Concrete in Aggressive Aqueous Environments
218-SFC Technical committee : sonic methods of quality control of fresh cementitious materials
Chairman : Pr. H. W. Reinhardt, Secretary : Pr. C. Grosse
Subject Matter: The properties of cement based materials in the fresh and hardening state are currently being measured with rather conventional methods. The methods are not objective because the results are highly dependent on the measuring device and the measuring procedure. A few examples: setting and hardening of cement is determined by the Vicat needle. It measures the penetration resistance of a standardized needle under a defined force. Begin and end of setting is defined in the appropriate cement standard (for instance EN 196). The value of these two points during the hydration of cement is rather arbitrary and only useful for comparison of cements. However, there is no valuable relation between the results of the Vicat test and properties of concrete in the hardening state. Questions like end of workability of a concrete mixture cannot be answered from the Vicat test.
Workability is measured by various test methods such as the slump test, the flow table test, the vebe time, the compaction test and the penetration test. None of these tests give an objective result. The change of workability is assessed from tests at various times. However, there is no clear relation between workability test and practical workability at the site, for instance during slip forming.
What would be needed are objective, science based test methods with continuous monitoring of material properties from the fresh until the hardened state. There is a need by material producers, suppliers, and users to control production, mix, and execution.
New testing methods are available which could be used. They are based on the propagation of waves through a medium with time (or state) dependent properties or on the refection coefficient. This wave reflection coefficient as well as the velocity of the waves is clearly related to the setting and hardening state. Some acoustical methods use compressional waves while others use shear waves. Features like damping, dispersion, energy transport, frequency spectrum can also be measured and depend on the composition of a mix. Changes of the mix, either as the material or the amount (concentration) of a component is concerned can be detected. After having carried out numerous measurements it should be possible to control production and mixing rather accurately.
Control of cement based materials during production and execution leads to higher quality, more uniformity, and thus better durability. The control of high performance products is necessary in the light of structural safety. The increasing use of blends of binders asks for more control than today.
During the RILEM TC 185-ATC several procedures in non-destructive testing of fresh and hardened cement based materials using different techniques like NMR, acoustic emission or based on electrical or thermal effects have been investigated. Since there are already standards for some of these methods, there is a need mainly for recommendations regarding acoustical methods demonstrating their ability to be a standardized. The groups dealing with acoustical methods for quality control of fresh concrete and cement will be approached for participation in the new TC. They will supply the latest developments and guarantee a successful work of the TC