EXPERIMENTAL PROGRAM GFRP dowel characterization

The research involved GFRP dowels measuring 34.9 and 38.1 mm (1.38 and 1.50 in.) in diameter (Fig. 1) and epoxy- coated steel reference dowels 28.6 mm (1.13 in.) in diam- eter. The GFRP dowels were made of continuous E-glass fibers in vinylester resin using the pultrusion process. The fiber content was 80.6% by weight. The physical and mechanical properties of the GFRP dowels were deter- mined using the appropriate test methods provided in CSA S807 (Canadian Standards Association 2010) and ACI 440.6 (ACI Committee 440 2008). Mechanical charac- terization included testing representative specimens of the GFRP dowel bars to determine transverse shear strength (ACI 440.3R [ACI Committee 440 2004], Test Method B.4), interlaminate shear (short-beam test) (ASTM D4475), flex- ural strength, and flexural modulus of elasticity (stiffness) (ASTM D4476). Table 1 presents the physical and mechan- ical properties of the GFRP dowels obtained from testing.

Test prototypes

A total of six JPCP prototypes were constructed and tested. The prototypes included two reinforced with GFRP dowels 34.9 mm (1.38 in.) in diameter, two reinforced with GFRP dowels 38.1 mm (1.50 in.) in diameter, and two reinforced with epoxy-coated steel dowels 28.6 mm (1.13 in.) in diam- eter. For each dowel type and diameter, one prototype was tested under monotonic load (Phase I), while the second one was tested using a cyclic-load scheme (Phase II). The test prototypes were 2440 mm long x 610 mm wide x 254 mm deep (96 x 24 x 10 in.). The slabs were cast with a 19 mm (0.75 in.) butt joint, and each test prototype had two dowel bars spaced 305 (12 in.) mm apart. Figure 1 shows the dimensions and details of the pavement prototypes.

The length of the specimens was selected based on finite-element modeling (Maitra et al. 2009), which simu- lated the experimental study conducted by the U.S. Naval Civil Engineering Research and Evaluation Laboratory (Keeton and Bishop 1957). This study revealed that the vertical shear force in a dowel beyond a distance of approxi- mately 1200 mm (4 ft) from the center of the load was insig- nificant. Thus, 2440 mm (8 ft) was selected as the total length for the jointed slab prototypes for this study. The geometry

and dimensions of the slab prototypes were consistent with jointed slab prototypes tested elsewhere (Eddie et al. 2001).

The prototypes were fabricated with a MTQ Type III-A concrete with a target 28-day compressive strength of 35 MPa (5.1 ksi), as specified in Standard 3101 for MTQ normal-density mass concrete (MTQ 2009a). The concrete mixture contained 380 kg/m3 _{(23.7 lb/ft}3_{) of GUb-SF}

cement, and had a water-cement ratio (w/c) of 0.42, with high-range water-reducing admixture to maintain a mixture slump of 120 ± 30 mm (4.7 ± 1.18 in.) (MTQ 2009a). The pavement prototypes were cast at the structural laboratory using three concrete batches. The average concrete strength of the concrete batches was 48.0 ± 3.5 MPa (7.0 ± 0.5 ksi) based on testing of three concrete cylinders (150 x 300 mm [5.9 x 11.8 in.]) from each batch.

Subgrade base layer

The granular base consisted of three 100 mm (4 in.) thick layers of limestone aggregate compacted using a 90 kg (198 lb) vibrating plate. The granular mixture was prepared according to AASHTO specifications (Class A). The gran- ular subgrade mixture consisted of 50% sand (0 to 5 mm [0 to 0.2 in.]), 20% 10 mm (0.4 in.) crushed rock (5 to 14 mm [0.2 to 0.6 in.]), and 30% 20 mm (0.8 in.) crushed rock (14 to 28 mm [0.6 to 1.1 in.]). Aggregates were dampened before placing to maximize the compaction efficiency. Once the base was completed, a thin layer of sand was applied to the final surface to provide contact between the concrete surface and subgrade. The base was extended by 300 mm (12 in.) on all sides to allow for load distribution and prevent failure of the base-layer container. The overall dimensions of the base layer were 1.52 m wide x 3.35 m long x 0.30 m deep (5 x 11 x 1 ft). Upon completing the base, the base modulus (stiffness) was measured using a Briaude Compacting Device (BCD), and was 110 MPa/m (4.9 ksi/ft).

