Practical evaluation methods for the rheological properties of fresh concrete, especially with regard to appropriate surface treatment

Project duration: 2014 to 2016

The aim of this research project is to develop methods for the rapid evaluation of the rheological properties of fresh concrete on delivery that are suitable for construction sites. In particular, the "stickiness" of a fresh concrete is to be recorded, which can have an unfavorable effect on subsequent surface treatment (e.g. smoothing) or even make adequate treatment impossible.
Nowadays, concretes with low w/b values are often used in concrete construction. This has a positive effect on their durability and the sustainability of the structures. The lower the w/c ratio, the denser the concrete structure, the durability is improved and the compressive strength of the concrete increases. However, due to the reduced water content, workability may be impaired. In recent years, it has been shown several times in practice that the concrete surfaces of freshly placed concrete could not be processed properly. The concretes for which subsequent curing of the concrete surface was a problem exhibited a high degree of "stickiness", although the consistency of the concretes on delivery corresponded to the specifications made in advance.
Within the scope of this research project, mortar and fresh concrete tests are scheduled, which will be examined with regard to surface finishing. For the time being, the specified combinations of binder and PCE-based superplasticizer will be investigated on the basis of mortars. For this purpose, the mortars are adjusted for a target consistency of 260 ± 10 mm depending on the superplasticizer dosage. Subsequently, an attempt is made to establish a relationship to the rheological parameters (yield point, viscosity) of the mortars by means of correspondingly modified test methods. For this purpose, in addition to a modified spreading measurement, the flow measurement in the inclined flow channel as well as the ball and cone sinking test are used. Finally, the surface workability of selected concretes will be examined on the basis of the preceding findings. For this purpose, among other things, the penetration test with the penetration device and the smoothing with a simulating trowel (controllable, powerful agitator with smoothing disc) under defined load with simultaneous recording of the torque will be used to assess the workability of the surface.