maxitfloor.com - Concrete Beams

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Concrete with levelling

Single-side Drying

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Concrete Beams

Drying of Concrete Beam Structures

The main factors and how they affect drying are given below:

Binder content	The greater the proportion of binder relative to water, the larger the amount of mixing water that can be bound chemically.
Temperature and ventilation	Air temperature should be > +20 °C. At lower temperatures the drying process is prolonged. The same applies if ventilation and rate of air change are reduced.
RH value of the ambient air	The higher the RH the longer the drying process. RH_AIRshould be around 50-60%.
Thickness of the construction	The thicker the construction the longer the drying process.
Single-sided or double-sided drying	With single-sided drying the measurement depth is 40% of the thickness of the construction. For double-sided drying the comparable figure is 20%. Single-sided drying includes slabs on the ground, permanent formwork or coverings that form an air gap. Double-sided drying includes intermediate floors without permanent formwork
Damp arising externally	Seasonal effects produce major variations in RH values.
Aftercuring	Necessary aftercuring (spraying with water) extends the drying process.
Humidification after curing	This will extend the drying process unless chemical drying can be applied.
Underfloor heating	Changes the humidity profile. Humidity is redistributed, producing higher RH in the upper layers.

Calculation of the Drying Time
The following parameters constitute the data required to calculate the drying time of concrete:
The chosen concrete quality (water/cement ratio, water/binder ratio). Climatic conditions – temperature and RH of the air. Thickness, and single-sided or double-sided drying. Chemical setting as a function of time, that is affected by temperature and the dimensions of the construction. Water vapour permeability, that depends on concrete quality, curing conditions and the RH in the material (membrane curing produces a less permeable concrete). Equilibrium relationship between moisture content and RH in the pores of the material (sorption curve). This is affected by whether the material is in a drying out or humidification phase.
The difficulty of determining these parameters with sufficient precision, coupled with the complexity of the calculation model required to predict exact drying times, means that the calculation programmes currently available for concrete should be viewed as aids, giving a rough estimate of the order of drying times, rather than exact values. For this reason, on-site measurements should always be carried out. One of the most widely used calculation programmes is Torka S version 2.0, 2002, produced to estimate drying times for concrete made with building cement. The programme is used as a tool to ensure that an economic concrete quality can be selected based on drying capabilities. The theory and development of the programme were carried out by the Department of Building Materials and Physics of Building at LundsTekniska Högskola. No calculation programme for the drying of levelling compounds currently exists. This is mainly because the drying process is further complicated when levelling adds an extra layer on top of the concrete. The general principles on that Torka S, for example, is based no longer apply in such cases. However in order to provide users of maxit's levelling compounds with drying data, a comprehensive testing project is currently in progress.
High-performance concrete
This is a concrete with a low water/binder ratio (< 0.4), achieved by adding superplasticisers. The cement chemically binds the greater part of the mixing water. Since there is no accepted method of measuring surface moisture, it is recommended that the concrete surface be kept completely free from moisture and allowed to dry for at least four weeks before possible covering (Rapp, T, RBK 010402). The impermeability of the surface is so high that moisture from adhesives is not absorbed; high-performance concrete should therefore be covered with levelling compound before applying adhesive to the final covering.