Frost heave testing

What does a frost heave test measure?

It measures how much a prepared specimen of the material rises when it is frozen from the top with water available at its base, recorded in millimetres. That upward movement is the same mechanism that lifts and cracks a pavement over a cold winter, as ice lenses form in the material and draw more water up behind them. The test is looking for that behaviour in the laboratory before the material goes into the ground.

Which standard is frost heave testing carried out to?

It is normally carried out to BS 812-124, the method named in the Specification for Highway Works. Clause 801 of Series 800 sets the acceptance criterion, classifying material as non-frost-susceptible where the mean heave is 15 mm or less. Your own specification may set a different limit or reference a different document, and we work to the one named in your contract.

When does frost susceptibility actually have to be proven?

When the material will sit close enough to the finished surface for frost to reach it. The Specification for Highway Works requires material within 450 mm of the designed final surface of a road or paved central reserve to be non-frost-susceptible, reducing to 350 mm on sites where the Mean Annual Frost Index is below 50. Below that depth the requirement usually falls away, so the construction thickness above the material matters as much as the material itself.

What actually happens to the specimens during the test?

Specimens are compacted to a reference density and water content established beforehand, usually by the vibrating hammer method, then conditioned and set up in a refrigerated cabinet with their bases standing in water at a controlled temperature. The tops are held under freezing conditions while the bases are kept unfrozen, and the rise of each specimen is measured through the freezing period. The reported result is the mean heave of the set.

How long does frost heave testing take?

Longer than anything else in a subbase suite, and it needs planning into the programme rather than squeezing in at the end. Before the freezing stage can start, the material has to be graded, checked for plasticity and compacted to a determined reference density and moisture content, and the specimens then need conditioning. Allow weeks rather than days overall. We confirm the expected timing for the agreed scope before the work is booked.

What makes a material frost susceptible in the first place?

Fine material and the ability to draw water upwards. A material with a significant proportion of silt-sized particles has pores fine enough to feed ice lenses by capillary action but coarse enough to let water move, which is the worst combination. Clean crushed rock with few fines rarely heaves, while chalk, some limestones, soft brick and any material with a dirty fines fraction are the usual culprits.

Can we demonstrate compliance by material type instead of testing?

Sometimes. Some specifications and overseeing organisations accept certain well established materials as non-frost-susceptible on the basis of type and previous test evidence, and a supplier may already hold a current frost heave certificate for the source, which saves repeating the test. The other route is to design the frost susceptible material out of the top 450 mm altogether. Whether either is acceptable is a decision for the overseeing organisation rather than the laboratory.

What do you need from us before the specimens can be made?

A representative bulk sample large enough to prepare a full set of compacted specimens, plus enough material for the grading, plasticity and compaction work that has to come first. That means a substantial quantity rather than a bucketful, taken as increments from several points across the stockpile or the delivery instead of one scoop from the face. A shortfall means a second trip for material, and on this test that costs real programme time.

How is the result reported and what counts as a pass?

The report gives the heave measured for each specimen and the mean heave for the set, in millimetres, together with the density, moisture content and grading the specimens were prepared at. Against the Specification for Highway Works, a mean heave of 15 mm or less classifies the material as non-frost-susceptible. The preparation details matter as much as the number, because the result only applies to material in that condition.

What are our options if the material comes back frost susceptible?

Either keep it away from the frost zone or change the material. In practice that means increasing the thickness of the layers above it so it sits below the specified depth, replacing it with a material shown to be non-frost-susceptible, or reworking the source to reduce the fines that are driving the heave. Which of those is workable depends on the levels, the design and the programme, so it is a decision to take with your designer.

Does a frost heave certificate have to come from an accredited laboratory?

Often, yes. Some specifications require frost heave testing to be carried out by a laboratory holding current UKAS accreditation for that method, so it is worth reading the clause before you place the work. Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so ask us to confirm the position for the testing you need.

What do you need in order to quote for frost heave testing?

The material type and source, the specification or clause it is being tested against, how many samples are involved, the site postcode and the date the result is needed. Cost is driven mainly by the number of samples and the preparation each one takes, because the grading, plasticity and compaction work all sit in front of the freezing stage. Tell us the programme and we will be straight about whether it can be met.