Betonin koostumuksen vaikutus sen tiivistettävyyteen: Raportti tilaustutkimusprojektista

Fresh concrete may contain up to 20% of entrapped air before compaction. The amount of air depends not only on the properties of the concrete but also on the mould and the reinforcement. The purpose of compaction is to remove the entrapped air as well as possible. It is assumed that entrained air is not significantly impacted by the compaction.

Lately, it has been noted that the compaction of concrete might not be always sufficient. This phenomenon has been observed in the cases where half–full-sized test structures has been cast. The density of the drilled cores has been notably lower (100...150 kg/m3) compared to the laboratory specimen used in quality control. Correspondingly, the estimated air content of the hardened concrete has been higher than the target level and the compressive strength of the concrete structure has been measured to be notably lower than in the laboratory specimen.

The purpose of the contract research project "Good vibrations", carried out in Aalto University, Department of Civil Engineering, was to investigate how the composition of concrete affects the degree of compaction and how much compaction different concrete types require. The research was based on the test structures cast in the ready-mix plant. Drilled cores were extracted from the hardened test structures and their densities and compressive strengths were analyzed. In addition, the compaction in the mould was recorded from two angles. Realized vibration times and the removal of the entrapped air was estimated from the recordings.

The results show that the compaction fills the mould easily but removing the entrapped air is challenging. After normal vibration times, the remaining amount of entrapped air was generally 1...4% more than in the laboratory specimen. This corresponds to a reduction of the compressive strength about 5...20%. In addition, the workability of concrete, low water-cement ratio or high superplasticizer amount seem not to have a clear effect on the degree of compaction. It was noted that vibration may easily cause segregation of the concrete. Especially, air-entrained concretes with high workability are susceptible to segregate even with short vibration times. However, with common vibration times this risk is moderately low as well as the segregation seems to have little effect on the average strength of the test structures. The sensitivity of air-entrained concrete for segregation requires further research.

The practical goal of the project was to develop instructions for compacting the concrete. In this report, a four-step process was formulated where a poker vibrator is used for compaction.