Designing grout mixes Tested, custom-designed grout mixes provide consistent quality By Bruce A. Suprenant and Jeffrey L. Groom ant a grout mix with adequate strength, pumpability, and low cost? Then head to your nearest testing laboratory and ask for a grout mix design. A lab will help you establish design criteria and make trial batches to meet those criteria at the lowest material cost. Then you can have the grout delivered ready to use by a ready mix company. Although some contractors still try volume proportioning in the field, most specifications don’t allow this anymore. W Design criteria Unfortunately, most specifiers and contractors don’t give adequate attention to the design criteria for a grout mix. As with a concrete mix, the design criteria for a grout mix are specific for each job. A grout mix that worked well on the last job may not be suitable for the next job. Don’t go overboard on the design criteria, though. Use two, or at most, three grout mixes on a single job. Any more than this requires careful planning and attention to make sure the right mix is placed in the right location. Consider the following design criteria. Compressive strength. Most specifications require grout to have compressive strength equal to or exceeding the specified compressive strength of the masonry, from, but not less than 2000 psi. The compressive strength is measured in accordance with ASTM C 1019 (Ref. 1). Durability. The durability of grout is seldom an issue. The grout usually is protected from moisture saturation and thus not susceptible to freeze-thaw deterioration. If freeze-thaw is a problem, add an air-entraining admixture. To minimize rebar corrosion, con- sider using a corrosion-inhibiting admixture (calcium nitrite). Grout space. According to ASTM C 476 (Ref. 2), grouts are classified as fine or coarse depending on the maximum aggregate size. If the maximum aggregate size is 3⁄8 inch or larger, then the grout is classified as coarse; less than 3⁄8 inch is classified as fine. Grout must flow easily into confined spaces, so the smaller the space, the smaller the grout’s maximum aggregate size must be. However, coarser grout is more economical. It shrinks less and requires a smaller proportion of cement. Therefore, coarse grout is preferred wherever conditions allow it. Table 1 shows recommended grout types for various grout spaces. Based on this table, two different grout mixes probably would be used for a project that required grouting of both a collar joint and cells of masonry units. Consistency. Grout needs to be very fluid to be pumped or poured, to flow around reinforcing steel, and to fill a cavity leav- ing no voids. Although a flow cone or flow table can be used to measure consistency, most test labs use a slump cone. The grout slump should be between 8 and 10 inches. Use about an 8-inch-slump grout for masonry units with low absorption and about a 10-inch-slump grout for masonry units with high absorption. Be prepared to adjust the grout’s consistency for masonry units of different absorption capacities and rates, for varying temperature and humidity, and for different size cavities. Cavity size affects the surface area that comes in contact with the grout. The greater the surface area, the more water will be absorbed. Material selection The contractor selects a ready mix producer who chooses the suppliers of cementitious materials, aggregates, and admixtures based on product quality, service, and price. The contractor then must arrange to have material samples delivered to the test lab. If the ready mix producer changes material suppliers during the project, then a mix design should be made with any new materials. The test lab, if necessary, can check each product to make sure it conforms to ASTM standards. Cementitious materials. Use portland cement meeting ASTM C 150 (Ref. 3) or blended cements meeting ASTM C 595 (Ref. 4). Almost all masonry grouts contain fly ash as a partial replacement for portland cement. Fly ash contributes strength, acts as a waterreducer, and costs less than cement. ASTM C 595 permits up to 40% fly ash by weight of the cementitious materials. Aggregates. Aggregates used in grout must conform to ASTM C 404 (Ref. 5). This standard defines gradation for both fine and coarse aggregate and sets limits on deleterious substances (lightweight and friable particles), organic impurities, and soundness. Although ASTM C 404 limits the maximum aggregate size to 3⁄8 inch, some engineers allow up to 3⁄4-inch aggregate for grouting columns, pilasters, or large voids. The larger aggregate takes up more volume, reduces grout