Capping Damaged Slabs

Project Prep

Different levels of slab damage require different solutions from a thin resurface to a complete tear-out and re-pour. However, there’s a lesser-known, third option for damage between 1/2” and 2” deep – capping.

The process of capping is essentially the same as pouring a new slab, meaning good results are easy to achieve at a fraction of the cost of a new pour or specialty resurfacing mixes.

1. Ensure air temperature will remain between 40°F – 90°F for 24 hours before and after the project.
2. Estimate the amount of material required. One 60# bag of Sand Mix will cover about 5 ft² at 1” deep.
3. Use the Resurfacing Calculator  for fast results.
4. Make sure your slab is clean and free of any loose concrete – a pressure washer is a good way to get it ready for resurfacing.
5. Build a form around the area to be repaired to simplify the project.
6. The top of the form boards will indicate the height of the capped slab – anywhere from 1/2” to 2” higher than the area to be repaired.
   1. For example, if capping a sunken area of sidewalk, the top of the form should be level with the adjoining sidewalk slab.
7. Confirm overall slab level by leveling edge to edge and corner to corner.
8. Apply a bond enhancer such as Bond Strength Primer, Concrete Glue or Bonder & Fortifier to the repair area and allow to dry to a tacky texture.

Mixing

Capping a slab with Sand Mix adds a minor step to mixing, replacing part of the mix water with a bond-enhancing admixture to achieve ultimate performance.

1. When using bond-enhancing admixtures, DO NOT bring the slab to SSD before or after application—water can act as a bond breaker in this instance.
2. Wearing respiratory protection, pour Sand Mix into a 5-gallon bucket or wheelbarrow. If using a drum mixer, add a small amount of head water before adding the dry mix.
3. Replace part of the mix water with a bond-enhancing admixture.
   1. If using Bond Strength Primer, use 1 part primer to 2 parts water.
   2. If using Concrete Glue, use 1 part primer to 1 part water.
   3. If using Bonder & Fortifier, use undiluted straight from the bottle.
4. Mix for roughly three minutes until water is fully incorporated.
5. When correctly mixed, Sand Mix should hold together when packed into a ball.

Application

It’s time to pour! This stage is simple but ensure you move quickly. Results are best achieved with at least one assistant.

1. Pour concrete into your form and begin consolidating with a trowel.
2. Continue pouring until concrete reaches the top of the forms.
3. Select a very straight piece of scrap lumber to use as a screed—ideally wide enough to span the entire slab.
4. Drag the screed backwards in a back-and-forth sawing motion across the slab to level the surface.
5. Capped area should be no higher than the adjoining surfaces—Sand Mix cannot be feather-edged.
6. Use a bull float—or hand float for smaller repairs—to smooth out the surface. Technique is important here – keep the leading edge slightly elevated on the push, lift the handle to elevate the trailing edge on the return pull.
7. Take a break! Floating brings bleed water to the surface which must evaporate before moving on to final finishing.

Finish

Bring it home! Once the surface bleed water has evaporated and the concrete is thumbprint hard, you’re ready to move into final finishing.

1. Run an edging tool between the slab and form to encourage a clean release and create a rounded edge. Run the edger back and forth until the edge is smooth and fully separated from the form.
2. Cut control joints between the capped slab and adjoining slab(s) using a groover tool and a 2×4 as a straight edge.
3. Use a magnesium (mag) float to put a final smooth finish on the slab. This step will even out any imperfections left from bull floating, edging, control jointing, etc.
4. Mag floating is complete when there is a slurry of cement brought back to the surface—the effect looks like the bleed water brought to the surface after bull floating.
5. A hard troweled finish can be achieved by making additional passes with a steel trowel, allowing the slab to harden slightly between passes.
6. A broom finish—popular in outdoor slabs—is achieved by dragging a concrete broom across the slab. Make light, smooth strokes—just enough pressure to make an impression on the surface.