Anaclet Turatsinze

Strain-softening of concrete in uniaxial compression

0025-5432/97

Influence of autogenous cracking on the durability of repairs by cement-based overlays reinforced with metal fibres

The focus of this research is a better understanding of the structural behaviour and durability of metal fibre reinforced concrete repairs. A total of 9 overlaid specimens, with the overlay on the tension face, were tested under deflection controlled three point bending. Three types of overlays were tested: one made with plain mortar (M0) and two with fibre-reinforced mortars (M1 and M2). Mortar M1 was reinforced with 40 kg.m−3 of 30-mm-long hooked-ends steel fibres. Mortar M2 was reinforced with 20 kg.m−3 of 30-mm-long amorphous metal fibres. During the preparation of the repair layer, the upper face of the repair was notched at mid-length using a shaped PVC plate, fitted to the mould. Because of the specimen curing conditions, the associated drying shrinkage had initiated a crack at the notch tip before the mechanical tests started. In such conditions, the debonding mechanisms are quite representative of the debonding of in situ repairs. It is shown that the debonding of fibre-reinforced repairs is significantly delayed in comparison with plain repairs. This better behaviour is due to the fibres’ ability to restrain the crack opening within the repair layer, which in turn helps to reduce the stresses at the base-repair interface.RésuméL’objectif principal de cette recherche est de mieux comprendre les mécanismes qui gouvernent la durabilité des réparations par rechargements minces adhérents en béton renforcé de fibres métalliques. Au total 9 éprouvettes composites simulant une structure réparée ont été testées en flexion trois points, la couche de réparation étant en zone tendue. Trois compositions du rechargement ont été étudiées; une non fibrée (M0) et deux autres renforcées de fibres métalliques (M1 et M2). M1 est renforcée avec 40 kg.m−3 de fibres métalliques tréfilées à crochets de 30 mm de long. M2 est renforcée avec 20 kg.m−3 de fibres métalliques amorphes de 30 mm de long. Par une plaque en PVC terminée en biseau, une encoche est réservée au milieu de la face de la réparation au moment du coulage. Les éprouvettes sont conditionnées de telle sorte que, avant les essais mécaniques, elles présentent une amorce de fissure de retrait à la pointe de l’encoche. C’est une configuration réaliste par rapport à une réparation réelle dans les conditions normales d’untilisation. Il a été constaté que, dans ces conditions, le décollement des rechargements renforcés de fibres est significativement retardé par comparaison avec le cas des rechargements non fibrés Ce meilleur comportement est dû à la capacité des fibres à limiter l’ouverture des fissures, avec pour effet de limiter les contraintes à l’interface support-rechargement.

Rubberised concrete for the design of pavement on soil

Cement-based materials suffer from low tensile strength and poor strain capacity. They are brittle and highly sensitive to cracking, notably to cracking due to length changes whatever the original cause of the length change. This study shows that a partial replacement (by volume) of natural aggregates by rubber aggregates obtained by grinding of used tires is a suitable solution to improve the strain capacity before the macro-crack formation, conferring to the rubberised concrete a reduced propensity for shrinkage cracking. Moreover, the modulus of elasticity is advantageously reduced and it can be predicted using mix laws such as Hashin-Shrikman low bound considering rubberised concrete as a two-phase composite. Despite its low strength, the elastic quality index (EQI) of rubberised concrete remains in the range of values used for the design of pavements on soil. Besides this potential in terms of pavement application, it is also an interesting way to utilise industrial by-product and consequently a contribution to keep environment clean.

Durability of an Overlay-Old Concrete Interface: The Role of a Metal Fibre Reinforcement

Abstract The proposed paper focuses on the debonding propagation along an overlay-substrate interface, notably on the damage of the interlocking between the two faces of the interface under static loading. The induced shrinkage length changes of the overlay and the substrate and its influence on the interface debonding are taken into account. The work associates experiment and simulation approaches with the purpose to clarify the role of fibre reinforcement on the interface debonding mechanism. Two types of overlay materials (OM), a fibre reinforced mortar (FRM) and a plain one (PM), are investigated. Direct tensile tests on notched OM specimens were firstly conducted to obtain the tensile strength and the residual normal stress - crack opening relationship. Drying and autogenous shrinkage of OM have been evaluated. The debonding opening - residual normal tensile stress relationship was investigated by static tensile tests. Three-point flexural static tests were then performed on composite substrate-overlay specimens to evaluate their structural behaviour, in particular the durability of the interface. The debonding interface propagation was monitored using a video-microscope with an enlargement of x175. Relying on the identified and quantified parameters, modelling of the above mentioned static tests was carried out by the finite element method using CAST3M code developed in France by CEA (Commission for Atomic Energy). The shrinkage effect was taken into account by using moisture diffusion equations and a relationship between shrinkage strain and water content variation. The model predictions showed a good agreement with the experimental results and proved the important role of fibre capacity to restrain the crack opening by transferring stresses through the crack.

