SCP Tech Brief: Concrete Cracking
An unfortunate truth in concrete construction is that concrete cracks. There is no magic technology that solves all forms of concrete cracking. When properly used, Spray-Lock Concrete Protection (SCP) products may help reduce the chances of some forms of cracking, but not all. In this technical brief, you will learn how to distinguish between different types of common cracks and SCP’s products effects, if any, on those types of cracks.
Cracking is caused by many different reasons, but all cracks in concrete are caused by movement of the concrete in a way that exceeds the concrete’s ability to resist the forces generated. Concrete is strong in compression, but relatively weak in tension, flexure, and torsion. Some cracking mechanisms cause the concrete to move differently from the top to the bottom, for instance, generating stress at the interface between the two movement planes. In general, concrete cracks due to external loading, thermal gradients, moisture gradients, or in response to a chemical reaction (internal or external).
Cracking Due to External Loading
When loads exceed concrete’s ability to resist the forces applied cracks may occur. An example is negative-moment cracks above beams in an elevated deck caused by flexural forces between the beams moving in a downward direction. The concrete remains stationary above the beams while the concrete between the beams moves, sometimes resulting in cracking. Another example is cracking in a slab on grade that is exposed to traffic loads along an edge, breaking the concrete in a half-moon shape. SCP products do not help resist cracks due to external loading. Proper design to resist forces applied, including the strength of concrete and reinforcement sizing and positioning are the keys to limiting cracking due to external loading.
Plastic Shrinkage Cracking
Sometimes the surface of a freshly placed concrete slab dries out before the remainder, forming a “crust” that tends to crack. The cracks that form are typically shallow, run parallel to each other, and do not intersect the edges of the slab. The cracking typically appears in the time period just before finishing operations begin up to final troweling. These cracks are known as plastic shrinkage cracks because they happen when the concrete is still relatively plastic – it has not yet reached final set.
Plastic shrinkage cracks can occur when weather conditions cause rapid evaporation of bleed water before it can be replaced naturally by the subsurface concrete. Low relative humidity, high winds, and high concrete temperature can all contribute to plastic shrinkage cracking.
The good news is that plastic shrinkage cracks are not usually structural problems. The bad news is that they are cosmetically unappealing in instances where the concrete is exposed. Weather conditions conducive to plastic shrinkage cracking of concrete can be readily predicted by using several weather-related websites and/or commercially available instruments. There are several recommendations that help prevent plastic shrinkage cracks from ACI, NRMCA, and other sources, including erecting wind breaks, misting or fogging the slab with water, placing concrete in the early morning hours, lowering concrete temperatures, and using micro fibers. SCP products have little to no effect on plastic shrinkage cracking since most plastic shrinkage cracks occur before it is the proper time for SCP product application.
Plastic Settlement Cracking
Also known as subsidence cracking, plastic settlement cracks appear over embedded items such as reinforcing steel as concrete settles or subsides. Plastic settlement cracking is caused by insufficient consolidation, high slumps (overly wet concrete where aggregate segregation is occurring), or lack of adequate cover over embedded items. These types of cracks are recognizable by their resemblance in number and spacing to the reinforcing steel pattern below in the slab. SCP products have no effect on plastic settlement cracks. Plastic settlement occurs due to the lack of proper consolidation, high slumps, or inadequate cover over rebar, all of which are outside the
parameters that SCP technology affects. Plastic settlement can be of particular concern on deep (> 12”) reinforced concrete slabs.
Crazing is the development of a network of fine random cracks on the surface of concrete caused by differential
shrinkage of the surface layer. These cracks are rarely more than 1/8 in. (3 mm) deep and are more noticeable on steel-troweled surfaces and when concrete is wet. Crazing is most often caused by a higher water to cement ratio at the surface of the concrete as a result of over-t
roweling, sprinkling water on the surface of the concrete during finishing operations, or finishing concrete while bleed water is still present.Because the shrinkage that occurs in the top surface differs from the substrate concrete, SCP products can do little to mask or alleviate that difference. Although SCP technology can reduce drying shrinkage as a whole, crazing is caused by the differential between the top surface and the rest of the concrete. For instance, if the top surface of untreated concrete is expected to shrink 0.08% and the bottom of the concrete is expected to shrink 0.04%, SCP technology may reduce both values, but a difference will still occur, and therefore crazing may occur if the appropriate conditions exist.
Settlement cracking is caused by the loss of base or subgrade support of the slab. Concrete is strong in compression, but relatively weak in tension or flexure. When support is lost, concrete may “settle” along with the base or subgrade material. These cracks often indicate a significant structural issue that should be addressed and are recognizable by the vertical displacement from one side of the crack to the other. Settlement cracking occurs due to loss of support beneath the slab. SCP products do not provide sufficient additional flexural strength to counteract this type of failure.
Drying Shrinkage Cracking
Drying shrinkage cracks are caused by the change in volume of concrete associated with the loss of some of the water in the concrete due to evaporation. When concrete is first placed, it is typically at its greatest volume. Only a fraction of the water used in concrete is consumed by the cement hydration process. Much of the remaining water leaves the concrete, causing the concrete to shrink. When concrete is restrained by the ground, embeds, re-entrant corners, etc., tensile forces develop that can exceed the concrete’s ability to withstand them, and cracks form. Contraction joints are generally introduced to concrete to provide vertical planes of weakness that allow the concrete to form cracks in predetermined straight lines.
Drying shrinkage cracks are most likely to form at or near sources of restraint such as turn-down footings, depth changes, plumbing, and other penetrations. SCP products can reduce drying shrinkage of the concrete significantly by trapping some of the water inside the concrete and filling capillary voids, but drying shrinkage cracks will still occur near these sources of restraint.
SCP technology can significantly reduce drying shrinkage, but sources of restraint and improper jointing can still cause cracking.
Please note that there are many other types of concrete cracks that may occur. The preceding descriptions are the most commonly encountered early-age cracks in concrete construction. The use of SCP products cannot guarantee crack-free concrete. While proper slab design and installation methods can help reduce the chances for random cracking, ACI states that cracking can still be expected in at least 2% of all concrete panels, even where all work is executed properly.
For further information, many sources exist on concrete cracking and the mechanisms involved. SCP recommends information from ACI, PCA, and NRMCA as sources for further reading. Please contact SCP Tech with any questions at SCPTech@SprayLock.com.