What is expanding grout?
Expanding grout, also called non-shrink grout (in Dutch zwelmortel), is a pourable grout that expands slightly as it cures, so that shrinkage after hardening is zero or even negative. It is usually a ready-mixed blend of Portland cement, fine quartz sand and chemical admixtures that cause the slight expansion. You only add water.
Ordinary mortar and concrete shrink as they dry. Under a column or machine base this leaves a gap, so the load is no longer transferred over the full area. Expanding grout prevents this: it fills the space completely and stays in full contact.
Applications
- Base plate grouting under steel columns, base plates, machinery, rails, bridge bearings and precast elements.
- Anchoring threaded rods, anchors and bolts in drilled holes or pockets in concrete.
- Filling joints between precast units and between precast units and foundations.
- Filling tolerances, for example under the sole plate in timber-frame construction, and sealing gaps against draughts and vermin.
- Rapid-setting versions are also used to plug leaks in basements.
Properties
These vary widely between products, so always check the technical data sheet. Depending on the product, expanding grout is non-shrink, watertight, oil-resistant and resistant to frost and de-icing salts. Compressive strength ranges from about 10 N/mm² to well over 50 N/mm² depending on the product. Working time is typically around half an hour to three quarters of an hour.
Tips and common mistakes
- Substrate: clean, sound, load-bearing and thoroughly pre-wetted beforehand (no standing puddles).
- Measure accurately: use the specified amount of water. Extra water makes pouring easier but reduces strength and can cause segregation.
- Pour from one side: under a base plate, pour from one side so that air can escape on the other side.
- Temperature: work within the range stated by the manufacturer (often roughly 5 to 35 °C).
- Curing: protect the exposed surface from rapid drying by sun and wind.
Related terms
- Grout
- Base plate grouting
- Shrinkage
- Shrinkage crack
- Swellable waterstop
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