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Figure 2 compares the growth of the shrinkage stresses versus the tensile strength of the concrete. If contraction joints are not in place before shrinkage stresses exceed the tensile strength of the concrete, random cracking occurs.

The rate at which shrinkage stresses grow primarily depends on the rate of moisture loss from the concrete by subbase absorption and surface evaporation, rate of concrete cooling and the number and severity of concrete restraints. Whereas, the rate of strength gain is dependent on the rate of the chemical reaction between the portland cement (cementitious materials) and water. Strength gain is dependent on the amount and type of cement and admixtures in the concrete and the placing and curing temperature of the concrete. Cooler concrete temperatures delay the rate of strength gain. Concrete strength gain can be seriously delayed when placing concrete on cold subbases or during cold weather conditions. Also, concrete surface temperatures can drop significantly late in the afternoon when the sun sets causing both delayed setting and thermal contraction.

Delayed concrete strength gain typically delays the saw cutting of the contraction joints. If this happens, the risk of out-of-joint cracking significantly increases because shrinkage stresses can easily exceed the tensile strength of the concrete before contraction joints are installed.

Joint sawing

It is critical to saw cut contraction joints before the shrinkage stresses exceed the tensile strength of the concrete. Of course, the concrete must have enough strength so the sawing operation does not break or pull aggregate particles free from the concrete along the cut, creating jagged and rough edges. Sawing too early causes saw edge damage called raveling, and sawing too late causes random cracking, as illustrated in Figure 3.

Factors that affect the sawing window include concrete mixtures that have high water demands, rapid or retarded rates of hardening, sudden temperature drops or rain showers, sudden temperature rises, high winds and low humidity, cold temperatures and cloudy skies, hot temperatures and sunny conditions, high friction between subbases and slabs, and dry and porous subbases. If any of these or a combination of the conditions exists, the sawing window can be affected so that contraction joints cannot be installed before the shrinkage stresses exceed the tensile strength of the concrete. This is especially true when the rate of concrete hardening has been delayed by cool or cold weather conditions so that saw cutting is also delayed. In these cases, early-age random cracking is likely to occur while waiting on the concrete to become hard enough to sawcut without excessive raveling. In these situations, the concrete must be protected from moisture loss and temperature drops. Also, consider accelerating the rate of strength gain so saw cutting can begin sooner without excessive edge raveling.