Rubberised concrete: from laboratory findings to field experiment validation

Abstract Cement-based materials suffer from low tensile strength and limited strain capacity. They are brittle and highly sensitive to cracking, and such characteristics are the cause of the main distresses that limit the sustainability of their applications. Laboratory findings showed that the use of rubber aggregates obtained by grinding end-of-life tyres was effective in reducing the tendency of concrete cracking. This paper focuses on validating these findings in actual field conditions. For this purpose, control and rubberised concretes were produced using an industrial concrete plant and then used for the construction of prototype pavements. In equivalent conditions of construction, including length, the monitoring of the field experiment over a period of more than one year showed that the pavement constructed using control concrete tended to crack more due to shrinkage than the pavement constructed using rubberised concrete. Such experimental findings show that the use of a concrete incorporating rubber aggregates from used tyres may be an appropriate solution for sustainability, for economy and for saving non-renewable natural resources.

CEMENT-BASED THIN BONDED OVERLAYS: NUMERICAL STUDY OF THE INFLUENCE OF BOND DEFECTS AND FIBRE REINFORCEMENT

ABSTRACT The aging of concrete structures raises the problem of repairs. Thin bonded concrete overlaying can be a suitable technique for the rehabilitation of large area structures. A variety of structures or elements are concerned, for instance slabs on grade (mainly industrial floors), pavements, bridge decks, walls and tunnels. Toppings and linings are also relevant to the same issue. The aim of the overlay may be to retrofit a damaged surface, to improve the mechanical capacity of a structure by increasing its thickness, or to do both. The durability of an overlay relies on the durability of the bond with the substrate. In fact, the real issue is this bond. Its strength and durability often remain more or less unpredictable, much more in situ than in laboratory conditions, because of the inherent variable conditions on the work site. Especially, poor preparation of the substrate to be overlaid or its contamination are the cause of local bond defects. Relying on FEM numerical modeling, a quantitative evaluation of the incidence of such bond defects on the further debonding of the overlay is proposed. The modeling tool had been previously validated with experimental results in the absence of bond defects. In the part of the work reported in this paper, bond defects of different sizes were addressed. In addition to the defect size, the influence of the strain softening behavior of the debonding interface, due to bridging and interlocking, and the influence of fiber reinforcement in the overlay by fibers were investigated. The results of this study stresses the importance of taking into account the bridging and interlocking along the debonding interface. They also confirm the beneficial role of fiber reinforcement in the overlay. At last, they demonstrate that, owing to bridging and interlocking mechanisms along the interface, the effect of an initial bond defect vanishes soon while debonding propagates.

CEMENT-BASED MATERIALS INCORPORATING RUBBER AGGREGATES: SHRINKAGE LENGTH CHANGES

ABSTRACT Cement-based materials are brittle. As a consequence to their poor straining capacity and their sensitivity to shrinkage, they generally present cracking detrimental to the durability of structures. Nowadays, a solution to prevent or to delay the shrinkage cracking remains a research issue. Fibre reinforcement, restraining the crack opening, is one of the most documented way to partly reach this objective. This paper focuses on a second option to decrease the brittleness of cementious materials: the incorporation of low modulus aggregates. The study aims to design a composite exhibiting a high straining ability before macrocracking localisation. It has been assumed that incorporating aggregates with low deformation modulus should succeed with the challenge. Rubber aggregates were chosen. They confer to the work a second facet: the opportunity to recycle rubber tyres, fulfilling a demand of clean environment conservation. The results presented compare the properties of a plain mortar with the ones of two mixes obtained by partially replacing the sand aggregates by rubber aggregates. Two ratios of sand replacement, 20 and 30% by volume, were investigated. In both cases (natural sand and rubber aggregates), a maximum grain size of 4 mm was used. Previous results had shown that rubber aggregates are strongly detrimental to the composite strength. In return, the modulus of elasticity of the mortar incorporating rubber aggregates is substantially decreased and its straining capacity before failure is significantly increased. On another hand rubberised mortars suffer higher length changes due to shrinkage than plain mortar. In order to weigh up benefits and deficits, ring tests have been carried out and their results clearly demonstrate the benefit: the straining capacity enhanced by rubber aggregate substitution widely offsets the additional shrinkage length changes. The future prospects are the combination of the beneficial effects of both the fibre reinforcement and the rubber aggregate substitution to design a cimentitious composite exhibiting enhanced ductile failure.

