One of the advantages of building with structural concrete is its repairability. The appearance of unexpected cracking, or other deterioration of the concrete is often the first sign that repairs are needed. In reinforced concrete such cracking is caused most often by the expansive forces created as rebar corrodes. Typically this is the result of inadequate cover, cracking construction joints, or permeable concrete. Other types of concrete deterioration can be the result of exposure to severe weather or chemicals, impact, overloading, or under design. In other cases, there are underlying soil conditions or ground water issues that can be contributing to the concrete problems.
The first step in developing a concrete repair plan is determining why the repair is necessary. Repairs may be needed to improve appearance of the concrete member or structure, to provide better protection of the embedded reinforcement, to meet load carrying requirements, or all three.
Some concrete repairs are primarily structural while others are primarily cosmetic. A structural repair, as defined in ACI 564R-04, “Concrete Repair Guide,” is one that “reestablishes or enhances the structural capacity of a member.” Other repairs that are deemed nonstructural often are made for aesthetic reasons. But these also may affect long-term structural integrity, such as when surface repairs are made to replace or renew the concrete cover protecting steel reinforcement.
For anything more than minor repairs, the contractor needs to work with engineering support. This will ensure that the proper cause of the concrete deficiency is identified and that the repair plan will have the intended effect. Additionally, ACI 564R-04 outlines general considerations for undertaking concrete repair as well as specific information about methods and materials.
Clues for the trained eye
An experienced investigator can learn a great deal about what's going on with various structural elements by observation. Telltale crack patterns, such as what may be observed on the upper surface of a parking deck around a column, can indicate potential punching shear failure or reinforcement that is too low. On columns, beams, or other vertical surfaces, diagonal cracks may be the result of shrinkage. However, they also may indicate excessive shear loads.
Excessive deflection, such as sagging cantilevers, should raise a red flag. Remember that relieving the load—both live and dead loads—must occur before any repairs are made. Repairs made without transferring those loads to temporary support while the work is done and the concrete hardens do not restore the member's load-carrying capacity.
Cracks that are curved typically indicate some underlying problem, such as differential settlement under a slab on grade. Basically any strange type of crack should raise a red flag and be investigated further.
When concrete spalls, it happens for a reason. Most often spalling indicates rebar corrosion, but may come also from aggregate problems such as alkali silica reaction. In either case, simply patching the surface will not provide a long-term solution.
Understand the repair
Most repairs involve several components, each of which must be treated appropriately for the repair to be successful.
Rebar. Expose all corroded surfaces. Clean and coat before placing new concrete. Then remember that the concrete cover that is placed over the rebar is what will provide protection for the future against water penetration and further corrosion.
Again, note that just patching up the concrete over rusty rebar makes the situation worse. The new patch may eliminate drainage paths that previously allowed the concrete to dry and the corrosion to slow.
Concrete. Proper surface preparation is absolutely essential. All unsound material must be removed without introducing additional cracking in the remaining substrate. ACI 564R-04 provides a good discussion of various demolition methods falling under the general categories of blasting, cutting, impacting, milling, and hydrodemolition.
Form-and-pour cementitious formulations provide the most predictably successful concrete repair materials. Unless some specific performance is necessary—such as exceptionally fast setting or placement without forms—experienced repair contractors generally try to match the existing concrete and tend to use simple, cementitious-based materials in lieu of more exotic repair materials.
Surface treatment. Remember that if you seal up a concrete surface that moisture has been flowing through, the pressure remains. For example, if you seal a retaining wall to eliminate seepage, the pressure that builds up is either going to have to be relieved in some other fashion or the lateral load will have to be supported. In colder regions, it also can lead to freeze/thaw damage and a failure of repairs.
Source: CONCRETE CONSTRUCTION MAGAZINE
Publication date: February 1, 2008 By Tom Klemens
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