2.2.1 Dovetailed cross section fibre (DT) The fibres tested herein are different in size and material composition; three fibres are tested; one of which makes use of a dove tailed (DT) fibre shape where the stretching and diameter change produces a gripping effect in the fibre dovetails and this permits greater stress transfer once the initial bond is broken. These fibres are referenced DT (Dovetail fibres). This DT fibre technology is compared against two different synthetic structural Type 2 fibres. The variables of the Type 2 fibres were, length, profile, polymer type and cross sectional area. Due to the complex shape of the fibres, the nominal diameters of the fibres are stated, not taking into account the undulations or dovetail features, both of which can provide additional load transfer beyond the first sign of fibre slippage when under load. The pulltruded polypropylene DT fibres were 2.0mm diameter and 60mm in length and manufactured from polypropylene having a modulus of elasticity of 3500 N/mm2. This fibre has the same material properties as Type B fibres used within this research. The design makes use of the contraction of the fibre material to enhance the fibre to cement paste mechanical bond when a tensile load is applied. The gripping effect is shown in Figure 1 when the polypropylene fibre is under load, and the black arrows indicate a gripping effect due to the Poisson effect. This was measured in Richardson et al [2]. Figure 1 - Cross sectional area of DT fibre under load. (Source [21]) Figure 2 shows the end detail of the DT fibre at x 100 magnification. The dovetail fibre features are clearly visible within Figure 2 and as the inset design detail by Thomas et al [21]. The DT fibres can be used as a direct replacement for other synthetic fibre types in concrete. Figure 2 - DT fibre 2mm diameter (Inset source – [21]) 2.2.2 Comparative fibre details (Types A and B). Two different structural types were used in conjunction with the DT fibres to compare against the DT fibres, as well as to compare the relative performance between themselves. The indented/crimped fibres (A) are 50 mm x 0.941 mm nominal diameter polyethylene Type 2 fibres that have a melting temperature of 164 °C and an elastic modulus of
The (elastic – brittle) beams had zero residual strength following the initial crack and the DT 20 and DT 10 B (elastic – plastic) beams displayed strain softening following the initial cracking of the concrete. The DT 10.A beams displayed strain hardening characteristics following the initial crack as displayed in Figure 8. The DT 20 beams showed signs of strain softening immediately after the initial cracking of the concrete, then the dove tailed fibre effect can be seen which was indicated by a rise in the load transfer for an average extension of 0.3mm, thereafter acting as a non linear hinge. The fibres gradually de-bonded and failure, by pull out, was observed in the tension zone below the neutral axis. Figure 8 – Load deflection performance 4.3 Toughness The CMODj values are units of flexural strength given as N/mm2. To achieve a satisfactory performance in accordance with BS 14651, CMOD 1 must be either 1.5 N/mm2 or above and CMOD 4 must achieve a minimum value of 1 N/mm2. All of the fibre concrete results exceed the minimum values laid down in the BS for fibre concrete as displayed in Table 4. Table 4 – CMOD data Beam ref Flexural strength at CMOD 1 (CMOD 0.5mm) Flexural strength at CMOD 2 (CMOD 1.5mm) Flexural strength at CMOD 3 (CMOD 2.5mm) Flexural strength at CMOD 4 (CMOD 3.5mm) Fibres spanning rupture plane P1f 0 0 0 0 0 P2f 0 0 0 0 0 P3f 0 0 0 0 0 P4f 0 0 0 0 0 P5f 0 0 0 0 0 P6f 0 0 0 0 0 Mean DT 20.1F 2.57 2.61 2.48 2.34 39 DT 20.2f 4.38 4.19 4.00 3.78 55 DT 20.3f 4.10 3.81 3.17 2.95 52 DT 20.4f 5.94 5.93 5.77 5.54 74 DT 20.5f 4.47 4.34 4.10 3.84 46 DT 20.6f 3.69 3.67 3.64 3.33 48 Mean 4.19 4.09 3.86 3.63 52
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