9-4 : GROUT FOR CONCRETE MASONRY

Introduction

Grout is a fluid cementitious mixture used to fill masonry cores or cavities to increase the structural performance of masonry. Grout is composed of cement, aggregate, and sufficient water to allow ease of placement and ensure complete filling of the grout space. The high initial water content of grout compensates for absorption of water by the concrete masonry units after placement. Thus, grouts gain high strength despite the high initial water to cement ratios.

Grout is often used to structurally bond separate wall elements together. This is most commonly seen in reinforced construction, where grout is used to bond the steel reinforcement to the masonry, allowing the two elements to act as one unit in resisting loads. Sometimes the collar joint of a double wythe masonry wall is grouted as one of the requirements for the two wythes to act compositely.

Grouting also increases the net cross-sectional area of masonry and allows walls to carry higher compressive and shear loads. It increases a wall's stiffness, increasing resistance to lateral loads as well. Masonry cantilever retaining walls are often solidly grouted to increase the wall's weight, and hence the resistance to overturning.

This TEK includes information about: the types of grout; requirements for each; grout admixtures; and procedures for testing grout. Information on grout mixing and placement is contained in Grouting Concrete Masonry Walls (ref. 1).

Specifying Grout

Grout for use in concrete masonry walls should comply with Standard Specification for Grout for Masonry, ASTM C 476 (ref. 5). The Standard defines two types of grout: fine and coarse. Fine grout contains sand as its only aggregate, while coarse grout allows pea gravel, or other acceptable aggregate, in addition to the sand. Aggregates for grout must comply to Standard Specification for Aggregates for Masonry Grout, ASTM C 404 (ref. 4), which includes requirements for grading, impurities, soundness, and methods of sampling and testing.

There is no appreciable difference between the strengths that can be attained by fine and coarse grout; the choice of grout type depends mainly on the minimum clear cross-sectional dimensions of the grout space and the grout pour height. Specification for Masonry Structures (ref. 2) includes criteria for the use of fine vs. coarse grout based on these parameters (see Table 1).

Coarse grout is typically preferred, since it is more economical to produce. Standard C 476 allows grout to be specified by either a proportion specification or by strength requirements. The proportion specification mandates grout proportions by volume of materials, as shown in Table 2.

When using the proportion specification, material volumes should be measured consistently throughout the job to properly control and maintain proportions of grout materials. The materials are then mixed for at least 5 minutes in a mechanical mixer with sufficient water to bring the grout to the desired consistency. All grout should be of fluid consistency to ensure that the grout space is completely filled. Specification for Masonry Structures requires a slump between 8 and 11 in. (203 and 279 mm), and the Uniform Building Code (ref. 9) requires a slump between 8 and 10 in. (203 and 254 mm). Studies have shown that adequate grout compressive strengths are achieved even when using such grouts in wet concrete masonry units.

When grout strength is specified, the grout strength should be about the same as the net area compressive strength of the concrete masonry units, but never less than 2000 psi (13.8 MPa) at 28 days. When grout strength is specified, the grout must be sampled and tested in accordance with Standard Test Method for Sampling and Testing Grout, ASTM C 1019 (ref. 7).

Admixtures

A variety of admixtures are available to achieve desired grout properties. ASTM Standard C 476 requires admixtures to be included in the specification or approved by the purchaser. In addition, antifreeze compounds, used to lower the freezing point of grout, are prohibited by the Standard. If temperatures are expected to be below 40oF (4oC) or masonry unit temperatures are below 40oF (4oC), the cold weather construction procedures of Specification for Masonry Structures should be followed to ensure masonry is not adversely affected. Accelerators are sometimes used to reduce these protection requirements.

Admixtures containing chlorides should not be used in grout. Chlorides will corrode steel reinforcement and can contribute to efflorescence in the wall.

Several admixtures are available which provide a combination of desirable characteristics, such as shrinkage compensating, plasticizing, and retarding. As with any admixture, manufacturer's directions and quantities should be carefully followed.

Superplasticizers

Superplasticizer admixtures are used to reduce the water content of the grout while maintaining high flow consistency. Superplasticizers are seldom used in grout for several reasons. First, grout is placed in absorptive molds (i.e., concrete masonry units), which provide a natural reduction in water content, unlike concrete which is placed in nonabsorptive molds, where water reduction is desirable to increase strength. Second, grouts containing a superplasticizer admixture must be placed within a limited amount of time after the admixture is added. This time limit may not be practical for all jobs.

Superplasticizers are, however, sometimes used when the masonry units and mortar contain an integral water repellent admixture, to compensate for the lower water absorption of the units and mortar. It has been demonstrated that there is enough hydrostatic pressure from the grout to overcome the lower absorption of units containing certain integral water repellents, although manufacturers should be consulted for more information.

Accelerators

Accelerating admixtures increase the heat of hydration of grout. They are used in cold weather to decrease the grout setting time and increase the rate of strength gain. This increased heat of hydration does not eliminate the need for cold weather protection requirements, but does reduce the length of time that a wall must be protected.

Shrinkage Compensators

Shrinkage compensating admixtures cause a slow controlled expansion of the grout which is intended to offset grout shrinkage due to the initial water loss to the masonry units. The expansion improves bond between the grout and the masonry units. These admixtures may be especially useful for high-lift grouting, where a large volume of grout is placed and consolidated at one time.

Retarders

Retarding admixtures are used in hot weather to keep the grout workable long enough for placement, consolidation, and reconsolidation. They may also be used when the grout cannot be placed right away, as may be the case when the grout will travel a long distance to the jobsite. Retarded grout should not be retempered with water.

