By Jacob L. Borgerson and Karla Salahshour, Contributing Authors
Concrete will crack. It might crack because of plastic shrinkage, drying shrinkage, or induced loads, in addition to others.
While there are many sources and causes of concrete cracking, plastic shrinkage cracking represents one of the most common sources of this deterioration in America’s roads and bridges. Since it’s such a concern, it’s important to understand the causes, mitigation, and repair techniques of plastic shrinkage cracking of concrete.
Causes
Shortly after concrete is placed, water will mitigate to the surface. This mitigation is called bleed water.
Bleed water manifests at the surface because the denser constituents (e.g., cement, rock, sand) settle and the water rises. Plastic shrinkage cracks occur in early-age concrete when the rate of surface moisture loss is greater than the rate of bleed water rising to the surface (i.e., the surface “dries out”).
Rapid loss of moisture from the concrete will then cause shrinkage at the concrete surface and therefore tensile stresses.
The concrete surface will crack if the concrete has not achieved sufficient strength to resist these stresses.
Most notably, hot, sunny, dry, or windy conditions contribute to the loss of concrete surface moisture. Other factors that influence the bleeding of concrete also can potentially contribute to plastic shrinkage cracking. Examples include: the water-cementitious materials ratio, the use of supplementary cementitious materials, the dosage of chemical admixtures, and the temperature of the fresh concrete.
Concrete with slower setting times may cause higher plastic shrinkage due to the increased period that the concrete is in a plastic state.
Bleed water capacity is the volume of bleed water that the concrete releases to the surface. If the concrete does not have the bleed water capacity to replace the moisture loss during this extended time period or preventive measures are not taken, the concrete will crack.
Accordingly, as the evaporation rate increases during the day due to weather conditions (temperature, humidity, etc.), so will the propensity of the concrete to crack.
As the term indicates, these types of cracks occur when the concrete is still in a relatively plastic state and, consequently, has very little tensile strength.
Plastic shrinkage cracks typically appear parallel to each other and are often oriented perpendicular to wind direction. The cracks are typically 6-to 18-inches in length and spaced 1-to 2-feet apart.
As such, the cracks are discontinuous (often with overlapping crack tips) and do not typically extend to the edge of the concrete surface. In some instances, the crack pattern may appear more random in orientation. These cracks are relatively shallow, usually less than 2 inches.
If cores are extracted from the concrete element at the location of the cracking, the nature of the cracks can be further examined.
Cracks can form through the cement paste and at interfaces between paste and coarse aggregate. Specifically, the cracks pass around the coarse aggregate particles and display tearing of the paste, which indicates that the cracks formed before the paste had developed sufficient strength.
Mitigation
The risk of plastic shrinkage cracking can be reduced using an evaporation retardant, fogging, and/or wind breaks. Depending on the ambient conditions, some approaches are more effective than others.
The “Guide to Hot Weather Concreting,” which is in the American Concrete Institute’s (ACI) manual of concrete practice, recommends implementing evaporation control measures when the conditions exist for evaporation rates to be greater than 0.2 pounds/square feet/height.
The evaporation rate of surface moisture can be estimated using the Menzel Formula as given in the “Guide to Hot Weather Concreting.”
Table 1 provides an example of applying the Menzel Formula to determine the evaporation rate of surface moisture for unprotected concrete. This example illustrates that as the concrete and air temperatures increase and the relative humidity decreases, the evaporation rate will increase.
The advantages of applying an evaporation retardant have been known for over 40 years. They can reduce the evaporation rate of surface moisture by 45%.
Evaporation retardants are applied while the concrete is in a plastic state. These products are intended to form a thin monomolecular film that reduces moisture loss from the concrete surfaces.
These products are typically applied across the concrete surface with a low-pressure hand-held sprayer. In particularly hot and windy conditions when evaporation rates are high, the products may be applied multiple times during the placement to mitigate crusting of the concrete surface and the resulting plastic shrinkage cracking.
These products should only be used in accordance with the manufacturer's instructions.
In some instances, fogging can be effective in reducing the premature drying of the concrete surface and, consequently, mitigating the risk of plastic shrinkage cracking.
Fogging can be accomplished by gently misting water through a sprayer over the concrete surface. Water should not be directly applied to the concrete surface as this could ultimately cause a weak concrete surface. The intent of this activity is to increase the relative humidity of the air over the concrete surface.
A higher relative humidity directly over the concrete will consequently slow the evaporation of moisture from the concrete surface and reduce the risk of plastic shrinkage cracking.
As noted, the wind velocity at the concrete surface is a significant factor contributing to plastic shrinkage cracking. Depending on the nature of the concrete placement, wind breaks can be effective at decreasing wind velocity and/or deflecting wind away from the surface of the concrete.
Wind breaks are temporary and can be as simple as plastic sheeting fastened and erected to stakes at a height of approximately 2-to 4-feet.
Repair
Plastic shrinkage cracks are aesthetically undesirable but rarely structurally significant. They can, however, allow aggressive chemicals to penetrate the concrete faster and can also become worse with drying shrinkage, thermal movement, and loading.
“Control of Cracking in Concrete Structures,” in the ACI’s manual of concrete practice, provides guidance for the acceptable widths of cracks for reinforced concrete under service loads.
If the cracks are relatively small, and depending on the exposure and service conditions, crack repair may not be necessary.
For concrete roadways and bridges with frequent traffic, neglecting crack repairs may cause future maintenance problems. Fortunately, plastic shrinkage cracks can typically be repaired and, if done properly, they usually do not present significant performance problems in the future.
Plastic shrinkage cracks can typically be filled using a gravity-fed epoxy. Cracks can be treated individually or by flooding the surface. Gravity-feeding epoxy can be effective for shallow cracks but may not penetrate the full depth of deeper cracks.
Any sealing material that remains on the deck surface can be covered with a layer of sand for skid resistance and excess material will eventually wear off. If grooves will be installed, as is the case for many bridge decks, the cracks should be filled prior to saw cutting the grooves. RB
Jacob L Borgerson, Ph.D., is a consultant with Wiss, Janney, Elstner Associates, Inc., where he provides concrete expertise in multiple areas. Karla Salahshour is a petrographer with Wiss, Janney, Elstner Associates, Inc.
