Why Concrete Cracks

Concrete develops cracks at quite the high frequency. The reasons for these imperfections are wide ranging. It is nearly impossible to produce concrete that stays free from cracks for a long period of time. The only way to prevent concrete from cracking is to leave it in the truck.  This is primarily due to the fact that concrete shrinks and expands as moisture and temperature change. The majority of concrete shrinks rather than expands. When concrete shrinks, it tends to crack at a fairly early age. While the resulting cracks are ugly and often a hassle to maintain, they do not impact the integrity of the concrete as a walking surface.

Intentional Cracks

There are control joints in slabs of concrete and concrete driveways that are pre-planned cracks. These joints function as a weak point to accommodate for shrinking concrete. This way, the concrete at the control joint is intentionally cracked instead of allowing for random parts of the concrete slab to crack over time, often at remarkable (and undesired) depths. Those who install concrete actually expect it to crack along these control joints at some point in the future.

Temperature Induced Cracking

When the temperatures rise and falls through the seasons, concrete changes in unison. The winter's freezing and thawing cycle is the cause of many concrete cracks. Concrete actually freezes around 18 degrees and then cracks when it thaws out towards the end of winter and the early spring. It is prudent to apply sand onto concrete surfaces instead of salt. Salt actually heightens the free and thaw cycle of concrete and exaggerates the cracking.

It is not just the harsh winter temperatures that cause concrete to crack. It also cracks when exposed to intense sunlight and heat. When temperatures rise, concrete expands and actually moves. Then when the sun goes down and temperatures drop, the concrete shrinks back to its initial size as it was exposed to the extreme heat. This problem can be alleviated with a high quality acrylic silicone sealer that is solvent based. Or, one can hire a concrete contractor to apply an epoxy injection or dry packing / routing technique to fortify the areas of the concrete that have begun to crack.

The Loss Of Moisture

When concrete is fresh in its plastic state, it can shrink when it suffers a fast loss of moisture. This can result from alterations in external temperatures, humidity and even the strength of the wind. Concrete is especially sensitive when it is freshly placed as its moisture can evaporate quite quickly and easily. When the moisture evaporates quicker than the “bleed water" (excess water) from the mix can replace it, the top level of the concrete shrinks.  In turn, this shrinkage causes a stress between the top level of concrete and the concrete below. This causes superficial cracks that typically appear on the very top of the concrete. Yet these cracks can eventually transition into full fledged cracks that extend deep into the concrete's layers.

This problem can be prevented with fog nozzles that saturate the air directly above the concrete's surface. Other techniques include the use of waterproof paper, wet burlap, plastic sheets or a liquid membrane that covers the top and decreases the amount of evaporation that takes place. Wind breaks can help by reducing the impact of wind. Sun shades should also be applied to drop temperatures to acceptable levels to avoid shrinkage cracking.

Hardened State Cracking

After concrete is initially placed, it can still suffer from “hardened state cracking” in a later stage. This drying shrinkage cracking typically occurs when too much moisture is lost from its cement paste. This can be avoided by altering the concrete mix's proportions to decrease the amount of water and increase the other components. Or, the reinforced steel can be modified. Some have found that control joints are effective at inducing cracking on predetermined weak points to prevent random and deeper cracks from forming elsewhere.

Increte of Houston has provided service to the Greater Houston area for over 30 years. We can answer any and all of your concrete questions. Call us at 281-499-3990 or send us an online message here. We are always happy to help!


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