Stress-strain relationship of fibre reinforced concrete exposed under elevated temperatures

A research had been done to study the stress-strain relationship of fibre reinforced concrete (FRC) exposed under elevated temperature. The FRC used were single fibre concrete and hybrid fibre concrete which is combination of two different properties of fibres i.e. steel (SF) and polypropylene (PPF) by applying fibres volume fraction at 1.5%. At the same time, the fibres proportion of steel-to-polypropylene ranged in the following percentages: (100-0), (75-25), (50-50), (25-75) and (0-100). Cylinder samples of 150 mm diameter x 300 mm high were used and subjected to a compressive load to determine the relationship. All samples were casted and then water cured for 28 days before exposing them to the desired temperature i.e. 200ºC, 400ºC, 600ºC and 800ºC for 1 hour. For the control specimens, the cylinders were left at room temperature (27ºC) until the test day. Before placing the cylinders into the compression testing machine, they were left to cool naturally. All test results were tabulated and the stress-strain relationships were compared between the variations of the elevated exposure temperature. The findings show that the addition of fibres only improve the Elastic Modulus of concrete at room temperatures (27ºC), but when exposed under elevated temperatures, the Elastic Modulus decreased especially above 400º. High temperature

Toughness properties of steel-polypropylene fibre reinforced concrete under elevated temperature

Application of fibres in concrete which is also known as fibre reinforced concrete (FRC) has been found to improve the energy absorption or toughness of the material. However, only little information on FRC toughness is found when it is exposed to high temperature such as under prolonged heat or fire. The main objective of the study is to evaluate the toughness behaviour of FRC by experimentation when exposed to elevated temperature. The fibre used in the experimental work is steel (ST) or polypropylene (PP), and also the combination of both fibres. The fibre dosage varied between the ST and PP summarised as (100-0), (75-25), (50-50), (25-75) and (0-100). Meanwhile, the total volume fraction, Vf is fixed at 1.5% and they are exposed to elevated temperature at the following degree; room temperature (27 C), 200 C, and 400 C. The research shows that the addition of fibres in concrete enhanced the FRC toughness, however, it reduces as the exposure temperature increases

Finite element modelling of interface shear strength at concrete-to-concrete bond

Interface shear strength between concrete layers cast at different times plays an important role to provide monolithic behavior of composite concrete. In this paper, a computational modeling approach is used to study the concrete-to-concrete bond behavior between two concrete layers cast at different times; concrete base and concrete topping. The compressive strength of the concrete base is 40 N/mm2, while the concrete topping is 25 N/mm2. Finite Element Analysis (FEA) package ABAQUS 6.12 is used to model the bond interaction of concrete-to-concrete layers, which is then verified with the experimental test. Four specimens with different types of surface textures are - -brushing in transverse direction and projecting steel reinforcement crossing the interface. Failure of the bonded interfaces is modeled with cohesive zone model (CZM) approach with zero thickness interface element where the governing parameters are - meanwhile, the projecting steel surface is modeled with Modified Drucker-Prager/Cap-Plasticity Model (CPM) approach with 1 mm thickness of interface element. The parameters used in the analysis include interface shear strength, fracture energy and elastic shear stiffness for CZM approach. The CPM parameters for modeling projecting steel surface are cohesion, interface friction angle, cap eccentricity parameter, initial cap yield surface position, flow stress ratio, yield stress at interface. The study shows that the difference between the modeled and experimental results is relatively small and therefore shows the capability of the finite element analysis to carry out interface analysis

Interface shear strength of concrete-to concrete bond with and without projecting steel reinforcement

