Moisture in Concrete

Question 1

Explain the concept “excessive initial moisture” for concrete by using the following concepts:
Wo= amount mixing water (kg/m3) put into the concrete at the mixing of concrete
delta Wo= amount of water added to the concrete during the moist-curing (kg/m3)
Wn= the amount of chemically bound during the hardening
W=the final state as regards the moisture content of the concrete if the surroundinghas a specific relative humidity

Wb= excessive initial moisture (kg/m3)

Answer 1

Wd=Wo+deltaW-Wn-W

Question 2

what can you say about the amount of chemically bound water in concrete of grade K25 compared to the amount chemically bound in K40

Answer 2

The amount chemically bonded in K40 is much higher (71 l/m3). In K15 42l /m3

Question 3

What is self-desiccation

Answer 3

It is when the cement takes care of all the excessive initial moisture

Question 4

Why is it of importance that concretes of higher grades are not moist-cured by pouring water but instead cured by the use of membrane liquids

Answer 4

Because the advantage from an excessive initial moisture point of view will be completly lost

Question 5

Regard a slab on ground which is membrane cured. Name five important factors that have an impact of the drying out time of this slab.

Answer 5

1. Concrete grade, a higher grade gives a reduction of drying time needed
2. Drying climate. RH and the temperature are of great importance
3. Slab thickness. The slab thickness is very decisive
4. The properties of the underlaying insulation material
5. permanent form or not

Question 6

What can you say about the drying out time for a slab of low grade concrete cast on a permanent form

Answer 6

It has a longer dry out time than a higher class. The drying out time will be 60 days