Validation of international code-type concrete elastic modulus estimation methods

The elastic modulus of concrete is utilized in the design of reinforced concrete structures, including in predicting creep deformation. This elastic modulus can be estimated, using models contained in national design codes, by considering one or more properties (usually compressive strength). The proposed paper assesses the accuracy of eleven empirical elastic modulus estimation models, when compared with the actual values measured on a range of concretes under laboratory controlled conditions. The equations considered are those contained in BS 8110 (1985), SANS 10100 (2000), SANS 10100 (2000) Modified, ACI 209 (1992/2008), AS 3600 (1988, 2001 and 2009), CEB-FIP (1970, 1978 and 1990), EC 2 (2004), GL 2000 and 2004), GZ (1993) and RILEM Model B3 (1995). The test results indicated that the discrepancies between the measured and estimated values were only significant in the case of the SANS 10100 (2000) Modified method (P = 3,1 %) and the CEB-FIP (1970) method (P = 2 %). The most accurate methods were the SANS 10100 (2000) and AS (2009) which both yielded a coefficient of variation (ωj) of 9,3 %. The least accurate method was the CEB-FIP (1970) which yielded a coefficient of variation (ωj) of 22,7 %. Furthermore, the test results of this research were used to establish which factors influence the elastic modulus of concrete. It was found that the concrete density, the density of the included aggregate and the coarse aggregate content separately correlated significantly with the elastic modulus (P ≤ 3 %)

Evaluation of the creep coefficients of international concrete creep prediction models

Abstract: Creep of concrete is an important design consideration. National design codes therefore provide empirically based models for the estimation of creep deformation. Such models estimate a creep coefficient (φ) and an elastic modulus (E) of the concrete, both of which are used to predict the creep strain at any age. This paper assesses the accuracy of the creep coefficients (φ) predicted by fourteen “design code-type" models, with a view to ascertain whether the estimated φ or E is responsible for the inaccuracy of some of the models. The models considered are those contained in SANS 10100 (2000)/BS 8110 (1985), SANS 10100 (2000) Modified, ACI 209 (1992), AS 3600 (2001 & 2009), CEB-FIP (1970, 1978 & 1990), the Eurocode EC (2004), Gardener and Lockman (2000 & 2004), Gardener and Zhao (1993) and the RILEM B3 (1995) methods. Laboratory creep tests were conducted on concrete prisms covering a range of mixes. The measured φ values were statistically compared to those predicted by the models considered. The results indicated that, for the range of concretes tested, the CEB-FIP (1990) method yielded the most accurate predictions of creep coefficient, giving the lowest overall coefficient of variation (all) of 27,7 %. The least accurate method was the CEB-FIP (1978) which yielded an overall coefficient of variation (all) of 112,5 %. Furthermore, the accuracy of the predicted φ values correlated highly significantly (P = 0,001 %) with the accuracy of the predicted creep magnitudes. The results of this investigation led to recommending the SANS 10100 (2000)/ BS 8110 (1985) model for predicting creep coefficients for South African conditions

Evaluation of the creep coefficients of the fib 2010 and RILEM B4 concrete creep prediction models

Abstract: Creep of concrete is an important design consideration. National design codes therefore provide empirical based models for the estimation of creep deformation. Such models generally estimate a creep coefficient () and an elastic modulus (E) of the concrete, both of which are used to predict the creep strain at any age. This paper assesses the accuracy of the creep coefficients () predicted by the relatively new international fib Model Code 2010 (MC 2010) and RILEM Model B4 using a laboratory test programme. The measured creep coefficient () values were statistically compared to those predicted by the models considered. The MC 2010 (2012) Model, which yielded an overall coefficient of variation (ωall) of 44.9 %, was found to be more accurate than the RILEM Model B4 (with a (ωall) of 103.3 %). Both the models validated were found to yield less accurate creep coefficients than their respective predecessor models

Validation of international concrete creep prediction models by application to South African concretes

Creep deformation of concrete is often responsible for excessive deflections at service loads which can compromise the performance of a structure. National design codes therefore provide prediction models for the estimation of creep deformation. These models are empirical-based. This paper assesses the accuracy of six international code type models, when compared with the actual strains measured on a range of South African concretes under laboratory control conditions. The models considered are those contained in AS 3600 (2001), AS 3600 (2009), Eurocode EC 2 (2004), GL (2000), GL (2004) and GZ (1993). The results indicate that for the range of concretes tested, the GL (2000) model yielded the most accurate predictions, giving the lowest overall coefficient of variation (ωall) of 31,9%. The least accurate method was the AS 3600 (2009) which yielded an overall coefficient of variation (ωall) of 74,7%. This paper also recommends a new approach to assessing the accuracy of creep models

