QUESTION: An architectural residential driveway I poured two years ago consisted of an exposed aggregate finish with a stamped border and a stamped circle in the middle of the driveway. The slab was 6 inches thick with #4 rebar at 18 inches on-center in both directions. Sawcut joints with depths of about 1¼ inches were installed.
Now, there are several out-of-joint cracks. Most of the crack widths at the surface are less than 0.04 inches. They distract from the natural beauty of the driveway. I’ve sealed some of the cracks with a clear silicone product.
My client is now asking if his driveway should be removed and replaced. I do not want to tear out and replace the driveway because of these random cracks. Should we replace the driveway? Are there better products that I can use to seal or repair these cracks?
ANSWER: Cracking is an intrinsic property of concrete. It is a brittle material and when thermal and dry concrete shrinkage combine with restraint from the base and fixed objects, tensile stresses occur. When restrained-shrinkage stresses exceed the tensile capacity of the concrete, it cracks. So expect some out-of-joint cracking, even using the best construction practices.
For typical slab-on-ground construction, there are two options for controlling restrained-shrinkage cracks. One consists of tooling or saw cutting contraction joints into the slab. Sometimes called control joints, these joints are actually weakened sections that crack and open to relieve the restrained-shrinkage stresses. The other consists of installing steel reinforcement to control the width, not the location, of the cracks. Combining both options can provide excellent crack control.
Often, reinforced slabs on ground will develop narrow-width, out-of-joint cracks because the slab failed to see the contraction joints, the reinforcement passing through the contraction joints restrained the joints from properly cracking and opening, or both.
For your driveway, the reinforcement is controlling crack widths and growth, providing load transfer and maintaining vertical surface alignment across the cracks and joints, and maintaining the structural integrity of the driveway. Therefore, the driveway is not deficient and should not be replaced. While unsightly, these restrained-shrinkage cracks are controlled and should not affect the longevity of the driveway, especially if they are repaired.
Crack repair
Unrepaired cracks are prone to surface discoloration caused by salts from within the concrete being transported and deposited along the crack boundaries. After it enters the cracks, water picks up free salts from the concrete. Because of surface evaporation, salt-laden water moves upward and evaporates, leaving behind a deposit of salts that noticeably discolors the surface.
More unsightly than discoloration is edge spalling. This occurs when cracks are exposed to heavy vehicular traffic and harsh freeze/thaw conditions. Crack edges are easily broken off due to ice jacking or pressures associated with ice growth within cracks. As crack widths at the surface grow and edges become more ragged due to edge spalling, cracks become more unsightly and prone to continual deterioration.
Repairing or sealing cracks can minimize both surface discolorations and edge spalling. Depending on the repair material and technique used, sealing may also improve the appearance or hide the cracks. However, it is nearly impossible to make cracks disappear.
Before starting any crack repair, establish the objectives. For this driveway, objectives should include sealing to minimize water and other debris from entering the cracks, minimizing edge spalling from frost damage, and improving the concrete’s appearance.
Decide if the cracks are active or dormant. For active cracks, future movements are anticipated and repair materials should be elastomeric or flexible. Often, active cracks are really acting as contraction joints, and the repair material must be soft and flexible to accommodate future crack movements.
The material’s ability to stretch is typically given by the manufacturer as the joint movement capacity. For silicones and polyurethanes, common values for joint movement capacities are ±50 percent and ±25 percent, respectively. Reported capacities are based on the manufacturer’s recommended sealant reservoir dimensions (width and depth). For a 3/8-inch reservoir width, a typical polyurethane sealant should tolerate a crack width growth up to 3/32 inches without tearing.
Sealant reservoirs are cut or routed using special crack-routing blades, either square- or v-shaped. Cutting the sealant reservoir will increase the width of the crack at the surface of the slab and make the repair more noticeable. Repairing or sealing active cracks is essentially the same as preparing and sealing contraction joints.
Dormant cracks
For dormant or stable cracks, future movements are not anticipated, so repair materials can be harder and less flexible. Harder repair materials provide more edge support for the cracks than softer materials. More importantly, sealant reservoirs are not required so repairs are less noticeable. Only the loose or damaged concrete along the crack edges must be removed so the repair material can bond to sound concrete.
Your cracks are essentially dormant because of the steel reinforcement, so it’s not necessary to treat the cracks as working contraction joints. Selecting the repair material and technique can be primarily based on viscosity, color, UV stability, and potential for hiding cracks.
For your client’s driveway, a clear silicone sealer will work. However, there are other products that are specifically designed for repairing cracks in decorative flatwork that can create more aesthetically acceptable repairs. Consider using one of the new products that is harder than silicone sealants, can provide more crack edge support, is colored, and allows granular materials to be mixed into or topically added to the repair material. Type “concrete crack repair” into to a search engine and you’ll find many websites offering different materials and crack repair options.
Written by Kim Basham, PhD, PE, president of KBE Engineering, who specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. Email [email protected].
