Many years ago when my young son was sitting on my knee, he asked three questions. Why is the sky blue? Why do birds fly? Why does concrete crack? The first question I looked to physical science for the quick answer. The second question I am still working on. And as for the third question, we in the concrete industry are slowly getting our arms around it.
This much we think we know: when concrete is restrained from movement and exposed to a force greater that its ability to resist, cracking will occur. Concrete can be restrained from movement in three ways:
- Being physically affixed to a given point or position;
- Being restrained by friction between the concrete and the sub-base over which it is placed; and
- The internal structure of the concrete itself. If concrete is allowed freedom of movement both internally and externally, cracking will not and cannot occur. The forces that can cause cracking in concrete are the result of either an applied load or volume change of the concrete.
With a common understanding of the mechanism of cracking we can now consider how cracks occur. The "how" goes to the relationship between the forces that can produce cracking and the restraint that is required for cracking. It is well understood that when an applied load is placed on a concrete member that is not strong enough to resist or transfer the load, the member will crack and fail.
Less obvious are the conditions that cause a flat slab to crack. There must be a force and a resistance to that force as a condition for cracking. As the slab begins maturing, passing from the plastic state to a hardened mass, both hydration and drying are occurring. The drying process can continue for many months and even years. Drying, or the loss of moisture, is also a loss of volume. When a slab loses volume and there is a resistance to movement, tensional forces can develop within the concrete and can cause cracks.
One source of the resistance to movement is the result of friction that develops between the concrete and the sub-base. If these tensional forces are not addressed, the slab will crack more or less at regular intervals. Under normal conditions the distance between cracks will vary from about 8 to 20 feet. By placing a system of joints in the concrete, tensional stress can be relieved and random cracks can be prevented. A (shrinkage) control joint is an engineered weakness in the slab which allows the slab to crack where we want it. While beyond the scope of this offering, current ACI documents address the spacing and construction of control joints.
Of equal importance with joint spacing is the timing of joint placement. Drying shrinkage occurs at its highest rate within the first few hours after placement. If the sawing of joints is delayed, the concrete will be predisposed to crack in places other than the sawed joint. This can occur within a few hours after final set. The sawing of joints should begin as soon as the concrete can be sawed without causing the edges to ravel. Waiting until the next day, in most cases, will be too late. A direct consequence of delaying the sawing of joints is random, erratic cracking. It should be noted that these erratic cracks might not appear for days, or even weeks.
While a well-designed system of joints can prevent the random development of structural cracks, it cannot protect the concrete from plastic shrinkage cracks and drying shrinkage cracks. These cracks must be addressed by either reinforcing the cement paste or by controlling the rapid evaporation of moisture from the surface of the concrete. Plastic shrinkage cracks are those cracks that occur while the concrete is still plastic, before setting. Drying shrinkage cracks occur after the concrete has taken set. Synthetic fibers placed in the concrete have been found effective in reducing drying shrinkage cracks but have little effect on cracks that form while the concrete is highly plastic. During the early stages of hydration, the cement paste is very weak. Synthetic fibers afford an elongated bond plane to which the cement paste can attach itself. Fibers also provide additional strength to the very weak cement paste. This additional strength helps control cracks after the concrete has taken final set through the next few days. With the development of tensile strength as the result of good curing procedures, the concrete will be able to better resist cracking.
From the time concrete is poured until setting occurs, the only real protection that can be used to reduce plastic shrinkage cracks is to control the amount of moisture lost. According to ACI 305, "Hot Weather Concreting," when the evaporation rate approaches the bleeding rate, precautions should be taken. ACI 305 also states that the bleeding rate can vary from 0 to over 0.2 lb/ft2/h and that plastic shrinkage cracks can occur at almost any time. The most practical and economical method to control plastic shrinkage is the use of L&M's E-Con evaporation control agent. Unlike other methods that only add water to the surface of the concrete, E-Con temporarily retards the evaporation rate of vital internal moisture within the concrete during placement, reducing shrinkage and preventing plastic shrinkage cracks.
Concrete also cracks for many other reasons, such as alkali-silica reaction, freeze-thaw, delayed ettringite formation and others. But these are subjects for another time. For now, I'm back to working on why birds fly. My grandchildren are now asking the questions.
See related article in September 2001 issue of Concrete News, regarding Drying Shrinkage of Concrete; Carl Bimel, author.
© 2002 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2002.