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Is Fly Ash permitted as fill under concrete slabs?

March 24, 2020

A recent question raised with HIA’s Building Services team, was whether ‘fly ash’ is permitted as fill under a concrete slab.

Let’s have a look and work through the question.

What is Fly Ash?

Fly ash is a by-product of coal combustion in power stations and can be used to improve the overall performance and quality of concrete.

Concrete-grade fly ash is a supplementary cementitious material that delivers improved later-age strength, workability and enhances the durability properties of concrete. It is commonly used as a 20% – 30% cement replacement in concrete.

Requirements for fill under concrete slabs

The foundations of a building are critical to its successful performance. As such, the soil must have the strength or bearing capacity to carry the building load with minimum movement.

The minimum bearing capacity (soil strength rating) may depend on the site conditions. The soil may be naturally undisturbed or be disturbed by building work. Where soil is disturbed by building work, the bearing capacity can be dramatically altered.

This is typically the case for sloping sites where cut and fill procedures are used. In these situations the soil needs to be consolidated, generally via compaction, to achieve the required bearing capacity.

The National Construction Code NCC) Volume Two and AS 2870 Residential Slabs and Footings provide details for the type of material to be used as fill and the compaction of the fill and bearing strength.

NCC Volume Two

 Requires the fill to be controlled fill or rolled fill and either be of sand or clay.

For Controlled fill:

Sand fill up to 800 mm deep must be well compacted in layers not more than 300 mm deep by vibrating plate or vibrating roller.

Clay fill up to 400 mm deep must be well compacted in layers of not more than 150 mm by a mechanical roller.

Rolled fill:

Sand fill up to 600 mm deep must be compacted in layers of not more than 300 mm by repeated rolling by an excavator or other suitable mechanical equipment.

Clay fill up to 300 mm deep must be compacted in layers of not more than 150 mm by repeated rolling by an excavator or similar machine.

There is no prescriptive option in either the NCC Acceptable construction provisions or AS 2870, for the use Fly Ash or any other material other than sand or clay.

Where a layer of material is required to be placed over the controlled or rolled fill or on a site where no fill is required to provide a level base for the slab panels a level layer of clean quarry sand must used with a depth of not less than 20 mm. Alternatively a layer of graded stone that is used as a termite management system may be substituted for the sand.

AS 2870 requirements

Filling used for the support of a slab shall be controlled or rolled fill. The installation of the fill is similar to the NCC provisions sand fill shall not exceed 800mm and be compacted in layers not exceeding 300mm.

For non-sand fill (clay) the maximum depth is 400 mm and be placed in layers not exceeding 150mm.

So can Fly Ash be used?

Where a concrete slab has been designed using the Deemed-to-Satisfy acceptable construction provisions or the prescriptive requirements of AS 2870, then the answer is NO Fly Ash cannot be used as filling or a base to site the slab panels or slab beams on.

However, it may be considered based on a first principles engineering design approach or a Performance Solution, subject to acceptance by the relevant building surveyor/certifier.

For further information on the NCC or relevant Australian Standards contact HIA's Building Services team on 1300 650 620 or email