Dampness Measurement in Concrete Floors

Complex structures like buildings incorporate a number of porous materials like brick, plaster, mortar, timber and concrete. All these materials hold some water within them. This water is mobile and can be transported into the atmosphere and into adjoining materials.

It follows that a "wetter" material will have a higher water or water vapour content. The water vapour pressure will be higher and this will drive the water vapour towards areas where the water vapour pressure is lower (or less water is present). In other words, water vapour will diffuse from a damp area into a dry area. This will continue until such time as equilibrium is achieved or until no further migration can take place.

It is necessary to measure and reduce the amount of excess water in concrete after curing for a number of reasons:

1. To determine the time to dry the concrete sufficiently in order to apply a surface covering.
2. To reduce corrosion of reinforcing steel bars.
3. To reduce swelling and biological decay of wood based floor coverings.
4. To prevent alkali attack on polymer based floor adhesives.
5. To predict the final mechanical and physical properties of the concrete.
6. To reduce the extent of alkali silica reaction and carbonation.
7. To study the drying out of various grades of concrete.

To date a number of methods are used to measure the "moisture content", namely:

The gravimetric or weight loss method. This relies on a sample being taken from the concrete structure, normally dried in an oven, and the weight loss recorded as a percentage of the final dried sample weight. Apart from not being a site measurement it is difficult to collect a sample without weight loss. Also precautions must be taken to collect a representative sample; for example, a small sample of concrete with a high proportion of large aggregate particles will lead to errors in measurement.

Drying is the most commonly used analytical method for calibration purposes. As discussed earlier, moisture may be present in a material in several different forms, and the loss of water due to heating/drying will depend on the temperature used. A high temperature may remove more bonded water causing discrepancies in measurement. In general this method is justified only when water is the sole constituent which is volatile and there is no degradation of the solid part of the material at elevated temperatures.

The electrical conductivity method. This relies on the electrical conductivity of the sample increasing with larger amounts of water present. A method exists where two holes are drilled in the concrete and the electrical conductivity is measured using a liquid electrode poured into these holes, as shown in the figure in the previous column. This method also relies on the electrical conductivity calibration being available for the particular mix/type of concrete used. The main advantage of this type of measurement is that it is instant and the instrument does not require frequent recalibration and is relatively rugged. The disadvantage is that there is a limit to the depth of measurement. In practice this is possible for sand/cement mixtures but is a lot more difficult with aggregates or where additives are used and the ratio of sand/cement/aggregate is not known.

The relative humidity method. The amount of free water available at any time will dictate the humidity level of the air in close contact with the concrete, and hence measurement of the relative humidity of the air in close proximity with the concrete can be used as a measure of its free water content. This method is preferable because:

1. The humidity value obtained is universal and does not have to be related to the different materials or the size shape or type of samples.
2. It is not influenced by the composition of the concrete or any additives/chemicals present nor the presence of steel reinforcing bars in the concrete.
3. It is a non contact measurement.
4. Temperature variations and their effects are negligible as the measurement is made within the concrete at the concrete temperature.
The measurement accuracy depends very much on the humidity measuring capability of the instrument/s used. This paper describes the technique and equipment for measuring the high humidities that normally exist in cementitious materials like concrete.

When all capillary water is evaporated diffusion of water below the surface takes place by water molecules changing from the liquid to the vapour phase and diffusing through the pores and capillaries within the concrete. Vapour diffusion is a slower process and this is why the drying out rate is very much slower at the end of the drying period. If part of the drying surface is sealed with an impervious membrane for a while and then the humidity of the air between this membrane and the surface of the concrete is measured, then the value of relative humidity of this air reflects the amount of free water at the surface, but does not necessarily reflect the availability of water well below the surface which could eventually be transported to the surface. This indeed forms the basis of the British Standard method of measurement (BS 8203).

Measurements can also be made by removing a small representative sample which can subsequently be placed in an airtight container into which a measuring probe is also placed. After humidity equilibrium is achieved the humidity value can be read. The advantage of this method is that the measurement can be done off site minimising on site time.

An alternative method is to drill a hole in the concrete and place a humidity measuring probe within this hole, sealed from the outside atmosphere to measure the humidity at depth. The advantages here are:

1. The air inside the hole will then be in good humidity equilibrium with the water/water vapour in the concrete. Although sufficient time should be allowed when the hole is initially drilled for the concrete to settle after the physical and thermal shock of drilling, the time taken to make subsequent measurements is relatively short.
2. The measurement can be made within the thickness of the concrete where the sampling point is more representative of the bulk than at the surface and if the core of the structure has been dried down to a reasonable level of humidity, the possibility of diffusion to the surface is greatly reduced.
For the measurement of humidity a number of devices which could be inserted into a hole drilled into the concrete have been available for some time. However, all these devices suffer from a number of faults like instability, inaccuracy, sensor failure and sensor drift due to contamination requiring frequent recalibration and servicing/ replacement. These problems are particularly serious at the high humidities that exist in the boundary layer adjacent to most concrete surfaces. Hence the measurement has to be done with the highest possible repeatability and accuracy with systems which have inherently low drift and do not require frequent calibration.

What is required is a reliable instrument capable of measuring humidity with the highest possible accuracy. It is well known that an excellent method of measurement of moisture content or humidity in the atmosphere is the measurement of dewpoint by the use of dew formation on a chilled mirror, where the measurement accuracy is a function of the mirror's temperature sensor performance rather than the measurement or electrical parameters of the absorptive properties of chemical films.

For the measurement of humidity, two parameters must be known, the ambient temperature and the dewpoint temperature of the atmosphere. The ambient temperature measurement is reasonably straightforward and a number of devices are available which give good accuracy and repeatability.

However, good dewpoint measurement has until now been available only using instruments which require the air sample to flow through a measuring head. This makes measurement impossible in a static environment such as the void within a concrete structure.

The instrument and measuring technique described here combine the precision and repeatability of chilled mirror dewpoint measurement with the operational simplicity and size of hand held hygrometer probes. The probe had its own expanding ring which could trap the air in the void where the measuring tip was placed.

All the materials used within the measuring tip are specially selected for their low thermal conductivity and low moisture absorption properties, for example stainless steel, delrin and solid gold for the mirror.

None of these components are prone to deterioration or require replacement from normal use. The length of probe available is such that the tip can be placed at the centre of a fairly large thickness of concrete. Such a probe coupled with an instrument which could measure on site, without mains if necessary, and could log results for inspection later.

The above figure shows an averaged profile of humidity/moisture distribution for several types of concrete structures after some drying.