Grout is a highly fluid mixture of cementitious materials, aggregate, and water, used to solidly fill spaces around reinforcement and anchorage. Grout is required to be placed at a slump of 8 to 11 inches – a very wet mix – to flow into the grout space. Excess water in the mix does not adversely affect grout strength as it is absorbed by surrounding masonry. Masonry grout may be mixed on site or delivered in ready-mix trucks.

Benefits & Techniques

Grout is used to bond reinforcement to surrounding masonry, increase fire ratings and thermal mass, and improve sound dampening qualities.

ASTM Requirements for Grout

ASTM C476, Standard Specification for Grout for Masonry, lists grout requirements. All grout is to have a slump of between 8 and 11 inches, and a compressive strength exceeding 2,000 psi. Fine grout (sand aggregate only) may be used in all applications and is required when the grout space is small. Coarse grout contains aggregate up to 3/8” size and is an economical mix commonly used for reinforced masonry construction. See the Building Code Requirements and Specification for Masonry Structures Specification Table 7 for grout space requirements.

Common formulations for site-mixed grout are:
Fine Grout - 1 part cement to 3 parts sand (by volume)
Coarse Grout - 1 part cement, 3 parts sand, and 2 parts gravel

Placing Grout

Low lift grouting: For low lift grouting, walls are built to 5 feet or less, and vertical reinforcement is then placed into hollow cells or reinforced cavities. Grout is placed in one continuous pour to the top of the wall and consolidated before building an additional 5 feet of masonry.

High lift grouting: Walls may be built up to 24 feet tall, before grouting, when following the high-lift method. Cleanouts are left at the base of the wall at each bar position to facilitate inspection and also for removal of mortar debris from the base of each grouted cell. Cleanouts are formed by leaving out a masonry unit, a face shell, or part of a face shell. Following inspection, vertical reinforcement is installed, cleanout covers put in place and braced, and the wall grouted.

Grout lifts have historically been limited to 6 feet, meaning that grout is poured and consolidated in lifts of 6 feet or less until the full wall height is filled. Recent versions of the Building Code Requirements and Specification for Masonry Structures Specification (Section 3.5 D) now permit story-height grout pours of up to 12’-8” so long as the masonry is at least 4 hours old, the grout slump is between 10 and 11 inches, and there are no intermediate bond beams within the grout pour.

Consolidating Grout

Once placed in the wall, grout must be consolidated using mechanical vibrators whenever it is placed in lifts of more than 12 inches (see Building Code Requirements and Specification for Masonry Structures Spec 3.5). Grout is consolidated initially, during placement, for proper flow and to remove any entrapped air from the pour.

Water absorbed by surrounding masonry units causes freshly placed grout to shrink and grout is required to be re-consolidated to remove shrinkage cracks and arching voids. The time delay for reconsolidation varies, but usually grout should be reconsolidated 5 to 15 minutes after initial placement.

Self-Consolidating Grout

Self-consolidating grout (SCG) is a specially formulated product designed to completely fill grouted spaces in reinforced masonry construction. While somewhat more expensive than standard masonry grout, SCG speeds placement and, because no consolidation is required, can save costs by reducing grouting crew sizes. The 2008 Building Code Requirements and Specification for Masonry Structures code contains material and placement requirements for SCG.

Specifying Grout

Quality Assurance & Testing

The Building Code requires that grout proportions be monitored on the jobsite. For site-batched grout, the inspector should record the volumes of cement, sand, and gravel being added to the mixer. Quality assurance of ready-mix grout is accomplished by review of the supplier’s certificates. Testing for slump and compressive strength, though not required by Code, should follow the requirements of ASTM C1019 - 11.

Nondestructive methods such as ultrasonic velocity testing, impact echo, microwave radar, and infrared thermography are often used to inspect grout placement after the wall is grouted. These methods are able to locate voids and cracks in the grout, and identify if the grout is placed at the desired locations.

Infrared Thermography