ACI 209R-92 (ECR values)

This was the only creep and shrinkage model implemented in the RAM Concept prior to Version 8 Update 2. This model will automatically be selected when opening any RAM Concept file that was created in one of those prior versions.

The ACI 209R-92 creep model utilizes the creep factor that is input in the ECR box in the Load History / ECR tab of the Calc Options dialog. The input creep factor should represent the final ultimate creep value and should take into account concrete mix, environmental considerations, etc. and can reflect any considerations required by regional building codes. The modification factor γ_la to account for initial load application times other than 7 days is automatically included in RAM Concept ’s load history calculations and should not be incorporated into the input creep factor.

The ACI 209R-92 model calculates creep strain based upon the modulus of the mean concrete strength at time of loading. Since Concept calculates curvatures based upon the concrete modulus value at 28 days, an adjustment is needed to convert creep strains to the code model values. A rigorous conversion in RAM Concept is not possible, however, because the load history curvature calculations are non-linear. Instead, RAM Concept applies an approximate adjustment, which recognizes that creep strains are proportional to the inverse of the elastic modulus. For the ACI 209-92 model, all creep strains are modified by the ratio of the mean elastic modulus at 28 days (E_cmt28) to the mean elastic modulus at the time of loading (E_cmt0). For this conversion, RAM Concept assumes that the code calculated or directly input Ec value is an experimental value and can be considered a 28-day mean modulus as a result. The adjustment is based on Equations A-16 and A-17 that are referenced for the ACI209R-92 model in ACI 209.2R-08 and calculated as follows:

Where, a and b are constants that are functions of the cement type and type of curing (moist curing assumed).

The ratio of the code calculated or directly input E_c value to the code calculated or directly input E_ci value is used as an upper bound for this calculated adjustment.

The ACI 209R-92 shrinkage model for moist curing is implemented. This model utilizes the shrinkage strain that is input in the ECR box in the Load History / ECR tab of the Calc Options dialog. The input shrinkage strain should represent the final ultimate shrinkage and should take into account concrete mix, environmental considerations, etc. and can reflect any considerations required by regional building codes. The adjustment factor for curing times different than 7 days is automatically calculated by the program using ACI 209.2-08 Equation A-6 and the input Cure Duration. For very low Cure Duration values, the adjustment factor approaches infinity. To avoid undefined values, the adjustment factor calculated using Equation A-6 is limited to a maximum value of 5.

ACI 209.2R-08/GL 2000

This is the default creep model for new models that are created when any of the following design codes are selected for use: ACI 318, IS 456, and CAN/CSA A23.3. The GL2000 model was developed by Gardner and Lockman and is summarized in ACI 209.2-08.

The Basic Creep Coefficient in the Load History/ECR tab of the Calc Options dialog represents the constant (2) in the first term of ACI 209.2R-08 Equation A-103. When "Code" is selected for the Basic Creep Coefficient parameter, RAM Concept automatically uses this value for the coefficient. A different value for the basic creep coefficient can also be input by the user. Whether the Basic Creep Coefficient is set to "Code" or input by the user, RAM Concept automatically calculates the adjustment terms associated with the second and third terms of Equation A-103, which use the input Relative Humidity, the selected Exposure, and the geometry of each cross section. If a value of 0 is input for the Basic Creep Coefficient, then the effective creep coefficient including all adjustment factors is assumed to be 0.

The ACI 209.2R-08/GL 200 model calculates creep strain based upon the 28-day mean concrete elastic modulus. Since RAM Concept assumes that the Ec value is a 28-day mean modulus as a result, no adjustment for the elastic modulus is required at early ages.

AS 3600-2018

This is the default creep model for new models that are created when any of the AS 3600 design codes are selected for use.

