How to specify perlite concrete in a BOQ: clauses, density and what to verify

A good specification survives contact with the site. A vague one gets substituted, and the first you hear of it is when something fails. This page sets out what a perlite concrete specification needs to carry so that what gets laid is what you drew.

Name the density, not just the material

Writing lightweight concrete in a BOQ is an invitation to substitute. There are a dozen materials that qualify, with very different behaviour, and the contractor will quote the cheapest one that fits the words.

Specify the oven-dry density in kg per cubic metre. That single number fixes the thermal performance, the dead load and the strength together, and it is testable on site with cubes.

The clauses a specification needs

• Material: perlite concrete using construction-grade expanded perlite aggregate with cement and an air-entraining admixture

• Density: the oven-dry density required, in kg per cubic metre

• Thickness: average and minimum thickness, remembering that minimum section is about 25 mm

• Thermal requirement: the thermal conductivity or the thermal resistance the build-up has to achieve, so the grade can be verified rather than assumed

• Finish: levelled to falls where applicable, smooth monolithic finish ready to receive the specified topping

• Testing: thermal conductivity to ASTM C518-21, and cube density on site

• Placement and supervision: who supervises the first batches and how the mix is controlled

A sample line

Providing and laying insulating perlite concrete screed to falls, using construction-grade expanded perlite aggregate with cement and air-entraining admixture, to achieve an oven-dry density of 400 kg per cubic metre, average thickness as per drawing, minimum 25 mm, finished smooth and monolithic to receive the specified waterproofing system. Thermal conductivity to be verified to ASTM C518-21.

Adjust the density and thickness to your own design. The structure of the clause is the part that matters.

What to ask the supplier for

• Thermal conductivity test reports to ASTM C518-21 for the grade you are specifying, not for a lighter grade

• Written confirmation that the design mix will be used as tested

• Confirmation of who will supervise the first batches on site

• A BOQ format and specification wording you can drop into your document

If a supplier quotes you a conductivity figure, ask whether it was measured at that density or interpolated between tested grades. Both are legitimate answers. Only one of them has a report behind it.

Four mistakes worth avoiding

• Specifying a density without stating the thermal resistance the build-up must achieve, which leaves nothing to verify against

• Specifying a section thinner than about 25 mm, which the material cannot be laid at

• Expecting the screed to act as waterproofing. It does not, and your waterproofing system stays exactly as specified

• Specifying a light insulating grade to save money against brickbat coba. On installed cost it does not; the commercial case belongs to the heavier screed grades

Want the wording checked?

Send us the clause you have drafted or the build-up you are working on, and we will return the specification wording and a BOQ format that matches it.

Write to info@theperlite.in or call +91 98600 63626.

Kelite, a product by Keltech Energies. Specified, engineered and executed by Perlite India.