Concrete Properties for LRFD
Description
Definition of the material properties for a section or beam is a three step process:
- Add the material type (concrete / reinforcing steel / etc.)
- Define a set of properties for that material type (characteristic strength etc.)
- Allocate the material properties defined in 1 & 2 to the relevant parts of the section or beam section component.
This form performs the second of these 3 stages for concrete. As data is entered the program checks that values are within the expected range for concrete. This check will highlight any values in error by a factor of 10 or more. The displays for Strength Limit State and Service Limit State are to the same scale, and are updated as the values in the fields are changed.
For the Strength Limit State, the stress-strain distribution is assumed to be parabolic, in accordance with the provisions of A5.6.2.1 of AASHTO LRFD 9. The parabolic shape used is generally accepted as the closest shape in substantial agreement with test results which is practical for design purposes.
Form Graphic
Related Topic
Field Help
Compressive strength f'c
Enter the compressive strength at 28 days for the concrete. A warning will be issued if the value entered is outside the limits in A5.4.2.1 of AASHTO LRFD 9, but the value will not be rejected.
Strength LS stress/strain distribution
To select a stress / strain curve for the concrete, click on this field to display a drop down list containing 3 options as follows:
Default 1
This is based upon the ULS stress strain curve defined in the UK bridge design code BS 5400, with due allowance made for the strengths based upon cylinder crushing tests and not cube strengths. Also the maximum strain will be taken as 0.003, and not 0.0035.
Default 2
This is based upon the ULS stress strain curve defined in the Hong Kong Highways Department design code, with the same allowances as described for Default 1 above.
Define
This may be used to define a stress / strain curve of the same form as the ones above (i.e. parabolic up to the maximum stress) but with different parameters.
Modulus of Elasticity - Ec
Enter the modulus of elasticity of the concrete under short term (instantaneous) loading.
The default value is calculated from the formula given in A5.4.2.4 of AASHTO LRFD 9, based on the compressive strength specified above.
As elastic modulus, shear modulus and the Poisson's ratio are related, a change of this value will affect one of the three elasticity parameters, in this case the shear modulus.
Modulus of Elasticity - Long Term (LRFD 7 only)
Enter the modulus of elasticity of the concrete under long term loading. The default value is one half of the short term elastic modulus value, which gives an effective modular ratio of 2n as defined by A5.7.1 of AASHTO LRFD 7.
Poisson's Ratio, ν
Enter the value for Poisson's ratio. The default value is typical for this type of material. As elastic modulus, shear modulus and the Poisson's ratio are related, a change of this value will affect one of the three elasticity parameters, in this case the shear modulus.
Shear Modulus, G
Enter the shear modulus value. As elastic modulus, shear modulus and the Poisson's ratio are related, a change of this value will affect one of the three elasticity parameters, in this case the Poisson's ratio.
Compressive stress limit factor
Enter the stress limit factor for concrete in compression. This value will be used for calculating the permissible limiting stress in prestressed members.
For temporary stresses before losses, the limit is given in A5.9.2.3.1a of AASHTO LRFD 9 as 0.65 for pre-tensioned or post-tensioned members and 0.60 for other concrete components.
The limit is given in A5.9.2.3.1b of AASHTO LRFD 9 as 0.60 due to permanent and transient loads and during shipping and handling, or 0.45 due the sum of effective prestress and permanent loads.
Default value is 0.60.
Modulus of Rupture
The modulus of rupture must be entered as a negative value or zero. The default value is obtained from A5.4.2.6 of AASHTO LRFD 9 based on the value for normal weight concrete for the entered value for compressive strength.
Coefficient of thermal expansion
Enter the coefficient of thermal expansion for use in temperature gradient calculations.
The default value is 0.0000108 /°C as suggested in A5.4.2.2 of AASHTO LRFD 9 for normal density concrete.
The corresponding value for low density concrete is 0.000009 /°C.
Correction Factor for Source of Aggregate, K1
Enter the correction factor for source of aggregate as defined in A5.4.2.4 of AASHTO LRFD 9.
The default value is to be taken as 1.0 unless determined by physical test and as approved by the authority of jurisdiction.
Maximum aggregate size
The maximum aggregate size specified here is used to determine the crack width spacing parameter sxe in A5.7.3.4.2 of AASHTO LRFD 9.
Density
Enter the density of the concrete. This is used in the calculation of self-weight moments, and in the calculation of mass. It is also used to select the appropriate resistance factor f for the shear calculations from A5.5.4.2 of AASHTO LRFD 9.
The default value is 0.145KIP/FT3 (which is equivalent to 2400 kg/m3 or 23.56 kN/m3) as given in Table 3.5.1-1 of AASHTO LRFD 9 for normal density concrete.
Where the actual density is greater than this the tension stresses for pre-tensioned beams at transfer may be unduly conservative.
The value may depend upon the type of aggregate, the amount of reinforcement, and the degree of compaction.
A value less than or equal to 0.135KIP/FT3 (21.21 kN/m3) defines a lightweight concrete and gives the option to define Splitting Tensile Strength for the calculation of Concrete Density Modification Factor.
Concrete Density Modification Factor
Enter the concrete density modification factor as described in A5.4.2.8 of AASHTO LRFD 9.
The default value is to be taken as 1.0 for normal density concrete.
Specify Splitting Tensile Strength?
Check this checkbox to enter splitting tensile strength for the calculation of concrete density modification factor according to A5.4.2.8 of AASHTO LRFD 9.
The default value is non-checked.
Splitting Tensile Strength of lightweight concrete, fct
Enter splitting tensile strength for the calculation of concrete density modification factor according to A5.4.2.8 of AASHTO LRFD 9.
The default value is as suggested in the National Cooperative Highway Research Program Report 733 "High-Performance/High Strength Lightweight Concrete for Bridge Girders and Decks":
-0.25*sqrt( f'c ).
Property Name
The program will supply a reference name which should provide a nearly unique identifier to the material property set. This value may be edited to give a more readily recognizable name if required.