Behaviour of fibre reinforced concrete slabs

This paper presents a comparison of the properties of concrete slabs when two types of fibres are added. One specimen had no fibres and acted as a control specimen. The remaining four specimens had steel and polypropylene fibres added in the volumetric ratio of 0.5% and 1.0%. The dimensions of the slab specimens were 820×820×80 mm and were supported by four rollers at their edges. A displacement controlled point load was applied at the centre of the slabs. The ductilities of the tested slabs were calculated and compared so as their load carrying capacities. Results of the experimental programme were compared with theoretical predictions. Based on the results of the experimental programme, it can be concluded that the addition of 1% by volume steel fibres had the best effect on the ductility of the slabs. 2%). This study has shown that the compressive strength of HSC improved when the fibres were added in the concrete and the maximum strength happened when 1.5% fibres was added in the concrete. The splitting tensile strength also has improvement (98.3% improvement at 2% fibre) Gunneswara and Seshu (2003) studied the behaviour of the torsion of steel fibre reinforced concrete members. The volume fraction of the fibre varied from 0% to 1.2% at an equal interval of 0.3% and the strength of the concrete was 20, 30, 40, 50 MPa. 20 beams were cast for testing (100×200×2000 mm) under pure torsion. The beams were supported by two rigid supports at the each end to simulate a simply supported beam. The twist arms were placed at either supports of the beam. Loads were applied on the two twist arms. For each load increment the corresponding twist per unit length was measured until the specimen failed. All the beams failed with a single potential crack with the volume fraction 0.3 and 0.6%. The beams failed suddenly and violently and these beams got separated into two pieces. But the fibrous beams with volume fraction of fibres 0.9 and 1.2% were not separated into two pieces showing a better ductility. According to the comparison, the ultimate torsional strength, torsional toughness and torsional stiffness increased due to the addition of steel fibres. The improvement of torsional toughness has higher improvement in higher grades of concrete than low grade of concrete. Nataraja et al. (2005) tested steel fibre reinforced concrete (SFRC) mixes with fibre volumes of 0.5%, 1.0% and 1.5% and 0% (plain concrete as a reference) and concrete strengths of 30MPa and 50MPa, in order to develop and validate a mix proportioning method for SFRC mixes, and to determine the impact resistance of steel fibre reinforced concrete. Nataraja et al. (2005) cast six cubes (150×150×150 mm) and six discs (150 mm diam×64 mm thick) for each of the eight mixes, giving a total number of 96 samples for all of the mixes. The steel fibres that were used were 40 mm long, 1 mm diameter round crimped steel fibres with ultimate tensile strength of 1010 MPa. The aspect ratio of the fibres used was 40. An initial mix design was prepared with a water cement ratio of 0.5, from which two other mixes were designed using the same slump of the initial mix. Three tests were then undertaken with the eight mixes, those being a workability test, an ultrasonic pulse velocity test and an impact test. The workability test involved a slump test and a Vee-Bee test. The results of these tests showed that workability decreases significantly with the addition of fibres, although Nataraja et al. (2005) also notes that a slump test is not a good measure of workability for SFRC and that a needle vibrator will assist in placement. The ultrasonic pulse velocity test measures the uniformity of the concrete and therefore the quality of the concrete. The test indicated that the samples were of good quality and showed that the strength of the concrete increases with an increase in fibre volume, albeit marginally, as concluded by Nataraja et al. (2005). The impact test measured the impact resistance of the samples. A weight of 4.5 kg was dropped from a height of 0.45 m onto a steel ball of 64 mm diameter, which rested upon the centre of a concrete disc sample. The relative performance of the samples was measured, by comparing the number of required blows to cause visible cracks in the top surface of the disc and the number of blows required to cause the specimen to fail. The results of the test indicated that while the plain concrete sample failed in a brittle manner with only a few blows to crack the samples and cause failure, the SFRC samples had in the order of 40% to 60% increase in crack resistance compared to the plain concrete. SFRC also had up to 25 times the impact resistance of plain concrete. The conclusion of the study was that post crack resistance in SFRC is 50% higher than that of plain concrete and that the addition of steel fibre to concrete significantly improves impact resistance. Choi and Li (2003) compared the failure mechanisms of ring type steel fibre reinforcement with straight hooked-end steel fibres. Flexural tests under 3-point loading were carried out with samples of varying ring diameter (20, 30, 40, 50 and 60 mm) and fibre diameter (0.4, 0.5, 0.8 and 1.2 mm) for the ring steel fibre reinforced concrete (RSFRC) and fibre length (45 and 60 mm) at 0.6 mm fibre thickness for the straight hooked-end steel fibre reinforced concrete (SHSFRC). Concrete strength of 33.9 MPa at 28 days was used. Beam samples (350×100×100 mm) were loaded at a rate of 0.01mm/min until the midspan deflection reached 2.0 mm. The results of the test showed that the ring type fibres failed by one of three modes: tensile fibre rupture after yielding, concrete fracture (in a conical shape) and separation of the fibre and matrix. The SHSFRC