Non-shrink grout is a cementitious or resin-based grouting material specifically engineered to compensate for the natural shrinkage that occurs as ordinary cement mixes dry and hydrate. When you grout a column base plate with ordinary cement mortar, the mortar shrinks as it cures — pulling away from the underside of the base plate and creating a gap through which load cannot be transferred. Under a heavily loaded column, this gap causes point loading at the plate edges rather than uniform stress distribution, potentially causing cracking in both the plate and the concrete foundation below.

Non-shrink grouts solve this problem by incorporating expansion mechanisms — typically gas-generating chemicals, expansive cements, or shrinkage-compensating admixtures — that cause a small controlled expansion in the plastic state that exactly offsets subsequent drying shrinkage. The result is a grout that maintains full contact with both the base plate above and the concrete foundation below throughout its service life.

Step 1: Surface Preparation

The concrete foundation surface must be clean, sound, and free from oil, dust, laitance, and any previous grout. Use mechanical scabbling, grit blasting, or high-pressure water jetting to expose clean aggregate and create a mechanically keyed surface. A surface roughness equivalent to CSP 3–4 (Concrete Surface Profile) is ideal. Remove all loose material and blow out with compressed air before grouting.

Step 2: Pre-Wetting

Thoroughly saturate the concrete substrate with clean water for a minimum of 6 hours before grouting (overnight is ideal for large base plates). Remove standing water immediately before grouting but leave the surface in a saturated-surface-dry (SSD) condition. Dry concrete will absorb water from the fresh grout, affecting workability and potentially causing plastic shrinkage. A damp substrate improves adhesion and prevents premature stiffening.

Step 3: Formwork

Install grout shuttering on three sides of the base plate, leaving the fourth side open for pouring and the opposite side as a vent. Ensure shuttering is sealed at the base and around anchor bolt sleeves to prevent grout leakage. For fluid grout, a head (hopper or funnel) at the pour side helps maintain continuous flow pressure under the plate. Seal any gaps in the formwork with foam tape or rapid-setting repair mortar.

Step 4: Mixing

Add approximately 80% of the required clean water to a mixing drum, then add the full bag of grout powder. Mix for 3–5 minutes with a slow-speed mechanical mixer (paddle type, not drum mixer). Add remaining water gradually to achieve the required consistency. Never exceed the maximum water addition — over-watering reduces strength, increases shrinkage, and causes bleeding. Discard any grout that has begun to stiffen before placement.

Step 5: Placing

Pour grout continuously from one side only — pour from the same point, allowing grout to flow across the full width and out the vent side. Do not pour from multiple points simultaneously as this traps air under the plate. Maintain continuous flow until grout appears at the vent. For large base plates, use a pump to push grout from one side rather than gravity pouring. Maintain a positive head (excess grout height above the plate level) until initial set to compensate for settlement.

Step 6: Curing

After initial set, cure the exposed grout surface with wet hessian or curing compound for a minimum of 7 days. Protect from direct sun and hot, dry, or windy conditions which cause rapid evaporation and surface cracking. In hot weather (above 35°C ambient), add ice to the mixing water to extend pot life and reduce evaporation. Do not apply load until the required minimum strength is achieved and confirmed.