U.S. Department of Transportation
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-04-122
Date: February 2005
Plastic shrinkage cracking is an early-age pavement distress that forms before the set of a freshly placed concrete pavement. Plastic shrinkage cracks are the short irregular cracks that form on the fresh surfaces of concrete (see figure 18). They can be from few centimeters to just under 1 m long. The crack spacing is irregular, varying from a few centimeters to 0.6 m apart. Plastic shrinkage cracking is caused by the rapid loss of water from the surfaces of the fresh concrete. The cracks form when the rate of evaporation is greater than the concrete's bleeding rate. According to experience and previous research, conditions where the evaporation rate of a pan of water in excess of 1.0 kg/m2/hr will cause cracks to form in concrete of the same temperature.(6) Caution should be exercised when the evaporation rate exceeds 0.5 kg/m2/hr.
Figure 18. Plastic shrinkage cracking in concrete pavement.
With the loss of water from the pavement surface, there is a volumetric contraction of the fresh concrete. The shrinkage occurs primarily in the paste, with the aggregate acting only as restraint. These differential volume changes can induce tensile stresses in the pavement, and can subsequently cause cracks to form. The fresh concrete does not have sufficient strength to resist these capillary stresses within the fresh paste. There is currently no way to predict with certainty when plastic shrinkage cracks will form.
The method currently used to predict evaporation rate was developed by C. Menzel in 1954. It uses air temperature, relative humidity, concrete temperature, and wind velocity to determine if the evaporation rate is high enough for plastic shrinkage cracks to form. When the rate of evaporation is 0.5 kg/m2/hr, cracks can occur. When the rate exceeds 1.0 kg/m2/hr, precautionary measures are mandatory. This procedure originally was developed based on the rate of water evaporated from a standing pan of water.
Evaporation of water from the surface of freshly placed concrete is primarily due to climatic conditions. Typically, plastic shrinkage cracking occurs when construction takes place in hot weather. The climatic conditions that are suitable for their formation are:
These three conditions increase the rate of evaporation from the surface of the concrete pavement. Water also can be extracted from the slab by absorption or suction into the subbase and/or formwork. This can aggravate the water loss and promote additional cracking.
Plastic shrinkage cracks do not always form during the hot weather months. Other factors in addition to climate can influence the behavior of the fresh concrete. Some cracks are caused by incorporating new materials into the concrete, such as excessive fines, admixtures, and fiber reinforcement. Fines have a greater water demand and can affect the bleed water rate. Admixtures, such as superplasticizers and retarders, also affect the plastic state of the concrete by making the concrete experience less bleeding, and delay the set. Polypropylene fibers have been effective in delaying plastic shrinkage cracking. Similarly, the size of the concrete structure influences plastic shrinkage, since the slab depth is related to bleed rate.(23) These factors influence plastic shrinkage cracking, and the method to predict crack formation should account for these different material constituents and size
The loss of water from the surface of the concrete must be minimized to prevent plastic shrinkage cracking. One option is to moist cure the surface of the concrete immediately after placement, and to continue to do so for at least 24 hours. The most effective method is to keep the surface of the pavement wet. Other options are to erect wind barriers around the pavement or to erect sunshades to protect the surface from heat.
To improve current curing practices, the Federal Highway Administration (FHWA) recently sponsored a research project intended to provide guidelines for curing of PCC pavements (PCCP).(24) These guidelines include recommendations on selecting curing methods, curing application, curing duration, and temperature management issues.
Another ongoing FHWA project aims to develop a Microsoft® Pocket personal computer (PC)-based system with guidelines on curing of PCCP using concepts in the FHWA curing guidelines study and in HIPERPAV II. In addition to guidelines on selecting, applying, and timing curing methods, the Pocket PC system will have the capability to monitor real-time concrete temperature for determining concrete maturity and predicting concrete strength, among other features.
