Batching

The concrete was produced using the concrete mixing station at the DTI laboratories. The main challenge of the concrete production was to comply with the requirement of less than 1% batching deviation on the individual constituent. In particular the water content was a challenge and a special procedure for controlling aggregate moisture content was established.

In connection with the production of each batch of concrete the moisture content in the individual aggregate fraction was determined by micro wave drying on three samples of approximate 1 kg each taken directly from the material actually batched, i.e. from the material on the conveyor belt. The adopted procedure also allowed to batch exactly the nominal amount of aggregate as material could be added or removed from the conveyor belt to equal precisely the desired amount. The total time required to obtain the moisture content of four aggregate fractions was approximately 25 minutes. During this time the aggregate was resting in the closed confines of the mixer, i.e. no moisture loss took place. From the measured moisture contents the exact amount of water needed to obtain the w/c-ratio was calculated and subsequently batched.

The maximum deviation on the w/c-ratio was 0.86 % corresponding to values between 0.397 - 0.403 for a nominal value of 0.400, i.e. roughly 10 times better than what is typically achieved by industrial concrete mixing stations.

Based on the data for the +60 batches of concrete produced, the minimum, maximum, average and standard deviations obtained for the individual constituent.

The mixing station is equipped with a 375/250 liter counter-current mixer, five aggregate silos and four powder silos from Haarup A/S, and process control software from Skako A/S. Prior to initiation of the project all weight cells were calibrated using traceable weights. All materials were batched based on weight.