Consultant's Corner: Repairing Cracked Concrete
By: Mark LeMay, A.I.A.
Last spring, my colleague, Steve Lucy, P.E., wrote in this space about the evaluation of concrete structures. He rightfully pointed out that besides death and taxes – concrete cracking is pretty much a sure thing. Cracks in concrete can be the result of issues during placement of the concrete, or conditions that develop after the concrete has cured. So what are some of the forces and mechanisms that cause concrete to crack? And what is the appropriate course of action to avoid problems down the line?
Once cracks in concrete occur, they become a conduit for moisture to infiltrate the concrete structure; or, in the case of slabs-on-grade, the underlying sub-grade. The assessment of your concrete structure by a structural engineer will help you to determine the probable causes of the cracks and whether you are dealing with cracks that are structural or cosmetic in nature. An engineer can also determine if the cracks are active (experiencing movement), or static (not moving).
ALL CRACKS ARE NOT CREATED EQUALLY
There are a variety of causes for concrete cracks. Shrinkage, curing, finishing, support and stress all result in different kinds of cracking and require various solutions.
Shrinkage - Typically, concrete “shrinks” during its hydration (curing) process at the rate of approximately ¼” for every 100 feet of length. To help control where concrete will crack, it is customary to create a grid pattern of weakened planes in the surface of the concrete, often called “control joints.” The most common method used to accomplish this “controlled cracking” is by sawcutting the surface shortly after the concrete is placed and finished. If the concrete is not sawed at the proper time, or if the cuts are not at least one-third the depth of the concrete, random cracks may develop (see photo at right).
Poor Placement of Reinforcing Steel - Reinforcing steel placed too close to the surface of the concrete can also result in cracking (see photo at left). Typically, for structural columns, beams, girder and walls, 1 ½” to 2” of concrete cover is necessary to avoid issues.
Differential Curing - In order for concrete to reach its maximum potential strength, it is extremely important to cure newly placed concrete properly. This allows the complete mass of concrete to hydrate (cure) at the same rate. If the exposed surface of newly placed concrete is exposed to dry or windy conditions, excessive, rapid drying of the surface can occur. The internal stress caused by the different curing rates within the concrete matrix can result in surface cracking.
Improper Finishing - In most instances, improper finishing occurs on slabs. Overworking the concrete surface during finishing can bring too much of the cement paste to surface, and result in a weak surface (see photo at right). Craze-cracking, or “alligator” cracking is an indication that the slab surface was overworked during the finishing operation.
Lack of Adequate Support - Slabs-on-grade are designed to be fully supported by the subgrade at all times. Improper compaction of the subgrade could lead to future settlement of the subgrade, and a loss of support for the concrete.
Excessive Stress - Movement of the structure, such as excessive bending, deflection or twisting, is the primary cause for stress-related cracks in concrete. Movement could be the result of natural causes, such as wind pressures, earthquakes, or expanding/contracting soils. Other causes could include excessive loading, impact or – heaven forbid – inadequate design.
Structural Cracks - Now that is a serious sounding phrase! Cracks that could potentially lead to a partial or complete failure of a column, beam, joist, slab or wall endanger the health, safety and welfare of a structure’s occupants and neighbors. Commensurately, cracks falling into this category need to be addressed in an expeditious manner, to maintain (or possibly restore) the structural integrity of the concrete structure (see photo at left).
Cosmetic Cracks - Most “moving” cracks are cosmetic, but still possess the potential for deleterious structural effects if not addressed. Cracks that permit water to contact reinforcing steel will, over time, lead to corrosion, and the undeniable result – an increase in volume of the rusting steel, and spalling of the concrete surface. In many instances, spalled concrete is the result of unrepaired cracks in concrete, and an indication that proper maintenance was not performed. It is the classic case of “Pay me now, or pay me later.” For this reason, concrete exposed to the elements should be diligently protected against this moisture intrusion.
Open cracks and control joints in slabs-on-grade that permit water infiltration to the supporting subgrade can result in differential swelling and shrinkage. Swelling causes an upward movement of the slab, which can lead to significant cracking (see photo at right).
SO HOW DOES A BUILDING OWNER OR PROPERTY MANAGER DEAL WITH CONCRETE THAT IS CRACKED?
That depends. Repairs to structural cracks should be determined by a Structural Engineer, and could include pressure injection of an epoxy liquid into the crack, pinning or stitching across the crack, and/or the application of a surface-applied composite strengthening material.
Moving cracks should be routed and sealed to keep water from penetrating the surface of the concrete, contacting the reinforcing steel, and/or percolating into the sub-grade. Control joints should be cleaned of all existing joint materials and packed with a closed-cell backer rod. Moving cracks and control joints should be sealed with a high-quality joint sealant using proper joint design (see detail above). Two-component urethanes can provide up to 50 percent movement capability, and silicones and silyl terminated polyethers can accommodate movement up to +100 percent/-50 percent. In traffic areas, it is very important to tool the sealant below the stop surface of the concrete. Failure to do so may cause the sealant to be pushed up above the surface of the concrete and exposed to tire contact when the crack/joint is at its narrowest width. Cracks and joints should not be sealed if they cannot be dried sufficiently to permit the installation of a flexible joint sealant.
Leaking cracks in underground (see photo at right) or water-retaining structures should be pressure injected using an expanding polyurethane foam or gel material that will form a flexible, watertight seal. Staggered holes are drilled at a 45° angle to intersect the crack at the mid-depth of the concrete member. One-way injection ports are set; the crack is flushed with water, followed by injection of the repair material. Leaking joints should be thoroughly cleaned and packed using oakum rope soaked in expanding polyurethane foam. Pressure injection behind the rope seal may also be necessary to further seal the joint against water penetration.
At interior locations, where small, hard-wheeled vehicles are used (such as warehouses), it may be necessary to seal cracks and joints in concrete slabs using a semi-rigid material (+/- five percent movement), such as a polyurea or a semi-rigid epoxy joint filler. In these instances, the material is placed flush with the top surface of the concrete to help support and protect the edges of the concrete from impact damage.
If unraveling of the concrete has already taken place along the edges of the crack or control joint, it may be necessary to remove a one inch deep strip of concrete along each side, and replace it with an “elastomeric” repair mortar (available from parking garage expansion joint manufacturers) prior to filling the crack or joint.
Help Is Available
Addressing concrete cracks can be difficult with all the variables at hand. The International Concrete Repair Institute (ICRI) is an organization dedicated to the advancement of knowledge in the concrete repair industry, and seeks to increase the potential for the execution of appropriate, durable, and aesthetically-pleasing repairs. Over the years, ICRI has developed technical guidelines to assist owners, engineers, contractors and material suppliers in selecting and specifying repair techniques and materials for a wide range of concrete-related topics. Their published guidelines include No. 03734 – Guide for Verifying Performance of Epoxy Injection of Concrete Cracks, and No. 03738 – Guide for Selecting Grouts to Control Leakage in Concrete Structures. Information on these and many other concrete repair resources are available on the ICRI website – www.icri.org.
Mark LeMay, A.I.A., is Project Manager in charge of the Condition Assessment Group for Jaster-Quintanilla, a structural and civil engineering and land surveying firm, with offices in Dallas, Austin, Houston and San Antonio. He has been a registered architect in the State of Texas since 1982. Mark can be reached at 214-752-9098 or at email@example.com.