What is a Non-Shrink Grout?

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.

Types of Non-Shrink Grouts

1. Cementitious Non-Shrink Grouts

The most widely used type for general structural grouting. These are pre-blended, single-component, Portland cement-based grouts with controlled expansion additives. They are mixed with water only and can be placed at different consistencies — fluid (self-levelling), flowable, or trowelable — by varying the water addition. Key examples in the Fosroc range include:

• Fosroc Conbextra GP1 — General purpose grade, >30 MPa at 28 days, gaps up to 50mm
• Fosroc Conbextra GP2 — High-strength precision grade, >50 MPa at 28 days, gaps up to 100mm
• Fosroc Conbextra GP3 — Rapid-hardening grade, >20 MPa at 3 hours, for fast-track applications

2. Epoxy Grouts

Three-component systems (epoxy resin, hardener, and quartz aggregate filler) offering very high compressive strengths (>70 MPa), excellent chemical resistance, and very low creep under sustained load. Used for precision grouting of vibrating machinery such as compressors, generators, and milling machines where long-term dimensional stability under dynamic load is critical. Epoxy grouts are significantly more expensive than cementitious alternatives and require experienced application — mixing temperature, component proportions, and pot life must all be carefully managed.

How to Select the Right Grout Grade

Application	Gap Thickness	Loading	Recommended Grout
Standard building column bases	25–50mm	Normal structural	Conbextra GP1
Heavily loaded industrial columns, crane rails	25–100mm	High structural / impact	Conbextra GP2
Fast-track construction, early loading needed	25–50mm	Standard structural	Conbextra GP3
Vibrating machinery, compressors, generators	25–75mm	Dynamic / vibratory	Epoxy grout
Precast element joints	10–30mm	Standard structural	Conbextra GP1 or GP2

Step-by-Step Application Guide

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.

Common Grouting Mistakes to Avoid

• Over-watering the mix: The single most common cause of grout failure. Follow the manufacturer’s water addition exactly — add a small amount extra if needed but never exceed the specified maximum
• Dry substrate: Always pre-wet the concrete foundation or the grout will lose water to the substrate, stiffen prematurely, and develop lower strength than design
• Pouring from multiple points: Always pour from one side only to avoid trapping air pockets under the base plate
• Grouting in extreme temperatures: Above 35°C or below 5°C, additional precautions are needed. Use chilled water in hot weather; use warm water and insulate in cold weather
• Loading too early: Wait until the grout achieves the specified minimum strength before applying structural load — check manufacturer data for minimum loading times