Abstract. This paper deals with the use of embedded piezoelectric transducers to monitor the ultrasonic P -wave velocity evolution during the setting and hardening phases of concrete since casting time. The main advantage of the technique is the possibility to overcome the limitations of traditional methods which do not allow to apply specific mechanical boundary conditions during the measurement. The embedded transducers are based on the "Smart Aggregates" concept previously developed at the University of Houston, Texas. Two piezoelectric transducers are embedded in a prismatic mold and the evolution of the P -wave velocity is recorded for the first 24 hours in concrete after casting time. The results are very promising and show a good agreement with classical ultrasonic tests using external transducers.Confidential: not for distribution.

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The design of concrete structures is based on calculation rules, which often do not take into account the very early age behaviour of the material. However, during this period, structural concrete is subjected to strains due to the hydration process of cement. If these strains are restrained by concrete itself or surrounding boundaries, stresses start to build up that can lead to the formation of cracks. Among the parameters involved in the stress build up, the stiffness evolution is of major importance. This paper reports the use of eight different techniques aimed at stiffness evolution assessment, applied on the same concrete mix, in a round robin experimental test within three laboratories. The observations are compared after having expressed the results at the same equivalent age. Both the loading stress rate and amplitude are observed to have an effect of limited importance on the determination of the quasi-static elastic modulus, which might be explained by very short term creep. Ultrasonic measurements provide values of E-modulus that are higher than the values provided by the quasi-static tests at the time of the concrete setting. Similar mechanisms associated to very short term creep could explain the difference between the quasi-static and high-frequency elastic modulus.

Monitoring the evolution of an early age set of parameters on concrete is necessary to predict the early age behaviour of structures. The difficulty lies in the fact that this monitoring must be automatic because the concrete hardening process takes place over a long period after the casting. This paper presents a new methodology and an apparatus, specifically designed at IFSTTAR, to monitor the hardening process of a concrete. Mainly, the Young’s modulus can be monitored in compression. Measurements start soon after having cast the concrete and the sample temperature is completely controlled so that the concrete maturity is well mastered. The performances of this apparatus, obtained on an ordinary concrete, are compared to more classical measurements using an extensometer mounted on the sample just after the setting time and to ultrasonic measurements. In these cases, the temperatures were not controlled and results have to be expressed in equivalent time. A comparison with another method developed and used at ULB by using the same concrete, in the frame of a joined cooperation between our two laboratories is achieved. This test set up is based on the so called Temperature Stress Testing Machine (TSTM). This device has been specifically designed with a control of the concrete maturity by the use of a dummy specimen only submitted to free deformations (thermal, shrinkage). The TSTM allows compressive and tensile testing starting just after the setting time. In addition, concrete properties, such as compressive and tensile strength, have been characterized at early age. These values have been used for the design of the loading histories applied in the automatic tests. The heat released by the cement hydration has also been measured in order to express the results on a maturity scale

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h i g h l i g h t s Mass loss and relative humidity are measured during concrete drying. Though invert analysis, robust material transport properties are identified. The results are independent on the choice of the desorption isotherm model. The contributions or vapor and liquid water to drying depend on the choosen model.