Tech Talk: Concrete Homes: controlling cracks
By: Ed Sauter, CFA Technical Director
Part III: This six-part series focuses on details of today’s concrete homes. Over the past two years, this column has provided considerable information on the general benefits of concrete homes, the various methods used for construction and the performance characteristics. We now focus on strengthening the understanding of the decisions, details and results that can affect the quality achieved in the above-grade concrete home industry delivered by removable forms (RCFs). Since strongly entering the market nearly a decade ago, this method of construction offers an ever-expanding variety of architectural and practical construction solutions for today’s homeowner and designer.
Cracking in concrete is the number one concern in calls fielded by the CFA Technical Staff. It is also the area of greatest misunderstanding. In this article, I hope to shed some light on the causes of cracks, methods of reducing their frequency and width, and what to do when they appear. Notice I said when they appear. Two things can be said about concrete—its gets hard, and it cracks. Cracks in concrete, however, are not a serious fault unless they allow water to penetrate or they are unintended structural cracks in unreinforced walls.
Basic concrete technology
First, however, a very brief treatise on concrete. Concrete is a very strong material in compression. The 28-day compressive strength of concrete we use in everyday residential applications typically ranges from 2,500 pounds per square inch (psi) to 4,000 psi. In certain applications (high-rise buildings, etc.) special concrete mixes can attain strengths in excess of 10,000 psi. Concrete, however, is relatively weak in tension. In general, the tensile strength of concrete is about 10 percent of the compressive strength or 250 to 400 psi. This characteristic impacts nearly all concrete walls because they act like a simply supported beam when load is applied.
Most concrete walls are designed to span from top to bottom—from floor to floor, floor to footing, or roof to floor. The concrete on the side of the wall where the load is applied is placed in compression and the opposite side of the wall is in tension. If the tensile strength of the concrete is exceeded, a crack will occur. If the wall is reinforced with steel, the steel will carry the tensile component of the applied load. Because steel is very strong in tension compared to the concrete (40,000 psi to 60,000 psi), it does not take much volume of steel to carry the load. Structural reinforcement is placed in the wall to span between the points of restraint or attachment. Structural reinforcing steel is spaced closer than shrinkage steel and the location in the wall is critical (usually close to the inside face, opposite the applied loads such as earth or wind). Most residential concrete walls that are 9 feet tall or less do not have structural steel. The thickness of the concrete, 8 inches to 12 inches, is sufficient, even in tension, to carry the horizontal loads from all but the most aggressive soils.
Temperature and shrinkage cracks
Applied load is not the only force that will cause concrete to crack—in fact, it is usually not the reason residential walls crack. Most cracking in residential walls, both below and above grade, is caused by temperature change and by shrinkage as moisture evaporates from the concrete. Temperature and shrinkage cracks are similar in their manifestation, and methods for reducing the number and width of cracks are similar.
Temperature cracks can occur at any time, even after the concrete has reached its design strength. The cause is differential temperature in different parts of the wall. The exterior of a south-facing wall may be significantly higher than the interior in an air-conditioned, uninsulated structure. The reverse can be true in winter. A similar condition exists from top to bottom of a wall, in particular a basement wall, where the top might be exposed to negative-20-degree temperatures while the bottom sits comfortably at 50 degrees. Temperature differentials can induce tremendous stress in concrete elements. Little can be done in an uninsulated wall other than the installation of steel to more evenly distribute the stress and keep the cracks tight.
Most shrinkage cracks occur before the concrete has reached its full strength. Concrete is designed based on the strength it should attain in 28 days, but it takes time to reach that strength, and the concrete can be vulnerable during that period. Tensile force is applied to concrete as it contracts due to the evaporation of unused water. As the concrete attempts to shrink, cracks can occur. One method of limiting shrinkage cracks is to strictly control the amount of water used in the mix. It takes only a small amount of water for hydration, the process that makes concrete harden, to occur. Additional water is added to make the concrete flow-able. After the concrete has been placed and its initial set has occurred, excess water evaporates, leaving voids in the concrete. The concrete attempts to contract to fill the voids left by the water, resulting in shrinkage. The concrete is still very weak in tensile strength at this stage (50 psi to 100 psi—10 percent of the 500-psi to 1,000-psi compressive strength), which makes if very vulnerable to cracking. Most shrinkage cracks occur perpendicular to the length of the wall. The reason is that the wall is typically much longer than its height plus the wall, in particular the bottom, is constrained from movement by the footing. The vertical dimension of the wall is much shorter (thus less total shrinkage) plus the top is not constrained—it is free to shrink.
