Cracking in Plastic Concrete

By Adam Neuwald

Millions of people are burdened by cracking and discomfort caused by dry skin each year. The lips, hands and feet are typically the most susceptible extremities because they are regularly exposed to the elements. Conditions with low humidity and drying winds remove moisture from the skin leaving behind layers of dry skin cells. In the end, a simple smile or a clenched fist may exceed the tensile strength of the dried skin, which would lead to cracking.

Just as one may use lotion or lip balm to reduce the natural drying of skin during the winter and summer months, one must also take precautionary measures to prevent fresh, or plastic, concrete from cracking at an early age.

What causes plastic shrinkage cracking in concrete?
There are multiple forms of shrinkage that may occur in concrete. These include plastic shrinkage, drying shrinkage, autogenous shrinkage and carbonation shrinkage. It is important to understand the causes and effects of each type of shrinkage, but this article addresses only plastic shrinkage.

Similar to your skin, plastic shrinkage cracking occurs in freshly placed concrete when moisture evaporates from the top layer of concrete faster than it can be replaced by bleed water. A negative capillary pressure will develop within the pore structure of hydrating concrete as water evaporates from the capillary pores.

This negative pressure results in the development of an overall compressive force within the dehydrating surface, ultimately causing the top layer of fresh concrete to shrink. The underlying concrete does not shrink at the same rate as the dehydrating surface layer, and this restraint causes tensile stresses to develop within the surface layer. Once the fresh concrete’s tensile strength is exceeded, shallow cracks of varying depth and length develop throughout the surface.

Plastic shrinkage cracking will not occur in all concrete surfaces. Products most susceptible to plastic shrinkage cracking are those with a large surface area exposed to certain environmental conditions. According to the Portland Cement Association, the most common conditions that cause plastic shrinkage cracking are:

Low air temperature
High concrete temperature
Low humidity
High wind speed

The use of supplementary cementitious materials (SCMs) may also increase the potential for plastic shrinkage cracking. A majority of SCMs are finer than portland cement and ultimately have a higher surface area, requiring additional mix water to sufficiently coat the cementitious materials. This increase in fine material will often reduce the amount of bleed water that will reach the concrete’s surface. Concrete containing SCMs may be susceptible to plastic shrinkage cracking if the evaporation rate exceeds 0.1 lbs/ft2 per hour, while concrete containing silica fume is even more susceptible as bleed rates are reduced.

Identifying plastic shrinkage cracking
Plastic shrinkage cracks may range in length from an inch to a few feet depending on the size of the product and severity of evaporation. Cracks usually form perpendicular to the direction of drying wind, if present, and are usually spaced at regular intervals ranging from a few inches to a foot in typical precast surfaces. Cracks may also appear in a polygon pattern.

Plastic shrinkage cracking may be confused with settlement cracking as both are similar in appearance. Although plastic shrinkage cracking and plastic settlement cracking are often addressed separately in literature, they often occur in unison. Due to the natural force of gravity, concrete will continue to settle following placement and consolidation. Tensile forces will develop in the upper layer of concrete as it settles around reinforcement or at locations where there is a sudden change in cross-sectional thickness. Cracks are typically straight and located above reinforcing steel with minimal concrete cover.

Hairline plastic shrinkage cracks in the surface of architectural concrete are often referred to as crazing. These cracks are visible to the unaided eye, but are not measurable by ordinary means. Crazing cracks may collect dirt, which compromises the aesthetic value of architectural concrete. Crazing can often be avoided by utilizing concrete with low cement contents and by following proper consolidation and curing procedures.

Effects of cracks
Plastic shrinkage cracking does not initially affect the structural capacity of a precast product, but it may lead to accelerated corrosion of imbedded reinforcing steel, compromising the structural capacity of the product at a later age. Plastic shrinkage cracking and settlement cracking provide a direct route for deleterious substances to penetrate the concrete, negatively affecting the concrete’s durability and performance in corrosive environments. The importance of crack width on long-term durability has long been recognized by many design codes as stated in ACI 224, “Control of Cracking in Concrete Structures,” and shown in Table 1. If plastic shrinkage cracking has occurred, the surface of the concrete may require a sealant to enhance durability, depending on its intended use.

Prevention of cracks
Plastic shrinkage cracking often occurs during the summer months when hot drying winds are present or during the winter months when the difference between the concrete temperature and ambient temperature promotes rapid evaporation of bleed water. Several simple precautions may be taken to prevent the formation of plastic shrinkage cracking. Pay attention to weather conditions and keep an eye on the surface of products following casting. Products with large exposed surface areas are most susceptible to plastic shrinkage cracking such as panels, slabs or tank bottoms.

During the hot summer months it may be beneficial to keep aggregate stockpiles shaded and wet, especially lightweight aggregates. Using cold water or ice may also aid in keeping the concrete temperature low. If possible it is best to cast products indoors both during the summer and winter months as drying winds will often lead to plastic shrinkage cracking.

Make sure to close large overhead doors or install wind screens to prevent winds from drying the surface. Curing compounds, poly sheeting (preferably a light or opaque color) or curing blankets can prevent the evaporation of water from the surface layer of concrete. Fogging systems that maintain a high relative humidity above the concrete surface are also effective in preventing plastic shrinkage cracking. Fog sprays can be created with a pressure washer in combination with an atomizing nozzle. Curing compounds, poly sheeting or blankets should be placed over the concrete surface after fogging has ceased.

Proper precautions should also be taken to prevent plastic shrinkage cracking in patching materials. Prior to placing the patching material, thoroughly saturate the area that is to be patched with water or coat it with a bonding agent. Non-shrink grouts have been successful in producing crack-free patches.

Follow the manufacturer’s instructions for applying and curing the patch. If using a simple mortar mix, proper curing procedures should alleviate the potential for plastic shrinkage cracking. A wet piece of burlap covered with plastic should be sufficient for curing small patches. Large patches may require advanced curing methods.

The use of fibers
The use of polypropylene fibers may improve concrete’s resistance to both plastic shrinkage cracking and plastic settlement cracking. Fibers will increase the stiffness of the mixture, thereby minimizing differential settlement. Research also suggests that fibers have the ability to replenish bleed water to the drying surface through preferential bleeding channels created along each fiber. Fibers will also increase the tensile strain capacity of plastic concrete, reducing the potential for plastic cracking. Fibers reduce the length and width of cracks by bridging the crack, preventing it from spreading. Fibers will also increase the handling durability of freshly stripped products, reducing the number of spalls caused by forklifts and chains.

Conclusion
Surface cracking in plastic concrete can often be avoided through the use of proper mix designs and curing procedures. Not all plants will experience problems with plastic shrinkage cracking, but all should know how to prevent it. It is important to pay attention to changes in weather, especially when casting products outdoors. High winds and low humidity create an ideal environment for plastic shrinkage cracking. The use of fibers, fog systems, wind screens or simply casting products indoors should aid in reducing the likelihood of plastic cracking. Some precasters do not take proper precautionary measures until it is too late. Each plant should include a section on preventing plastic cracking in its quality control manual.

Many people carry a stick of lip balm with them through the winter months, whether they have chapped lips or not. In just the same manner, the successful precaster will be proactive and know how to prevent plastic cracking whether they have a problem or not.