Post-crack (or post-peak) flexural response and toughness of fiber reinforced concrete after exposure to high temperature

Generally, the performance of most materials is characterized by parameters based on the mechanical properties such as strength, strain, or stiffness etc. However, in fibre reinforced concrete (FRC), unlike other materials, strength or stiffrress alone is not sufficient to characterize its behaviour, and the value of toughness is often used instead. In this study, two different methods (ASTM C1018 and JSCE SF-4) are used to measure the toughness of steel and polypropylene fibre reinforced concrete subjected to bending. Results indicated that in the JSCE method, the information obtained by only one specified deflection toughness seemed to be insufficient in reflecting the characteristics of the load-deflection curves of both FRCs. On the other hand, in the ASTM method, the obtained information using the four toughness values at different deflections appeared to better clariff the characteristics of both FRCs.

Toughness evaluation of steel and polypropylene fibre reinforced concrete beams under bending

Behavior and Design of High-Performance Fiber-Reinforced Concrete Coupling Beams and Coupled-Wall Systems

Concrete is considered as a brittle material, its enrichment with distributed short kenaf fibres is believed to increase the toughness of matrices. This is achieved by prohibiting the concrete from being cracked and propagated. Kenaf is a natural fibre has typically some benefits such as being renewable, eco-friendly, biodegradable and locally accessible as compared to other types of fibres used for concrete reinforcement. This study was conducted experimentally to investigate the characteristics of kenaf fibre reinforced concrete (KFRC) materials and to determine the performance of kenaf fibre reinforced concrete beams under static and dynamic loadings. The basic concrete materials used in the study are 10 mm aggregate, ordinary portland cement and clean water. Mixing procedures were evaluated to produce KFRC materials with different chopped fibre lengths (10 mm, 15 mm, 20 mm, 25 mm and 30 mm) and fibre volume fractions (0.5%, 1%, 1.5% and 2%). At the preliminary stage, the alkaline treatment test was carried out on the kenaf fibres. Then, the raw materials and concrete properties were investigated by a series of physical and mechanical property tests on fresh and hardened concrete to identify the optimum characteristics of fibre length and content to be used in the concrete mixture. The study also investigated the structural behaviour of KFRC beams, in which the samples were undertaken by monotonic bending load and repeated bending load tests under four points loading system until failure. The load-deformation behaviour of the beams was observed and monitored during testing. Results from the study of alkaline treatment on kenaf fibres found that the best condition was 5% NaOH in three hour emerging time. The study also found that the optimum length of kenaf fibres was 20 mm and the optimum volume fraction yields the value of 1%. From the KFRC beam tests, it was found that the KFRC beams exhibited better performance as compared to normal concrete beams. The best static and dynamic behaviour was observed for the beam using KFRC in tension zone and plain concrete in compression zone, with the ultimate bending load of 5.9% higher than normal concrete beam after the flexural test and 15.6% higher than normal concrete beam after the repeated load test. By observation, the number of crack formation in tension zone increased by 40% and crack spacing was less by 15% as compared to normal concrete beams. The total energy absorption from the load-deflection behaviour of KFRC beams until ultimate failure was 77% higher than normal concrete beams. The relationships between KFRC material and structural performance against fibre lengths and volume fractions have been developed based on the non-linear numerical models and proposed for the analysis and design of KFRC. Conclusively, KFRC material and structure exhibit appreciable tensile, flexural, and impact strength under static and dynamic loadings compared to normal concrete.

Performance of kenaf fibre reinforced concrete under static and dynamic loadings

Fire has been always a major concern for designers of steel and concrete structures. Designing fire-resistant structural elements is not an easy task due to several limitations such as the lack of fire-resistant construction materials. Concrete reinforcement cover and external insulation are the most commonly adopted systems to protect concrete and steel from overheating, while spalling of concrete is minimised by using HPFRC instead of standard concrete. Although these methodologies work very well for low rise concrete structures, this is not the case for high-rise and inaccessible buildings where fire loading is much longer. Fire can permanently damage structures that cost a lot of money. This is unsafe and can lead to loss of life. 

In this research, the author proposes a new type of main reinforcement for concrete structures which can provide better fire-resistance than steel or FRP re-bars. This consists of continuous braided fibre rope, generally made from fire-resistant materials such as carbon or glass fibre. These fibres have excellent tensile strengths, sometimes in excess of ten times greater than steel. In addition to fire-resistance, these ropes can produce lighter and corrosive resistant structures. Avoiding the use of expensive resin binders, fibres are easily bound together using braiding techniques, ensuring that tensile stress is evenly distributed throughout the reinforcement. In order to consider braided ropes as a form of reinforcement it is first necessary to establish the mechanical performance at room temperature and investigate the pull-out resistance for both unribbed and ribbed ropes. Ribbing of ropes was achieved by braiding the rope over a series of glass beads. Adhesion between the rope and concrete was drastically improved due to ribbing, and further improved by pre-stressing ropes and reducing the slacked fibres. Two types of material have been considered for the ropes: carbon and aramid. 

An implicit finite element approach is proposed to model braided fibres using Total Lagrangian formulation, based on the theory of small strains and large rotations. Modelling tows and strands as elastic transversely isotropic materials was a good assumption when stiff and brittle fibres such as carbon and glass fibres are considered. The rope-to-concrete and strand-to-strand bond interaction/adhesion was numerically simulated using newly proposed hierarchical higher order interface elements. Elastic and linear damage cohesive models were used effectively to simulate non-penetrative 'free' sliding interaction between strands, and the adhesion between ropes and concrete respectively. Numerical simulation showed similar de-bonding features when compared with experimental pull-out results of braided ribbed rope reinforced concrete.

Numerical modelling of braided fibres for reinforced concrete

FIBERS OF PRINCIPAL CURRENT INTEREST FOR USE AS REINFORCEMENT IN PORTLAND CEMENT PRODUCTS INCLUDE ASBESTOS, GLASS, PLASTICS AND STEEL. CONSIDERABLE RESEARCH IS NOW DIRECTED TOWARD THE DEVELOPMENT OF MATERIALS AND MANUFACTURING METHODS THAT WOULD ALLOW THE SUBSTITUTION OF GLASS FIBERS FOR ASBESTOS FIBERS IN PORTLAND CEMENT PASTE AND MORTAR. PLASTIC FIBERS MAY SERVE A USEFUL PURPOSE AS ENERGY ABSORBERS IN CONCRETE THAT IS SUBJECT TO EXPLOSIVE LOADING. BECAUSE OF COST, THE USE OF STEEL FIBERS IN PORTLAND CEMENT PRODUCTS WILL PROBABLY BE LIMITED TO SPECIAL APPLICATIONS WHERE CONVENTIONAL REINFORCEMENT IS NOT SATISFACTORY. /AUTHOR/

G E Monfore

Papers

A review of fiber reinforcement of portland cement paste, mortar, and concrete