The Basic Creep Coefficient in the Load History/ECR tab of the Calc Options dialog represents the basic creep coefficient in AS 3600-2018 Table 3.1.8.2. When "Code" is selected for the Basic Creep Coefficient parameter, RAM Concept automatically calculates this coefficient using the concrete strength and interpolating between the tabulated values. A specific value for the basic creep coefficient can also be input by the user. Whether the Basic Creep Coefficient is calculated or input by the user, RAM Concept automatically calculates the code defined adjustment factors k2, k3, k4, and k5. k2 is calculated for each time step using the equation in Table 3.1.8.3. k3 is calculated using the time input for the Initial Load Application in the Calc Options dialog. k4 is determined using the selections for Environment and Exposure and the geometry of each cross section. k5 is determined based on the concrete strength of the section. The load history calculations in RAM Concept exclude the adjustment factor k6, which account for creep nonlinearity at high compressive stress. The "Max. Fa/Fc Ratio" for each cross section is reported in the Load History Analysis Table for each load history step.

The AS 3600-2018 model calculates creep strain based upon the mean modulus of elasticity at 28 days (E_c), which is a function of the mean in situ compressive strength (f_cmi). When calculating the mean elastic modulus, RAM Concept internally adjusts from the input characteristic cylinder strength to the mean in-situ. In the event that cylinder strength is determined experimentally (f_cm) by the user for use in a RAM Concept model, we recommend inputting the equivalent characteristic cylinder strength f’c found from Table 3.1.2 in the Material Specification. Similarly, in the event that the elastic modulus is determined experimentally for use in a RAM Concept model, we recommend finding an in-situ modulus and entering that value as the Specified Ec in the Material Specification.

The AS 3600-2018 shrinkage model distinguishes between autogenous shrinkage and drying shrinkage. RAM Concept calculates and tracks each over time using separate time curves. The Basic Autogenous Shrinkage Strain in the Load History/ECR tab of the Calc Options dialog represents the final autogenous shrinkage strain defined by the term ε_cse* in Equation 3.1.7.2(4). This strain can be calculated by the program or input by the user. RAM Concept automatically calculates the final Autogenous Shrinkage Strain using equation 3.1.7.2(2). The Basic Drying Shrinkage strain in the Calc Options dialog represents the basic drying shrinkage strain defined by Equation 3.1.7.2(5). Like the Basic Autogenous Shrinkage Strain, this strain can be calculated the program or input by the user. Note that the term ε_csd.b should be input for Basic Drying Shrinkage Strain, and not ε_csd.b*

Eurocode 2-2004

This is the default creep model for new models that are created when any of the Eurocode 2 or BS 8110 design codes are selected for use.

The Basic Creep Coefficient in the Load History/ECR tab of the Calc Options dialog represents β(fcm) defined by Eurocode 2-2004 Equation B.4. When "Code" is selected for the Basic Creep Coefficient in the Load History / ECR tab of the Calc Options dialog, RAM Concept automatically calculates this coefficient based on the concrete compressive strength. A specific value for the basic creep coefficient can also be input by the user. Whether the Basic Creep Coefficient is calculated or input by the user, RAM Concept automatically calculates the code defined adjustment factors to account for time after loading (Equation B.7), relative humidity (Equation B.3a or B.3b) and cement type (Equation B.9). The load history calculations in RAM Concept exclude the adjustments to account for nonlinearity at high compressive stress (Equation 3.7) and the effect of temperature on concrete maturity (Equation B.10). The "Max. Fa/Fc Ratio" for each cross section is reported in the Load History Analysis Table for each load history step.

The Eurocode 2-2004 model calculates the creep coefficient based upon the tangent modulus, E_c. Since RAM Concept assumes that the calculated or input E_c value is the secant modulus, E_cm, an adjustment is needed to convert creep strains to the code model values. This is adjustment factor is taken as 1.05 based on Clause 3.1.4 (2).

The Eurocode 2-2004 shrinkage model distinguishes between autogenous shrinkage and drying shrinkage. RAM Concept calculates and tracks each over time using separate time curves. The Basic Autogenous Shrinkage Strain in the Load History/ECR tab of the Calc Options dialog represents the final autogenous shrinkage strain defined by Equation 3.12. The Basic Drying Shrinkage strain in the Calc Options dialog represents the basic drying shrinkage strain defined by Equation B.11. Like the Basic Autogenous Shrinkage Strain, this strain can be calculated the program or input by the user.

The relationship between the input parameters in the Load History/ECR tab of the Calc Options dialog and the code model equations are summarized in the following table for each creep/shrinkage model. RAM Concept uses the tabulated value when “Code” is selected for the respective input parameter.