Introduction
Concrete cracks are a common issue that can occur in driveways, garage floors, patios, sidewalks, and other areas. Cracks can range from minor hairline fractures to wide gaps and faults in the concrete. Left unaddressed, cracks will continue to expand and cause further deterioration. Repairing cracks is important for aesthetic reasons and to prevent long-term damage.
Small cracks are inevitable as concrete ages, shrinks, and settles. However, major cracks or faults indicate underlying problems that need to be addressed. Causes can include ground movement, freeze/thaw cycles, improper concrete installation, excessive loads, and poor drainage. Identifying the root cause will help determine the right repair strategy.
Repairing cracks properly involves assessing the type and extent of damage, preparing the area, sealing, filling, and reinforcing cracks, and applying protective sealants. With the right materials and techniques, concrete cracks can be repaired to restore appearance and structural integrity. This guide covers step-by-step instructions for effective DIY concrete crack repair.
Types of Cracks
Concrete cracks can be categorized into a few main types based on what causes them:
Shrinkage Cracks
Shrinkage cracks occur as concrete dries and shrinks during the curing process. They are often very fine and barely visible. Shrinkage cracks are unavoidable but can be minimized by proper concrete mix design, placement, and curing techniques.
Settlement Cracks
Settlement cracks form when the soil or base under the concrete settles unevenly, causing stress in the concrete. Settlement cracks often run vertically or diagonally, following the direction of movement. Improper soil compaction is a common cause.
Impact Cracks
Impact cracks are caused by an applied force, such as heavy loads rolling over the concrete, hammer blows, or objects falling on the surface. They often appear as star-shaped cracks radiating out from the point of impact.
Crazing Cracks
Crazing cracks are very shallow, hairline cracks that develop across the entire surface of concrete. They are caused by a combination of concrete shrinkage, poor finishing, and rapid evaporation during curing.
Corrosion Cracks
Corrosion cracks occur when the steel reinforcement bars inside the concrete rust and expand, causing pressure. These cracks run parallel to the steel bars. Moisture penetration and chloride attacks are common causes.
Freeze-Thaw Cracks
Freeze-thaw cracks happen when water inside the concrete expands as it freezes and thaws. Over time, this action causes cracks and flaking on the surface. Concrete that lacks proper air entrainment is susceptible.
Causes of Cracks
Cracks in concrete can occur for various reasons both during and after installation. Understanding the root causes is key to properly repairing and preventing future cracks.
Some common causes of cracks include:
Drying Shrinkage
Concrete shrinks as it dries and hardens after installation. If restrained too early, this natural shrinkage can cause cracks due to internal stresses. These cracks often show up within the first few weeks after pouring.
Freeze-Thaw Cycles
In cold climates, water can seep into small pores and cracks in concrete. When the temperature drops below freezing, the water expands as it turns to ice and exerts pressure, forcing cracks to widen over time.
Load Stress
Concrete slabs and structures can crack under loads that exceed their load capacity. Vehicular traffic, heavy equipment, changes to the foundation or soil, and seismic activity can all cause load-related cracks.
Corrosion of Rebar
The steel rebar inside reinforced concrete can corrode when exposed to water and air over time. As the rebar rusts and expands, it causes cracks and spalling in the concrete.
Settlement
If the soil or base under a concrete slab settles unevenly, it can cause cracks due to shifting stress. Settlement cracks often follow a line or pattern.
Poor Jointing
Concrete naturally expands and contracts with temperature changes. Without proper joints, this movement can cause cracks at weak points. Control joints are saw-cut or tooled into concrete to guide cracks along desired lines.
Proper crack repair starts with identifying the root cause. This helps address underlying issues and prevent continued cracking problems in the future.
Assess the Damage
Before repairing concrete cracks, it's important to thoroughly assess the damage to determine the best repair method. Here are some key things to check:
- Width and depth of the crack - Measure the width at the surface and the depth. Narrow cracks less than 1/4 inch wide may only require sealing, while wider cracks likely need filling and reinforcement. Deeper cracks suggest a more serious problem.
- Location of the crack - Note where the crack begins and ends. Cracks near the edges or joints of concrete slabs are more common, while cracks in the middle may indicate a weaker spot. Check if the crack follows the line of rebar reinforcements.
