Investigation of Concrete Segregation by Ultrasonic Pulse Velocity

Segregation is the separation of the components of fresh concrete or mortar, which can be caused by bad proportioning, insufficient mixing or excessive vibration. There are several tests which can be measured on hardened concrete or at the beginning of hardening. These are generally based on the percentage of the Fine aggregates between the top and the bottom of the samples. Concrete segregation has been tested trough three usual techniques, sieve, column and ultrasonic pulse velocity. Correlations were performed between various segregation indexes (sieve segregation index, column resistance index and ultrasonic resistance index) to test whether the ultrasonic method is efficient compared to the traditional methods. The fresh concrete segregation was firstly assessed on 17 different samples by the sieve segregation index test. The same 17 fresh samples were pre-hardened for an hour in vertical channel (dimension of 100 x 100 x 500 mm), and sub-samples from bottom and top were tested by counting Fine aggregates. Ultrasonic velocities were previously measured on the same samples and possible differences between the top and the bottom were assessed. Six concrete mixtures from the entire samples presented sieve segregation index lower than 15% and segregation resistance by counting course aggregates higher than 95%. These six stable samples have an ultrasonic segregation index approaching 100%. The remaining samples are found to be unstable concretes: sieve segregation index higher than 15%; resistance index "𝒇" lower than 65% and an ultrasonic index of segregation "u" lower than 80%. The ultrasonic pulse velocity method could be a quick and easily alternative in testing segregation of both fresh and hardened concrete.