Shear capacity of concrete slabs reinforced with glass-fiber ... Journal... of series C slabs. Material

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  • 83PCI Journal | Summer 2012

    Corrosion of steel in reinforced concrete structures is one of the main factors limiting the service life of bridge decks and parking structures. Chlorides from deicing salts or a marine environment act as catalysts for the corrosion of steel in concrete. Corrosion mitigation requires expensive maintenance, repair, or replacement. The use of glass-fiber-reinforced polymer (GFRP) bars as internal reinforcement is a possible solution to corrosion of steel bars. In addition to their noncorrosive properties, GFRP bars have higher strength than steel bars and are light and easy to handle, which makes them attractive as reinforcement for certain concrete elements, such as slabs. However, GFRP bars have different mechanical properties from steel; GFRP bars behave in a linear elastic manner until rupture, which makes concrete members reinforced with GFRP bars vulnerable to brittle failure.

    Considerable research has been undertaken to investigate both flexural and shear performance of GFRP-reinforced concrete structures. Despite the differences in material properties compared with steel bars, the prediction of flexural capacity using the strain compatibility approach is still effective. The behavior of lightweight concrete slabs reinforced with GFRP bars without shear reinforcement is a topic of active research. Prediction of shear capac-

    ■ This paper reports an experimental investigation of the flexural and shear performance of concrete structures reinforced with glass-fiber-reinforced polymer (GFRP).

    ■ Simply supported slabs of both normalweight and lightweight concretes with compressive strengths in excess of 8000 psi (55 MPa) were tested.

    ■ Modified compression field theory first- and second-order equations can provide accurate yet conservative predictions of the behavior of GFRP-reinforced concrete despite the differ- ences in mechanical properties between GFRP and steel.

    ■ The predictions are less conservative for lightweight than for normalweight concrete.

    Shear capacity of concrete slabs reinforced with glass-fiber-reinforced polymer bars using the modified compression field theory

    Ruifen Liu and Chris P. Pantelides

  • Summer 2012 | PCI Journal84

    ity is essential in the design of GFRP reinforced concrete members, as Guide for the Design and Construction of Structural Concrete Reinforced with FRP Bars (ACI 440.1R-06)1 recommends that such members be designed as overreinforced, making them vulnerable to shear failure. There is little research available on GFRP-reinforced slabs constructed with high-strength normalweight or light- weight concrete.

    The modified compression field theory (MCFT) is an analytical model with 15 equations that produce accurate estimates of shear strength for steel-reinforced concrete members.2 Bentz and Collins3 reduced the MCFT equations to two, which still accurately estimate the shear strength of steel-reinforced concrete members.4 Hoult et al.5 found that crack widths are affected by both a size effect and a strain effect regardless of the type of reinforcement used; they also showed that the two MCFT equations proposed by Bentz and Collins work equally well in predicting the shear capacity of normalweight concrete slabs reinforced with steel or FRP reinforcement.

    Sherwood et al.6 demonstrated that the width of a member does not affect the shear stress at failure for steel-rein- forced concrete members, which indicates that the MCFT could be used for both beams and slabs. Bentz et al.7 found that despite the brittle nature of the reinforcement, FRP- reinforced large concrete beams behave similarly in shear to steel-reinforced concrete beams. In this paper, a series of 20 tests is presented to investigate the influence of slab width and depth, slab span, concrete compressive strength, and type of concrete (lightweight versus normalweight) on the shear strength of GFRP-reinforced slabs. The maxi- mum deflection of the slabs under service loads satisfied the American Association of State Highway and Transpor- tation Officials’ AASHTO LRFD Bridge Design Specifica- tions8 in the tests for the slabs designed for flexure accord- ing to ACI 440.1R-06 guidelines (Pantelides et al.9).

    Experimental program

    Twenty slabs were tested to investigate the behavior of GFRP-reinforced concrete slabs constructed with high- strength normalweight or lightweight concrete. The construction variables included unit weight and compres- sive strength of concrete, slab span and depth, slab width, and reinforcement ratio. Four series of slabs were built with different dimensions or reinforcement ratios. Figure 1 shows the top and bottom reinforcement for series A and B slabs. Series A and B slabs have the same width (2 ft [0.6 m]) but different spans and depths. Series C slabs have the same reinforcement, thickness, and span as series A slabs, but their widths are 6 ft (1.8 m). Series D slabs have the same dimensions as series C slabs, but series D slabs have a GFRP reinforcement ratio approximately half that of series C slabs.

