AESTHETIC REPAIRS TO CONCRETE WITHOUT COATINGS by Michael P. Edison

Although concrete is often perceived to be a structural material, valued for its performance rather than for its appearance, in some applications it must meet both functional and aesthetic requirements. While this is obviously the case in architectural concrete applications, where color and texture of the concrete are an integral part of a building's design, aesthetics may also be of importance for repair projects involving a much wider range of buildings and structures. In particular, concrete buildings and structures that undergo repair may become unsightly as a result of the repair process, creating an aesthetic issue that was not a concern at the time of original construction. A common approach to addressing aesthetic issues has been to apply decorative coatings after repairs are complete, restoring a uniform appearance. For owners who are committed to long-term retention and maintenance of their properties, however, this approach has the negative consequence of introducing a recurring maintenance cost for periodic repainting. Eventually, after some number of repainting campaigns, it will also require that multiple coats of paint be stripped prior to further repainting, representing a significant additional cost.

An alternative is to undertake aesthetic repairs without the use of coatings. While this approach requires greater skill and care and somewhat increases initial repair cost, it saves the cost of an initial coating application and eliminates the cyclical maintenance costs associated with periodic reapplication. Aesthetic repairs involve the use of custom-matched repair materials, typically matching both the matrix color and aggregates in the original concrete. The process

begins by collecting samples of the existing concrete and cleaning them prior to color-matching. The samples should be chosen carefully in order to represent the full range of colors, textures and

surface conditions present in the building or structure. Matching the samples should then begin by identifying a suitable binder for the repair system.

The owner of these architectural concrete buildings in New York City avoided the more than half-million dollar cost of coating by using color-matched repair mortars in 1995. The cost of subsequent recoating maintenance has also been avoided.

For most repairs to modern concretes, a combination of suitable white and grey portland cements can be used to replicate the original binder (Ref: ASTM C150, ASTM C1157). It should be noted, however, that most 19th Century and some early 20th Century concretes were based on binders other than portland cement, such as Natural Cement (Ref: ASTM C 10). Repairs to these older concretes are best performed in-kind, using a Natural Cement binder to better match both appearance and performance. Portland cement was predominant in the 20th Century, however, and is the most likely candidate for use in repairs to 20th and 21st Century concretes. Because cements vary widely in color, it is important to select a cement that is as close as possible to the matrix color in the existing concrete. In some cases, it may be best to examine several different brands of cement. Because ordinary portland cements have trended darker in color in recent years, it may be necessary to blend with white portland cement to achieve a suitable lightness/darkness "value". Once a suitable cement has been identified, the entire quantity required should be purchased at one time, as there is no guarantee that different lots of cement will match each other, even when purchased from the same manufacturer. Often, this type of matrix-matching can be performed without the use of pigments, assuming the original concrete was also not pigmented. Small quantities of pigment can be used to slightly adjust matrix colors, if necessary, and use of excessive amounts of pigment should be avoided. ASTM C979 limits pigment additions to 10% of cement weight for iron oxide pigments and 2% of cement weight for carbon black, as levels in excess of these limits can result in significant strength reduction and increased water demand and shrinkage. Full color saturation, the point at which further pigment additions will have little or no further impact on concrete color, is typically reached at about half these maximum levels, however. While concentrations close to the saturation point may be appropriate for matching pigmented architectural concretes, much lower levels would typically be used to adjust the match to an unpigmented matrix. Furthermore, excessive reliance on pigments to achieve final color can result in artificial appearance, and significant differences between the wet and dry appearance of repairs as compared with the original concrete. The use of admixtures in aesthetic repair formulations must be carefully considered at the outset. Some admixtures, notably latex modifiers, can have a significant impact on color and appearance, so if they are to be used, they must be included in all color matching trial formulations. Only UV-stable modifiers should be used, such as those based on 100% acrylic polymers, or the repair color will not remain stable. Other admixtures, such as air entraining or set accelerating additives, may also impact color. Matching of aggregates is of immediate importance in situations where there has been sufficient erosion of the cementitious matrix to expose some of the original aggregate. Over time, as erosion progresses on both original and repair surfaces, well-matched aggregates are an important factor in color-match retention. In most cases, sand and stone were local to the project site, and it may be possible to locate sources of similar materials. The older the structure, the more challenging this may prove, as quarries and river beds

that may have served as aggregate sources for older, historic structures may no longer be accessible. Precast concrete, particularly those with decorative exposed aggregates, can sometimes be exceptions to the rule, as these materials may have been transported from a greater distance. Once the required components have been identified, trial mixes must be made and cured under conditions representative of those that will be encountered on the project site. Water addition levels and curing must be controlled and consistent. Surface finishing techniques, such as scrubbing to expose some of the aggregates, are time sensitive and should also be demonstrated as part of a mockup process. In some cases, surface set retarders are used to facilitate exposure of embedded aggregates. Sufficient time must be allowed for the test mixes to cure and develop their final color before final recipe selections are made. For muted colors in unmodified matrices, a minimum of 7-10 days' curing is recommended, and test areas should be fully dry when observed. Small specimens of latex-modified mixes can sometimes be force-cured with warm air after a much shorter curing period, allowing earlier determination of final color. While this process can seem costly and complicated for some projects, for others the replication of the original mix can be a straightforward process. When original ingredients are easily sourced, as may be the case for more recent construction, replication mixes utilizing the same ingredients may be all that is required. Even more complex matches can be cost-effective, however. Particularly when repairs are being made to buildings and structures of significant scale, the cost savings associated with the elimination of coating application and maintenance can be an order of magnitude greater than the investment in producing aesthetic repairs. The process can also be simplified by having a company that specializes in the production of custom-matched repair materials perform the color-matching work. Mock-ups should be installed prior to large scale repair, so that adjustments may be made before approving a final recipe. Once a formulation is approved, the custom repair system manufacturer can produce the approved materials in the required volume with a high level of accuracy and consistency. Michael P. Edison is a chemical engineer and President of Edison Coatings, Inc. in Plainville, CT. For more than 30 years, the Company has been engaged in the production of custom-matched repair systems and coatings for concrete, stone and masonry.