Stéphanie Staquet

Monitoring of the ultrasonic P-wave velocity in early-age concrete with embedded piezoelectric transducers

This note deals with the use of embedded piezoelectric transducers to monitor the ultrasonic P-wave velocity evolution during the setting and hardening phases of concrete subsequent to casting time. The main advantage of the technique is the possibility of overcoming the limitations of traditional methods which prevent the application of 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 h in concrete after the casting time. The results are very promising and show a good agreement with classical ultrasonic tests using external transducers. (Some figures may appear in colour only in the online journal)

Determination of time-zero and its effect on autogenous deformation evolution

ABSTRACT The aim of this study is the determination of time-zero, a reference time from which the autogenous deformation is expressed, and the effect of this choice on the evolution of autogeneous deformation of concretes with different slag proportions (0 and 71%). The determination of time-zero is based on the setting evolution measured by means of several experimental methods (mechanical, electrical and ultrasonic). Autogenous deformation is also measured with a new shrinkage testing set-up developed at the Université Libre de Bruxelles. Following the experimental results it appears that a time-zero equal to the final setting is the most adequate choice for classic concretes. For slag cement concretes, characterized by an expansion of their cement matrix at early age, time-zero should be taken equal to the time of the expansion peak.

Behaviour of slag cement concrete under restraint conditions

ABSTRACT Some constructions built with slag cement concrete exhibit cracking at early age due to restrained shrinkage. To determine the slag effect on cracking, this study focuses on the autogenous deformation evolution of concretes characterized by different percentages of slag (0 and 42% of the binder mass) under free and restraint conditions by means of the TSTM device (Temperature Stress Testing Machine). Despite the fast kinetics of its autogenous deformation, the cracking appears later for the slag cement concrete than for the Portland cement concrete. This behaviour is related to the swelling of its cementitious matrix at early age and its large capacity for relaxing the stresses.

Decoupling Thermal and Autogenous Strain of Concretes with Different Water/Cement Ratios During the Hardening Process

At early age, the autogenous strain and the coefficient of thermal expansion (CTE) are two of the most important concrete properties that are responsible for volume changes of cement-based materials and therefore for possible cracking in concrete structures (especially massive structures). Autogenous strain and CTE are dependent on the water/cement (W/C) ratio. A new experimental approach was developed to define both properties as well as the initial and final setting time with a single test. Every two hours, thermal variations of 3 °C are applied on a concrete sample with the device so-called BTJADE (from the French acronym BeTon au Jeune Age, Deformation Endogene). The initial setting time is the moment when the value of the CTE no longer corresponds to a liquid. The final setting time corresponds to a zero value of the derivative of the autogenous strain before the swelling of the concrete. Results of the initial and final setting time are compared to the evolution of the transmission of the ultrasound P- and S-waves. Three different compositions are investigated to define the influence of the W/C ratio (0.4, 0.5, and 0.6), and a fourth composition, for which the binder is composed of only 25 % cement, is also used in order to validate all observations. A new model expressed in function of the equivalent time or the advancement degree of reaction (defined with isothermal calorimetry) is proposed for computational modelling of the autogenous strain and the CTE. Effect of the W/C ratio and the physical mechanisms are highlighted.

Innovation for Railway Bridge Decks: A Prebent Steel-VHPC Beam

An extensive experimental study has been carried out in the framework of the French National Project MIKTI devoted to innovative composite structures to extend the Belgian technique of composite prebent beams called “preflex” beams, to very high performance concrete (VHPC). By comparison with experience gained from the Belgian precast industry, the main advantage of using VHPC with silica fume is to reduce the prestressing losses of the system, thanks to a significant decrease of the creep in addition to the possibility of optimizing the weight and the span of the beam. Major issues of the research were to validate a design method at serviceability state related to concrete cracking and at ultimate limit state corresponding to instability of the steel girder and the preservation of the composite behaviour of the beam even after a large number of live load cycles. The analysis of the global response of two 13 m long beams monitored during several months allowed to conclude the feasibility to optimize a prebent beam with a VHPC.

