Frost heave susceptibility
The frost heave test described in BS 812-124 (as amended by MCHW Clause 801.8) is costly and time consuming and is not suitable for routine on-site control checks. The test is primarily intended as a method to establish whether an aggregate from a particular source is likely to be frost susceptible when used in road construction. Material for the frost heave test must be representative of the source or sub-grade encountered. Authorities usually maintain a list of “Approved Suppliers of Non-frost Susceptible Materials” and should have knowledge of frost susceptible sub-grades in their locality. The following notes on identification of potentially frost susceptible material are for guidance but are not exhaustive:
- Clay materials can be regarded as non-frost susceptible, particularly when the plasticity index is greater than 15%. Clay/silt mixtures are more difficult to assess and are likely to be of marginal frost susceptibility.
- Silts, particularly those with more than 10% passing a 0.063 mm BS sieve size, are likely to be frost susceptible.
- Cohesive/granular materials will often be frost susceptible; the quantity and type of granular aggregate and, to a lesser degree, the silt fraction are the controlling factors. If the aggregate is frost susceptible it is very likely that the mixture will also be frost susceptible.
- Granular materials with more than 10% passing a 0.063 mm BS sieve size have a high potential for frost susceptibility, and granular materials with more than 12% passing 0.063 mm are likely to be frost susceptible.
- All crushed chalks are frost susceptible. The amount of frost heave will increase with the saturation moisture content of the chalk.
- Oolitic and magnesium limestones are likely to be frost susceptible, particularly those where the aggregate saturation moisture content exceeds 3.5%.
- Hard carboniferous limestones are unlikely to be frost susceptible unless they have been contaminated with clay or have more than 12% passing 0.063 mm.
- Crushed granites will only be frost susceptible if the percentage passing 0.063 mm exceeds 12% and is plastic.
- ‘As dug’ sands and gravels are frequently frost susceptible especially if the percentage passing 0.063 mm BS sieve size is greater than 12% or if it is plastic. Sands and gravels won by “wet working” techniques are unlikely to be frost susceptible unless contaminated by a clay or a high silt fraction.