Concrete Cracks in New Homes
Every year the Greater Ottawa Home Builders Association office receives calls of concern about cracks that have appeared in the basement walls or floors of a new home. This column is intended to explain some of the basic principles of concrete performance and educate new home owners on how to recognize normal from abnormal cracking. The first and most important thing to understand about concrete is that it comes with two guarantees: it will get hard, and there will be cracking. It is important to note that normal shrinkage cracking is the most frequent cause of consumer concern in the residential construction industry.
According to information provided on the Concrete Foundations Association of North America website, the most significant problem is that of consumer perception: that if cracking has occurred, the product has somehow failed. Customers need to be educated as to what might occur, what can be done about it, and what is considered acceptable.
Article 1.11 in the TARION Warranty Corporation (www.tarion.com) performance guidelines covers it this way:
Condition: Cast-in-place concrete foundation wall is cracked.
Acceptable performance/condition: Cracks resulting from normal shrinkage of materials are acceptable; cracks in excess of 6mm in width are not acceptable.
Warranty: One year – work and materials – Cracks resulting from normal shrinkage of materials caused by drying after construction are excluded from the statutory warranty.
Action: Walls with cracks exceeding the acceptable condition shall be repaired.
Remarks: Concrete walls naturally crack during curing due to shrinkage. Where lateral or vertical movement is evident, further investigation may be required. The colour and texture of a repaired area may not match the surrounding concrete.
While cracking cannot be prevented, it can be significantly reduced or controlled if preventative measures are taken. Cracking can occur before or after concrete hardens. It can be the result of one or a combination of factors, from drying shrinkage, thermal contraction, restraint (internal or external), to shortening, subgrade settlement, and applied loads. Read on to obtain a more in-depth explanation of these factors, and possible remedial action to be considered.
The Concrete Foundations Association of North America website carries the following detailed descriptions of contributing factors to concrete cracking:
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. Plastic-shrinkage cracks are most common in slabs and are relatively short cracks (may vary in length from a few centimeters up to 3m (10 ft) apart) 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. These cracks often penetrate to mid-depth of a slab.
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.
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 inch in every 10 feet of length. One method to accommodate this shrinkage and control the location of cracks is to place construction joints at regular intervals.
The major factor influencing the drying shrinking 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.
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. 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.
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.
In next month’s column we’ll discuss the most effective repair techniques for the main types of concrete cracks.