Base plate grouting is completely filling the gap between two structural elements — usually beneath the base plate of a steel column or a machine — with a purpose-made pourable grout. The point is that the load is transferred evenly across the whole bearing area, rather than through a few small packing shims. The Dutch term for the operation is ondersabelen.
Why it is necessary
A steel column is first levelled on shims or levelling nuts. That leaves a gap of typically 20-50 mm between the base plate and the concrete. Without grouting:
- the entire column load bears on a few small shims, risking local crushing of the concrete
- water can stand in the gap, corroding the base plate and holding-down bolts from below
- the lateral bedding that makes the connection stiff is missing
Where it is used
- Steel columns on pad foundations and concrete slabs
- Heavy machinery and plant requiring a large bearing area
- Noise barriers, bridge parapets and portal frames in infrastructure work
- Wall panels and precast concrete units set on a prepared base
Method and pitfalls
- Use a non-shrink or shrinkage-compensated grout. Ordinary mortar shrinks as it hardens, leaving the gap you were trying to fill. For narrow gaps or heavily loaded plates, a two-component resin grout is often used instead
- Get the flow right. The grout should run under the plate by itself; pour from one side so air can escape at the other
- Clean and pre-wet the substrate with cement-based grout — dry concrete draws water out of the mix
- Form the edges and finish slightly above the underside of the plate, with a small outward slope so water runs off
- It is not always required. On faces where moisture drains freely and full bearing is not called for, grouting may be omitted — that follows from the structural design, not from convenience
- Do not load until the grout has reached the strength stated by the manufacturer
Related terms
- Non-shrink grout
- Base plate
- Shim
- Pad foundation
- Holding-down bolt
- Propping
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.