shrinkage, and requires less cement for equivalent strength. Some grout pumps, however, may not pump a 3 4⁄ -inch rock mix. Admixtures. ASTM C 476 doesn’t allow the use of admixtures in grout unless specified by the engineer or architect or approved by the contractor. Admixtures, however, can facilitate construction by accelerating grout set in cold weather and retarding grout set in hot weather. To maintain the water-cement ratios and high strength, some engineers specify a superplasticizer instead of adding water to increase slump. Shrinkage-compensating admixtures or grouting aids are the most common grout admixture. After placement, grout shrinks from 5% to 10% as the surrounding masonry units absorb water from the grout. To minimize this volume loss, a shrinkage-compensating admixture may be added to the truck mixer at the jobsite. These blended admixtures expand the grout, retard its set, and act as a water-reducer. This combination minimizes grout volume loss and gives workers more time to vibrate the grout before it stiffens. Table 2 shows the admixtures typically used for grouting, their applications, and costs. Trial batches After the architect or engineer sets the design criteria and the contractor obtains the material samples, the testing lab suggests mix proportions that meet the design criteria. Then the lab makes trial batches to determine the most economical grout mix. ASTM C 476 serves only as a rough guide to mix design. It expresses proportions in ranges to account for material differences throughout the United States. The test lab bases its trial mix proportions on experience. Though ASTM C 476 lists grout proportions by volume (Table 3), most specifications don’t allow batching of grout by volume in the field. Today, test labs and ready mix companies batch by weight. Volume proportions can be changed to equivalent weight proportions (pounds per cubic yard) by using the specific gravity of each ingredient. Don’t experiment at the jobsite A mix design that uses three trial batches costs from $800 to $1,200. Some contractors try to avoid a mix design to save $1,000, but they often pay for it later with a grout that has low strength and doesn’t pump. One ready mix supplier tried volume proportioning a grout mix in accordance with ASTM C 476 but found that the mix segregated when pumped. Don’t try to design a mix at the site; pay a test laboratory for a mix design. You’ll sleep a lot better. Bruce A. Suprenant is a consulting engineer and adjunct associate professor at the University of Colorado at Boulder. Jeffrey L. Groom is a senior engineer at CTL/Thompson, a materials engineering and testing laboratory in Denver. Race St., Philadelphia, PA 19103. 2. ASTM C 476, Standard Specification for Grout for Masonry, ASTM. 3. ASTM C 150, Standard Specification for Portland Cement, ASTM. References 4. ASTM C 595, Standard Specification for Blended Cements, ASTM. 1. ASTM C 1019, Standard Method of Sampling and Testing Grout, ASTM, 1916 5. ASTM C 404, Standard Specification for Aggregates for Masonry Grout, ASTM. A Mix Design Example A ready mix company requests a grout mix for filling block cells. The design criteria are: minimum 2500-psi compressive strength at 28 days, 8-inch slump, 580 to 700 pounds of cementitious materials, 40% fly ash replacement of total cementitious materials, and total entrapped air from 0.5% to 1.0%. The ready mix company submits these materials for use: Type I-II portland cement, Class C fly ash, sand meeting gradation re q u i rements of Size No. 1, and coarse aggre g a t e meeting gradation re q u i rements of Size No. 8. The test lab mixes and evaluates three trial batches (Table 1). All three mixes meet the design criteria. However, the lab recommends a compressive strength overdesign of 1200 psi. (Although overdesign is required for concrete mixes, no standards have been established yet for grout.) So now the grout must achieve at least 3700 psi. Tr i a l Batches 2 and 3 exceed this s t rength; Trial Batch 1 does not. To determine the minimum amount of cementitious materials (the most costly ing redient) needed to achieve 3700 psi, the strengths of the trial batches are plotted against their cementitious contents on a graph (Figure 1). Based on the resulting curv e , the lab determines that at least 625 pounds of cementitious material are needed to re a c h 3700 psi. It thus re c o m m e n d s the mix described in Table 2. 6. ACI 530.1/ASCE 6, Specifications for Masonry Structures, American Concrete Institute, P.O. Box 19150, Detroit, Ml 48219. PUBLICATION #M910218 Copyright 1991 The Aberdeen Group All rights reserved