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Single-side Drying

Single-sided Drying of Cement-based Levelling Material

For single-sided drying of levelling compound, these are the issues that apply to the drying processes affecting normal drying and self-drying levelling compounds:

Normal drying levelling compound – design factors relating to moisture; calculation of drying times

Research shows that for thicknesses up to 30mm, drying times are closely proportional to thickness, while for thicknesses over 30mm drying times increase at a greater rate. As with concrete, drying time also increases with lower temperatures and higher RH levels. In Figures 1 and 2 below, drying times (with single-sided drying) to 90% and 85% RH are plotted for various thicknesses and temperatures.

Figure 1: Drying times to 90% RH for levelling compounds Floor 4310 and Floor 4150 applied at different thicknesses.

Figure 2: Drying times to 85% RH for levelling compounds Floor 4310 and Floor 4150 applied at different thicknesses.

The drying times for layers of different thicknesses are based on laboratory measurements in the standard laboratory climate for levelling materials, that is 23±2°C and 50±5% RH. Since the climatic conditions at building sites can vary significantly, it is necessary to make corrections for the local temperature and RH when estimating drying times that apply on site. Establishing the correct drying time can be done from, for example, the graphs for thicknesses above 10mm. The effect of different temperatures can be read straight from these graphs. The correction for differences in ambient RH is made by applying the following factors:

Ambient RH	Correction factor
70-80%	1.5
30%	0.85

Example 1
Material:

Floor 4310

Application thickness: 30mm

The levelling operation is to be done in late summer/early autumn when experience shows that indoor RH is likely to be 70-80% over long periods. Temperature in the building will probably be around 10°C since no heating will have been installed. Based on the suppliers of the adhesive and the floor-covering, RH under the covering after laying should not exceed 85% (CM < 3%) for cementitious levelling/screed products.

Calculation: Calculation: From Figure 2, drying time before laying the covering is 50 days for a 3 cm thickness at 10°C. Since ambient RH in the room will be around 75%, the calculated drying time needed is 50 * 1.5 = 75 days.

Example 2
Material:

Floor 4310

Application thickness: 30mm

The levelling operation is to be done in late winter when the building is heated to 15°C. Since it is cold outdoors, indoor RH can be estimated to be around 30%. The floor is to be covered with linoleum using an alkali-resistant flooring adhesive. The surface RH requirement is therefore 90% (CM < 3.5%).

Calculation: From Figure 1, drying time for a 3cm thickness at 15°C and 50% ambient RH is 31 days. Correcting for the low winter ambient RH, the drying time to 90% surface RH becomes
31 * 0.85 = 26 days.

Note: The above data for calculating drying times for maxit levelling compounds Floor 4310 and Floor 4150 is based on the information available today. This information is constantly being updated, so please contact maxit about the design of levelling layers according to moisture conditions.

Self-drying Levelling Compounds

Levelling compounds intended for early covering are based on having a binder content so high that internal chemical drying continues even after loss of moisture to the environment through the covering has ceased. Drying before covering is intended mainly to ensure that the surface of the levelling layer will be dry and able to accept the water in the adhesive. The average RH in the levelling layer at the time of covering is, therefore, high; >95% (CM > 5%). The RH level, however, falls as the chemical processes of hydration use up the water. Accordingly, there is no point in measuring the RH between the concrete and the levelling material at the time of covering. However the RH in the underlying construction should be measured by an authorised damp-control engineer

The way that self-drying levelling compounds work is the same in principle as for fast-drying (high-performance) or self-drying concretes. The difference is that levelling materials are low alkaline since they are based on aluminous cement. This means that stuck-down floor-coverings can tolerate an RH level higher than 85-90% in the levelling layer for the first few months. For concrete, that is highly alkaline, a similarly early application of a flooring adhesive would produce a high level of emissions resulting from alkaline breakdown, chiefly of the flooring adhesive.

Note: Materials that can be classed as immediately moisture-porous such as wooden floors must always be given appropriate moisture protection.