A study of some factors affecting bond in cementitious fiber reinforced repairs

Mechanical properties of steel fibre reinforced and rubberised cement-based mortars

Use of steel fiber reinforced mortar for seismic strengthening

In this study, experimental and numerical procedures are proposed to predict the debonding failure of concrete elements strengthened with fiber-reinforced polymers (FRPs). Such debonding is modeled as a damage process, which takes place in a band along the bond line (crack band). Three-point bending tests were designed to obtain the softening curve of the crack band. The numerical simulations are conducted using a plastic-damage model. In this approach, the damage resulting in debonding is defined using the softening curve of the crack band. Numerical results are validated against experimental results obtained from single-lap shear tests. The numerical models were capable of predicting the experimentally observed load versus strain behavior, failure load, and failure mechanism of the single-lap shear specimens. The predictive capabilities of the numerical approach presented here were further investigated by means of a parametric study of the single-lap shear test. Results from this study indicate the appl...

Damage Approach for the Prediction of Debonding Failure on Concrete Elements Strengthened with FRP

This study presents experimental procedures conducted to characterize the bond between concrete and fiber reinforced polymeric laminates. The experimental characterization was aimed at obtaining the softening of the concrete-epoxy interface (CEI) formed during the installation of these laminates. In particular, splitting tensile and three-point bending tests were used to determine the tensile strength ( ft ) , the size-effect fracture energy ( Gf ) , and the cohesive fracture energy ( GF ) of two concrete-epoxy interfaces. The sensitivity of these properties to the type of epoxy, specimen geometry, and surface conditions was also investigated experimentally. From this study, it was found that the tensile strength ( ft ) and the cohesive fracture energy ( GF ) of plain concrete and the two concrete-epoxy interfaces under consideration are similar in magnitude. Conversely, the size-effect fracture energies ( Gf ) of plain concrete and a CEI are up to 64% different. Experimental results indicate that the con...

Experimental Characterization of Concrete-Epoxy Interfaces

The ageing of concrete structures creates the need for their repair. The thin bonded overlay technique is widely used; it is particularly suitable for the repair of large concrete areas. It is appropriate for slabs on grade (mainly industrial floors), pavements, bridge decks, walls and tunnels. Toppings and linings are similar applications. The aim of the overlay may be to smooth a damaged surface, and/or to improve the mechanical capacity of a structure by increasing its thickness, or to provide additional cover for corrosion protection. The durability of such repairs or overlays depends on the durability of their bond with the base structure; it is usually the critical aspect. Work, carried out in Laboratoire Materiaux et Durabilite des Constructions (Laboratory for building Materials and Constructions Durability) in Toulouse, France, is part of a larger programme started in 1990. The programme objective is to achieve a thorough knowledge of the debonding mechanisms of an overlay. Design rules and practical recommendations eliminating or drastically reducing durability hazards are expected to be developed. The work reported here focuses on the debonding propagation along an interface, notably the major influence of the interlocking between the two faces of the debonding interface. The study relies on finite elements modelling and on the comparison with experiments. Having evidenced the major role of interlocking in the example of a crack propagation, the paper focuses on the actual case of an interface. Debonding propagates in mixed mode, mode I combined with mode II, bringing an extra degree of complexity. Beyond the problem of the mixed mode, an important issue is how interlocking changes while debonding propagates. Indeed, mode I debonding opening damages interlocking for both mode I and mode II. A mode II slip has the same coupled effect on interlocking. A very significant problem is taking into account the coupling of mode I and II displacements to evaluate the actual value of the interlocking at any step of the propagation. The involved parameters have been identified and a method is proposed to take into account the coupling. The comparison with experimental results is promising. This research opens a new approach on the mechanism of the propagation of a debonding along an interface. The next step will consider evolution of the interlocking in the case of fatigue loading.

Interface Between an Old Concrete and a Bonded Overlay: Debonding Mechanism

Cement-based bonded overlay is a frequently used technique for smoothing a damaged surface and/or restoring or improving the mechanical capacity of a structure by increasing its thickness. It is well established that the durability of such a repair is limited by its debonding from the substrate. Whatever the original cause of this debonding, there is general agreement that cracks cutting the repair layer (or any discontinuity such as joints or boundaries) are systematically involved. In situ repairs have demonstrated that reinforcement with commonly used contents of fibres effectively improves the durability of the repair. However, numerous trials under conventional laboratory conditions have failed to confirm the beneficial effects of the fibre reinforcement and three times as much fibre has been required to produce laboratory effects similar to those found in the field. This paper aims to explain this discrepancy. One part of the explanation may be that cracking due to length change (pre-cracking) is not likely to affect small laboratory specimens. Also, most of the laboratory tests have used monotonic loading (or monotonic straining) cases while fatigue loading would constitute a more realistic test. When such corrections are taken into account, a better understanding is obtained of the actual role of fibre reinforcement on the durability of cement-based repairs, as shown by the results and analysis presented here.

