Bernard Espion

Measurement of time-dependent strains of concrete: Prepared by subcommittee 4: Standardized test methods for creep and shrinkage

1359-5997/98

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.

Nonlinear analysis of framed structures with a plasticity minded beam element

Abstract A general method for geometrical and material nonlinear analysis of planar framed structures is presented. The theoretical prerequisites prior to the development of a displacement type approach for a beam element in plasticity are established. The shape functions and stiffness matrix for a nined.o.f. beam element are evaluated. The generalised updated Lagrangian formulation is used to describe the equations of equilibrium and the predictions of a computer program are compared with examples ranging from buckling and elastica problems to experimental plasticity in steel and in reinforced concrete.

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.

Structural weaknesses of the Hennebique early reinforced concrete system and possible retrofitting

Abstract One of the most popular reinforced concrete patterns used in many countries until the First World War was the system of François Hennebique (1842–1921). The best known characteristics of the Hennebique design, which was used in buildings as well as in bridges, are the monolithic structure with continuous T-beams reinforced with bent-up bars overlapping the supports and the use of flat open U-shaped stirrups as transversal reinforcements. The reasons for the success of his company have been extensively studied. However, so far, the assessment of the carrying capacity of the system remains incomplete. The first part of this paper deals with the results of experimental bending tests up to failure performed on three full-scale T-beam segments removed from a narrow gage railway viaduct built in Braine-l’Alleud (Belgium) in 1904. They aim at identifying the mechanisms of failure. The second part of the paper presents retrofitting actions to counteract the observed principal structural weakness—the too short overlapping length of the tensile rebars over the supports.

Domination of commercial patents in the evolution of early reinforced concrete. Case-study of the region of Brussels

The advent and early developments of reinforced concrete were related to national patenting. This paper proposes an in-depth study of the logic of reinforced concrete patents at the turn of the 20th century, based on the case of Belgium. Before the First World War, a considerable number of systems were patented by private inventors for commercial purposes. Patents on reinforced concrete constitute today a primary source of information, both for their technical content and for the assessment of the market penetration of the innovative material. The scientific reliability of these patents is variable and ranges from the rational to the unrealistic. Propagation of reinforced concrete occurred following international trends, such as Hennebique or Monier, or by local inventors, mainly building contractors. Reinforced concrete started to be considered as a structural material after the French engineering standard of 1906, adopted by Belgium. Moreover, examining these patents helps to understand the structural specificities of the early phase of reinforced concrete. Therefore, this study enhances the conservation process of such construction.

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.