    Material properties

    The normalweight concrete used in this study was ready- mixed concrete incorporating a 3/4 in. (19 mm) crushed limestone. The specified compressive strength of both normalweight and lightweight concretes was 6000 psi (41 MPa); however, several batches were cast at different times and consequently the concrete compressive strength for the normalweight concrete at the time of testing ranged from 8500 psi (59 MPa) to 12,600 psi (87 MPa) and for lightweight concrete from 8100 psi (56 MPa) to 10,900 psi (75 MPa).

    The lightweight concrete used was sand-lightweight con- crete, which had a coarse aggregate (expanded shale) size of 1/2 in. (13 mm). The unit weight of the sand-lightweight concrete used was 123 lb/ft3 (1970 kg/m3).

    The GFRP bars used for construction were no. 5 (16M) bars. The tensile strength of the specific lot of GFRP bars used in these tests was 103,700 psi (715 MPa), and the modulus of elasticity was 6280 ksi (43 GPa), as determined from tensile tests of the bars according to ACI 440.3R-04.10

    Table 1 shows the concrete compressive strength at the time of testing, the actual reinforcement ratio, and the bal- anced reinforcement ratio.

    Test setup and procedure

    All slabs were tested as simply supported members on two reinforced concrete beams (Fig. 2). Elastomeric pads 6 in. (150 mm) wide and 2 in. (50 mm) thick were placed on the supporting beams so that the slabs could rotate freely near the support without coming into contract with the beams.

    The load was applied using a hydraulic actuator through a 10 in. × 20 in. × 1 in. (250 mm × 500 mm × 25 mm) steel bearing plate for all slabs, which simulates the area of a double-tire truck load on a bridge deck.8 The steel bearing plate was placed directly on the concrete surface of the panels. The wider panels are subjected to a combination of one-way shear and punching shear. The load was applied as a series of half-sine downward cycles of increasing am- plitude without stress reversals. The load application was displacement controlled at a constant rate of 0.2 in./min (5 mm/min). The loading scheme was intended to simu- late repeated truck loading applied to the slab of a precast concrete bridge deck.

    Test results

    During testing, all slabs developed flexural cracks at low loads and additional diagonal cracks as the loads increased. Ultimately, the slabs failed in diagonal tension (Fig. 3). After formation of the critical diagonal crack near one of

  • 85PCI Journal | Summer 2012

    the two supports, the concrete crushed on the compression face of the slabs. All slabs failed the same way regard- less of concrete type (normalweight or lightweight), slab dimensions, or amount of reinforcement. In a few tests, a few GFRP bars in the top mat near the outer edges of the slab snapped and sheared off after the ultimate load was reached, shortly before the ultimate deflection (Fig. 3). This occurred after the concrete cover had spalled off and

    the bars were exposed, and was the result of the GFRP bars trying to carry the compression forces arising from the applied load. The GFRP bars in the bottom mat did not fracture in any of the tests even though they experienced significant tensile strain and deformation. Table 1 shows the concrete compressive strength at the time of testing, the actual reinforcement ratio, the balanced reinforcement ratio, and the experimental shear capacity.

    Figure 1. Dimensions for top and bottom glass-fiber-reinforced polymer reinforcement mat for slabs. Note: no. 5 = 16M; 1 in. = 25.4 mm; 1 ft = 0.305 m.

    6 ft

    2 ft 8 ft 2 ft

    18 N

    o. 5

    a t 4

    in .

    24 No. 5 at 6 in.

    A

    2 in

    .3 in. 3 in.

    2 in

    .

    A

    Support

    6 ft

    2. 12

    5 in

    . 1

    in .

    4 in.

    C ov

    er

    2 in.

    9. 25

    in .

    4 in. Section A-A

    Series C slabs

    6 ft

    2 ft 8 ft 2 ft

    8 N

    o. 5

    a t 8

    in .

    18 No. 5 at 8 in.

    A

    2 in

    . 4 in. 4 in.

    2 in

    .

    A

    6 in

    . 6

    in .

    Support

    6 ft

    2. 12

    5 in

    . 1

    in .

    8 in.

    C ov

    er

    5 in. 9. 25

    in .

    8 in.

    Section A-A

    Series D slabs

    2 ft

    2 ft 9 ft - 6 in. 2 ft

    6 N

    o. 5