Concrete Cracks

Evaluation of Foundation Cracks

Various foundation problems can create unsightly foundation cracks. Shape, pattern, relationship to wall discontinuities and angles, placement of wall penetrations, frequency of occurrence, correlation with cracks in floors, and location in the wall (center, corner), as well as age of wall, length, width, continuity, relation to site conditions (rock, blasting, depth of backfill) are all suggested areas to look at when evaluating foundation cracks. Most foundation cracks occur within the first 5 years of being built. Usually these cracks are not structural problems.

• The Five W's (Why, Where, When, What and Who)
  Why, where and when does concrete crack, what is the solution and who can repair concrete cracks.
• Types of Cracks Temperature and Shrinkage cracks, Hairline cracks, Settlement cracks, Structural cracks, Vertical cracks, Horizontal cracks, Diagonal cracks and Structural cracks.
• Application Any type of concrete structure such as foundation walls and floors, concrete slab, around sewer pipes and tie rods, concrete driveway and pathway, concrete patio decks, garages and more.
• Concrete Crack Injection Procedures Easy step-by-step procedures to fully inject a concrete crack, around sewer pipes, conduits or tie rods using Flexomeric Concrete Crack Repair Kits.

Concrete is the product of mixing cement and water with aggregates (sand and gravel or crushed stone). To better understand concrete cracks and why they occur, it's important to understand concrete's basic ingredients.

Basic Components of Concrete

Cement - The foremost ingredient of concrete, cement acts as a binder when it is combined with water in a process called hydration. The resulting paste coats the fine and coarse aggregates to make a strong composite once the paste sets and hardens. Along with a variable of factors, the strength and durability of concrete is influenced by the quality of cement used.

Water - Water is required to activate the binding property of cement and to provide workability with the concrete such that it can be poured into holes. One other important variable involved with the appearance and strength of concrete is the amount of water used in the mix expressed as the water to cement ratio: All else being equal, the lower the water to cement ratio, the stronger the concrete and the lower the chances of cracking to occur.

Aggregates - Concrete is a composite of binder and filler. Water and cement form to create a binder, while aggregates make up the filler. Aggregates are categorized as coarse if they measure greater than 4.75mm and fine if they measure less than that. Using this standard, gravel and crushed stone are considered coarse while sand is considered fine. To achieve both workability with concrete as well as strength and durability, a certain mixture of fine and coarse aggregates has to be achieved. Concrete becomes more workable when smooth and rounded aggregates, like sand and gravel from riverbeds, are employed. But concrete has to be strong and durable as well, and the elongated, rough texture and angular profile of crushed stone make for stronger concrete although they require more cement to bind them together.

Because of various considerations with construction practices, exposure to elements, and the chemical and physical properties of components used, concrete cracking is bound to occur. Cracking, however, can be minimized by understanding its causes and by devising ways to mitigate it.

Concrete Cracking in Plastic and Hardened State

Generally, there are two types of concrete cracking: One occurs when concrete is in its plastic state, and the other when it is in its hardened state. In both instances, poor construction practices can also greatly contribute to the occurrence of cracking and construction failure.

Plastic state cracking can either be due to shrinkage or settlement, whereas hardened state cracking can either be due to drying shrinkage or thermal stress.

Plastic State Concrete Cracking

Plastic shrinkage cracking often results from rapid moisture loss. To mitigate this type of cracking (as cracks of this type often develop into full-depth cracks), fog nozzles can be employed to saturate the air above the concrete surface as high humidity and air temperature can contribute to rapid moisture loss. The use of plastic sheets or wet burlap on top of the concrete surface also adds moisture, whereas waterproof paper and sun shades slow down moisture evaporation from the finished concrete product.

Settlement cracking arises from concrete settling in its plastic state after initial placement. Vibration in the formwork also contributes to this type of cracking. To minimize voids when the mix is first placed, proper vibration should be introduced, and the flowability of the mix should be decreased (because concrete that is too pasty may result in cavities). Rigid formwork that resists vibration should also be used along with increased concrete cover.

Hardened State Cracking

Drying shrinkage cracking that happens when cement is in its hardened state occurs when moisture loss decreases the volume of concrete and the adjacent joints do not resist such shrinkage. To prevent this type of crack from occurring, the aggregate should be increased and the water content decreased, and the reinforcing steel and control joints should be carefully detailed so they are able to support areas that are prone to cracking.

Thermal stress cracking ensues when the concrete's surface area is exposed to different temperatures, as what usually happens when certain portions of the surface are completely insulated and some are partially protected. To prevent this from happening, allowances for movement of the concrete as it hardens should be considered as the formwork is put in place since a certain degree of temperature variation, especially in massive building projects, inevitably occurs.

Construction Practices and Concrete Cracking

Poor construction practices also result to premature cracking in the finished concrete product. Often, this happens when too much water is added to the mix to make concrete more workable (or to save cost on cement), or when curing is prematurely terminated or hastened to meet deadlines. Shortened curing period often results to shrinkage when concrete is at its weakest. Improper placement of structural steel and other structural detailing can also result to concrete settling in its weakest spots, resulting to preventable cracks.