Fly Ash

Fly ash is a by-product of coal combustion, and is not often thought of as an admixture in the same sense as chemical admixtures. Fly ash can be used in grout as a pumping aid, to provide a greater slump with less water. Fly ash can also replace some of the portland cement in the grout mix, which has an economic advantage since fly ash costs less per ton than portland cement.

Quality Assurance

Grout Slump

Two field tests are commonly performed for grout, the slump test and the compressive strength test. The slump test measures the consistency of the field grout batch. Standard Method of Test for Slump of Portland Cement Concrete, ASTM C 143 (ref. 8) is used to test the slump. Grout slump should be between 8 and 11 in. (203 and 279 mm) to facilitate complete filling of the grout space and proper performance. When the rate of water loss is high, due to higher temperatures and/or the use of highly absorptive masonry units, it is desirable to maintain a higher slump. When water will be absorbed at a slower rate, a lower slump is desirable.

To test slump, the cone, shown in Figure 1, is dampened and placed on a flat, moist, nonabsorbent surface, and filled with grout in three layers. Each layer is rodded 25 times with a round steel tamping rod to consolidate the grout.

Figure 1 �Measuring Grout Slump

The second and third layers are rodded through the depth of the layer and penetrating into the layer below. After the top layer is rodded, excess grout is struck off flush with the top of the cone. The mold is immediately removed, and the slump measured by determining the distance between the top of the cone and the displaced original center of the top surface of the specimen, as shown in Figure 1.

Compressive Strength Testing

When grout compressive strength testing is required, the procedures of Standard Test Method for Sampling and Testing Grout, ASTM C 1019 (ref. 7) are used. The number of grout samples to be tested should be specified before the start of construction. Three specimens comprise one sample. This test method provides criteria for producing a grout specimen to be tested. The grout specimen is formed in a mold made up of concrete masonry units with the same absorption and moisture content characteristics as those being used on the job (see Figure 2).

Figure 2�Grout Mold for Compressive Strength Testing

The mold space should simulate the grout location in the wall. Thus, if the grout is placed between two different types of masonry units, both types should be used to construct the mold. The masonry units remove excess water from the grout to more closely replicate the grout strength in the wall.

For this reason, concrete test methods should not be used for grout. Grout cubes or cylinders formed in nonabsorptive molds will give unreliable results. For example, compressive strength results of 2 x 2 in. (51 x 51 mm) grout cubes formed in accordance with ASTM C 780 can be expected to be 20-30% lower than the same grout tested in accordance with ASTM C 1019. Similarly, 3 x 6 in. or 6 x 12 in. (76 x 152 mm or 152 x 305 mm) grout cylinders can be expected to produce strengths 30-40% lower than the same grout tested in accordance with ASTM C 1019, as described below.

First, a square piece of wood (with dimensions as specified in ASTM C 1019), which acts as the base for the specimen, is placed on a level surface where the block mold will remain undisturbed and protected from freezing and extreme variations in temperature for 48 hours. Permeable paper, such as paper towel, is taped to four masonry units to break the bond between the grout specimen and the masonry units, but still allow water to be absorbed by the masonry units. The units are then arranged around the piece of wood to form the mold, as shown in Figure 2.

Grout is poured into the mold in two lifts. Each layer is rodded 15 times, distributing the strokes uniformly over the cross-section of the mold, to eliminate any air bubbles. When rodding the upper layer, the rod should penetrate about 1/2 in. (13 mm) into the bottom layer. After the second layer is rodded, the top of the cube is leveled with a straight edge and immediately covered with damp fabric or similar material to promote curing. After 48 hours, the molds are dismantled and the specimens are carefully packed for transport, keeping them damp, and shipped to the laboratory for testing.

The lab should store the specimens in a moist room prior to testing. When tested, the specimens should be capped in accordance with the applicable provisions of Standard Method of Capping Cylindrical Concrete Specimens, ASTM C 617 (ref. 3), and tested according to Standard Method of Test for Compressive Strength of Molded Concrete Cylinders, ASTM C 39 (ref. 6). Figure 3 shows some ASTM C 1019 test results for grouts with various aggregate to cement ratios. Although, in general, a lower aggregate to cement ratio produces higher grout strengths.

Figure 3 shows that a direct relationship between the two does not exist.

Figure 3�Relationship Between Grout Mix and Compressive Strength

References

1. Grouting Concrete Masonry Walls, NCMA TEK 3-2. National Concrete Masonry Association, 1996.
2. Specification for Masonry Structures, ACI 530.1-95/ASCE 6-95/TMS 602-95. Reported by the Masonry Standards Joint Committee, 1995.
3. Standard Practice for Capping Cylindrical Concrete Specimens, ASTM C 617-94. American Society for Testing and Materials, 1994.
4. Standard Specification for Aggregates for Masonry Grout, ASTM C 404-95. American Society for Testing and Materials, 1995.
5. Standard Specification for Grout for Masonry, ASTM C 476-95. American Society for Testing and Materials, 1995.
6. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, ASTM C 39-96. American Society for Testing and Materials, 1996.
7. Standard Test Method for Sampling and Testing Grout, ASTM C 1019-89a(1993)e1. American Society for Testing and Materials, 1993.
8. Standard Test Method for Slump of Hydraulic Cement Concrete, ASTM C 143/143M-97. American Society for Testing and Materials, 1997.
9. Uniform Building Code. Whittier, CA: International Conference of Buil

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