A study on the punching shear behavior of 8 slabs with recycled aggregate concrete (RAC) was carried out. The two main factors considered were the recycled coarse aggregate (RCA) replacement percentage and the steel fibre volumetric ratio. The failure pattern, load-displacement curves, energy consumption, and the punching shear capacity of the slabs were intensively investigated. It was concluded that the punching shear capacity, ductility and energy consumption decreased with the increase of RCA replacement percentage. Research findings indicated that the incorporation of steel fibres could not only improve the energy dissipation capacity and the punching shear capacity of the slab, but also effectively improve the integrity of the slab tension surface and thereby changing the trend from typical punching failure pattern to bending-punching failure pattern. On the basis of the test, the punching shear capacity formula of RAC slabs with and without steel fibres was proposed and discussed.

Tab.5 The load and deflection of the slabs punching failure

Fig.7 The relationship between slab load and reinforcement strain. (a) No. S-1–S-5 of RAC0; (b) No. S-1–S-5 of RAC30; (c) No. S-1–S-5 of RAC50; (d) No. S-1–S-5 of SFRAC50-0.5%; (e) No. S-1–S-5 of SFRAC50-1%; (f) No. S-1–S-5 of RAC100; (g) No. S-1–S-5 of SFRAC100-0.5%; (h) No. S-1–S-5 of SFRAC100-1%; (i) No. S-3 of RAC; (j) No. S-3 of RAC50 with steel fibres; (k) No. S-3 of RAC100 with steel fibres

Fig.8 The relationship between slab load and concrete strain. (a) RAC0; (b) RAC30; (c) RAC50; (d) SFRAC50-0.5%; (e) SFRAC50-1%; (f) RAC100; (g) SFRAC100-0.5%; (h) SFRAC100-1%; (i) No. C-1 of RAC; (j) No. C-1 of RAC100 with steel fibres

Fig.9 $P-\Delta $ curves. (a)$P-\Delta $curve of 8 recycled concrete slabs; (b)$P-\Delta $curve of RAC without steel fibres; (c)$P-\Delta $curve of RAC50 with steel fibres; (d)$P-\Delta $curve of RAC100 with steel fibres; (e)$P-\Delta $curve of steel fibre reinforced RAC with ${V}_{f}=\mathrm{0.5}\%$; (f)$P-\Delta $curve of steel fibre reinforced RAC with ${V}_{f}=\mathrm{1.0}\%$

Fig.10 $P-\Delta $ curve and equivalent ductility line

specimen

displacement ductility coefficient

energy absorption ${S}_{\Delta}(\mathrm{k}\mathrm{N}?\mathrm{m})$

RAC0

1.7706

6.6210

RAC30

1.7170

5.0504

RAC50

1.7132

4.8520

SFRAC50-0.5%

2.3500

10.2135

SFRAC50-1%

2.4917

9.6608

RAC100-0%

1.6502

4.3504

SFRAC100-0.5%

1.6908

5.1055

SFRAC100-1%

2.3655

9.4675

Tab.6 Displacement ductility coefficient and energy absorption

specimens

cube

cylinder

side (mm)

strength grade

200

150

100

C20-40

C50

C60

C70

C80

compressive strength

0.950

1.000

1.050

0.800

0.830

0.860

0.875

0.890

Tab.7 Concrete compressive strength relative value of different shapes and sizes specimens [^{31}]

Tab.8 Punching calculations of the steel fibres reinforced recycled concrete slab

Fig.11 The contrast between ${P}_{u}^{\mathrm{c}\mathrm{a}\mathrm{l}}$ and ${P}_{u}$

The following abbreviations and symbols have been used in this paper:

RAC

=

recycled aggregate concrete

RCA

=

recycled coarse aggregate

SFRAC

=

steel fibre recycled aggregate concrete

LWAC

=

lightweight aggregate concrete

NAC

=

natural aggregate concrete

HRB

=

hot-rolled ribbed bar

LVDTs

=

linear variable differential transformers

h

=

the depth of slab (mm)

h_{0} P_{u}

=

the effective depth of slab (mm) the failure load (kN)

$\Delta $ ${\mu}_{\Delta}$

=

the deflection at the center of slab (mm) the displacement ductility coefficient

${\Delta}_{\mathrm{0}}$

=

the deflection corresponding to the failure load (mm)

${\Delta}_{y}$

=

the nominal yield deflection calculated by the method of equivalent energy (mm)

${S}_{\Delta}$

=

the energy absorption of slab punching failure mode ($$\mathrm{kN}?m$$)

V_{Rd,c}

=

the design value of the punching shear resistance of a slab without punching shear reinforcement along the control section considered (MPa)

C_{Rd,c}

=

a parameter in Eq. (1)

${\gamma}_{c}$

=

The partial factor for concrete

k_{1} r_{1}

=

a parameter in Eq. (1) the reinforcement ratio for longitudinal reinforcement

d

=

the depth of slab (mm)

${\rho}_{ly}$

=

a parameter related to the bonded tension steel in y- direction

${\rho}_{lz}$

=

a parameter related to the bonded tension steel in z- direction

${f}_{ck}$

=

the characteristic compressive cylinder strength of concrete for 28 days (MPa)

${\sigma}_{cp}$

=

The compressive stress in concrete from axial load or pre-stressing (MPa)

${\sigma}_{cy}$

=

the normal concrete stress in the critical section in y- direction (MPa)

${\sigma}_{cz}$

=

The normal concrete stress in the critical section in z- direction (MPa)

${\mu}_{\mathrm{1}}$

=

the basic control perimeter (mm)

${\nu}_{\mathrm{min}?}$

=

a parameter in Eq. (1)

${\beta}_{p}$

=

the influencing factor of the steel fibre on reinforced RAC

${V}_{\mathrm{f}}$

=

the volume of steel fibres

${l}_{\mathrm{f}}$

=

the length of steel fibres (mm)

${d}_{\mathrm{f}}$

=

the diameter of steel fibres (mm)

${P}_{u}^{\mathrm{c}\mathrm{a}\mathrm{l}}$

=

the calculated value of concrete slabs’ punching shear capacity (kN)

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