Durability of bonded cement-based overlays: effect of metal fibre reinforcement

Throughout time, the use of lignocellulosic resources has been implemented in the development of building materials. Among these resources, natural fibers are used as mineral binders reinforcement due to their specific mechanical properties. This experimental investigation focused on effect of flax and hemp fiber reinforcement on the resistance of pozzolanic-based mortars to cracking due to restrained plastic shrinkage. Results were compared with polypropylene fiber reinforcement and with control mortar without fibers. The quantity of fibers added to the mortar mix were respectively 0.25% - 0.5% by mass of binder for polypropylene fibers and 0.5% - 1% by mass of binder for flax and hemp fibers. All fibers have a similar length of 12 mm. The cracking sensitivity was evaluated based on two different methods: the first consists in casting the mortar in a metal mold with stress risers whose criteria are inspired by the ASTM standards. The second consists in pouring the mortar on a brick support. In order to assess the effect of fibers on cracking due to restrained plastic shrinkage, the number of cracks, total crack area and maximum crack width within the first 6 hours after casting were determined using digital image correlation (DIC). Results showed that the flax and hemp fibers were more effective in controlling restrained plastic shrinkage cracking compared to polypropylene fibers. With a natural fiber of 1% by mass of binder, maximum crack width was reduced by at least 70% relative to control mortar based specimens. Natural fibers show great ability to propensity for cracking due to restrained plastic shrinkage; so that, they could be an alternative and ecological solution for polypropylene fibers.

Effect of Natural and Polypropylene Fibers on early Age Cracking of Mortars

Considering the current energy environment, both efficient and environmentally friendly solutions have to be developed for building construction. Bio-based building materials offer new perspectives through their insulating and natural humidity regulation capacities. Nevertheless, these materials are as complex as they are promising, and grey areas still remain regarding their behavior. Their water sorption and desorption curves recorded in experimental work demonstrate a hysteresis phenomenon and, although plausible hypotheses have been formulated in the literature, there is currently no consensus on its causes. Furthermore, it is important to emphasize that no reference considers the hydrophilic nature of the resource. Yet, this is a specificity of raw material coming from the plant world. In this context, this paper explores the microstructure and chemical composition of plant aggregates to propose a new explanation for the hysteresis. It is based on recent work demonstrating the existence of differentiated hydrogen bonds between the water sorption and desorption phase in cellulose. Obviously, hysteresis also has an origin at the molecular scale. Lastly, the hypothesis put forward here is supported by the swelling of bio-based materials that has been observed at high relative humidity, and this study aims to identify a link between the mechanics of bio-based materials and their hygroscopic behavior. A swelling/shrinking is macroscopically observed. Combining the fields of chemistry, physics, and civil engineering allowed us to demonstrate that it comes from a molecular-scale hydromechanical coupling. This is a major breakthrough in the understanding of bio-based composites.

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Microstructure of Bio-Based Building Materials: New Insights into the Hysteresis Phenomenon and Its Consequences

Given their specific properties, their natural and renewable sources and their low environmental impact in production, natural fibers offer an opportunity for the development of eco-friendly cement-based composites. The main objective of this experimental work is to evaluate the resistance to the impact load of mortars incorporating natural fibers or polypropylene fibers at 28 days. The assessment was carried out according to an experimental protocol developed in our laboratory. The method consists in dropping a metallic ball on a square shaped specimen of 30x30x2 cm3 to determine the energy supported by each sample. For each specimen, the number of blows required for the first crack initiation and for the total collapse of specimen are detected using a device allowing to measure the speed of ultrasonic waves. The device was fixed on the specimen itself. In order to fulfill the mechanical identity card of the composites, flexural and compression tests were also carried out at 28 days. In this experimental protocol, the pozzolanic binder was considered with different fiber percentages of polypropylene (0.25% and 0.5% by mass of binder) and of natural fibers (0.5% and 1% by mass of binder). All fibers have a length of 12 mm. Results show that natural fiber reinforcement could be considered as an ecological alternative to polypropylene fiber one to improve the resistance of mortars to impact loads.

Vincent Sabathier

Papers

Interface Between an Old Concrete and a Bonded Overlay: Debonding Mechanism

Durability of bonded cement-based overlays: effect of metal fibre reinforcement

Effect of Natural and Polypropylene Fibers on early Age Cracking of Mortars

Microstructure of Bio-Based Building Materials: New Insights into the Hysteresis Phenomenon and Its Consequences

Natural Fibers vs. Synthetic Fibers Reinforcement: Effect on Resistance of Mortars to Impact Loads