Applicability of ultrasonic measurement on the monitoring of the setting of cement pastes: effect of water content and mineral additions

Until now, standardized methods developed for the detection of initial and final setting time of cement paste were destructive and non-continuous. The well-established ultrasonic pulse transmission velocity (UPV) method can be correlated to the setting process of cement-based materials by the measurement of P-waves. However, S-waves seem to be better indicators of the setting of cement-based materials, because they are more sensitive to the solid matrix connectivity and less dependent to the air content. A new methodology, based on the combined monitoring of P-waves and S-waves, has already been developed to determine the setting times of mortar and concrete. The purpose of this study is to extend this methodology to cement pastes. Ordinary portland cement pastes with three water/cement ratio between 0.3 and 0.5 have been investigated. In addition, a portland cement, blast-furnace slag and limestone filler blended cement paste with a 0.4 water/cement ratio is studied in order to validate all observations. This study shows that the dynamic elastic properties can be linked to the initial and final setting times of cement paste. The early age evolution of the elastic properties depends on the air content and the binder nature. The combined monitoring of P-waves and S-waves provide accurate indicators of the setting process.

Shrinkage of slag cement concrete in free and restrained conditions

Slag cement concrete offers many advantages that lead to its intensive use in the Construction Industry in Belgium but it would present a higher sensitivity to cracking at early ages than Portland cement concrete. This preliminary study focuses on the evolution of autogenous and drying shrinkage and their effect on cracking sensitivity at early ages of slag cement concretes. Moreover, the influence of an extra period of the curing time on the total shrinkage is also underlined. In this way, various slag cement concretes were studied and compared to reference Portland cement concretes characterized by similar rheological and mechanical performances in free and restrained conditions. This experimental study highlights the role of the autogenous deformations in the evolution of the total shrinkage of slag cement concrete at early ages. Actually, our experimental data show that its evolution in terms of magnitude and kinetics is very different from that of Portland cement concrete. It was also observed that the slag cement concrete under restrained condition cracks earlier (between 15 and 60 days) than Portland cement concrete (after several months). This experimental research has also shown that the total shrinkage of slag cement concrete can be decreased partially by prolonging the curing time.

Deviations from the principle of superposition and their consequences on structural behavior

The focus of this study is on deviations from the linear viscoelastic behavior of concrete occurring at high stress levels, at early age loading and in case of unloading implying strain reversal. Paper describes the series of creep tests that was performed on high strength concrete specimens undergoing creep under constant stress, followed by a period of recording of the creep recovery after complete unloading.The experiment data show that the creep recovery deviates strongly from the numerical predictions obtained by the application of the principle of superposition but seems to conform rather well to the recovery model proposed.

ANALYSIS OF THE CAMBER AT PRESTRESSING OF A NEW KIND OF COMPOSITE RAILWAY BRIDGE DECK

Publisher Summary Up to 300 composite railway bridge decks of a new kind, belonging to the trough type with U shaped cross section, have been constructed in Belgium for 10 years. They seem to perform according to expectations. However, this chapter has some concerns about the variability between the measured and computed camber of these bridge decks just after the transfer of pre-stressing. In order to explain this variability, the chapter makes a statistical analysis with a sample of 42 bridge decks using 10 variables like the concrete compressive strength, the heat cure, the type of steel girder, and the age of concrete at pre-stressing. The ratio between maximum tensile stress and yield strength in the steel girders at the pre-flexion and the type of steel girder are found to be the most significant variables to explain the variability. If the steel girder is hot-rolled and the tensile stress/yield strength ratio in the steel girders at the pre-flexion is high, then the difference between the measured and computed cambers is high too. So, the construction process of the steel girders is important because of its influence during the elastification phase. The purpose of the chapter is to report on this statistical analysis in order to explain the influence of each of these variables on the behavior of these composite railway bridge decks.

Mixture Proportioning for Crack Avoidance

In this chapter, the nature, the physical and chemical properties and the content of the concrete constituents will be discussed for the understanding of the way of making an optimized mixture proportioning of the concrete for massive applications where the temperature rise must be minimized.