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Materials Testing and Structural Investigations




ISAT (Initial Surface Absorption Test)


The ISAT test specified in BS 1881 part 5 was originally developed as a laboratory method of measuring the porosity of concrete.

Where carried out on oven dried concrete, the results are reasonably comparable between materials. Where the test is carried out on site the results are greatly influenced by the existing moisture conditions as well as surface cleanliness.

The test consists of the measurement of water flow into the test specimen through a known surface area. The contact area is defined by a plastic cell sealed onto the surface. Measurement of the volume flow is obtained by measurement of the length of flow along a capillary of known dimension.


The cell is manufactured from clear acrylic to allow observation of the water level and ensure the complete renewal of air. A clear reservoir is connected to the 'inlet' of the cell. The 'outlet' of the cell is connected to a capillary tube with an affixed scale. A valve is fitted to the inlet side to isolate the reservoir.

The cell size is normally 100mm diameter although for some specimens it is necessary to use a smaller cell. The British Standards require a contact area of not less than 5000 mm5 (80mm diameter) which cannot always be achieved on a small sample.


Before use the capillary tube is calibrated to determine the area of the cross section. The area of the cell is measured and the scale factor for the cell/tube combination is determined.

0.01 ml/m3/sec = 6.10-4 AreaCell length mm of tubing


The cell is clamped to the test surface so as to ensure an even pressure and good seal around the perimeter. If necessary the seal is improved with silicone sealant, 'Plasticine™' or 'Blu-tack™'. The capillary tube and reservoir are mounted 200mm above the cell.

After filling the cell completely the reservoir is closed off and measurements taken of flow along the capillary tube. Sets of readings are taken at 10 minutes, 30 minutes and 1 hour after the first wetting of the surface. There is also an optional 2 hour reading.

For each set of readings the reservoir is closed off and the flow distance along the capillary is measured for 5 seconds. The number of scale units determines the observation period.

Number of Scale Units moved in 5 seconds Period during which movement is recorded
Less than 32 minutes
3-91 minute
10-3030 seconds
More than 30Record initial surface absorption as more than 3.60ml/m5/sec


The initial surface absorption is then simply the number of scale units moved in 1 minute. If readings are taken for a longer or shorter period they must be converted to 1 minute duration.

The ISAT can also be calculated from first principles

ISAT = length. Atube .10 (where t is in seconds)

Acell .t Atube and Acell are in mm5, length is in mm.

Where the samples have been oven dried the initial surface absorption of samples may be comparable. Under site conditions the existing moisture level in the concrete restricts the absorption under test.

N.B. The results of such site tests must be treated with caution at best and may be meaningless at worst.

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