In structural situations, modulus of elasticity, or stiffness of the material, affects the load transfer. Stiffer materials deform less under a given load than lower modulus materials. Structural repairs require that the modulus of elasticity be similar to that of the concrete substrate, while nonstructural repairs can utilize a lower modulus. Low shrinkage compensation is desirable in the material to reduce cracking after placement, especially in large volume repair applications. Bond strength, permeability, water vapor transmission, sulfate resistance, abrasion resistance, tensile and flexural strengths may also be factors when choosing a repair mortar.
Often, even more than the repair mortar selected, proper surface preparation may be the single key factor to the success of the repair installation. All unsound concrete must be removed.
When preparing an area, simple configurations are best, preferably with square cut corners for load transfer. Avoid inside corners. Uniform depth of the area to be repaired is also desirable. Avoid feather edging, especially on any surface that will have traffic.
Cracks in concrete generally exhibit movement. Filling a dynamic or moving crack with a rigid repair mortar will only result in the area recracking. The movement must be stopped or relieved through a control joint to manage the cracking. Cracks can be welded together with an epoxy; however, failure to stop the movement or install control joints will also result in cracks reappearing.
Most concrete repair materials require the substrate to be SSD (Surface Saturated Dry). Prior to installing a cement-based repair mortar, the substrate should be saturated with water. However, puddling or standing water must be removed to keep the material-water-cement ratio intact.
Another key is the placement of a bond coat. To the saturated area, just prior to final placement, place a small amount of the mixed mortar onto the area and scrub it well with a stiff brush into the area. Do not add extra water or liquid to this scrub coat, as that will increase the water-to-cement ratio, creating a weak bond line at the repair.
While the scrub coat is still with moisture, install the repair mortar. Consult the product's data sheet for proper mixing instructions and depth of installation requirements, as well as any special application instructions, finishing techniques or curing procedures.
David L. DeBruler (DaveDeBruler@DaytonSuperior.com) is technical service manager for Dayton Superior Corp., Chemical Division, and J.V. "Van" Crary (VanCrary@DaytonSuperior.com) is marketing manager for Dayton's Restoration and Decorative Coating Programs.