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Concrete 101

What Are The Causes and Effects of Concrete Damage

Start by thinking about the basic characteristics of concrete which is strong in compression and weak in tension. This means a crack implies the concrete was in tension. A crack is always perpendicular to the direction of the tension. A standard shrinkage crack will be running diagonally from a re-entrant corner in a concrete slab. The concrete will shrink back in each direction from the corner and that diagonal crack is perpendicular to the direction of shrinkage. Look for a diagonal crack across the corner of a slab panel where it was run over by heavy traffic or the subbase was poorly compacted—the bending force creates tension across the top of the concrete slab.

Here are a Few Typical Concrete Problems and Their Causes:

Corrosion of Reinforcing Steel:

Steel rebar is protected inside concrete because the concrete is very alkaline which prevents rust. However if there are chloride ions in the mix, such as from deicing salts, the chloride destroys the “passivating layer” of alkalinity around the steel, allowing it to rust. Rust has greater volume than steel and the expansion presses against the concrete putting it in tension and causing it to crack and deteriorate. Chlorides get to the concrete through cracks or by simply penetrating through the concrete’s pore structure.

This doesn’t look so bad on the surface, but the rust tells us that the reinforcing steel is deteriorating. The only solution is to remove the unsound concrete and replace with new.

Disintegration Due to Freeze-Thaw:

Concrete is in fact porous, so if water gets in and freezes it will break off small flakes from the surface. The presence of deicing salts make matters worse. Called “scaling”, it can occur during the first winter and get worse over time. If severe, it can lead to complete failure of the concrete. Full air entrainment completely prevents scaling.

Alkali-Aggregate Reaction:

Alkali-aggregate reaction can create failure of concrete from the inside. AAR is a result of reactive aggregate in concrete that forms a gel around the aggregate particles. If the gel gets wet it expands and can destroy the concrete. There are now some lithium products on the market that can mitigate AAR.

Drying Shrinkage Cracks:

Most concrete has more moisture in it when placed than is consumed by the dehydration reaction. In the reaction, as water evaporates, the concrete shrinks—approximately 0.15 inches over 20 feet. If you could hold a slab up in the air while it shrank, it probably wouldn’t crack, but since it is on the ground it has friction on the subgrade and the shrinkage is resisted and cracks form. Joints in concrete control the drying shrinkage cracks.

Plastic Shrinkage Cracks:

When concrete is placed, if the surface is allowed to dry out before the concrete has gained strength, a pattern of cracks will form on the surface called crazing. These types of cracks are shallow and narrow and seldom a serviceability problem—although they can be objectionable to some owners, especially on decorative surfaces. The solution is proper curing and synthetic fiber reinforcement can reduce this.

Blisters / Delamination:

Bubbled concrete surfaces can be caused by prematurely finishing and sealing the concrete surface sealing in air and water. This can especially be a problem with air entrained concrete and heavy finishing equipment.

Cracks from Structural Loads:

Reinforced concrete works as follows: the steel reinforcement is completely useless until the concrete cracks. Until the concrete cracks the steel doesn’t start getting pulled to hold the concrete together. With no steel, inadequate steel, or loads too heavy (either during construction or in service), then the cracks can widen. For a slab on ground that may not be serious, but for a wall, beam or column, cracks may signal structural concerns.


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