Effect of Styrene-Butadiene Copolymer Coating on Properties of Rubberized Cement-Based Composites

Properties of improved strain capacity and high shrinkage cracking resistance make rubberized cement-based composites suitable for large surface applications such as pavements and thin bonded cement-based overlays. Microstructurally, the interfacial transition zone (ITZ) between rubber aggregate and cementitious matrix is different from that encountered with conventional aggregates. It is universally accepted a reduction in mechanical properties due to the low stiffness of rubber aggregates. However, transport properties are possible to be improved and comparative with conventional concrete or mortar if bonds between rubber particle and cementitious matrix were induced by pre-coating rubber aggregates to become hydrophilic and preventing air-entrapment phenomenon’s during mixing and placing composites. In this study, two distinct solutions were suggested to improve properties of composites such as using air-detraining admixture and styrene-butadiene copolymer as agent for rubber aggregate coating. Rubber particles with sizes 0–4 mm were incorporated in mortar as 30% sand replacement by volume. Microstructural analysis by Scanning Electron Microscopy (SEM) clarified the dense interface due to bonds generated by coagulation of the cement paste-polymer on the rubber surface. The initial experimental results showed a reduction in air permeability of composites modified with styrene-butadiene copolymer. As expected, this treatment method also contributed positively to mechanical properties of composites, especially direct tensile strength.

Modelling of the debonding of steel fibre reinforced and rubberised cement-based overlays under fatigue loading

The paper focuses on the propagation of debonding along an interface between a concrete substrate and a cement-based thin-bonded overlay under fatigue loading. The investigated overlay materials were fibre reinforced and rubberised cement-based mortars. Tensile tests were performed to obtain the residual normal stress–crack opening relationship for the overlay materials. The drying shrinkage of the overlay materials was characterised by tests on prismatic specimens that showed the evolution of drying shrinkage vs. the mass loss. The substrate–overlay interface was investigated by static tensile tests to provide the relationship between debonding opening and residual normal tensile stress. Its evolution under fatigue loading was assumed to follow a cyclic bridging law for plain concrete. Three-point bending fatigue tests were then carried out on repaired substrate to obtain information on the structural behaviour of the interface. The debonding propagation was monitored by a video microscope with a magnification of 175×. Relying on the identified and quantified parameters, the above-mentioned fatigue tests were modelled by the finite element method using the CAST3M code developed in France by Atomic Energy Commission. A comparison between model and experimental results shows good agreement and proves the important role of fibre reinforcement and of rubber aggregates on the durability of the repair with respect to debonding.

Effect of incorporating rubber aggregates and fiber reinforcement on the durability of thin bonded cement-based overlays

After certain period of time, the degradation of concrete structures is unavoidable. For large concrete areas, thin bonded cement-based overlay is a suitable rehabilitation technique. Previous research demonstrated that durability of such applications is always a problem and one of its main reason is debonding at interface. Laboratory and field researches show that fiber reinforcement in repair material can be a solution for controlling crack opening and also to enhance the durability of thin bonded cement-based overlays. In other respect, previous researches also show that by addition of rubber aggregates obtained from grinding of used tyres is also a suitable solution for improving strain capacity of cement based materials. This present research mainly focuses on synergetic effect of using rubber aggregates and fiber reinforcement in mortar as a composite for the repair work. For this study four mortar compositions to be used as overlay material were prepared: one control mortar, second with fibers at dosage of 30 kg/m3, third containing rubber aggregates replacing 30 % sand by equivalent volume and fourth one containing fibers and rubber aggregates. Direct tension tests were conducted in order to obtain the tensile strength, strain capacity, residual post peak behaviour of the repair material and bond tensile strength of the repair-substrate interface. Results showed that although by incorporating rubber aggregates in mortar reduce compressive strength and modulus of elasticity but improvement in straining capacity is observed. Moreover, fiber reinforcement in repair significantly improves residual post peak tensile strength.