nt and water with aggregate (sand, gravel, crushed stone) and other substances (admixtures) produces concrete. When the cement and the water are combined, the mixture begins to harden (hydration). The concrete becomes hard and solid with its compressive strength progressively increasing over time (curing period).

Anyone familiar with the use of concrete during construction can tell you that there are many variables involved with the finished concrete product that directly affects its appearance and strength. The most important of these variables include; a) type of cement, b) ratio of water to cement, c) type and size of aggregate, d) type and proportionate amount of admixture, and e) various conditions and/or actions occurring during the mixing, placement, finishing, and curing. One absolute in the use of concrete as a building material is the fact that a certain amount of cracking is bound to occur. As one job foreman aptly put it, "The only way to guarantee it won't crack is to leave it in the truck!"

The following discussion relating to some of the more common causes of cracking in concrete may serve to clarify, and hopefully eliminate, some of the associated reoccurring cracking failures.
Concrete cracking is categorized as occurring either in the plastic state or in the hardened state.

PLASTIC STATE CRACKING

Plastic Shrinkage cracking is produced when fresh concrete in its plastic state is subjected to rapid moisture loss. This may be the result of a combination of factors during placement and curing such as; a) air and concrete temperatures, b) humidity, and c) wind velocity. When the moisture from the surface of freshly placed concrete evaporates faster than it can be replaced by bleed water (excess water in the mix) the surface concrete shrinks. Due to restraint from the concrete below, the drying surface layer develops tension stresses resulting in shallow cracks of varying depth. These cracks may later develop into full depth cracks.

Recommendations for Mitigation when weather conditions are unfavorable for a pour (especially in the construction of slabs-on-grade and other flatwork) proper curing methods are essential in the reduction of plastic shrinkage cracking. The use of fog nozzles to saturate the air above the surface, the application of plastic sheeting, wet burlap, waterproof paper, or spray applied liquid membrane to cover the surface and reduce evaporation, wind breaks to subdue wind velocity, and sun shades to reduce surface temperatures will go a long way in the reduction of plastic shrinkage cracking.

Settlement Cracking occurs when concrete in the plastic state continues to settle after its initial placement and vibration in the formwork. Where the concrete is locally restrained from downward movement by reinforcing steel, formwork, or previous pours, voids and/or cracking develop adjacent to the restraining element.

Recommendations for Mitigation methods to reduce or eliminate settlement cracking include: a) proper vibration to reduce voids when the mix is first placed, b) decreasing the slump (flowability) of the mix, c) the use of more rigid forms, and an increase in the concrete cover over embedded reinforcement.

HARDENED STATE CRACKING

Drying Shrinkage Cracking is commonly associated with the loss of moisture from the cement paste constituent producing a corresponding decrease in volume (shrinkage), coupled with restraint by the subgrade or adjacent structural members.

Recommendations for Mitigation drying shrinkage can be reduced and controlled by; a) adjusting the proportions of the mix (increased aggregate and decreased water content), b) careful detailing of reinforcing steel (especially through joints) and c) the liberal use of control joints (to induce cracks to occur along predetermined lines of weakness formed in the hardened concrete).

Thermal Stress cracking results when temperature variations due to weather exposure or (in more massive concrete structures) different rates of dissipation of the heat of hydration cause differential volume changes. Thermal expansion or contraction in hardened concrete may cause cracking; especially where connections provide restraint and no provision has been made for the elongation or shrinkage of the member over time. Temperature variations in structural concrete cause deflection and rotation in structural members that, if restrained, can produce serious stress cracking.

Recommendations for Mitigation Designers should be aware of conditions where certain portions of a structure are exposed to temperature variations while other portions may be partially protected or completely insulated. Allowance for the movement induced by temperature gradients should be made by supplying properly designed joints allowing freedom of movement and correct detailing on the plans. These same joints will also alleviate cracks due to movement induced by creep in concrete, which results in a long-term increase in strain or elongation under sustained loading.

POOR CONSTRUCTION PRACTICES

Mention must be made of the many improper practices in the field during the placement and curing of the concrete that ultimately lead to cracking. The most common of these is the adding of water to improve workability of the concrete. This results in increased settlement of the mix, an increase in the drying shrinkage, and reduced strength. The lack of adequate curing, or early termination of curing allows for shrinkage at a time when the concrete has low strength. Other problems that may give rise to cracking are poor vibration or consolidation, improper placement of reinforcing steel, and in slabs the over spacing of control joints used to minimize the effects of random shrinkage cracking.

For further information about PACE Investigative Services, please contact Sherrie Henderson at 818-251-1200 or e-mail her at shenderson@hfinc.com.