A. Toumi

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.

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.