failed by one mechanism only, the pull out of fibres prior to yielding. Choi and Li (2003) observed from the results that the ring diameter of the steel fibre and fibre volume both affect the flexural toughness of RSFRC and that there is an optimum ring diameter for any given concrete mixture. Flexural toughness appeared to increase with decreasing ring diameter. The research also showed that RSFRC performed better (45% overall) than conventional SHSRFC. Khaloo and Afshari (2005) performed an experimental programme in order to determine the flexural behaviour of steel fibre reinforced concrete (SFRC) slabs. Fourteen concrete mixtures with four different fibre contents, two different fibre lengths and two concrete strengths were designed in order to compare and analyse the results. The strength of the concrete was 30 and 45 MPa and the fibre volumetric percentage was 0, 0.5, 1 and 1.5. The results of the experiments of Khaloo and Afshani (2005) have shown that the plain slabs failed suddenly at cracking load without any appreciable deflection warning but the SFRC slabs with fibres failed gradually after the concrete slabs cracked. The conclusions drawn from the experimental programme are: the ultimate flexural strength of SFRC slabs does not increase significantly when the fibres are added to the concrete but the energy absorption capacity of slabs increased remarkably. Secondly, the resisting load after cracking was relatively low in the low volume fibre. The range of volume from 0.75-1.75 was recommended. Thirdly, a design method was set up according to the comparison between experimental value and theoretical value. It is known that the concrete is a brittle material. Research on how to improve the performance of the concrete continues since the last four decades. In order to reduce its brittleness and improve its mechanical properties, fibres are added to reinforced concrete. Some experiments have already been done and most results have shown that the performance of the concrete can be improved by adding the extra fibres but the magnitude of the effect and optimised content of the fibre in concrete still need further study. This paper is a step in this direction. 3 EXPERIMENTAL PROGRAMME Five slabs were cast and tested. The dimensions of the slabs were 820×820×80 mm. Wooden forms were used to cast the slabs. One of the slabs was cast without fibres, two slabs had steel fibres at the percentages of 0.5 and 1.0 by volume and the remaining two slabs had polypropylene fibres with the same percentages of 0.5 and 1.0 by volume. The first slab acted as a reference slab to the other four slabs. All slabs were made without reinforcement. Table 1 shows the details of the tested specimens. As most literature indicate that the optimum percentage of fibres is 1, this study has limited the maximum fibre percentage to 1. The steel fibres were hooked at both ends and had a length of 35 mm and a diameter of 0.55 mm. The polypropylene fibres had a length of 19 mm. Table 1 Slabs’ details. Slab No Dimensions Fibre Type Percentage by Volume mm R 82 0× 82 0× 80 None 0.0 0.5PP Polypropylene 0.5 1.0PP 1.0 0.5SF Steel 0.5 1.0SF 1.0 The concrete was purchased from a local supplier. The reference specimen was first poured to the formwork. A predetermined volume of concrete was poured to the mixer in the lab and a predetermined volume of fibres was added. The mix was mixed thoroughly and the second specimen was poured. The pouring technique was repeated for the three remaining three specimens. Small and large cylinders as well as small beams were cast for each of the five different mixes. The small cylinders (100×200 mm) were used to test the compressive strength of the concrete mix, the large cylinders (150×150 mm) were used to test the splitting strength of the concrete mix and the small beams were used to test the flexural strength of the concrete mix. All tests were conducted based on the relevant Australian Standard. After casting, all the specimens (slabs, cylinders and beams) were cured by covering them with wet Hessian and plastic sheets for 14 days. After that they were left to cure at room temperature in the labs. Four triangle steel slices were installed at the four corners of each slab formwork. Before casting the concrete, the formwork was wiped with the lubricant to allow ease of removal. Compressive strength test, indirect tensile test and flexural strength test were carried on the concrete mixes. All these tests were undertaken using the relevant Australian Standards. The slabs were removed from the curing environment and fixed on the testing machine (see Fig. 1) and four corners of the slabs were seated on the roller points. Two LVDTs were also connected to measure the deflections. A laser LVDT was used to measure the deflection of the middle of the slabs. A point load was applied by actuator and a 75×75×50 mm steel block was placed at the slab centre. A computer scanned and stored the applied load and the deflection at every three seconds. Figure 1 Slab testing. For the plain slab, the loading rate was one millimetre per 60 seconds. Once the maximum load was achieved during the test, the applied load was then decreased rapidly and then the slab collapsed. For the remaining slabs (0.5PP, 0.5SF, 1.0PP and 1.0SF), the loading rate was 60 seconds per millimetre before the slabs achieved the maximum load and then 30 seconds per millimetre was used until the slabs failed. Once the maximum load was reached during the test, the applied load decreased quickly but the ultimate deflection increased. 4 EXPERIMENTAL RESULTS 4.1 Concrete Results The results of the compressive strength, indirect tensile strength and flexural strength of the different batches are shown in the Table 2. Table 2 Results of testing concrete.