If you have cracks or splits in a cement slab or on concrete steps, you can use concrete bonding adhesive to help cover them and repair the damage. It allows you to add or pour new cement onto an existing piece without having to first rough up the surface.
Without the adhesive, repairing concrete involves a lot more work. Using it is luckily not that difficult, and this how-to guide will give you everything you need to know.
Note: These steps outline the process known as a "paint-on" application. In most cases, this type of application will result in bond strength of about 450 psi. If you need industrial-strength repair, you can perform a "slurry coat" application by mixing the concrete bonding adhesive with Portland cement, which will result in bond strength of 1300 psi. However, this will take three weeks to fully cure and dry.
Clear the Surface
Remove any loose chips or stones from the surface area where you will be applying the concrete bonding adhesive. Also, make sure that the surface is relatively clean of dirt and debris by using a broom to sweep it away.
Prepare the Adhesive
Shake the container of concrete bonding adhesive vigorously for a couple of minutes before opening it. Then, pour a portion into a separate container and dilute with clean water; mix about one part adhesive with two parts water. The first application will serve as a primer for the final coat.
Prime the Damaged Concrete Surface
Take a paintbrush and apply the watered-down adhesive directly to the cracks or splits in the surface. Allow this to dry for a couple of hours, or according to the instructions included with your product, before continuing.
Apply the Concrete Bonding Adhesive
Once the initial primer layer has dried a bit, you can begin applying the final coat. Use the paintbrush again to apply the bonding adhesive—this time full strength—directly to the damaged concrete. When brushing it on, make sure to use a thick coat for the best results. Refer to the instructions on the container once again, and wait for the prescribed drying time.
Mix and Apply New Cement
If you'll be putting gypsum or finish plaster (instead of cement) on top of the concrete bonding adhesive, you should apply it while the adhesive is still tacky and not yet dry. Otherwise, continue with regular concrete when the appropriate dry time has passed.
Follow the directions on your new cement for mixing it, making sure you have a wheelbarrow or a bucket handy. Then, apply it to the top of your bonding solution. Smooth the surface with a trowel and allow it to sit and cure properly.
How to Use a Concrete Bonding Adhesive
How long does it take for concrete bonding agent to dry?
Concrete bonding adhesive takes two to three hours to fully dry, though the dry time varies based on how thickly the agent is applied and how humid the atmosphere is.
Is concrete bonding adhesive necessary?
Once dried, concrete has no natural bonding agents. To get the dry concrete to bond with fresh, wet concrete, you will definitely need a bonding agent.
Without this agent, the new concrete will not adhere properly and you will end up with a big mess on your hand.
Will new concrete stick to old concrete?
New concrete will not stick to old concrete without a bonding agent. With a good bonding agent, however, this is a fairly straightforward DIY project and the end result will look nice.
What happens if you don't prime concrete?
Concrete does not need to be primed before adding bonding agent, but you do need to prime concrete before you paint it.
What happens if you don't water concrete?
Concrete is wet when you place it because concrete must have moisture in order to spread and ultimately set properly. You need to add water to concrete because otherwise, it will dry too rapidly.
When concrete dries too quickly, it cannot set well enough to achieve its maximum strength. It is also highly likely to develop cracks that ruin the finish.