Temporary works crane platform aggregate explained
On a recent project, we were tasked with determining the requirements for a crane platform, in preparation for cast concrete floor sections to be lifted into place.
Following a genuine hands-on experience to squeeze the clay soil and understand the properties of the ground in the right spot for the crane, we specified 6F2 aggregate to form the platform.
Now, 6F2 or 6F2 Selected granular material (coarse grading) to give it its full name in accordance with Section 6 of the Specification for Highway Works, is just a posh way of specifying mixed coarse aggregate.
It gives the contractor freedom to source an economically available material and it is typically composed of crushed concrete, brick and mortar, usually the result of demolition projects, with all the scrap metal removed.
A similar material from a quarry might be known as 3 inch down crusher run or 3 inch crush; these traditional names giving an indication of its composition, with all the pieces of aggregate ranging in size from a maximum 75mm (3 inches) down to dust.
However, a recycled material is both better for the environment and more economical than material from a quarry.
In the Specification for Highway Works, 6F2 is defined as capping and is intended to be placed over the sub-grade to increase the strength of the subgrade, prior to placing the more expensive sub base and final surfacing.
6F2 has many other uses such as general backfill, piling mats or temporary platforms and is recognised for providing a hard wearing surface when the coarse material has been compacted properly.
We specify it regularly as an initial layer for temporary access roads and hard standings for temporary buildings, as well as for crane platforms.
The location for the crane was a relatively dry spot, which allowed us to use 62F. If the site conditions had been wetter, or the ground had been made soft by heavy plant repeatedly churning the soil, we would have considered specifying an even coarser material that would not require compaction.
According to the Specification for Highway Works, 6F2 has certain requirements such as grading (nothing to large or too small) and the Los Angeles Coefficient which is a measure of how robust the particles are and how much or little they degrade under load.
The requirements are specific but aimed to give the contractor a wide scope to use a variety of materials. However, there is some confusion in the industry, with some suppliers referring to any old material as 6F2.
This is not the case and care has to be taken to ensure that only appropriate material is used. Material such as this gets its strength from friction between the grains and too much fine material or inappropriate grading can result in loss of friction and hence loss of strength – another reason to ask an expert.
Regardless of the terminology, the product we specified was perfect for a temporary crane platform, of 225m2, constructed to a depth of 0.5m which required approximately 200 tonnes of 6F2 aggregate.
Although PDMA deals with designs for projects large and small, we always deliver the same attention to detail, whether it’s squeezing clay or specifying the hardcore. It’s what we do.