- What's underneath the crack - Try to determine if the crack extends fully through the slab or only partway into the thickness. Using a hammer to tap along the crack can help determine this. Knowing how deep it goes helps assess the repair needed.
- Cause of cracking - Consider factors like recent weather or earth movements that may have contributed to cracks. Understanding the cause will help prevent future cracks in the same spots.
- Is the crack active or dormant? - Active cracks that continue to shift and expand will require more extensive repairs. Dormant cracks that have stabilized may be easier to patch.
Thoroughly examining the crack while it's open allows the selection of the right concrete crack repair materials and methods.
Prepare the Surface
Before applying any repair materials, it's crucial to properly prepare the concrete surface. This will help the repair last longer and bond securely.
First, thoroughly clean the crack and surrounding area. Use a wire brush, broom, vacuum, air compressor, or water hose to remove all dirt, dust, oil, grease, and loose concrete. Getting down to clean, solid concrete is vital.
Next, etch the surface using muriatic acid or another concrete etchant. This opens up pores in the concrete to accept the repair material. Follow product directions closely. Rinse thoroughly after etching.
If rebar is exposed, sand and clean it to remove rust. Apply a rebar primer to prevent future rust.
Finally, dampen the area before applying repair materials. This helps the patch bond better. The surface should be damp but not dripping wet.
Proper surface prep requires elbow grease but is essential for durable, long-lasting concrete crack repairs. Don't skip this step!
Choose Repair Materials
When repairing cracks in concrete, you'll need to select the right materials for the job. Here are some of the most common options:
Epoxy
Epoxy is a two-part adhesive that is extremely strong, durable and waterproof once cured. It comes in tubes that must be combined before application. Epoxy is best for repairing non-moving cracks and works well in hairline cracks or larger gaps. It can be tinted to match the surrounding concrete. Epoxy has a fast cure time of about 30 minutes to 1 hour.
Polyurethane
Polyurethane is another two-part adhesive that expands slightly as it cures, allowing it to penetrate cracks. It is flexible, making it a good choice for active cracks that may continue to shift and settle. Polyurethane comes in pourable and injectable formulas. It has a longer cure time of around 24 hours.
Cementitious Products
Cement-based compounds like hydraulic cement, concrete repair mortar, or patching compounds are also commonly used. These are powder products that are mixed with water before applying. Cementitious fillers are inexpensive and bond well to concrete. However, they cure more slowly than epoxies and may be prone to cracking over time. They are best for non-moving cracks and voids. Allow at least 24 hours of cure time.
When selecting a repair product, consider the location and cause of the crack. Active cracks will require more flexible fillers like polyurethane. Inactive cracks and spalls can be repaired with rigid epoxy or cement. Read product specifications to choose the right concrete crack repair material for your needs.
Apply Sealant/Filler
When repairing cracks in concrete, it's important to apply the right sealant or filler material to bridge the gap and bond the concrete back together. Be sure to follow the manufacturer's instructions closely when working with any concrete repair product.
For narrow cracks, you can simply apply a concrete sealant by squeezing it directly into the crack using a caulking gun. Cut the tip of the caulking tube at a 45 degree angle to help direct the sealant into the crack. Completely fill the crack, moving down its length and working the sealant into the bottom. Once filled, use a putty knife or spatula to smooth the sealant with the surrounding concrete.
For wider cracks or damaged areas, you'll need to use a concrete filler or patching compound. Epoxy fillers work well for deeper repairs, while cement-based fillers are good for shallow patches. Scoop some of the filler onto a trowel and press it firmly into the crack, overfilling slightly. Then, smooth it flush with the surrounding surface using a trowel or float. Be sure to follow the specified drying time before moving on to the next steps.
Properly filling cracks and voids is crucial for restoring structural integrity and preventing further deterioration. Take care to fully pack the repair material into the damaged area. Follow all safety precautions when working with concrete sealants and fillers.
Reinforce the Repair
Reinforcing the repair is key to preventing future cracking in the same area. There are a few methods that can provide added strength and durability:
Fiber Mesh
Fiber mesh can be mixed into repair mortar or concrete to reinforce it. The fibers, which are often made of plastic or fiberglass, help hold the material together and control cracking.
