How to Grout

Proper grout placement starts long before the grout is mixed. Substrate preparation, baseplate setup, temperature control, mixing, placement, finishing, and curing all affect the final performance of cementitious grout. Use the following steps as a general guide for successful grout placement.

Preparation

Concrete Substrate

Proper grout placement starts long before the grout is mixed. Substrate preparation, baseplate setup, temperature control, mixing, placement, finishing, and curing all affect the final performance of cementitious grout. Use the following steps as a general guide for successful grout placement.

Before placing grout, the concrete substrate must be properly cleaned, roughened, and presoaked with water for 18–24 hours.

Cleaning and roughening help ensure proper bond between the grout and the substrate. Presoaking helps prevent a dry concrete foundation from absorbing or wicking water out of the grout mixture before final set. This is especially important when placing non-shrink grout at a stiff or “dry-pack” consistency, where a dry substrate may contribute to shrinkage.

Do not take any chances — always presoak the concrete. Remove excess water just before grout placement, leaving the surface damp but free of standing water.

Baseplates and Sole Plates

Baseplates and sole plates should be properly aligned, shimmed, and cleaned before grouting. Remove all grease, dirt, loose rust, and scale.

Air vents may be needed to eliminate trapped air pockets, especially under baseplates with shear keys or stiffeners that create compartments on the underside of the plate.

Baseplate height can vary depending on several factors, including plate size and whether the plate supports a dynamic vibrating machine or a static load, such as a structural steel column. In general, cement-based grout is not recommended for depths less than 25 mm (1 inch). For grout depths over 100 mm (4 inches), up to 50% by weight of 10 mm (3/8 inch) pea gravel may be added.

Forming for Cementitious Grout Placement

When forms are required, they should be sturdy, caulked, watertight, oiled, and built to allow approximately 25 mm (1 inch) of space around the edge of the baseplate.

Choose one side of the form as the placement side. On this side, the forms should be spaced farther away from the baseplate to accommodate a 6–8 inch (150–200 mm) elevated head box or flow box. This allows grout to be placed and funneled under the plate.

The minimum form elevation should be high enough to allow complete gravity filling under the baseplate. For long pours, provide expansion joints as needed.

Temperature Control

Follow applicable ACI guidelines for hot and cold weather concreting. Many of the same principles apply to cement-based, non-shrink grouts.

In cold weather, proper preparation may include:

• Warming the concrete substrate and baseplate to 4°C (40°F) or above.
• Storing grout in a warm area.
• Using warm mixing water up to 32°C (90°F).

In hot weather, common steps include:

• Using cold or iced mixing water.
• Storing grout in a shaded or cool area when possible.
• Cooling the baseplate with cold water.

In all cases, take steps to moderate temperature extremes before and during grout placement.

Grout Placement: Plan Your Pour

Consistency

The proper grout consistency depends on baseplate clearance, job complexity, and placement method. On a single project, grout may be placed at several different consistencies. Some areas may require dry-pack consistency, while others may need plastic, flowable, or fluid placement.

Choose a grout that remains non-shrink at all required placement consistencies. The grout should conform to the non-shrink requirements of ASTM C-1107 across the full range of possible consistencies, from stiff to fluid.

Mixing Large Batches

For large batches, mix grout in gasoline-powered, paddle-type mortar mixers. Do not use rotary drum-style cement mixers.

It is recommended to have more than one mixer on-site to support continuous mixing and rapid placement in case of mechanical issues.

Before beginning, prepare the mixer by mixing one very fluid bag of grout. This is often called “buttering” the mixer. Discard this butter batch before mixing grout for placement.

For fluid consistency:

1. Add two-thirds of the recommended water to the empty mixer.
2. Start the mixer.
3. Slowly add grout powder.
4. Mix to a doughy state.
5. Add additional water until the desired placement consistency is reached.
6. Mix for 5 minutes to activate all ingredients and achieve a uniform, lump-free consistency.

Always follow the manufacturer’s water requirements and mixing recommendations. For stiffer consistencies, add the total water amount at the beginning of the mixing cycle.

Mixing Small Batches

For one-bag batches up to 55 lbs., use a 5-gallon (19 liter) pail.

For fluid consistency:

1. Add two-thirds of the recommended water to the pail.
2. Slowly add grout powder to the water.
3. Mix with a 1/2 inch variable-speed drill and jiffy mixer attachment.
4. Mix to a uniform, doughy state.
5. Add additional water to reach the desired consistency.
6. Mix at medium speed for 5 minutes to activate all ingredients.

Do not exceed the maximum water requirements stated on the bag. Do not overmix. For stiffer consistencies, add the total water amount at the beginning of the mixing cycle.

Depth of Pour

For grout depths of 100 mm (4 inches) or less, SpecChem cementitious grouts should be used as packaged, with only the addition of water.

For large or deep grouting applications, it may be necessary to extend the grout with up to 50% by weight of 10 mm (3/8 inch) pea gravel. High-strength grouts often contain a high percentage of portland cement, which can generate significant exothermic heat during early hydration. For pours over 100 mm (4 inches), pea gravel can help dissipate heat without negatively affecting non-shrink or compressive strength characteristics.

If pea gravel extension is not appropriate for a specific installation, the grout may be placed in multiple 75–100 mm (3–4 inch) lifts. Consult the grout manufacturer to confirm whether this approach is suitable.

Place Continuously and Rapidly

Once the substrate and baseplate are prepared, the consistency has been selected, and the grout is mixed, placement should begin immediately and continue continuously and rapidly.

Begin placement from the selected placement side only. This helps avoid cold joints and reduces the chance of air entrapment.

Grout anchor bolt holes and keyways first. Use head boxes and grout pumps where they are beneficial.

Concrete vibrators, straps, and rods may be used sparingly to help move grout. However, excessive vibration can cause bleeding and segregation in fluid grout and should be avoided. When longer working time is needed, use a grout designed for extended workability, such as SpecChem Spec Precision Grout Premier.

Place grout at least 5 mm (1/4 inch) above the bottom of the baseplate to help ensure the void is completely filled. Cover immediately with wet rags or curing compound.

Finishing

Just before final set, exposed grout shoulders should be cut back at a 45-degree angle from the base of the plate to the concrete foundation. This shape helps transfer loads uniformly from the baseplate to the foundation.

Other finishing methods may be acceptable depending on the requirements of the project and the experience of the owner or engineer. If grout shoulders are cut, curing must resume immediately after trimming.

SpecChem SpecPoxy CJ is recommended where quick, easy trimming is desired.

In-Place Protection

Curing

Improper or nonexistent curing is one of the most common causes of hairline cracks and shrinkage in high-quality cementitious grout.

All cement-based grouts should be carefully and immediately cured with wet rags, covered with plastic sheeting, and later coated with a high-solids membrane curing compound. For stiff or dry-pack grouts, wet curing is especially important and may be critical.

In every case, the sooner curing begins, the better.

Temperature Protection

After placement, protect grout from temperature extremes during the first 48–72 hours. Freezing protection is especially important.

Grout temperature should be maintained at a minimum of 4°C (40°F) until the grout reaches 3000 psi (22 MPa).

Jobsite Vibration

After placement, reduce or eliminate excessive jobsite vibration for the first day, especially vibration caused by nearby operating machinery.

During cold conditions, eliminate external vibration until the grout reaches 3000 psi (22 MPa).