Concrete Surface Prep Part 1: Unsound Concrete

The problem with concrete is that you never know what it will do when you hit it with an abrasive blast.

Fines and cement paste break away easily enough, exposing the harder aggregate. Unlike steel, the compressive strength of concrete is not uniform, so blasting won’t impart a consistent surface profile or anchor pattern. A blasted concrete surface is a cratered landscape of cavernous pores and capillaries.

For proper bonding to occur, repair mortars, concrete overlays and coatings must penetrate and fuse into the substrate’s capillary network. When the network is blocked, blasting opens it up, exposing the aggregate and roughening the landscape.

But with no peaks and valleys to press tape to and measure by depth micrometer, how does the blaster know when he’s achieved the specified surface profile? How is it even possible to specify a profile for concrete?

There are other problems. Not only must a properly prepared concrete surface be adequately roughened, it must also be sound and contaminant-free. But due the relatively porous nature of concrete, contaminants often penetrate deep into the surface, weakening the substrate to a depth beyond the effective reach of abrasive blasting. For other applications, like decorative finishes, only the superficial, friable layer of hardened cement paste should be removed. What do you do when abrasive blasting is overkill or underkill?

This guide to concrete surface preparation walks you through the solutions to these problems and more. But first, let’s understand how and why concrete can fail, and how it can be made sound again.

When a coating, overlay or repair mortar is applied to unsound concrete, the bond between the two materials may hold but when pressured, unsound concrete will simply detach from the slab. Cracks, microcracks, blisters, scaling, spalling, or delamination are symptoms of unsound concrete that must be removed, then patched, before surface preparation can proceed.

Carbonation

Carbonation occurs when carbon dioxide penetrates the surface through pores and microcracks, and reacts with moisture and calcium hydroxide in the cement, forming calcium carbonate. This initially hardens the concrete, but it also reduces pH from 13 to 8. This reduction in alkalinity leaves embedded steel rebar vulnerable to corrosion.

Rebar Corrosion

Rust is a hydrated iron, and is more massive than the steel it replaces. The increasing rust mass creates tensile stress, causing concrete to crack and spall.

Chemical Attack

Sulphates leach into concrete through the groundwater, react with hydrated compounds and expand, causing mechanical failure. Chemicals can also soften, erode and dissolve cement paste.

Fire Damage

Concrete won’t burn, but high heat causes the concrete to lose the majority of its compressive strength, flexural strength, and elasticity. Expanding water vapor trying to escape can cause spalling.

Overloading/Impacts

Placing concrete under a heavy load, especially by impact, causes microcracking and cracking. Impact tools can cause extensive microcracking.

Over-wet Concrete

Too much water in the mix leaches cement to the surface. The surface will dry before setting occurs, causing shrinkage, cracks, reduced compressive strength, and laitance.

Visually inspecting the surface will reveal the worst of the damage: the cracks, spalling, scaling and crushed delaminations, but the full extent of the unsound concrete is not always apparent.

Perhaps the easiest thing to do is to tap around visibly weak areas with a hammer. If the head of the hammer bounces, the concrete’s compressive strength is good. If the hammer indents and pulverizes the surface, the concrete is unsound and should come out.

Dragging a screwdriver across the surface can also expose unsound concrete. If it leaves a shiny white streak, the concrete is sound. If it scratches off powder, the concrete is unsound.

Cases where pockets of air, water or unwetted material have formed under the surface are not so easy to detect: the surface may be sound enough to pass the hammer or screwdriver test, but structurally compromised. Listening to the sound made when tapping the concrete reveals areas of differential density. On large surfaces, these hollows can be detected by dragging a chain over the surface, or by using equipment that flogs the surface with an array of chains.

Once the extents of unsound concrete have been outlined, removal can begin.

Voids are not the only surface defect that require fixing. Projections that extend past the dry film thickness of a coating can cause the coating to fail, especially around sharp edges. When the overlay is a self-levelling compound, it may require an excessive amount of material to adequately cover projections. Mortar splatter, fins, ridges and other protrusions exceeding 1/16th inch should be ground down or chipped away.

Before patching with a cementitious product, the substrate should be in a saturated surface dry (SSD) condition. This entails spraying it with a hose then wiping it off, so that the pores are saturated but no free water remains on the surface.