Testing loads and procedures

The JPCP prototypes were tested under two different loading conditions: static (Phase I) and cyclic (Phase II). During Phase I, the prototypes were monotonically loaded to 200 kN (45 kip) to induce cracks at the joints. Thereafter, the load was released, and the prototypes were loaded again up to failure. During Phase II, the prototypes were subjected to 1 million cycles ranging from 10 to 50 kN (2.25 and 11.24 kip), followed by monotonic loading up to failure.

Table 2 summarizes the loading schemes, while Fig. 2 shows the test setup.

For static testing (Phase I), the monotonic load was applied with a stroke-controlled rate of 0.01 mm/sec (0.02 in./min) to allow for progressive contact and loading. The load was applied using a 1000 kN (225 kip) hydraulic actuator on one side of the joint over a loading plate of 306 mm (12 in.) in diameter. The prototypes were loaded up to 200 kN (45 kip), then the load was released. Thereafter, the prototypes were loaded again at the same rate until failure. E and LTE were calculated at an applied load of 40 kN (9 kip) (service load, which is equal to one half the equivalent axle load) from the deflection measurements of two linear variable differ- ential transducers (LVDTs) on both joint sides (loaded and unloaded).

For the fatigue testing (Phase II), the prototypes were tested up to 1 million cycles. The load followed a sinusoidal waveform that varied from 10 to 50 kN (2.25 to 11.24 kip). The minimum load (10 kN [2.25 kip]) was required to main- tain contact between the slab and loading plate and to mini- mize the impact on the subgrade. The maximum load (50 kN [11.24 kip]) was set to achieve the service load and keep 40 kN (9 kip) as the cyclic test amplitude, which is equal to one-half the equivalent axle load (service load). It should be mentioned that this loading scheme closely represents field conditions under which load is applied and removed as a vehicle approaches the joint or moves away from it. The load was applied with the same hydraulic actuator (1000 kN [225 kip]) with a load-controlled scheme. The loading

Table 1—Physical and mechanical properties of GFRP dowel bars

Physical properties Mechanical properties

GFRP dowel diameter, mm 34.9 38.1 GFRP dowel diameter, mm 34.9 38.1

Fiber type Glass E-type Transverse shear strength, MPa 184 ± 2 173 ± 3 Resin type Vinyl ester resin Short beam shear strength, MPa 61 ± 0 54 ± 2 Fiber content, % 80.7 80.6 Four-point flexural strength, MPa 1210 ± 50 1077 ± 61

Cure ratio, % 100 100 Flexural modulus of elasticity, GPa 50.3 ± 0.5 51.6 ± 0.8

Tg, oC 124 123 — — —

Moisture uptake, % 0.06 0.07 — — —

Notes: 1 mm = 0.0394 in.; 1 MPa = 0.145 ksi; 1 GPa = 145 ksi; °C = 5/9(°F – 32).

Table 2—Loading schemes of test prototypes

Phase Number and prototypes Loading scheme

I Static

One 34.9 mm (1.38 in.) GFRP

Monotonic to 200 kN (45 kip), unloading, monotonic reloading

to failure. One 34.9 mm (1.50 in.) GFRP One 28.6 mm (1.13 in.) epoxy-coated steel II Cyclic One 34.9 mm (1.38 in.) GFRP

One million cycles between 10 and 50 kN (2.24 to 11.24 kip) at 15 Hz. Thereafter, static testing

until failure. One 34.9 mm (1.50 in.)

GFRP One 28.6 mm (1.13 in.)

and unloading was applied at a frequency of 15 Hz. This frequency is equivalent to the time that a vehicle needs to cross the joint, assuming a speed of 65 to 80 kph (37.3 to 49.7 mph) (MTQ 2009b). Because the test prototypes did not fail after 1 million cycles, the prototypes were retested under monotonic static load until failure. Before cycling, as well as after predetermined sets of cycles (1; 1000; 10,000; 100,000; 500,000; and 1,000,000), the load cycling was interrupted, and a monotonic loading test up to 40 kN (9 kip) (service load) was conducted to assess joint performance.

TEST RESULTS