Diagonal Crack In Poured Foundation

CFA: Concrete Cracking Overview

Types of Cracks in Concrete

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A common adage is that there are two guarantees with concrete. One, it will get hard and two, it will crack. Cracking is a frequent cause of complaints in the concrete industry. The Concrete Foundations Association has produced a new flyer to help contractors educate their customers about the causes of cracks and when they should be a concern. A more detailed explanation of cracking is presented in this article.

Cracking can be the result of one or a combination of factors such as drying shrinkage, thermal contraction, restraint external or internal to shortening, subgrade settlement, and applied loads. Cracking can not be prevented but it can be significantly reduced or controlled when the causes are taken into account and preventative steps are taken.

Another problem associated with cracking is public perception. Cracks can be unsightly but many consumers feel that if a crack develops in their wall or floor that the product has failed. In the case of a wall, if a crack is not structural, is not too wide the acceptable crack of a crack depends on who you ask and ranges from 1/16 to 1/4 and is not leaking water, it should be considered acceptable. It is in the best interest of you, the wall contractor, to educate your customers that the wall will crack and when it should be a concern to them.

Cracks that occur before hardening usually are the result of settlement within the concrete mass, or shrinkage of the surface plastic-shrinkage cracks caused by loss of water while the concrete is still plastic.

Settlement cracks may develop over embedded items, such as reinforcing steel, or adjacent to forms or hardened concrete as the concrete settles or subsides. Settlement cracking results from insufficient consolidation vibration, high slumps overly wet concrete, or a lack of adequate cover over embedded items.

Plastic-shrinkage cracks are most common in slabs and are relatively short cracks that may occur before final finishing on days when wind, a low humidity, and a high temperature occur. Surface moisture evaporates faster than it can be replaced by rising bleed water, causing the surface to shrink more than the interior concrete. As the interior concrete restrains shrinkage of the surface concrete, stresses can develop that exceed the concrete s tensile strength, resulting in surface cracks. Plastic-shrinkage cracks are of varying lengths spaced from a few centimeters inches up to 3 m 10 ft apart and often penetrate to mid-depth of a slab.

Cracks that occur after hardening usually are the result of drying shrinkage, thermal contraction, or subgrade settlement. While drying, hardened concrete will shrink about 1/16 in. in 10 ft of length. One method to accommodate this shrinkage and control the location of cracks is to place construction joints at regular intervals. For example, joints can be constructed to force cracks to occur in places where they are inconspicuous or predictable. Horizontal reinforcement steel can be installed to reduce the number of cracks or prevent those that do occur from opening too wide.

The major factor influencing the drying shrinkage properties of concrete is the total water content of the concrete. As the water content increases, the amount of shrinkage increases proportionally. Large increases in the sand content and significant reductions in the size of the coarse aggregate increase shrinkage because total water is increased and because smaller size coarse aggregates provide less internal restraint to shrinkage. Use of high-shrinkage aggregates and calcium chloride admixtures also increases shrinkage. Within the range of practical concrete mixes – 470 to 750 lb/yd3 5- to 8-bag mixes cement content – increases in cement content have little to no effect on shrinkage as long as the water content is not increased significantly.

Concrete has a coefficient of thermal expansion and contraction of about 5.5 x 10-6 per F. Concrete placed during hot midday temperatures will contract as it cools during the night. A 40 F drop in temperature between day and night-not uncommon in some areas-would cause about 0.03 in. of contraction in a 10-ft length of concrete, sufficient to cause cracking if the concrete is restrained. Thermal expansion can also cause cracking.

Structural cracks in residential foundations usually result from settlement or horizontal loading. Most but not all structural cracks resulting from applied loads are nearly horizontal parallel to the floor and occur 16 to 48 from the top of the wall. They are much more prevalent concrete block construction. They can be brought about by hydrostatic pressure or heavy equipment next to the foundation.

Diagonal cracks that extend nearly the full height of the wall are often an indication of settlement. In either of the above conditions, an engineer should be consulted. Diagonal cracks emanating from the corner of windows and other openings are called reentrant cracks and are usually the result of stress build-up at the corner. Diagonal reinforcement at the corner of openings can reduce the instance of crack formation and will keep the cracks narrow.

Other procedures which can reduce cracking in concrete include the following practices.

Minimize the mix water content by maximizing the size and amount of coarse aggregate and by using low-shrinkage aggregate.

Use the lowest amount of mix water required for workability and placement; do not permit overly wet consistencies.

Use calcium chloride admixtures only when necessary.

Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks more important in slabs

Provide contraction joints at reasonable intervals, 30 times the wall thickness is a good rule-of-thumb.

Prevent extreme changes in temperature after placement and initial cure.

Properly place and consolidate the concrete.

Cracks can also be caused by freezing and thawing of saturated concrete, alkali- aggregate reactivity, sulfate attack, or corrosion of reinforcing steel. However, cracks from these sources may not appear for years. Proper mix design and selection of suitable concrete materials can significantly reduce or eliminate the formation of cracks and deterioration related to freezing and thawing, alkali-aggregate reactivity, sulfate attack, or steel corrosion.

