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Water absorption testing is conducted to conclude whether the ACM (asbestos containing material) is cement or asbestos insulating board. This will determine whether it should be dealt with under licensed or non-licensed conditions.

Asbestos insulating board (AIB) typically contains 15-25% amosite often with a small amount of chrysotile, in a calcium silicate matrix (some older boards may contain up to 40% asbestos and also contain crocidolite). AIB is readily broken and can give significant fibre release. Asbestos millboards may contain 37-97% asbestos with a matrix of clay and starch. Millboards have a low density and are easily broken. Both AIB and millboards have a lower density than typical asbestos cement products and will generally absorb more water (≥30%) than asbestos cement products. Both AIB and millboard were produced as flat sheets (or blocks) and were not profiled or shaped.

Water Absorption Testing Method

This water absorption testing method is suitable for use on high-density asbestos cement products and medium-density insulating boards. The method is capable of classifying asbestos-containing board materials as being; AIB if their weight increase from water absorption is 30% or greater or be classified as asbestos cement if their weight increase from water absorption is less than 30%. It is not suitable for use with low-density boards, lagging or friable materials.

All samples accepted by the laboratory must be suitably labelled and double contained in sealed airtight bags. No responsibility will be taken for samples that are hand-delivered out of hours or posted until they have been checked into the system. A receipt will be issued for all samples (or batches of samples) delivered during working hours to confirm safe custody by the office. Samples must be entered into the laboratory booking in system, which gives each sample a unique reference number.

This test should only be carried out on samples in which the presence of asbestos has already been established; for example using the PLM method. The test will be carried out on a suitably sized sample (i.e. a minimum of 3cm x 3cm or 9cm2) that is free of any adhering material (partially painted samples can be used but may need longer to absorb water). A description of the sample (including the presence of any adhering material) will be recorded. Reference to the presence of any adhering material that cannot be removed will be included in the report.

A representative sample of the material to be tested is dried and the weight determined. After weighing, the sample is immersed in water for at least 15 minutes and until there are no more visible signs of bubbles being formed. Excess water is removed from the sample and it is re-weighed. The percentage of water absorbed is calculated using the dry weight and wet weight of the sample.


Samples with one or more surfaces coated or covered with an impermeable layer (e.g. paint, bitumen, PVA glue etc.) will take longer to saturate, may not absorb the same amount of water as similar uncoated samples, and may be unsuitable for analysis using this method. The use of warm water (50-60 C) and or surfactants can increase the rate of penetration of water. However, scoring the coating before starting the test to allow water to penetrate may also be used to overcome this problem; if there is no possibility of sending in a sample that isn’t covered with an impermeable layer.

Age weathered board materials, which have, or are starting to delaminate will absorb more water than they would in their original state and the results may, therefore, be an overestimation of the true value. We recommend that delaminated samples are not submitted for analysis if possible.

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