Las múltiples identidades del taller didáctico

The first visible cracking load of all slabs tested is presented in Table 3–3. The steel reinforced concrete slab exhibited a higher first cracking load than slabs reinforced with BFRP owing to the higher axial stiffness of steel bars than that of BFRP bars. The amount of BFRP reinforcement at different locations for each slab tested has also affected the first cracking load; for example slabs C–B–OU and C–B–UO experienced the first crack at the lower reinforcement location.

Figure 3–8 sketches the cracks occurred in the continuous slabs tested. Slabs C–B–UU and C–B–UO had deeper cracks at the mid-span region than the rest of the slabs as they were under reinforced at the mid-span region. In general, the crack spacing and crack depth for all slabs reinforced with BFRP bars were clearly larger than these of the slab reinforced by steel due to the low elastic modulus of BFRP bars in comparison with steel bars.

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Figure 3–8: Crack patterns at failure of BFRP reinforced concrete continuous slabs Table 3–3: First cracking and total experimental failure loads of slabs tested

Slab Notation

First cracking loads,

: kN

Total experimental

failure load, 2 : kN

Observed failure mode

sagging hogging

C–B–OO 15.4 18.5 195.0 Flexure-shear failure at middle support

C–B–OU 17.5 13.9 140.0 Flexure-shear failure at middle support

C–B–UO 16.5 18.4 130.0 Flexure-shear failure at middle support

C–B–UU 18.6 14.4 128.0 Flexure-shear failure at middle support

S–B–O 16.2 N/A 84.8a Flexure-shear failure at end support

S–B–U 12.7 N/A 42.0a BFRP bar rupture at mid-span

C–S–UU 22.6 22.6 144.0 Flexural-Tension Failure at both mid-span

and middle support a

Just P for the simply supported slabs

Figures 3–9 and 3–10 illustrate the main crack width at both mid-span and middle support regions for all slabs tested, respectively. The control slab C–S–UU had considerably less crack width at both mid-span and middle support regions among all slabs tested due to the higher axial stiffness of steel reinforcement than that of BFRP reinforcement. For the BFRP continuous slabs, wider cracks at the mid-span region were observed in slabs C-B-UU and C–B–UO with under reinforcement ratio than the over reinforced BFRP slabs, C–B–OO and C–B–OU, at their mid-span regions. It was not expected that slab C–B–UU had less crack width at middle support region than slab

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C–B–OU. This may be as a result of local de-bonding between top BFRP bars and concrete.

Figure 3–9: Total applied load versus crack width at mid-span of all slabs tested

Figure 3–10: Total applied load versus crack width at middle support of continuous

slabs tested

Three different failure modes were observed in the experimental tests as shown in Figures 3–11 to 3–18, and summarised in Table 3–3 and explained below:

0 20 40 60 80 100 0 1 2 3 4 5 Mi d -spa n load: k N Crack width: mm

C–B–OU C–B–UU C–S–UU C–B–OO

C–B–UO S–B–U S–B–O

0 20 40 60 80 100 0 1 2 3 4 5 Mi d -spa n load: k N Crack width: mm

C–B–OU C–B–UU C–S–UU

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Mode 1: Combined flexural and shear failure–This type of failure was observed in BFRP slabs C–B–OO, C–B–UO, C–B–UU, C–B–OU and S–B–O. The failure initiated at the compression side of the middle support region, followed by a major, sudden diagonal shear crack at the same location for continuous slabs C–B–OO, C–B–UO, C– B–UU and C–B–OU as shown in Figures 3–11 to 3–14. However, the shear failure occurred close to the end support in case of S-B-O slab as presented in Figure 3–15. This is mainly attributed to the low modulus of elasticity of BFRP that significantly reduces the shear resistance of the BFRP slabs tested.

Middle support

Figure 3–11: Compressive flexural-shear failure at middle support of slab C–B–OO

Middle support

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Middle support

Figure 3–13: Flexure–shear failure at middle support of slab C–B–UU

Middle support

Figure 3–14: Flexure–shear failure at middle support of slab C–B–OU

Load cell

LVDT

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Mode 2: Conventional ductile flexural failure–This mode occurred due to yielding of tensile steel reinforcement followed by concrete crushing at both mid-span and middle support regions for the control slab C–S–UU as shown in Figures 3–16 and 3–17. Hogging flexural failure at the central support was observed earlier than that at the slab mid-span.

Mid-span

Figure 3–16 Flexural–tension failure at mid-span of slab C–S–UU

Middle support

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Mode 3: BFRP Bar rupture–This mode was experienced by slab S–B–U, that was reinforced with an under reinforcement ratio of BFRP bars at the mid-span region as shown in Figure 3–18. It was therefore expected that the strain in BFRP reinforcement would reach its ultimate limit, at the mid-span section, before the full exhaustion of the concrete ultimate strain, which usually results in such failure mode. It should be mentioned that rupture of BFRP bars was sudden and accompanied by a loud noise indicating a rapid release of energy.

Mid-span

Figure 3–18: BFRP bar rupture failure at mid-span of slab S–B–U