It has been reported that the drying behaviour of screeds with CEM II cements (Portland-composite cements) and CEM III cements (blast furnace cements) differs from screeds with Portland cements and that drying to a state ready for the application of floor coverings often takes longer than in screeds with Portland cement. Screed installerstherefore often tend to avoid using CEM II and CEM III cements, and instead opt for Portland cements. The present research project intended to systematically examine the effects of the cement type on the drying behaviour of cement-bound screeds. The particular aim was to establish whether the use of CEM II and CEM III cements really affects the drying behaviour of cement screeds.
The investigations into the drying behaviour of cement screeds and hardened cement paste revealed the following:
- Screed mortars with blast furnace cements presented higher moisture contents (CM moisture and retained moisture determined by means of oven drying at 105°C) than screeds with other types of cement after being stored for up to six months in a climate of 20°C/65% RH (Figure 1).
- Hardened cement pastes with blast furnace and Portland-slag cements displayed considerably lower mass losses when stored in climates of 20°C/65% RH and 40°C/30% RH, and mass constancy even after a few weeks in a climate of 20°C/65% RH, i.e. no further material moisture was released into the environment.
The equilibrium moisture content of screeds with blast furnace cement is higher than in screeds with Portland cement in the case of common relative humidities. Therefore, e.g. when using blast furnace cement, it is impossible to fall below a maximum value for the moisture content defined on the basis ofthe drying behaviour of screeds with Portland cement. At the same time, the mass and speed of the expected further moisture loss, which are ultimately key to the possible occurrence of damage in floor constructions, are generally lower when using blast furnace cements than when using screeds, e.g. with Portland cement. Consequently, reservations about blast furnace cements and some Portland-composite cements because of their drying behaviour seem to be unfounded.
A follow-up project aims to investigate whether measurement and analysis of relative humidity in cement screeds leads to an assessment of readiness for the application of floor coverings that takes better account of the characteristics of various types of cement in terms of their pore structure and sorption isotherms than the current method of determining moisture content in cement screed, which is expressed in CM or % by mass.
2. Concrete spalling due to fire exposure
When concrete components are subjected to high temperatures in the event of fire, this can result in the spalling of pieces of concrete. Explosive spallingis considered to be the most serious form,; this is when pieces of concrete break off violentlywith loud, explosion-like noises. This issue has recently gainedprominence, partly because of increased use of high-strength concretes, and partly because of regulations in [EN 1992-1-2] regarding structural fire design. The standard states that explosive spalling isunlikely to occur "when the moisture content of the concrete is less than k % by weight," whereby k must be defined on national level. A value of 3.0 wt% is recommended in the standard; k has been defined as 4.0 wt% for Germany.
Systematic studies on the influence of various concrete compositions and ambient conditions on moisture content and moisture distribution in concrete components had not been carried out until now. Predicting the moisture content in concrete components, which is highly dependent on the chosen concrete formula and the ambient conditions, was therefore generally only possible to a very limited extent. It was considerednecessary for the present research project to investigate what moisture contents and distributions typically exist in concretes of different compositions under common ambientconditions. At the same time, the aim was to examine what influence concrete composition and moisture content could have on the occurrence of spalling when the concrete is heated.
The present research project simulated typical environmental conditions with the aid of a computer and calculated the resulting moisture content in the concrete using appropriate software. As input variables for the simulations, the material parameters of
- moisture storage function/sorption isotherms,
- water vapour transmission properties and
- water absorption coefficient
were determined experimentally for concretes of different compositions.