Crack control concrete

 Cracking must be controlled and deflections must not be excessive  Cracks should not be unsightly or wide enough to lead to durability problems  Design for the serviceability limit

Design Procedures

 In order to satisfy the serviceability

limit states, a concrete structure must

be serviceable and perform to its

intended function throughout its

working life

 Cracking must be controlled and

deflections must not be excessive

 Cracks should not be unsightly or wide

enough to lead to durability problems

 Design for the serviceability limit states

involves making reliable predictions of

the time dependent deformation and

cracking of the concrete structure

 The non-linear behaviour that complicates

serviceability calculations is due to

cracking, tension stiffening, creep and

shrinkage

 Of these, Shrinkage is the most problematic

 Shrinkage causes curvature on

non-symmetrically reinforced sections

 Shrinkage causes time dependent

cracking; and

 Shrinkage causes a reduction in tension

stiffening with time

 All of these contribute to the long term

deflection and cracking of a RC member

 The width of a crack depends on:

 The quantity, orientation and distribution of the reinforcing steel

crossing the crack

 Concrete cover and member size

 The bond between the concrete and reinforcement

 The deformation properties of the concrete

Crack Control

Simplified Approach

 For reinforced concrete beams and slabs, cracking is deemed to be

controlled (crack widths will be less than 0.3mm) if each of the

following is satisfied:

 The quantity of the tensile reinforcement in a beam of slab

provides an ultimate limit strength at least 20% higher than the

cracking moment (with cs taken as 0)

 The distance from the side of soffit of the member to the centre of the nearest longitudinal bar doesn’t exceed 100mm

 The centre to centre spacing of bars near a tension face of a beam

or slab shall not exceed 300mm for a beam and the lesser of two

times the slab thickness and 300mm for a slab

 The stress in the tensile steel is less than a limiting value

Simplified Approach

 For members subject primarily to flexure, the calculated steel stress

caused by the serviceability design moment shall not exceed the larger

of the maximum steel stresses given in Tables 8.6.2.2 & 9.5.2.1

Simplified Approach

 For members subject primarily to flexure, the calculated steel stress

caused by the serviceability design moment shall not exceed the larger

of the maximum steel stresses given in Tables 8.6.2.2 & 9.5.2.1

AS3600 Refined Approach

 The final crack width is the elongation of the tensile steel over the final crack spacing minus the extension of the concrete caused by the

induced load plus the shortening of the concrete between the cracks

AS3600 Comparison to Experiments

Crack widths predictions at

~50-80% of ultimate load

Tension Chord Model

13

Tension Stiffening in SFRC/RC

Consider the response of 2 tension ties:

Cracking load remains about the same

Visibly reduced crack spacings and crack widths

Fibres can still be effective after yielding – but this

Tension Chord Model - RC

Tension Chord Model for SFRC

Tension carried across the cracks through the fibres

Consequently the strain in steel bars

is reduced

Instantaneous Crack Widths

s c i

Proposed SFRC Crack Width Model Results

Time Dependent Crack Widths

Time Dependent Crack Widths

Time Dependent Crack Widths

st s

3 13 48

 =2fct for short term calcs

=1fct for long term calcs

Design Example

 Determine the maximum flexural crack width of a FRC member

subjected to a moment of 25.2kNm at t = 250 days The member has the following properties

cc

2 sc

tc ct tc b

9.56 0.009 9.56 1 0.006 66.6 mm

tc ct tc b

250 66.6

9.56 462 205 66.6 9.56 1 307 66.6 45 3

250 66.6

9.56 462 205 66.6 9.56 1 307 66.6 45 3

Design Example

 Step 6: Due to reduction in Tension Stiffening

 Step 7: Combined Curvature

 Step 8: Calculate crack width

Concluding Remarks

 Provisions for the calculation of flexural crack widths are available in

AS3600

 Available models are typically built off the tension chord model

 These models can be readily extended to account for the presence of

fibre reinforcement – ongoing research at USYD