Column bases
Non-shrink grout volume and bags under base plates
- where
- grout per base (litres)
- base plate length and width (mm)
- grout extension beyond each plate edge (mm)
- grout gap (mm)
- total grout with waste (litres)
- number of bases
- waste allowance (%)
- yield per bag (litres)
Explain this equation
Estimates the volume of non-shrink grout under a set of column base plates and the bags to order. Use it before a grouting visit so you do not run short mid-pour.
L_p and B_p are the plate dimensions and e is how far the formwork stands out from each edge. g is the gap from the concrete to the underside of the plate. V₁ is the grout per base. n_b is the number of bases, w the waste allowance, V_t the total and Y the yield of one bag at the mixed consistency.
The grout fills a flat box under each plate, so volume is length times width times depth. Dividing by 10⁶ converts cubic millimetres to litres. Waste is added as a percentage, and the ceiling brackets round up to whole bags.
Sense check: a 450 mm plate with a 50 mm margin and a 40 mm gap takes about 12 litres, close to one 25 kg bag.
Watch out: bolt pockets and sleeves can take more grout than the gap itself. Yield depends on the water added, so use the figure for the consistency you will mix.
Volume of flowable non-shrink grout to fill the gap under column base plates, and the number of bags to order.
Enter your values to see the result.
Good to know
The gap is filled to the formwork, which usually stands a little outside the plate so grout can be poured in from one side and rise on the other. Hole and pocket volumes for sleeved bolts should be added if they are grouted at the same time. SCI notes typical gaps of 25–50 mm for orthodox bases. Yield per bag depends on the water content: flowable consistency gives more volume than trowellable, so use the figure for the consistency you will mix.
Common mistake
Ordering by plate area only and running short at the formwork margin and bolt pockets, which is the worst place to stop a grout pour.
Limits
Rectangular plates and a uniform gap. Bolt sleeves, shear key pockets and uneven concrete add volume.
Sources
- Fosroc Conbextra GP technical data sheet (25 kg bag yields about 13.25 litres flowable or 12 litres trowellable; underplate gap 10–100 mm in one pour; allow for wastage)
- SCI P178 Design for construction, column bases (typical 25 mm to 50 mm grout gap with non-shrink cementitious grout; sleeve about 3 × bolt diameter; top of bolt within +25 mm to −5 mm of specified level)
- BCSA National Structural Steelwork Specification, 5th edition (cl. 3.4.5 HD bolt holes 6 mm over bolt diameter; 8.1.6 steel packs; 9.2 foundation and HD bolt check before erection; 9.6 erected components)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.