Hi All,
I followed all the posts regarding the creep factor to adopt in RAM to cope with the different formulation of the creep model adopted in AS3600 and the creep model implemented in RAM Concept according to ACI 209R-92.
I compared the results for some test slabs using RAM and RAPT. All the correlation factors that have been proposed so far do not give the same convergence between RAPT and RAM for different slabs tested.
The limitation of the test is that RAPT is a frame analysis, not an FEA software, therefore only one way slabs can give reasonable comparative results.
I tested few slabs with different spans ratios to be sure that the results I compared have the widest range possible within the limitation mentioned before.
I then read in details the ACI 209R-92 creep code, and that's where I found the answer.
RAM implemented the ACI209R-92 creep model.
This model is based on a creep formula for Standard conditions where the average parameters νu=2.35,ψ=0.6, d=10 have been adopted.
These parameters vary as follows:
ψ = 0.40 to 0.80
d = 6 to 30 days
νu= 1.3 to 4.15
This leads to νt = t^0.6/(10+t^0.6)*2.35 γc
γc is a combination of modification factors to account for the following:
- Loading age
- Differential shrinkage
- Humidity
- Average thickness of the member
- Temperature
- Concrete slump
- Fine aggregate percentage
- Cement content
- Air content
The modification factor for loading time other than 7 days is γta = 1.5*ta-0.118. This is included in RAM and kicks in when you input a value other than 7 in time of loading textbox.
Because RAM calculates the long term deflection with the ACI creep model we should let RAM apply time of loading modification factor to account for a time of loading other than 7 rather than trying to find a magic correlation factor.
Because we want to be consistent with the creep model that is used to calculate final deflections (implemented in RAM), we therefore should calculate the final inelastic creep value from AS3600 for a load applied at 7 days and input 28 days as Initial loading application in RAM. Let RAM apply the correction factor according to ACI209R-92.
The creep coefficient (calculated at 7 days) is equivalent to use the 28 days creep value from Table 3.1.8.3 multiplied by k3(7)/k3(28) = 1.46/1.1 = 1.327 (prior to Amendment 2 the ratio was 1.75/1.1=1.6), and add 1 to account for the elastic strain (creep factor in RAM is )
The reason why we input a creep coefficient for a load applied at 7 days is that we want to input a creep factor consistent with the creep model used.
RAM will apply the correction factor according to ACI209R-92 for whatever Initial loading application we specify.
ACI209R-92 uses a formula for standard conditions as follows:
- load applied at 7 days,
- moist cured for 7 days at 23°C,
- type I and III cement,
- 6% air content,
- 50% fine aggregate,
- 280-446 kg/m3 cement content,
- Temperature 23°C,
- 40% ambient relative humidity,
- Min 150mm thickness,
- Stress/strength ratio = 50%
For Shrinkage: Use values from table 3.1.7.2