Composite concrete consists of two elements cast at different times which are the concrete base and concrete topping. To achieve composite action, interface shear strength must be sufficient to resist the sliding motion between the two concrete surfaces in contact. The interface shear strength is mainly depended on concrete cohesion, friction and dowel action. A total of 36 “push-off” tests were performed to study the interface shear strength and to assess the influence of surface texture and steel reinforcement crossing the interface. Three different concrete base surfaces are prepared which include smooth or “left as-cast”, roughened by wire-brushing in the transverse direction and steel reinforcement projecting from the concrete base. Eurocode 2 provides design equations for determining the interface shear strength with different surface textures and also the one where projecting steel reinforcement crosses the interface. The experimental results show that the transverse roughened surface produced the highest interface shear strength of 1.89 N/mm2 (sn = 0 N/mm2), 4.69 N/mm2 (sn = 0.5 N/mm2), 5.97 N/mm2 (sn = 1.0 N/mm2) and 6.42 N/mm2 (sn = 1.5 N/mm2) compared with the other surface textures. This proves that the increase in the degree of roughness contributes to higher concrete cohesion and friction coefficient. However, for the surface with projecting steel reinforcement, the failure is not sudden as experienced by the surface without one. This is due to the contribution of the clamping stress from the dowel action of the steel reinforcements. Meanwhile, for specimens without any projecting steel reinforcements, the interface shear strength depended solely on friction and concrete cohesion of the surface textures. The interface shear strength of surface with and without the projecting steel reinforcement can be predicted using the Mohr-Coulomb failure envelope. This paper also proposed design expressions for concrete-to-concrete bond on surfaces provided with and without projecting steel reinforcement that can be adopted in Eurocode 2

Finite element modeling of the interfacial behavior at surface roughness concrete without the projecting steel

The development of composite action between precast concrete slab and concrete toppings is important to provide a monolithic behavior of composite concrete. The current Eurocode 2 provides design expression for interface shear strength of both concrete base and concrete topping. In this paper, the finite element modeling is presented and calibrated with experimental results. The interface shear strength between precast concrete slab and cast-in place concrete topping slabs was evaluated through a set of 9 push-off experiments. Finite Element Modeling package ABAQUS 6.12 was used to model the interface bond of concrete-to-concrete layers. The push-off test specimens featured segments of precast concrete slabs sized 300 mm × 300 mm × 100 mm with a variety of surface textures including trowel finished, indented and wire-brush roughened. A cast-in place concrete was poured on top of the concrete base to form a 300 mm × 300 mm × 75 m concrete topping. Failure of the bonded interfaces was modeled with cohesive zone model (CZM) approach with zero thickess interface element. The parameters used in the analysis include interface shear strength, fracture energy and elastic shear stiffness. The study shows that the difference between the model and experimental results is relatively small and therefore shows the capability of the finite element modeling to carry out interface analysis

Mechanical and shrinkage properties of hybrid steel and polypropylene fibre reinforced concrete composite

An experimental study had been carried out to investigate the mechanical properties, expansion and shrinkage of fibre reinforced concrete composite (FRC). However, instead of using single type fibre of either steel (SF) or polypropylene (PPF), this study also combined the two types in one mix.The mechanical properties investigated in this study include compressive strength, splitting tensile strength and flexural strength. Three different FRC mix proportions and one normal concrete (control) were casted which includes (a) 75% SF, (b) 75% SF + 25% PPF, (c) 25% PPF, and (d) 0% fibre for control (PC). Meanwhile, the volume fraction, Vf for the FRC was fixed at 1.5% and the concrete strength was designed to achieve grade C60 at 28 days. The results show that the use of fibres in concrete decreased the workability of concrete. In addition, concrete mix with both SF and PPF produced the highest splitting tensile and flexural strengths by an increase of 75.9% and 86.5%, respectively as compared with the control. Furthermore, expansion and shrinkage of FRC was found to be less than the control. It can be concluded that the combined SF and PPF in concrete gives the most appropriate combination as regards to the highest flexural and splitting tensile strengths, and also reduced the shrinkage strain

Flexural behaviour of reinforced concrete beams with discrete steel - Polypropylene fibres

