Vaisala HM44 - Manual (Page 15)

Chapter 5 ____________________________________________ Humidity measurements in concrete
VAISALA __________________________________________________________________________ 13
CHAPTER 5
HUMIDITY MEASUREMENTS IN
CONCRETE
General about humidity measurements in a
drilled hole
The HMP44 probe measures humidity of concrete in a drilled hole. In equilibrium stage, the
humidity of a concrete and air humidity in a drilled hole become same.
Drilling changes the stage of the concrete. This is due to the fact that heat is formed during
drilling.(Meantime the humidity of the concrete changes, because of the temperature
changes). To reach the equilibrium stage after drilling takes about three days. When the
equilibrium stage has been reached, the air humidity in a drilled hole and the concrete
humidity become equal. Then it is possible to make the humidity measurement.
For getting more reliable results, it is recommended to have several measurement points.
Measurement depth
Depth of a drilled hole
When drying of a concrete slab takes place from both sides, the minimum depth of
measurement (d) is made 20 % of the depth of the slab. When drying occurs from one side
only, the measurement depth (d ) is 40 % of the depth of a slab. See Figure 5.1.
Humidity profile (Figure 5.1)
The humidity profile of the slab depends on the drying of the concrete, i.e. whether the
concrete dries in one or in two directions. In concrete slabs drying in two directions, the
humidity is highest in the middle. After coating, the humidity spreads evenly throughout the