The Water Test On Water-Retaining Structures (Repairs to Concrete Water-Retaining and Water-Excluding Structures)

Almost all new water-retaining structures, such as reservoirs, swimming pools, sewage tanks, etc., are required to pass a water test. Repairs are undertaken when the structure fails to pass the test. In the case of existing structures, when leakage is suspected, it's also usual to carry out a water test. If it fails the test, then investigations are put in hand to determine the location of the leaks, assess why they have occurred, and arrange for their repair. After the repairs are completed, a further test is carried out. Because of this close connection between water test and repairs, the author feels that some discussion on the water test would be useful.

The Code of Practice, BS 5337, states: ‘The engineer should specify the permissible drop in the surface level taking into account the losses due to absorption and evaporation. For many purposes the structure may be deemed to be watertight, if the total drop in surface level does not exceed 10mm in 7 days ... For open reservoirs tested in a similar manner an additional allowance should be made for loss due to evaporation.’ The Code recommends that roofs be tested for watertightness when the structure holds potable water. Domed roofs should be provided with a waterproof membrane.

The Code recommends that a flat concrete roof should be tested by “lagooning” the roof slab to a minimum depth of 75 mm for a period of 3 days. The roof slab can then be regarded as satisfactory if no damp patches occur on the soffit during this period.

It would appear from the wording of the Code that the intention is that the concrete roof slab would be watertight in its own right and that except for domed roofs a membrane is not required. The author considers that the conditions of the test are very severe as in practice it's virtually impossible to construct a flat roof slab so that no damp patches appear unless a membrane is used.

The author is in agreement with the principle of testing the flat roofs of structures holding drinking water before a waterproof membrane is laid on the roof slab. However, it's prudent to provide a membrane, and the object of the water test would be to detect any leaks which should then be repaired prior to the application of the membrane. Damp patches would under these circumstances not be considered as ‘leaks’.

Regarding damp patches, all engineers experienced in reservoir construction know that there is usually condensation on the soffit of the slab. The presence of these patches could cause a dispute to arise. Therefore the slab should be checked for condensation before the roof is ponded; even then, the presence of the water on the roof may result in a drop in temperature of the slab and if it then fell below the dew point, condensation would occur.

While the test is undoubtedly useful, the inherent difficulties mentioned above should be recognized and dealt with in a practical way.

For elevated structures, such as water towers and swimming pools above ground level, where the outside of the walls and the underside of the floor can be inspected, the usual test requirement of maximum drop in level over seven days can be substituted by a clause requiring that there should be no visible sign of leakage.

In considering the Code recommendations, it should be noted that they are not clear. Having said that the engineer should take into account absorption and evaporation, it limits the total drop in surface level. In the next paragraph it says that in the case of open reservoirs an additional allowance should be made for evaporation. In the opinion of the author it's better to avoid disputes on site and arrange in the test requirements for the practical measurement of evaporation from both open and closed structures.

Metropolitan Water Board (now Thames Water Authority) engineers have found that there can be appreciable evaporation in roofed service reservoirs.

A simple and effective method of measuring evaporation is to fill a steel/plastic drum with water to within about 50 mm - 75 mm of the top, and to anchor this in the water of the reservoir during the period of the test. It can be assumed that the drum is completely watertight and therefore any drop in level in the drum will be due to evaporation.

There are a number of theoretical arguments which can be used to show that the evaporation from the drum is different to that from the water surface of the reservoir. No doubt there is some difference, but this is likely to be smaller than the large discrepancy which would result from the use of a formula and the insertion of factors which have to be evaluated by guesswork.

When a water-retaining structure is being filled for the first time (this is usually for the water test), the water level should rise slowly. The author’s opinion is that a rate of 075 m/24 h should not be exceeded. The majority of structures of this type don't contain more than a 7 m depth of water so that such a structure can be filled in 10 days.

As soon as the structure is full to overflow level, which is usually above operational top water level, the level should be accurately recorded. If a high degree of accuracy is thought necessary, then a hook gauge can be used; otherwise the level can be recorded on the wall or other accessible part of the structure.

The level should be read daily at the same hour; the drop in level should reduce each day so that at the end of the initial ‘soakage’ period of 7 days, the recorded 24 h drop should not exceed about 2 mm, which is difficult to measure anyway. In certain cases, particularly when the structure has been under construction for a long period, in dry weather, the initial period may have to be extended to 10 or 14 days before relative stability is achieved.

The 7-day test period should then be started by recording the water level in the structure and in the barrel used to record the loss due to evaporation. It is advisable to check both levels each day at the same hour.

If the drop in level over the 7-day test period does not exceed the figure in the specification and there are no signs of seepage on any of the exposed surfaces, the structure can be assumed to have passed the test.

The question is what should this ‘figure in the specification’ be. As previously stated, the recommendations in the Code are ambiguous. Many experienced water engineers consider that the loss of water, excluding evaporation, should be appreciably less than 10 mm. It is better to decide on a figure excluding evaporation and insert this clearly in the specification. It is useful to realize the quantity of water lost by a drop in level of say 10 mm; this amounts to 1 mm of water surface. This is an appreciable quantity of water; seepage through fine hair cracks and slightly porous concrete would certainly not result in this amount of loss over a period of 7 days. It is therefore important to include a requirement in the specification that there shall be no visible signs of leakage on any exposed surface of the structure. This will make it clear that all such defects must be put right under the contract without extra payment.