Factors for compactibility and risk of segregation for concrete-Report for contract research project 'Compact Air'

Compaction of concrete allows the concrete to reach its full potential quality. Compaction is a two-stage process where the vibrations effect first fills the mould due to the liquefaction, and second, the vibrations repulse the entrapped air from the concrete. The purpose of the Compact Air project was to determine possibilities to reduce the amount of compaction pores and reduce the risk of segregation. Firstly, the samples from the project "Good Vibrations" were analysed using combination of capillary suction, pressure saturation, and digital image analysis methods. These three methods were also applied on normal production concrete structures, from which drilled cores where vertically extracted. Secondly, test structures were cast to examine where the compaction pores are created and how the reinforcement as well as the compaction power affect the compaction pores. Finally, a rheological model for concrete under vibration was created to further understand the factors affecting the compaction process.

The experiments did not reveal any single factor to improve the compactibility nor reduce the risk of segregation. However, the results demonstrated well that the workability and compactibility are not correlated phenomena. The casting of the test structures showed that even though the fluid concrete quickly filled the mould, the required compaction time remained in the same level when compared to the stiff concrete. According to the rheological modelling, the maximum density of concrete is a parameter that is controlled by the selection of compaction time, but it can also be increased by improving concrete properties. The visual estimation and modelling of the optimal vibration time suggest that the optimal vibration cannot remove all the entrapped air. Therefore, the compaction process is always a compromise between the degree of compaction and segregation.

The effects of the superplasticisers during the compaction are complex even though the superplasticiser had minor impact on the rheological properties of the concrete. Since superplasticisers increase the slump value of concrete by decreasing the yield stress, their effect on viscosity is relatively small, and thus, have little influence on the compactibility. While the vibration mechanically breaks down the cement particle interactions, the plasticisers permanently reduce those interactions of the cement particle. Due to the permanent loss of interactions, plasticised concretes are more susceptible for segregation during vibration when compared to the non-plasticised concretes. Moreover, lower viscosity of the cement paste increases the velocity of compaction pores raising upwards, reducing the required time for entrapped air to exit the concrete. The best compaction quality can be achieved with a moderate workability concrete. Importantly, the height of compaction layers, the distance of compaction points, and the compaction times affect greatly the overall compaction quality. As such, the whole compaction process has a significant impact on the quality of concrete.