Drying attempt

The kiln test is a test method for determining the water content of fresh concrete . The experiment is carried out to calculate the actual water-cement ratio . The process is used both in the concrete plant for checking the mix and on the construction site for quality monitoring. The determination of the water-cement ratio with the aid of the kiln test is regulated in the German standard DIN 1048.

Background and conduct of the experiment

Kiln test under construction site conditions: Kiln with concrete on a gas oven

When producing concrete in a concrete plant, the amount of cement is weighed out before adding it. The amount of water to be added is also known, but the actual water-cement value (w / c value) can deviate from the calculated value due to small residual amounts of water in the mixer, rain during transport or due to the moisture in the aggregate. To check the w / c value, the amount of water in the finished mixture is determined with the kiln test.

For this purpose, at least 5 kg of fresh concrete are weighed out in a metal bowl or pan and completely dried over an oven at 105 ° C with constant stirring. If the mass of the sample no longer changes (“drying to constant mass”), all of the water has evaporated. The difference between the mass before and the mass after drying corresponds to the water content of the concrete sample. ${\displaystyle m_{w}}$

In addition to the water content of the sample, the fresh concrete density is required for the subsequent calculation of the w / c value , which can be determined with the aid of a container with a known volume. ${\displaystyle \rho _{c}}$

Calculation of the w / c value

The water content of the sample (in kg), the mass of the sample (in kg, usually 5 kg), the fresh concrete density (in kg / m³) and the cement content of the concrete mix (in kg / m³ ) are included in the calculation of the w / c value ) a. ${\displaystyle m_{w}}$ ${\displaystyle m_{p}}$ ${\displaystyle \rho _{c}}$ ${\displaystyle z}$

${\displaystyle w/z={\frac {\rho _{c}\cdot m_{w}}{z\cdot m_{p}}}}$

To carry out the test correctly, two samples must be kilned. If there is a difference in the water mass of more than 20 g, a third test must be carried out. The w / c value is given as the mean of the two or three tests.

literature

• German Institute for Standardization (Ed.): DIN 1048-1: 1991-06 - Test method for concrete; Fresh concrete . Beuth Verlag, June 1991.

Individual evidence

1. ↑ ^{a b c d} Darrversuch. bim.de, accessed on August 19, 2013 . 
2. ↑ Determination of the water content (drying test). (No longer available online.) Archived from the original on August 16, 2013 ; Retrieved August 19, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.