There are several approaches that can be employed to reduce cracking from shrinkage. The first should be a concrete mix where no more water is used than what is necessary to place the concrete. Concrete is usually delivered to the job site with a slump of around 3 inches. This is more than enough water to support hydration, but it would make placement and consolidation of the concrete very difficult. The slump is typically increased to between 4 inches to 6 inches for most wall applications to aid in the placement of the concrete. The problem is, the more water that is added, the greater the potential for cracks. Additional negative impacts of adding excess water include a reduction in strength, less resistance to freezing and thawing, and greater porosity. Fortunately, modern chemistry has given us some reprieve from the problems associated with adding water. Chemical admixtures known as mid-range water reducing agents (sometimes called superplasticizers) can be added to the mix to increase the effective slump without the negative effects of adding water.
The second method used to reduce the effect of cracking is the use of temperature and shrinkage steel. This is reinforcement added to the lengthwise direction of the wall. The purpose is to take the shrinkage force once the concrete has cracked. While it may increase the structural performance of the wall, it is important to remember that this is not structural reinforcement. It is typically placed in the center of the wall. The ACI Residential Standard calls for three horizontal rods in an 8-foot-high wall and four in a 9-foot-high wall. Above grade walls will often have more shrinkage steel because control of cracking is a higher priority in above grade applications. If appearance is an issue, tight cracks can be hidden by a good elastomeric paint.
A third method of reducing shrinkage cracks, when they are the types that occur before design strength is reached, is proper curing. Unfortunately, most forms are removed from the wall within 24 hours of casting, exposing the wall to the wind, sun, and other elements that will accelerate evaporation during the critical early strength-gain period. Leaving the forms on longer, covering the wall with polyethylene, or spraying the wall with curing compound are all techniques that will trap moisture in the wall while the strength is increasing. The stronger the wall before significant evaporation occurs, the fewer the cracks. Keeping the forms on for a lengthy period is not practical in most instances since the contractor needs to use them for their next job. One trick might be to see if your wall can be poured on a Friday—there is a greater likelihood that the forms might stay on your wall an extra day or two.
While crack from temperature and shrinkage can be unsightly, they are rarely a problem in regard to structural performance of the walls
Many foundation walls are cracked during the backfilling operation. The basic foundation wall is designed to be supported at the top and bottom (see figure A).
Figure A: Typical top and bottom restraint (i.e. footing connection and floor deck) for residential foundation walls designed to resist soil pressures.
This occurs after the above grade portion of the structure is in place but many foundations are backfilled before that occurs. If the earth or equipment used in the backfilling operation applies too much force, the top of the wall can be pushed in, causing severe cracking or even failure. Foundation walls should not be filled more than 4 feet above the level of the footing (assuming that there is bottom restraint) unless there are counterforts, offsets, corners, adequate bracing (see Figure B),
Figure B: Temporary bracing applied to foundation walls can provide adequate support for soil pressures before floor deck is in place.
or the wall segments to be backfilled are short (10–12 feet or less). A recent study of the Concrete Foundations Association on failures of basement walls documented a failure rate of foundation walls of less than 0.05% (41 walls in 200,000 basements), and all but one of those was from backfilling without top restraint in place, or from driving heavy equipment too close to the foundation.
Other sources of cracking
Heaving and settlement can also cause cracking in walls, but it is very rare. Concrete walls perform somewhat like huge, deep concrete beams, and they are very resistant to vertical loads resulting from frost heave or from settlement unless the settlement occurs over a very large area of the foundation.
Cause for concern?
Near-vertical cracks from temperature and shrinkage are not a structural concern. They can be unsightly, and if they allow water to enter the structure, they must be dealt with. A horizontal crack in an unreinforced (no vertical reinforcement) concrete wall is a concern. If this occurs, the contractor or an engineer should be consulted. The best fix for this type of problem is an epoxy injection. When performed correctly, an epoxy injected wall is often stronger than the uncracked wall.
Ed Sauter, email@example.com, is Executive Director of the Concrete Foundations Association and the Concrete Homes Council.