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MTODOS NUMRICOS EN INGENIERA R. Abascal, J. Domnguez y G. Bugeda (Eds.)

SEMNI, Espaa 1999

1

ANALYSIS OF CONCRETE SLABS SUPPORTED ON SOIL

Joaquim Barros

Departamento de Engenharia Civil Escola de Engenharia da Universidade do Minho

Azurm, 4800 Guimares, Portugal e-mail: barros@eng.uminho.pt

Palabras clave: Material non-linear analysis, steel fibre reinforced concrete, fracture energy, strain softening, strain stiffening, soil load bearing capacity Resumen: A numerical model for material non-linear analysis of concrete slabs supported on soil is described in this work. In this model, the cracked concrete is regarded as cracks with concrete between cracks. The behaviour of the concrete between cracks is simulated by the conventional theory of plasticity. The behaviour of the cracks is defined by their constitutive laws using the concrete fracture properties. Smeared multifixed and rotating crack models are available in the model. Concrete can be reinforced with sets of smeared bars of different materials and geometric properties, as well as with discrete steel fibres. A tension-stiffening model that takes into account the concrete fracture properties and the reinforcement characteristics is used for the reinforced cracked concrete. Main effects of the fibre reinforcement are reproduced in the model, introducing the material fracture energy and a convenient softening law in the crack constitutive law, and using a new stress-strain relationship for the behaviour in compression. Soil supporting the concrete slab is simulated with springs perpendicular to the slab middle surface. The soil non-linear behaviour and the loss of contact between slab and soil are accounted for. Concrete slabs supported on soil are analysed with the model proposed. Safety of wire mesh reinforced concrete slabs and steel fibre reinforced concrete slabs is discussed. A comparison with conventional design methods is performed. The influences of the soil load bearing capacity, as well as the loss contact between soil and slab on the behaviour of slab-soil system are analysed.

Joaquim Barros

2

1. INTRODUCTION In the last two decades the increase on computer power and software facilities, as well as a

significant effort on the experimental research about material constitutive laws, contributed to the development of several computer programs for numerical simulation of concrete structures. However, numerical simulation of a slab supported on soil remains a difficult task. Accurate simulation of the behaviour of this kind of structures is only feasible if the numerical model takes into account the non-linear behaviour of the concrete, soil and reinforcement, as well as the soil-slab interaction.

In recent years, steel fibre reinforced concrete (SFRC) has been extensively applied in

industrial floors with recognised economical and technical benefits1,2,3. Some experimental research with concrete slabs on soil (or other similar support like rubber or cork layers4) has been performed in order to evaluate the enhance in the load carrying capacity and concrete cracking behaviour due to fibre reinforcement5. These tests revealed that steel fibres can replace the conventional reinforcement in industrial floors.

The first approach to design slabs on soil was proposed by Westergaard6, using the

elasticity theory. Since materials are assumed linear elastic and the design condition is based on concrete tensile strength, very thick plain concrete slabs are obtained with this approach. In the beginning of sixties Losberg7 and Meyerhof8 developed similar theories based on the yield line theory for reinforced concrete laminar structures. However, these theories are not able to reproduce the deformational behaviour of a slab-soil system until collapse load. Nowadays, design of SFRC slabs is usually performed with models devised for structures of conventional concrete, wherefore fibre-reinforcing effects are not simulated appropriately. Therefore, the numerical simulation of the behaviour of SFRC slabs supported on soil is an actual challenge for the computational mechanics community.

The present work aims to contribute to the on going research effort on the numerical

simulation of concrete slabs supported on soil. Concrete slabs can be reinforced with smeared steel roads and/or steel fibres. Main changes on concrete behaviour due to fibre reinforcement were evaluated from experimental research and introduced in the material constitutive laws9. A design of a concrete slab on soil using a linear elastic finite element computer code is compared with an analysis using the numerical model described in this work. The influence of simulating the loss of contact between the slab and the soil and the influence of the soil constitutive law in a material non-linear analysis of a slab supported on soil are discussed.

