ASTM C476 Masonry Grout Mixtures

If a building’s components include reinforced concrete masonry construction, those hollow concrete masonry units (CMU) will get filled with masonry grout. Contract documents often require masonry wall construction that is not reinforced masonry to be filled with masonry grout; the reasons for this include increased fire ratings, acoustical performance, thermal storage capacity, etc.

Masonry Grout Defined

What exactly is masonry grout? It is a cementitious mixture used to fill cores or cavities in the CMU during masonry construction, and this mixture is quite fluid. Masonry grout is composed of cement, aggregate, lime (optional), fly ash (optional), and sufficient water to allow ease of placement in the field to ensure that all grout spaces are filled. The grout mixture can include air-entraining admixtures, but ASTM C476, “Standard Specification for Grout for Masonry,” does not require air entrainment. Masonry grout should comply with the requirements specified in C476, including the proportion criteria specified in Table 1.

Masonry grout should comply with the requirements specified in C476, including the proportion criteria specified in Table 1.Masonry grout is neither concrete nor mortar; those materials have distinct differences in water content and composition. Concrete differs from grout because it contains a much coarser aggregate and a significantly lower water-cement ratio. Concrete is placed with a minimum amount of water into non-absorptive forms. Conversely, grout is poured with a significantly higher water-cement ratio into what are essentially absorptive forms – masonry unit cells. The initially high water-cement ratio of grout is rapidly reduced because in-place masonry absorbs much of the water. Mortar, on the other hand, contains finer aggregates and only enough water to make it workable.

Fine Aggregate Used To Produce C476 Grout

The TMS 402/602 masonry code adopts ASTM C476; therefore, TMS 402/602 incorporates the code compliance criteria stipulated in C476. C476 requires that the fine aggregate used in grout mixtures comply with ASTM C404, “Standard Specification for Aggregates for Masonry Grout.” Table 1 of C404 lists grading requirements for the fine aggregate identical to the grading requirements of masonry sand (ASTM C144); therefore, most grout mixtures use masonry sand (C144) as the fine aggregate. C404 allows aggregates of gradations outside of those in Table 1 as long as they comply with the gradation criteria in C404, Section 4.

Types of Grout and Material Proportions

C476, Table 1 allows for two grout mixture types – fine and coarse grout. Fine grout uses only cement and fine aggregate, with a maximum aggregate size of 3/8 inch. Coarse grout uses cement, fine aggregate, and coarse aggregate (crushed stone or pea gravel). The maximum aggregate size for coarse grout is 1/2 inch. All aggregates, fine or coarse, must conform to the requirements of C404. Table 1 of C476 shows that the aggregates must be measured in a damp, loose condition. Both fine and coarse grout proportions require that fine aggregate proportions range between 2 1/4 to 3 times the sum of the volumes of the cementitious materials. Coarse grout proportions for the coarse aggregate portion of the grout require that the coarse aggregate be between 1 – 2 times the sum of the volumes of the cementitious materials.

Concrete ready-mix suppliers often furnish project masonry grout. Although C476 grout should be proportioned by volume (damp, loose condition), ready-mix suppliers often use software that batches grout materials by weight. The masonry grout should be designed per C476 volumetric methodology and then converted to equivalent weight proportions compatible with concrete ready- mix batching and delivery systems. The software used to generate the grout design mix is sensitive to the properties of concrete, but grout is not concrete. This can lead to insufficient water to hydrate the cement in the masonry grout adequately, as well as a cement factor that is too low.

Grout – Slump and Water Content

Grout for masonry construction is a high-slump material with a flowable consistency to ease placement and facilitate consolidation. Both the C476 specification and the masonry code (TMS 402/602) require grout to have a slump between 8 and 11 inches. There are some instances during masonry construction that the masonry code requires a minimum grout slump of 10 inches (TMS 602, Article 3.5D). Grout must be fluid enough to flow into the smallest grout spaces and around obstructions, such as reinforcing bars, joint reinforcement, anchors, ties, and small mortar protrusions (fins).

Slumps of between 8 and 11 inches concern those in the concrete world who think in terms of low slumps and low water-cement ratios. The higher water content of grout is critical to ensure that in- place grout has sufficient remaining water AFTER absorption by the masonry units for proper cement hydration. There are often instances where the placement of a low-slump grout in the masonry wall results in a grout without enough water present to allow for proper cement hydration. This can directly result in lower compressive strengths than the code requires.

Use of Fly Ash in Masonry Grout Mixtures

In some cases, Section 3 of C476 allows fly ash and other supplementary cementitious materials (SCMs) in grout mixtures in amounts of up to 40% of total cementitious materials. Fly ash can be used as a pumping aid or to provide a greater slump with less water. Using fly ash to replace some of the cement in the grout mix also offers economic advantages. However, one caution regarding use of this option is that most fly ash grout develops compressive strength slower than straight cement mixes. Therefore, one should consider this during cold weather construction and where early grout strength development in masonry walls is critical. Currently, two types of fly ash are used in masonry grout: Class C and Class F. Class C fly ashes are typically found west of the Mississippi, and Class F is found east of the Mississippi.

Using Masonry Grout Specified By Compressive Strength

C476 and the masonry code require a minimum compressive strength of 2000 psi for masonry grout. C476 allows for the grout proportions to be established based on 28-day compressive strength when the strength tests are performed in accordance with ASTM C1019, and the grout mixture was mixed to a slump of 8 to 11 inches. The grout mixture shall comply with Section 4 of C476 and other applicable requirements of C476.

Grout Production Methods

Various grout production methods are addressed in Section 5 of C476, including site-mixed and ready-mixed grout. Whether the grout is mixed on-site based on volume methodology or mixed on- site from factory pre-blended cementitious materials and aggregate by adding water, the grout mix materials and proportions should comply with the applicable requirements of C476.

Grout Testing

The masonry code requires masonry grout testing in the field to ensure consistency of grout strength. Field samples of the grout should be tested periodically per ASTM C1019, “Sampling and Testing Grout.”