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Experimental study of punching failure in LWAC slabs with different strengths

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Abstract

The constant demand of LWAC applications in structural engineering increases the need of performing studies focused on the behavior of reinforced members produced with LWAC. Mainly, by combining reduced weight and good mechanical performance, LWAC is an efficient solution for flat slabs. The design methods of punching shear strength of LWAC slabs are commonly based on experimental studies of NWC. However, both the stress–strain relation and the distribution of internal stresses of LWAC are quite different from those of NWC, due to the stiffness compatibility between the binding matrix and the LWA, and due to the enhanced performance of interfacial transition zone LWA-matrix. This behavior influences the distribution of internal stresses of LWAC, when compared with NWC, and results in a linear stress–strain relation until around 90 % of maximum stress and a brittle failure after peak when unconfined (Costa in, Lightweight aggregate structural concrete: precast and strengthening of existing structures, 2012). This difference is ignored by the main structural concrete codes or the design expressions of NWC are modified by a corrective coefficient for LWAC, depending on its density. This paper presents an experimental study focused on punching capacity of LWAC slabs. Six slabs were produced with equal longitudinal reinforcement ratio and without shear reinforcement, varying the compressive strength of LWAC from 29 to 54 MPa. Based on the recorded data during the tests, the cracking and maximum loads, the displacements, rotations and stiffness, the failure modes and cracking patterns are presented and analyzed. Experimental results were compared with design predictions of main codes, namely, EC2, MC2010 and ACI318. The results revealed that the variation of LWAC strength influences the punching strength, but has no significant effect on the stiffness and on the angle of the main crack of punching cone. The evaluation of punching shear strength achieved by design methods is higher than the experimental results and, in the case of MC2010 with level I of approximation, is more than double. Excepting for EC2, the ratio between the maximum experimental punching strength and the corresponding code prediction decreases with the increase of LWAC strength.

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Abbreviations

b :

Width of a cross-section

d :

Effective depth of a cross-section

d g :

Maximum aggregate size

f ck :

Characteristic compressive cylinder strength of concrete at 28 days

f lcm :

Mean value of compressive strength of LWAC

f lctm :

Mean value of tensile strength of LWAC

f y :

Yield strength of reinforcement

f yd :

Design yield strength of reinforcement

k I :

Stiffness corresponding to state I

k II :

Stiffness corresponding to state II

m Ed :

Average moment acting per unit length

m Rd :

Average design resistant moment per unit length

r s :

Position where bending moment is zero with respect to the column axis

u :

Control perimeter

u exp :

Experimental control perimeter

u cod :

Control perimeter predicted by codes

A s :

Cross-sectional area of tensile reinforcement

C rd :

0.18/γ c and γ c is equal to 1.5

E :

Young’s modulus of the material

E s :

Young’s modulus of reinforcing steel

P :

Applied load

P actuator :

Load applied by servo-actuator

P cr :

Cracking load

P code :

Punching resistance predicted by codes

P exp :

Maximum punching resistance recorded experimentally

P max :

Maximum load supported

V Rd,c :

Punching resistance in reinforced concrete slabs, without specific reinforcement

α s :

Equal to 40 for interior columns, 30 for edge columns, 20 for corner columns

β :

Ratio of long side to short side of the column, concentrated load or reaction area

δ :

Vertical displacement

δ cr :

Vertical displacement at cracking load

δ max :

Vertical displacement at maximum load

ψ :

Rotation of the slab

ρ l :

Tensile reinforcement ratio, equal to A s/(b.d)

ρ :

Concrete density

θ :

Angle of the main crack

∅:

Diameter of a reinforcing bar

η 1 :

Correction coefficient

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Acknowledgments

The authors would like to express their gratitude to the Department of Civil Engineering of the Polytechnic Institute of Coimbra, for providing the conditions to carry out the experimental programme. Acknowledgments are also expressed to the following companies by the material supply: Secil, Saint-Gobain Weber, BASF, Argilis.

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Authors and Affiliations

  1. ICIST, Av. Rovisco Pais, 1049-001, Lisbon, Portugal

    R. N. F. Carmo & H. Costa

  2. Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199, Coimbra, Portugal

    R. N. F. Carmo, H. Costa & M. Rodrigues

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  1. R. N. F. Carmo
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  2. H. Costa
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  3. M. Rodrigues
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Correspondence to R. N. F. Carmo.

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Carmo, R.N.F., Costa, H. & Rodrigues, M. Experimental study of punching failure in LWAC slabs with different strengths. Mater Struct 49, 2611–2626 (2016). https://doi.org/10.1617/s11527-015-0671-x

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