This paper discusses the experimental results on the flexural test of concrete containing different proportions of steel fibre (SF) and polypropylene fibre (PPF). The flexural test was carried out under 4-point bending load and followed the relevant standards to FRC. Hooked-end deformed SF fibre with 60 mm length and fibrillated virgin PPF fibre with 19 mm length were used in this study. Meanwhile, the concrete was designed for high strength concrete of C60. The mixture included both single SF and PPF, and also the combination of both fibres; Control beam (PC), beam with 75%SF, beam with 75%SF + 25%PPF and beam with 25%PPF. The total fibre volume fraction (Vf) was fixed at 1.5%. The experimental results show that the percentage proportion of combined SF-PPF at 75-25% had the best performance for its flexural capacity. Mixture with single PPF was also found not effective in delaying the onset of tension cracks and to increase the tensile strength of the concrete. Experimental result also shows beam with 75%SF +25%PPF had their structural stiffness improved the most as compared with the others. For the compressive strength, beam with 75%SF + 25%PPF also revealed comparable performance with the control for high strength composite concrete

Comparison of bond stresses of deformed steel bars embedded in two different concrete mixes

Catenary action in a precast concrete building structural system is one of the ways to avoid progressive collapse. The key for catenary action to work successfully depends on the strength performance of longitudinal ties, which closely depends on the bond performance between the ties and concrete. This paper investigates the effectiveness of deformed steel bar as catenary tie in precast concrete beam-column connection under column removal scenario. The main objective of the experimental work is to improve the bond performance between deformed steel bar and concrete topping. The parameter considered in the tests is the types of concrete for the topping. The different concrete mixes are normal concrete of Grade 40 and steel fiber reinforced concrete (SFRC). A series of pullout test specimens are conducted to investigate the bond behavior between the steel ties and the surrounding concrete. The results show the comparison of bond stresses of embedded deformed steel bars in two types of concrete mix. The deformed steel bar with concrete fiber provides higher bond strength as compared to bond in normal concrete. Therefore, it is more suitable for effective catenary tie in precast concrete beam-column connection for maximum efficiency and deformability in order to minimize progressive collapse

Influence of rectangular steel splice-sleeve for precast concrete connection

Precast concrete building system has gained its popularity in Malaysia because of the many advantages such as high quality of structural components, less labour intensive at the construction site, and shorter completion time of a project. One of the constraints in precast concrete structures is to ensure that the connections are strong enough to ensure the structural integrity and robustness of the overall frames. In this study, a total of nine rectangular steel splice-sleeve connections were tested experimentally under incremental tensile loads. Two steel plates were inserted and welded to each end of the steel splice-sleeve. The steel plates act as shear key to provide the interlocking mechanism to the grout and to enhance the bond property between the grout and the splice. These plates were adopted to prevent the grout slippage from the sleeve. The grout strength, embedded steel bar lengths and the size of the steel sleeve splice were varied among the specimens to study their effect on the tensile performance of the connection. The results showed that the higher strength of grout, longer embedded length of steel bar and smaller size of the sleeve contributes to a higher ultimate tensile load

The chemical properties of seaweed for modify concrete

Seaweed is one of the natural sources usually use in industrial food, cosmetic, pharmaceutical and fertiliser. A natural polymer material such as seaweed is found to have excellent bonding mechanism and also the critical factor to achieve sustainability. Malaysia is also rich with seaweed species like Eucheuma Cottonii. The future study was carried out seaweed species from Malaysia especially seaweed from Sabah. This research aimed to investigate the relationship of a few chemical properties namely physical properties, metal element content, microstructure image and chemical component of seaweed type of Eucheuma Cottonii for modifying concrete. This research is mainly base on experimental works. That powdered seaweed from Eucheuma Cottonii was analysis using Energy Dispersive X-Ray (EDX), Scanning Electron Analysis (SEM) and Fourier Transform Infra-Red (FT-IR). All the procedure of the laboratory work complies with the specified and relevant standard. The research had shown that Euchema Cottonii powder content suitable for metal and component to bind together with concrete. Concrete with seaweed powder is alternative to become green construction material for sustainable concrete