Non-Shrink Grout: Definition, Shrinkage Compensation, and Specification for Base Plates

Non-shrink grout is a pre-blended mortar formulated to stay at or slightly above its placed volume while it hardens. Standard cement-and-sand grout loses volume as water evaporates and the cement hydrates. That grout shrinkage opens a gap between the grout and the steel or concrete above it. A gap under a base plate means the load no longer transfers evenly. The plate bears on only part of the grout, which concentrates stress, cracks the grout, and loosens anchor bolts over time. Under vibrating equipment, the problem grows quickly.

What Is Non-Shrink Grout and How Does It Work?

Non-shrink grout is a construction material formulated to maintain its original volume—or expand slightly—after setting. In structural applications, such as under a steel column base plate, even microscopic shrinkage creates a gap, preventing the load from being transferred effectively to the foundation. Non-shrink grout solves this by incorporating additives—expansive agents—that generate a controlled expansion during the plastic or hardening phase, effectively locking the material against the contact surfaces.

The performance of these materials is governed by rigorous standards, most notably ASTM C1107 (Standard Specification for Packaged Dry, Hydraulic-Cement Grout (Nonshrink)). To be classified as non-shrink, the grout must show no volume loss after hardening and must typically exhibit high compressive strength. Depending on the water-to-powder ratio, these grouts can be mixed to three distinct consistencies: plastic (trowelable), flowable (pourable), and fluid (self-leveling). Plastic mixes can reach 14,000 psi compressive strength, while fluid mixes may yield around 8,000 psi, both well above standard concrete strength.

Why Grout Shrinkage Matters in Structural Applications

Non-shrink grouts control volume through expansive agents and carefully graded aggregates. The mechanism behind this stability involves either gas-generating agents or crystallizing admixtures. Gas-generating systems release tiny bubbles—often hydrogen or nitrogen—that cause the wet mix to expand before it sets. Crystallizing systems form ettringite crystals during hydration, pushing the cement matrix outward to compensate for shrinkage. This engineering ensures that when a heavy machine vibrates or a building sways, the grout pad remains solid, providing a continuous load path from the equipment to the ground.

Types of Non-Shrink Grout: Cementitious vs. Epoxy

Selecting the right non-shrink grouting material is a critical engineering decision that depends on the load type, environmental conditions, and installation window. The two primary categories are cementitious (mineral-based) and epoxy (resin-based).

Cementitious non-shrink grout is the industry workhorse. It is composed of hydraulic cement, graded aggregates, and shrinkage-compensating additives. It is cost-effective, easy to use, and behaves similarly to concrete. It is ideal for static loads, such as building columns and precast wall panels. However, it can be susceptible to staining and chemical attack. High-tech manufacturers like Fritz-Pak produce specialized admixtures that transform standard mixes into high-performance non-shrink grouts, allowing for greater control over flow and set times.

Epoxy grout consists of a resin base and a hardener. It offers superior chemical resistance and typically higher early strength. More importantly, epoxy grouts are designed for dynamic loads—vibrating machinery or seismic environments—where cementitious products may crack under repeated stress. The trade-off is cost: epoxy formulations are significantly more expensive and require precise mixing and temperature control during installation.

Structural Grout for Base Plates and Anchor Bolts

Non-shrink grout is used under column base plates, machinery, and precast connections, and around anchor bolts, where full contact with the bearing surface is required. Cementitious non-shrink grout meeting ASTM C1107 reaches at least 5,000 psi compressive strength at 28 days. High-strength and precision grouts exceed this threshold, with some formulations achieving strengths above 8,000 psi depending on the consistency and water-to-powder ratio used during mixing.

Flowable vs. Plastic Grout Consistency

The consistency of non-shrink grout directly influences both placement method and final strength. Engineers must specify the appropriate consistency based on gap geometry and access constraints:

Grout vs. Mortar: Key Differences

While both grout and mortar are cement-based, their functions and compositions differ fundamentally. Mortar is designed to bond masonry units—brick, block, or stone—and contains lime or other plasticizers to improve workability and adhesion. It is not intended to bear concentrated structural loads.

Non-shrink grout, by contrast, is engineered for load transfer. It fills voids under bearing plates, anchors, and connections where dimensional stability and high compressive strength are critical. Unlike cosmetic tile grout, structural grout must meet strict performance standards, including volume stability, compressive strength thresholds, and often chemical resistance. The distinction is critical: specifying mortar where grout is required can lead to premature failure and safety hazards.

Surface Preparation and Saturated Surface Dry Condition

Success with non-shrink grout depends on strict adherence to mixing ratios and surface preparation, particularly achieving a Saturated Surface Dry (SSD) condition. The substrate must be free of dust, oil, and loose material, and pre-wetted to prevent rapid moisture loss from the fresh grout. If the substrate draws water from the grout too quickly, the expansive mechanism is compromised, and shrinkage may occur despite the product's formulation. Achieving SSD ensures optimal hydration, maximum bond strength, and the intended volume stability that defines non-shrink performance.