Concrete Restoration

Concrete is one of the most useful building materials, unique in how it provides structural strength, fire-resistance, and long-term durability in a cost-effective package. As buildings with concrete components age, they require maintenance to repair any deterioration in their original design and envelope waterproofing.

Concerns with concrete walls typically focus on cracks and visible spalling (delamination).

When concrete cracks, it’s often a result of the initial curing process (shrinkage) or caused by settlement in the months or years following. These cracks can allow moisture and water to enter the structure, causing damage. Most concrete buildings are built with a construction “cold joint” between the floors, and often these joints need to be sealed to maintain a waterproof envelope.

Additionally, since concrete is permeable, the natural process of moisture, air, and pollution moving through it can be a potential threat, affecting both the concrete and the reinforcing steel within, called “rebar.” As rebar rusts (oxidizes) it expands, cracking and weakening the concrete around it.

If enough deterioration occurs, it can affect the durability of the structure. The rusting rebar not only affects the steel but also weakens the structural strength of the concrete. Furthermore, cracks in the concrete can cause spalling, where pieces of concrete break off, creating a safety hazard from falling debris.

Concrete is one of the most durable building materials, but all concrete is permeable. It has tiny pores that allow moisture and air to get in and out. This moisture and air is a potential threat to the structural steel in the concrete, called “rebar” (which is short for “reinforcing bar”).

The rebar has some built-in protection that is provided by the concrete. Concrete is naturally alkaline and thus produces a film of oxides and hydroxides on the surface of the reinforcing steel. This durable and self-replacing film protects the reinforcing bar from air and moisture that migrate through the pores in the concrete. It can last for hundreds of years.

So what is the problem? Why might concrete deteriorate and “spall” (delaminate) on your building? The culprit is usually moisture, or more specifically, certain kinds of moisture.

Moisture from rain penetrates the surface layer of concrete. This is not, in itself, a problem. The damage comes when the concrete is attacked by either chloride – that is, the salts in the air combined with moisture (typical on buildings near salt water) – or “carbonation” – that is, the rain that is naturally acidic. One or both of these culprits break down the protective film surrounding the reinforcing bar. Typically, this happens when the reinforcing bar is too close to the surface of the concrete. Moisture will penetrate and affect the rebar if it does not have a sufficient layer of concrete to protect the steel. Optimally, the rebar should have a minimum of 1.5” concrete cover to protect it.

When acidic or salty moisture and the air gets at the rebar, the result is that the rebar oxidizes, or rusts. And as the rebar corrodes it expands – up to ten times the volume of the steel consumed. The pressure of the expanding bar spalls or breaks away the surrounding concrete.

The rusting rebar weakens the structural strength of the concrete and can even cause a safety hazard from falling debris.

Our typical repair process is as follows:

• Begin by identifying the areas needing repair, both visually and by sounding them out. The delaminated areas will sound hollow when tapped with a hammer.
• Using mechanical tools, chip out the delaminated concrete. If there is rebar that has significant exposure, remove the concrete around the rebar as well.
• Then “saw cut” the perimeter to create a square-edged repair area. This step allows a neat, sound patch with good repair strength, minimum depth, and no thin edges that will later fail.
• Next, mechanically clean the reinforcing steel, removing loose rust and scaling. If the rebar is too badly rusted, it should be replaced with new steel.
• To finish the concrete, begin by wetting the repair area (saturated surface dry) and apply a slurry coat of the repair mortar to the reinforcing steel and the area being patched. Follow this with a polymer-modified patching material, packed into the area and troweled. Once the repair is cured, grind the area as needed to match the adjacent finish.
• When the repair work is done, either paint or apply a clear sealer to these areas, to protect the concrete against future damage.

Cracks in concrete or stucco are often repaired as part of a larger restoration or painting project.

If the cracks are very thin, sometimes called hairline, they may be able to be coated with an elastomeric coating. Apply one or two coats of an elastomeric coating over the cracks, in a method called ‘stripe coating’. The elastomeric coating bridges the crack and remains flexible if the crack continues to have minor movement.

Wider cracks or those with ongoing movement need to be dealt with differently, usually by grinding the crack and applying a sealant (sometimes called caulking) to create a flexible waterproof solution.

The choice of sealant and method of application is critical. Reference your engineer’s specification or the sealant manufacturer for proper installation methods and sealant choices for the environment, compatibility, and movement anticipated.

Typically, the procedure is to grind out the cracks with a grinder. This opens up the width of the crack to a uniform depth and width and allows for proper sealant application.

Following this, clean the surface and apply a primer if required. Commonly, a bond-breaker tape is required at the bottom of the crack, to limit adhesion to the sides of the crack only.

Apply a sealant suitable to the crack size, filling the gap to the manufacturer’s specification. This sealant will remain flexible, and if the crack continues to have movement, it has enough elasticity to flex with the building movement.

After applying the sealant to the crack, smooth or “tool” the product to leave it flush with the surface.

In some applications, where the cracks are no longer moving, areas can be repaired using a cementitious material. The advantage of this option is that the cracks are less visible since the mortar can be tooled flush to the surface and then ground to a matching finish.