Joaquim Barros

3

2. NUMERICAL MODEL

2.1 Introduction

In the present work concrete cracking is simulated under the framework of smeared crack

concepts. Smeared crack concepts can be categorised into fixed, multifixed and rotating crack models9,10,11. These crack models are available in the computational code developed. According to the present model the total strain increment of cracked concrete, , is due to the strain increment in the fracture zone, cr (the width of the finite element over which the micro-cracks are smeared out) and to the strain increment in concrete between cracks, ep

co , = ep

co cr+ . (1) In order to simulate the progressive damage induced by plasticity and cracking, a plane shell element is discretized in layers throughout element thickness. Each layer is considered in plane stress state. The concrete shell can be reinforced with conventional smeared steel bars or/and steel fibres. The plane shell was formulated under the well-known Reissner-Mindlin theory12.

2.1 Concrete constitutive laws

For the concrete between cracks, stress and strain increment vectors are related by the constitutive law cocoD = (2) where coD is the concrete tangent constitutive matrix,

= co

s

combco

DD

D

, (3)

combD is the in-plane material stiffness matrix and

cosD is the out-plane shear stiffness matrix.

Joaquim Barros

4

2.2.1 - Linear elastic uncracked concrete

For homogeneous, isotropic and linear elastic materials, the submatrix combD in (3) is the elastic in-plane material stiffness matrix, co embD , ,

=

2100

0101

1 2,c

c

c

c

ccoemb

ED

(4)

where Ec is the instantaneous modulus of elasticity and c is Poissons coefficient. In this work, the material behaviour on transverse shear deformation is considered on linear elastic state. Therefore, the material stiffness matrix in shear has the form

==

1001

, cco

escos GFDD (5)

where cG is the concrete transversal modulus of elasticity and F is a correction shear factor

13. 2.2.2 - Linear elastic cracked concrete

For linear elastic cracked concrete (ecr), the submatrix combD in (3) is designated by co

ecrmbD , , defined by the following expression10

+==

co

emb

Tcoemb

Tcrcoemb

coemb

coecrmb

comb DNNDNDNDDDD ,

1

,,,,

(6)

where $N is the matrix that defines the direction of the cracks developed in a sampling point (see Figure 1), and $Dcr is the matrix which accounts for the constitutive law of the cracks. Each crack is governed by the following constitutive relationship

s D ecr cr cr= (7) where scr is the crack stress vector (see Figure 1)

[ ] s s scr nncr ntcr T= (8)

Joaquim Barros

5

ecr is the crack strain vector

[ ] e ecr nncr ntcr T= (9) and

DD

Dcr I

cr

IIcr=

00

(10)

is the crack material stiffness matrix, where DI

cr and DIIcr are the mode I and mode II stiffness

modulus of a smeared single crack, respectively. DI

cr is characterised by the fracture parameters, namely, the tensile strength, f ct , the fracture energy, Gf , the shape of the softening law and the crack band width10, bl . Considerable experimental and numerical work has been done to characterise the fracture parameters of plain concrete. However, for SFRC the research on its post-cracking behaviour is still scarce. Fibre reinforcement mechanisms are reflected, mainly, on the fracture energy and on the shape of the softening branch. The remainder fracture parameters are only marginally affected by fibre addition into concrete.

t

snn

+

crs crnt

ntcr

e crnnn

1x

2x

Figure 1 : Crack stress and crack strain components.

In order to evaluate the fracture energy and the shape of the softening diagram for SFRC, sets of four point bending notched beam tests were carried out under displacement control14. It was tested sets of specimens reinforced with 0, 30, 45 and 60 Kg/m3 of hooked ends Dramix steel fibres15. Based on the results obtained, the following expressions were proposed

ffo

f WGG

213.3953.19 += with 5.225.1 fW (11)

for concrete reinforced with ZP30/.50 fibres (30 mm of leng