An assessment of the accuracy of nine design models for predicting creep in concrete

Abstract: Creep of concrete is a complex phenomenon that has proven difficult to model. Nevertheless, for many reinforced and prestressed concrete applications, a reasonably accurate prediction of the magnitude and rate of creep strain is an important requirement of the design process. Although laboratory tests may be undertaken to determine the deformation properties of materials, these are time consuming, often expensive and generally not a practical option. In addition, this is not often an option at the design stage of a project when decisions about the actual concrete to be used have not yet been taken. National design codes therefore rely on empirical prediction models to estimate the magnitude and development of the creep strain. This paper considers the suitability of nine ‘design code type’ creep prediction models when compared with the actual strains measured on a range of concretes under laboratory control conditions. The concretes tested incorporate three aggregate types and two strength grades for each aggregate type. The results are compared with the predictions of creep using models contained in BS 8110 (1985), SABS 0100 (1992), SABS 0100 (1992) modified, ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970, 1978 & 1990), the RILEM Model B3 (1995) methods. The results indicate that the CEB-FIP (1970) and BS 8110 (1985) methods provide suitably accurate predictions over all the concretes tested. These methods yielded overall coefficients of variation of approximately 18 % and 24 %, respectively. The least accurate method was the CEB-FIP (1978) which yielded a coefficient of variation of approximately 96 %. The results of this investigation led to recommending the BS 8110 (1985) model for South African conditions

Modeling for assessment of long-term behavior of prestressed concrete box-girder bridges

Large-span prestressed concrete (PC) box-girder bridges suffer excessive vertical deflections and cracking. Recent serviceability failures in China show that the current Chinese standard modeling approach fails to accurately predict long-term deformations of large box-girder bridges. This hinders the efforts of inspectors to conduct satisfactory structural assessments and make decisions on potential repair and strengthening. This study presents a model-updating approach that aims to assess the models used in the current Chinese standard and improve the accuracy of numerical modeling of the long-term behavior of box-girder bridges, calibrated against data obtained from a bridge in service. A three-dimensional finite-element model representing the long-term behavior of box-girder sections is initially established. Parametric studies are then conducted to determine the relevant influencing parameters and to quantify the relationships between those and the behavior of box-girder bridges. Genetic algorithm optimization, based on the response-surface method (RSM), is used to determine realistic creep and shrinkage levels and prestress losses. The modeling results correspond well with the measured historic deflections and the observed cracks. This approach can lead to more accurate bridge assessments, which result in safer strengthening and more economic maintenance plans

Verification of the bursting and spalling formulas in the FIB model code by finite element analyses of anchorage zones of pretensioned girders

In order to predict the stress and possible crack distribution in the anchorage zones of pretensioned girders several models have been developed as can be found in the fib Model Code, the ASHTOO code or Eurocode 2. In this paper, the bursting and spalling formulas from the fib Model Code are evaluated by finite element calculations since some issues could be raised when applying the proposed formulas for industrial applications, especially for beams of limited dimensions. The effect of the upper strands, the assumed stress distribution at the opposite side of the equivalent symmetric prism, the stress transfer diagram along the strands and the effects of the strand position relative to the simplified resultant forces remain unclear. Accordingly two-dimensional finite element models were developed to gain insight into the bursting and spalling formulations from the fib Model Code. The numerical models render stresses and the stress flow results, which allow a more clear coupling to well-known strut-and-tie models. The results indicate that for various strand configurations, especially for small beams, the fib formulations may be too conservative

Predicting creep deformation of concrete : a comparison of results from different investigations

Abstract: Creep deformation of concrete is often responsible for excessive deflection at service loads which can compromise the performance of elements within a structure. Hence, the realistic prediction of both the magnitude and rate of creep strain is an important requirement of the design process. Although laboratory tests may be undertaken to determine the deformation properties of concrete, these are time-consuming, often expensive and generally not a practical option. Therefore, relatively simple empirically based national design code models are relied upon to predict the magnitude of creep strain. This paper reviews the accuracy of creep predictions yielded by eight commonly used international “code type” models, all of which do not consider the same material parameters and yield a range of predicted strains, when compared with actual strains measured on a range of concretes in seventeen different investigations. The models assessed are the: SABS 0100 (1992), BS 8110 (1985), ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970, 1978 and 1990) and the RILEM Model B3 (1995). The RILEM Model B3 (1995) and CEB-FIP (1978) were found to be the most and least accurate, respectively