Why Concrete Cracks and How to Address It Before Flooring Installation?

It is a common saying in the construction industry that there are two guarantees with concrete: it will get hard, and it will crack. Cracking is a natural characteristic of concrete, but when it comes time to install a final floor finish, those cracks must be managed correctly.

Ignoring cracks or attempting quick, superficial fixes before laying new flooring or applying a coating is a recipe for failure. This article explains why concrete cracks occur and the proper methods for repairing them prior to flooring installation.

Concrete cracks for several reasons, but the most common cause is shrinkage during the curing process. As concrete dries, the water evaporates, and the concrete mass shrinks slightly. Because the slab is restrained by the subgrade and its own weight, this shrinkage creates tensile stress. Concrete has high compressive strength but relatively low tensile strength, so when the stress exceeds the concrete's tensile capacity, a crack forms to relieve the pressure.

Other causes of cracking include:

• Thermal expansion and contraction: Temperature fluctuations cause concrete to expand and contract, leading to cracking if joints are not properly placed.
• Settlement: If the subgrade beneath the slab settles unevenly, the unsupported concrete above it will crack under the weight.
• Overloading: Exceeding the designed weight capacity of the slab, particularly with heavy machinery, can cause structural cracking.
• Premature drying: If the surface of the concrete dries much faster than the bottom (often due to wind or sun exposure during pouring), "plastic shrinkage" cracks can appear on the surface.

To manage natural shrinkage, concrete contractors cut control joints (also known as contraction joints) into the slab. These are intentional weak points designed to encourage the concrete to crack in a straight, hidden line within the joint rather than randomly across the floor.

When preparing a floor for polished concrete, an epoxy coating, or even rigid floor coverings like tile or timber, ignoring existing cracks is highly risky.

If you apply a rigid epoxy coating over a moving crack, the crack will simply "telescope" through the epoxy, ruining the finish and allowing moisture to penetrate. If you install large-format tiles over a moving crack, the tile or the grout will crack as the slab shifts. In polished concrete, untreated cracks can spall and chip under the friction of the grinding equipment, creating an unsightly and unhygienic flaw in the final surface.

Proper crack repair requires more than simply wiping a bit of filler over the surface. The repair method depends on the nature of the crack.

The first step in proper repair is to "chase" or route the crack. A professional providing crack repair in Christchurch will use a specialized crack-chasing diamond blade attached to a grinder (and a dust extraction vacuum).

This blade cuts into the crack, widening it slightly and creating clean, vertical sidewalls. This removes loose debris and gives the repair material a solid surface to adhere to. Trying to force filler into a tight, dirty crack is ineffective.

The choice of repair material depends on whether the crack is "static" (not moving) or "dynamic" (subject to movement).

For static cracks: High-strength, rigid epoxy fillers are typically used. These epoxies cure to be stronger than the concrete itself and can be ground perfectly flush with the slab. They effectively "weld" the crack together, providing excellent support for heavy loads and a seamless base for polished concrete or epoxy coatings.

For dynamic cracks and control joints: Polyurea or polyurethane joint fillers are often specified. These materials cure to a semi-rigid state. They are hard enough to support the edges of the joint under heavy traffic (preventing spalling) but remain flexible enough to accommodate the slight expansion and contraction of the slab.

The repair material is deliberately overfilled slightly above the surface of the slab. Once it has fully cured, a planetary grinder is used to grind the repair perfectly flush with the surrounding concrete. This creates a smooth, seamless transition that is ready for the final polishing stages or coating application.

While hairline shrinkage cracks are often cosmetic, wider cracks or cracks that show vertical displacement (one side is higher than the other) can indicate underlying structural or settlement issues.

Before undertaking any major flooring renovation, it is prudent to have the slab assessed by professionals who provide Concrete Sealing Services In Christchurch. They can identify the cause of the cracking and determine the appropriate repair methodology to ensure the longevity of the new floor.

Cracks in a concrete slab are a natural occurrence, but they must be addressed properly before any final flooring system is installed. Proper routing, the selection of the correct epoxy or polyurea filler, and precision grinding are essential steps.

Investing in professional crack repair provides a stable, seamless foundation that protects your investment in the final floor finish, ensuring it looks better and lasts longer.