What Causes Concrete to Crack?

To effectively reduce cracking at the re-entrant corner of the concrete slab, a stress reducing device is positioned along an interior surface of a form or stem wall defining a re-entrant corner so that the angle formed between the first and second legs of the body member is disposed substantially adjacent the angle produced by the form or stem wall defining the re-entrant corner and the arcuate concrete. Re-entrant corners will usually cause cracking. Proper planning of the joint pattern to eliminate re-entrant corners will correct this problem. Even though you isolate the column box as shown here the slab will still develop uncontrolled cracks. Re-entrant corners are corners that point into a slab. For example, if one were to pour concrete around a square column, you would create four re-entrant corners. Because the concrete cannot shrink around a corner, the stress will cause the concrete to crack from the point of that corner (See Figure 1). Re-Entrant Corner Cracks: Re-entrant corner cracks are a special kind of shrinkage that occurs when concrete is poured around a column with corners. They can also happen when the concrete is poured around a rounded object, like a manhole. Shrinkage cracks in a poured concrete foundation can be diagonal or vertical and are.

Posted By:Dynamic Concrete Pumping , Date: May 22, 2019

Reentrant Corner Crack

Concrete is a miraculous substance. Extremely hard and durable, it is an ideal material for construction, whether you are building the foundation of an apartment building or an outdoor pool. One drawback of concrete is that it has a tendency to crack. But why does concrete crack — and what can you do about it?

There are actually several common causes of concrete cracking. Here is a list of some of the basic types of concrete cracks, what causes concrete to crack in these ways, and what you can do about it.

Types of Concrete Cracks

• Shrinkage: The most common type of cracking in concrete, especially early on, is shrinkage. Concrete is a mix of cement and water. As the concrete hardens, which is caused by the cement and water forming bonds, some of that water escapes through evaporation. This reduces the size of the concrete slab, and since concrete is a very hard, tightly bound substance, this loss of material creates stress. This stress can lead to cracks in the concrete. Solutions to shrinkage cracks include reducing the amount of water in your concrete mix and cutting control joints in the slab so that if it does crack, it cracks along predetermined lines that do not significantly affect the integrity of the concrete.
  
• Re-Entrant Corner Cracks: Re-entrant corner cracks are a special kind of shrinkage that occurs when concrete is poured around a column with corners. They can also happen when the concrete is poured around a rounded object, like a manhole. When shrinkage takes place, the concrete cannot shrink evenly around the object. Instead, long cracks can result, radiating from said object. Properly cut control joints are the generally accepted method for warding off this issue.
  
• Expansion Cracks: In hot weather, bonds within the concrete can weaken and the concrete can expand, creating stress. If the concrete does not have room to expand, it will crack. You can prevent this by adding an expansion joint made of a compressible material, like tar-impregnated cellulose fiber, to allow some “give” in the concrete.
  
• Heaving Cracks: In very cold weather, the freeze and thaw cycle may cause the concrete to lift and heave, resulting in cracks. Avoid this by paving over soil that drains away moisture and will “give” with the slab, rather than tightly packed, unyielding soil, and by not pouring too close to large tree roots.
  
• Settlement Cracks: These usually occur if the concrete has been poured over ground that is not well-suited to hold it. Examples include soil where a nearby tree has just been removed and the roots are decomposing, or where a utility trench has been refilled but not properly compacted.
  
• Excessive Weight Cracks: Concrete, like anything else, has load limits, although they can be very high. If the weight you put on concrete exceeds this limit, expect cracks to form. This is rarely a problem in residential situations, but if you are working with very heavy equipment or other items, you may wish to verify that your concrete can bear it.
  

For more valuable tips on pouring concrete effectively, contact Dynamic Concrete Pumping today.

Whether you are a contractor, architect, engineer or simply a home owner with a concrete foundation you have most likely heard the adage that “all concrete cracks”. While this is true, there are measures that can be taken to greatly reduce the magnitude and frequency of cracks. To do this, one must have an understanding of the factors that lead to cracking. One such factor is stress concentrations in the concrete which are evident at re-entrant corners. Re-entrant corners are defined as any inside corner that forms an angle of 180° or less. In a solid object that is subjected to internal or external loads, re-entrant corners create high stress concentrations. If that solid object is concrete, which is strong in compression but weak in tension, then it will inevitably lead to a crack that will propagate at approximately 135° from the corner. Re-entrant corner cracks are especially prevalent in concrete slabs that are relatively thin in comparison to their plan size. In this article, I will focus on re-entrant corners in slabs-on-grade.

Examples of loads that can induce stress in concrete slabs include:

Shrinkage of the slab during the curing process when the concrete will shrink in volume as the chemical reaction between the cement and water takes place. Depending on the curing methods in place, the top and bottom surface of the slab will cure at different rates which induces stress in the slab.
Temperature changes. As with all materials, when concrete increases in temperature it will expand and when it decreases in temperature it will shrink. This expansion/shrinkage induces stress in the slab due to restraints such as friction with the bottom of the slab, stiffening ribs, piers etc.
External loads such as additional material or assemblies placed on top of the slab.

There are a number of measures that can be utilized to control re-entrant corner cracks including:

Reentrant Corner Bars

Contraction (Control) Joints: Place contraction joints at the re-entrant corner to create weak planes in the slab that will increase the possibility of cracks forming in the bottom of these contraction joints rather than at ~135° from the corner. Contraction joints can be formed by tooling the joints while the concrete is still plastic or with a saw after the concrete has set. It is important that contraction joints are placed as soon as possible before re-entrant corner cracks begin to form.
Construction Joints: Placing a construction joint 90° to the interior corner eliminates the re-entrant corner and thus the stress concentration.
Wet Curing. Wet curing of the slab will slow down the curing process and will create a more uniform cure rate between the top and bottom of the slab. This has the effect of reducing but not eliminating internal stresses. Wet curing can be accomplished by ponding the slab, utilizing foggers to maintain a humid environment on the slab, or by applying a chemical curing compound. In my experience, ponding of the slab is the most effective means of wet curing however it is typically the least practical.

Re Entrant Corner Crack Download

Water to Cement Ratio: The primary ingredients in concrete are cement, water, fine aggregate and course aggregate. The water chemically reacts with the cement to bind the aggregate in a solid matrix. To fully hydrate cement, a water to cement ratio (w/c ) of 0.26 is required. Additional (free) water is added to the mix to increase the workability of the concrete. As more free water is added to the mix, it increases the shrinkage of the concrete because the free water will eventually evaporate out of the concrete. For our slab-on-grade design, Dudley Engineering typically specifies a maximum w/c ratio of 0.45. Additional workability can be achieved by adding water-reducing admixtures or superplasticizers to the mix.
Fly Ash: Fly ash is a recycled material that can be utilized in limited quantities to replace cement. Replacing a small portion of the cement with fly ash can have the benefit of reducing the expansion of the concrete during curing.
Concrete Additives: There are chemical admixtures which can be added to the concrete mix that reduce the shrinkage rate of the concrete. Recently, on a post-tensioned slab-on-grade foundation that was intended to remain exposed, Dudley Engineering specified a shrinkage-reducing admixture in the concrete. This, along with other measures listed above, has produced a slab that is showing no signs of visible cracks.
To have a slab-on-grade foundation that is relatively crack free even at re-entrant corners, a combination of the solutions addressed above should be utilized. In addition to having a more aesthetic slab, it will also exhibit better structural performance throughout the life of the structure.