Strength modeling of high-strength concrete with hybrid fibre reinforcement

American Journal of Applied Sciences, Feb, 2009 by A Ravichandran, K Suguna, P.N Ragunath

INTRODUCTION

With the development of high grade cement and availability of proper mineral admixtures and chemical admixtures, it has become quite common to adopt concrete with compressive strength of 60 MPa and above. The resulting structures are slender. Development of high strength concrete has made it possible to build taller and long span structures in concrete that was not otherwise possible (1), (2). But it has been proved that the comparatively higher compressive strength of HSC is an attractive profit from engineering characteristics and economic point of view whereas, the strength behaves against the ductility of concrete by welcoming brittleness pronouncedly (3). To improve the ductility of HSC, a strategy is to introduce steel or polymeric fibres in HSC which results in development of near isotropic material with reasonable tensile strength and greater toughness which prevents the initiation and propagation of cracks (4-8).

It has also been shown recently (9-12) that by using the concept of hybridization with two different fibres incorporated in a cement matrix, the hybrid composite can offer more attractive engineering properties because the presence of one fibre enables the more efficient utilization of the potential properties of the other fibre. However, the hybrid composites studied by previous researchers were focused on hybridization of steel, polypropylene and carbon fibres. The mechanical properties of hybridization of steel and polyolefin fibres in HSC at different volume fraction (0.5, 1.0, 1.5 and 2.0%) have been studied previously are available limited. Therefore the objective of this research is to determine the basic properties of hybrid fibre reinforced high strength concrete in terms of compressive, splitting tensile and flexural tests in comparison with the plain high-strength counter part and establishes models for predicting the behaviour of HSFRC and HYFRC under compression, splitting tension and flexure.

MATERIALS AND METHODS

The cement used in concrete mixes was ordinary Portland cement 53 grade as per IS 12269-1987. The sand used was local river sand with specific gravity of 2.54. The coarse aggregate was crushed stone with size of 20 mm and 10 mm and specific gravity of 2.67. Silica fume used was a commercially available CONPLAST (MS) in densified form which improved concrete properties in fresh and hardened states. To improve the workability of concrete, a high-range water-reducing admixture [hyper-plasticizer] was used during mixing operations.

The properties of the steel and polyolefin fibres are shown in Table 1 and the concrete mix proportions used in the test program are presented in Table 2.

Preparation of test specimens: In the preparation of concrete, the constituent materials were initially mixed without fibres. The fibres were then added in small amounts to avoid balling of fibres and to produce concrete with uniform material consistency and good workability. The steel fibre reinforced concrete and hybrid fibre reinforced concrete [combination of steel-polyolefin fibres] at different volume fraction of 0.5, 1.0, 1.5 and 2.0% specimens were placed into moulds and a vibrator was used to decrease the amount of air bubbles. The specimens were remolded after 24 h and then placed in a curing tank for 28 days and the specimens were removed from curing tank and allowed to air dry 12 h prior to test.

[TABLE 1 OMITTED]

Table 2: Concrete mix proportions used in the testing program

            Material                Quantity

53 Grade cement (Kg [m.sup.-3])         450
Sand (Kg [m.sup.-3])                    780
Coarse aggregate (Kg [m.sup.-3])
20 mm size                              680
10 mm size                              450
Water (Kg [m.sup.-3])                   160
Silica fume (Kg [m.sup.-3])              25
Hyper plasticizer (Kg [m.sup.-3])        25
Slump                                100 mm

Table 3: Mechanical properties of fibre-reinforced concretes

Fibre volume (%)                 Compressive strength (MPa)

                                  Measured          Strength
Steel          PO   Total          value           effectiveness (%)

0.0           0.0    0.0            61.2               0.00

0.5           0.0    0.5            63.5               3.76

0.4           0.1    0.5            62.2               1.63

0.3           0.2    0.5            61.6               0.65

1.0           0.0    1.0            67.1               9.64

0.8           0.2    1.0            66.3               8.33

0.6           0.4    1.0            63.1               3.10

1.5           0.0    1.5            69.9              14.20

1.2           0.3    1.5            67.5              10.30

0.9           0.6    1.5            65.2               6.54

2.0           0.0    2.0            69.8              14.10

1.6           0.4    2.0            69.2              13.10

1.2           0.8    2.0            68.8              12.40

Fibre                Splitting tensile            Modulus of
volume (%)            strength (MPa)             rupture (MPa)

                     Measured   Strength       Measured  Strength
Steel   PO   Total   value      effectiveness  value     effectiveness
                                 (%)                      (%)

0.0     0.0   0.0    4.86         0.00           7.30        0.00

0.5     0.0   0.5    7.67        57.82           9.60       31.51

0.4     0.1   0.5    7.90        62.55           9.72       33.15

0.3     0.2   0.5    7.39        52.06           9.35       28.08

1.0     0.0   1.0    9.90       103.70          11.80       61.64

0.8     0.2   1.0   10.00       105.76          12.00       64.38

0.6     0.4   1.0    8.90        83.13         11.25        54.11

1.5     0.0   1.5   11.26       131.69         12.89        76.58

1.2     0.3   1.5   11.45       135.60         13.00        78.08

0.9     0.6   1.5   10.80       122.22         12.54        71.78

2.0     0.0   2.0   11.70       140.74         13.10        79.45

1.6     0.4   2.0   12.50       157.20         13.80        89.04

1.2     0.8   2.0   11.64       139.51         13.16        80.27