Placement of concrete pavements during hot weather conditions when the temperature of the air exceeds 32 °C, may be undesirable with respect to pavement behavior. During hot weather concreting, the cement hydration is accelerated by the temperature of the air and the initial high temperature of the mix components. Depending on the cement composition, cement fineness, and admixtures used, the accelerated cement hydration may result in significantly higher heat development during the first hours after placement. This increased hydration also reduces the set time and complicates the paving operations, delaying the time for proper curing. The higher heat development in the concrete mix increases the loss of moisture in the concrete, increasing drying shrinkage. Undesirable hot weather conditions can be compounded further with the use of high heat cements, high cement contents, and certain admixtures. In addition, drastic temperature drops during the first days after concrete placement may significantly increase the tensile stresses in the pavement. If precautions are not taken to minimize the above situation, excessive stresses in the concrete pavement may develop that can result in what is commonly known as thermal shock, or random, uncontrolled cracking.
Although the strength of the concrete develops faster due to the higher hydration rate, the long-term strength is usually lower than that of concrete hydrating at a lower temperature. After the first 72-hour period, it has been found that the early-age effects (accelerated hydration and rapid strength gain) become minimal.
For the case of JPCP, thermal shock may occur in the form of random cracking before, or even after, the time when joints are sawed. Although these cracks may be tight initially, they may extend to full depth, affecting the structural integrity of the pavement. Traffic loads and subsequent temperature fluctuations usually will increase the extent and deterioration of the pavement, providing poor performance in the long term.
As figure 19 illustrates, thermal shock may be observed in CRCP in the form of very closely spaced cracks. In addition, the cracks tend to meander more than cracks developed during placements at lower temperatures. Also, cracks occurring during the first few hours tend to be wider than those occurring at later ages. The formation of longitudinal cracks is another typical distress associated with high temperature placement (see figure 20). According to experience, closely spaced transverse cracks and longitudinal cracks due to thermal shock are more prone to develop into spalling and punchout distresses in the long term as a consequence of traffic loadings and climate.
When significant changes in temperature are expected during the construction of concrete pavements, it is important to assess the risk of damage to the pavement, as well as measures that would keep the stresses in the concrete at an acceptable level. Alternatives such as modifying the temperature of the mix or curing methods to insulate the pavement from the environment may be used to control the pavement temperature and excessive moisture loss.
The initial temperature of the mix can be reduced in several ways, such as cooling down the mixing water, sprinkling or fog spraying the aggregates, or maintaining the aggregates in shade storage. In addition, minimizing the concrete hauling time will reduce the time that the concrete mix is exposed to hot weather before placement. Scheduling placing operations during times of the day, when climatic conditions are not as critical, will also minimize the risk of developing extremely high temperatures in the concrete mix.
Protecting concrete against moisture loss during the curing period to avoid excessive drying shrinkage can be accomplished by increasing the application rate of the curing compound, or by using curing methods that provide moisture insulation such as polyethylene sheeting. If drastic temperature drops are expected, a combination of curing methods such as polyethylene sheeting and cotton mats may be necessary to keep moisture in the concrete, and to provide a more uniform curing temperature. If curing procedures are not performed on time, excessive moisture loss in the pavement may not be avoided.
Recommended precautions to avoid thermal shock could also include the use of low heat cements and supplementary cementitious materials (SCM) such as fly ash. Retarding admixtures also may be used to help minimize the water demand during hot weather concreting. Some retarding admixtures possess water-reducing and set-retarding properties. Other types of admixtures may help prevent drying of the surface by increasing early bleeding. However, caution must be taken that the admixtures do not reduce the tensile strength or tensile strain capacity of the concrete.(25)
Figure 20. Longitudinal crack in CRCP due to thermal shock. Crack is enhanced for clarity.
An important factor that can determine the potential for early-age cracking in newly constructed JPCP is the timing of the joint sawing operations. The purpose of joints in a jointed concrete pavement is to control the location of the cracking that will naturally occur during the life of the pavement. The majority of this cracking will occur during the early-age period as a result of restraint to volumetric changes. If the joints are not sawed early enough, uncontrolled cracking can result. This may lead to undesirable long-term performance due to poor load transfer and spalling. Therefore, sawcutting operations should be performed as soon as possible following construction to minimize the potential for uncontrolled cracking. However, when sawcutting begins is also constrained by the time required to gain sufficient PCC strength to support the weight of the equipment (and operator), as well as the forces introduced by the cutting blade during the cutting operations.