To use fiber mesh, first prepare and clean the repair area. Mix the concrete or repair mortar, then add the fiber mesh per the manufacturer's instructions and thoroughly combine. Apply the fiber mesh-reinforced material to the repair area. Smooth and finish the surface. The fiber mesh will provide internal strength to reduce cracks.
Rebar
You can drive rebar, or steel reinforcing bars, into holes drilled in the existing concrete around the repair area. This helps tie the old and new concrete together.
Drill holes just wider than the rebar, angled toward the repair area. Clean dust from the holes. Epoxy the rebar into the holes. Make sure the rebar extends at least 3 inches into solid concrete on all sides of the crack/repair. Then apply the concrete or repair mortar over the rebar. The embedded steel helps strengthen the area.
Bolts
For larger repairs, you can use bolts anchored into the existing slab as reinforcement. Drill holes on both sides of the crack and epoxy threaded rods or bolts into the holes. Use washers and nuts to tighten the bolts, closing up the crack. Apply repair material over the tightened bolts. Once cured, the bolts will provide strength across the repaired section.
Seal and Protect
After repairing cracks in concrete, it's important to seal and protect the surface. This will help prevent future cracking and deterioration.
Sealers
Applying a concrete sealer is highly recommended after crack repairs. Sealers penetrate into the concrete to provide a protective barrier against moisture and deicing salts. This helps reduce freeze-thaw damage. Acrylic, polyurethane, and epoxy-based sealers are common options. Select a water-based sealer for easier application. Solvent-based sealers may last longer. Apply the sealer per manufacturer's instructions using a paint roller or sprayer. Two coats are usually needed for best protection.
Protective Coatings
For added protection, concrete coatings can be applied over the sealer. Epoxy, polyurethane, and acrylic coatings add a layer of flexible and waterproof protection. These coatings help resist damage from sunlight, chemicals, impacts, and other exposure. They provide aesthetically pleasing, enhanced durability compared to bare concrete. Make sure the concrete is clean before application. Follow manufacturer instructions for applying the coating with a roller, brush, or sprayer. Anti-slip additives can be included for improved traction. Recoat every few years or as needed to maintain protection.
Properly sealing and coating repaired concrete will maximize the longevity of repairs and prevent recurring cracks and deterioration. Use quality products and apply them correctly for best results.
Prevent Future Cracking
Taking steps to prevent future cracking in concrete can save time and money on repairs down the road. Here are some tips:
Install Control Joints
Control joints are planned cracks that are put in place where you want the concrete to crack. They help relieve stress and control where cracks occur. Control joints should be installed to divide concrete slabs into sections no larger than 15 feet by 15 feet. Use a groover or saw to cut the joints into the slab within 24 hours of the pour. Cut the joints about 1/4 the depth of the slab.
Proper Concrete Curing
Allowing concrete to cure slowly helps prevent cracks. Keep the slab moist for 3-7 days after pouring by misting it with water, covering it with damp burlap sacks, or applying a curing compound. This allows the concrete to hydrate thoroughly and gain strength evenly. Rushing the curing process can lead to cracks.
Use Reinforcement
Adding reinforcement materials helps strengthen concrete and resist cracking. Steel rebar is commonly used, as well as wire mesh, synthetic fibers, or glass fibers. The reinforcement holds the concrete together when shrinkage stresses occur. Follow recommended spacing and depths when installing reinforcement.
Control Concrete Shrinkage
As concrete dries and cures, it shrinks slightly. To minimize shrinkage:
- Use less water in the concrete mix
- Add air-entraining agents to improve workability
- Avoid using high-shrinkage aggregates like limestone or bluestone
- Control joint spacing and depth
With proper materials, curing, and joints, it's possible to minimize cracks in concrete. However, some hairline cracks may still occur due to the nature of the material. Addressing them promptly can help prevent further damage.
For more DIY tips and tricks, DIYwithHelp has got you covered…
If you need some professional guidance for your project, be sure to sign up for a free account with DIYwithHelp.com. Once you have an account, you can search our site (or fill out a project request form) and connect with expert tradespeople who are willing to lend their advice over scheduled video-calls.