For more information, refer to Design and Control of Concrete Mixtures, EB001, and Diagnosis and Control of Alkali-Aggregate Reactions in Concrete, IS413.

Foundation Crack Repair. If your home has a basement, chances are you have a crack in the foundation, through which water will leak eventually.

An overview of cracking behavior in concrete. Concrete Foundation Association PO Box 204 113 W First Street Mt. Vernon, Iowa 52314.

Tremendous forces can build up inside the wall due to any causes of cracks. When the forces exceed the strength of the concrete, cracks will develop. Each of these causes normally leave a signature in the type of crack it creates. Majority of these cracks are of little concern by themselves.

Shrinkage cracks in poured concrete foundation can be diagonal or vertical and are usually uniform in width. Sometimes these cracks have a V-shape less common, with the top of the crack looking larger and the crack getting smaller as it travels towards to floor and diminishing or stopping before reaching the bottom of the foundation wall. If the crack reaches the bottom, the crack might damage the building s footings and the crack might have a major impact on the foundation structure. Shrinkage cracks should not be ongoing nor of structural significance, though they may invite water entry through the wall.

In poured concrete foundations, shrinkage cracks are usually due to conditions at original construction: poor concrete mix, rapid curing or possibly other conditions. In any case, concrete shrinkage causes the concrete to develop internal stresses. Unless control joints were included in the wall or floor slab design, these stresses will cause the wall or floor is likely to crack in a classic concrete shrinkage pattern as the concrete cures.

Hairline cracks may develop in concrete foundations as the concrete cures. Hairline cracks do no cause problems with the stability of the foundation but do cause leakage problems. If the cracks appear shortly after pouring the concrete foundation, concrete may have been mixed poorly or poured too quickly. In poured concrete foundations, hairline crack frequently appear in the center of the walls because the wall corners have a greater stability.

Settlement cracks may appear when the underlying ground has not been compacted or prepared properly or if the subsoil was not of the proper consistency. A settlement crack may also appear as a random crack above areas where the soil of the subgrade was uneven after the concrete was poured. Settlement cracks are usually wider at the top of the crack than the bottom as the foundation bends over a single point, allowing differential settlement. This type of crack is usually continuous, and may occur multiple times in a wall.

Temperature and Shrinkage cracks are often vertical or diagonal. You typically see these cracks formed from a corner of a window, beam pocket, step down, or other opening. Cracks of this type are called reentrant cracks. These are very common and, unless they leak or show significant lateral displacement, are of no structural concern.

Vertical cracks often appear in multiples multiple cracks in one or more area. Vertical foundation cracks in poured concrete foundations that tend to appear nearly straight or wandering, generally even in width, intermittent, or more often straight is caused by shrinkage/thermal and are generally low risk. If there is significant vertical dislocation or signs of ongoing movement, you should consult a structural engineer. If the cause is shrinkage, it is probably less of a concern than if due to settlement. A vertical foundation crack due to earth loading or frost would be unusual. A larger vertical crack can occur when the construction contractors incorrectly prepare the concrete footings and when the wall had poor steel reinforcement as the workers poured the concrete for the wall. Cracks can also occur during the footing settlement.

Horizontal cracks found in the center of the wall are most likely caused by an applied load such as back fill around foundation compacted improperly or too soon, earth compacting as it settles, hydrostatic pressure against foundation due to high water table and poor drainage against the foundation wall, or heavy equipment operated too soon or too close to the foundation wall. Horizontal cracks found high up on the wall are most likely caused by frost damage. Sometimes these fine cracks are not noticed for years. In a case like this, a structural engineer should be consulted.

Diagonal cracks that are almost the full height of the foundation wall are usually caused by settlement. When there s a settlement problem with footing on one side of the wall, this settlement can also cause a diagonal crack. In a case like this, a structural engineer should be consulted. If the diagonal crack is wider at the top than at the bottom, then it may be caused by expansion clay soil or frost damage. If the crack is wider at the bottom than the top, there is likely a problem with the settlement beneath. Some uniform diagonal cracks are simply caused by shrinkage and will only cause water leakage problems. Diagonal cracks that emanate from a corner of a window and other openings are called reentrant cracks and are usually the result of stress built-up at the corner.

Structural cracks in residential foundations usually result from horizontal loading or settlement. Most but not all structural cracks resulting from applied load hydrostatic pressure or heavy equipment around foundation wall are nearly horizontal and appear 16 to 48 from the top of the wall. They are more common in block foundation walls.

Next time you re in your basement, take a look around at your foundation walls. You are bound to find a crack. Why. Cracks in basements happen.

The Foundation Crack Bible How to Identify, Diagnose Evaluate Types of Foundation Wall or Slab Cracks: this article provides a dictionary of different patterns.

Tremendous forces can build up inside the wall due to any causes of cracks. When the forces exceed the strength of the concrete, cracks will develop.

Do-It-Yourself Foundation Concrete Crack Repair Kits Foundation Crack Repair Made Easy for Homeowners. If your a homeowner that has cracks in your concrete foundation.