Recycled aggregate constituent testing

What does a recycled aggregate constituent test actually tell me?

It tells you what the material is made of, as a percentage by mass of each constituent type. The coarse particles are sorted into concrete and mortar, unbound aggregate and natural stone, clay masonry, bituminous material, glass, floating material and everything else, then weighed. That breakdown is what a specification compares against its limits, because two crushed materials with the same grading can behave very differently depending on what went into them.

Which standard is recycled aggregate constituent testing carried out to?

It is usually carried out to BS EN 933-11, the classification test for the constituents of coarse recycled aggregate. The method covers particles between 4 mm and 63 mm, so it describes the coarse fraction rather than the fines. Where your specification names a different method or a different set of categories we work to that instead, so send us the clause you are being held to.

What do the categories Rc, Ru, Rb, Ra, Rg, X and FL mean?

They are the constituent classes used to describe a recycled aggregate. Rc is concrete, concrete products and mortar; Ru is unbound aggregate, natural stone and hydraulically bound aggregate; Rb is clay masonry such as brick and tile, calcium silicate units and aerated non-floating concrete; Ra is bituminous material; Rg is glass; X is everything else, including clay and soil, metal, wood, plastic, rubber and gypsum; FL is floating material.

How are the different constituents separated and measured in the laboratory?

The sample is dried, washed and sieved to remove everything finer than 4 mm, then sorted particle by particle by hand into the constituent classes, and each class is weighed. Results are reported as a percentage by mass of the test portion. Floating material is the exception. It is separated in water, surface dried and measured by volume rather than mass, because a small weight of polystyrene or wood occupies a lot of space.

Why does the constituent split decide where the material can be used?

Because the limits in a specification are set on constituents, not just on grading. A recycled Type 1 unbound mixture, for example, is normally held to no more than 1% other material of Class X, up to 50% bituminous material and up to 25% glass. A material that is mostly crushed concrete with clean brick can meet those limits, while the same grading carrying a few percent of wood, plaster or soil will not.

What happens if the foreign material content comes back over the limit?

The material fails against that clause and cannot be used in that position until it is dealt with. In practice the options are to reprocess the stockpile, screen the contamination out and retest, blend it with cleaner material or accept it for a use where the limits are less demanding. Class X is usually the one that catches people out, because a small volume of plasterboard, wood or plastic goes a long way.

How big a sample does a constituent test need?

A constituent test needs a substantial sample: BS EN 933-11 sets a minimum test portion mass that rises with the top size of the material, so a coarse recycled aggregate needs tens of kilograms. That is the portion after reduction, so the bulk sample coming in needs to be larger again. Tell us the top size and we will confirm the quantity, because sending too little is the most common reason a constituent test has to be repeated.

How should a stockpile be sampled so the constituent result is representative?

Take a number of increments from different points and different heights in the pile and combine them into one bulk sample, rather than filling a bucket from the nearest face. Recycled material segregates badly, and the fines and the occasional lump of timber do not sit where you would expect. Sampling off the loading shovel as the pile is worked, or from a conveyor, gives a fairer picture than digging into the toe.

Is a constituent test on its own enough to show crushed material meets Type 1?

No. Constituent classification proves what the material is made of, but a Type 1 unbound mixture also has to meet the grading envelope and the requirements for resistance to fragmentation, magnesium sulfate soundness, plasticity of the fines, sulfate content and, where it sits close to the finished surface, frost susceptibility. We normally run constituent classification alongside grading as a minimum, because together they tell you most of what you need early on.

How are the results reported, and how long does the test take?

The report gives the percentage by mass of each constituent class, with floating material given as a volume, alongside the sample reference and the size fraction examined. Where your specification sets category limits we can present the result against them so the comparison is obvious. The sorting is labour intensive rather than slow, and we confirm the expected reporting date for the agreed scope before the work is booked.

Is the constituent classification method on your UKAS schedule?

Accreditation covers particular methods rather than everything we do, so it is a question to ask before booking rather than to assume. Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996, and accreditation is specific to the listed methods and scope. Ask us to confirm whether the constituent testing you need is covered, and the report will state the status of the result.

What should I send you to get a price for constituent testing?

The site postcode, the material and its source, the specification you are working to, approximate quantities and your preferred dates. Cost is driven by the number of samples, the quantity of material to be sorted, whether we collect the samples or you deliver them and what else is being tested at the same time. Hand sorting a large test portion takes time, so top size matters.