly wet consistencies.

4. Avoid calcium chloride admixtures.

5. Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks.

6. Provide contraction joints at reasonable intervals, 30 times the slab thickness.

7. Provide isolation joints to prevent restraint from adjoining elements of a structure.

8. Prevent extreme changes in temperature.

9. To minimize cracking on top of vapor barriers, use a 100-mm thick (4-in.) layer of slightly damp, compactible, drainable fill choked off with fine-grade material. If concrete must be placed directly on polyethylene sheet or other vapor barriers, use a mix with a low water content.

10. Properly place, consolidate, finish, and cure the concrete.

11. Avoid using excessive amounts of cementitious materials.

12. Consider using a shrinkage-reducing admixture to reduce drying shrinkage, which may reduce shrinkage cracking.

13. Consider using synthetic fibers to help control plastic shrinkage cracks.
See PCA's publication "Concrete Slab Surface Defects: Causes, Prevention, Repair" (IS177) for a full discussion on the causes of types of cracking, how to minimize cracks and proper procedures for dealing with cracking that can not be eliminated with the proper use of control joints etc.

Other sources for information on the cracking of concrete include:

ACI 224R (American Concrete Institute Committee 224). Although the report does not address the topic of what magnitude of cracking is acceptable in plain concrete (non-reinforced concrete) it does give reasonably clear guidance on acceptable crack widths in reinforced concrete which is more critical than plain concrete. The tolerable crack width values for reinforced concrete are included in Table 4.1 of ACI 224.

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