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

The influence of aggregate stiffness on the measured and predicted creep behaviour of concrete

A project report submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1998Aggregate stiffness is known to influence the magnitude of creep of concrete. The purpose of this research project was to quantify the influence of aggregate stiffness on the measured and predicted long-term creep behaviour of plain concrete. Basic and total creep tests were conducted on concrete specimens of two different strength grades for each of three different commonly used South African aggregate types (quartzite, granite and andesite). In addition, elastic modulus tests Were conducted on cores of the aggregate types considered. The test results revealed that no correlation exists between the creep of concrete and the stiffness of the included aggregate. These results appear to be attributable to the stress-strain behaviour of the aggregate/paste interfacial zone, in the case of aggregates with an elastic modulus in excess of 70 GPa. The experimental basic and total creep values from this investigation were compared with those predicted for each mix at the corresponding ages by the BS 8110 (1985), ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970), CEB-FIP (1978), CEB-FIP (1990) and the RILEM Model B3 (1995). This comparison indicated that the results predicted by each model vary widely and that no correlation exists between the magnitude of the aggregate stiffness and the creep strains predicted by each model.MT201

The influence of aggregate stiffness on the drying shrinkage of concrete

The drying shrinkage of concrete has a negative effect on its structural performance. In cases where the concrete is restrained, which includes most structures, shrinkage may result in appreciably sized cracks. These cracks affect the aesthetics of the structure and have negative durability related implications. This paper discusses the results of an investigation which was aimed at quantifying the influence of aggregate stiffness on the drying shrinkage behaviour of plane concrete. The experimental programme included measurements of drying shrinkage on concrete specimens of two different strength grades for each of three different commonly used South African aggregate types (quartzite, granite and andesite) under controlled conditions. In addition, elastic modulus tests were conducted on cores of the aggregate types assessed. The test results indicated that, for each aggregate type, the specimens with the higher water cement ratio exhibited less drying shrinkage than those with the lower water cement ratio. Furthermore, the concretes containing quartzite aggregate exhibited less shrinkage than both the granite and andesite concretes, at both water cement ratios. A positive correlation (r = 0.84) was established between the Relative Shrinkage Coefficient (which accounts for different water/ cement ratios) and the elastic modulus of the aggregate

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 %)

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

Determining soil plasticity characteristics from physico-chemical properties

Numerous pedological soil classification systems have been developed worldwide. These include an internationally accepted system and various national systems, some of which have been incorporated into databases which include maps. Such information is used primarily for agricultural purposes. Various physical and chemical soil properties are used for classifying soils according to these pedological systems. This paper proposes an approach, based on an extensive research project, which may be used to statistically determine the plasticity characteristics of soils from the physical and chemical properties that are used by pedological classification systems, such as the South African System. These plasticity characteristics may be used to establish the engineering soil classification groups which may, in turn, be used as a means of rapidly determining the general suitability of areas for proposed developments, particularly during the preliminary stages of transportation route locations and township developments, with a resultant saving of time and mone

Effect of hydrometer type on particle size distribution of fine grained soil

Abstract: The particle size distribution of soils, including clay content is of utmost importance in the field of Geotechnical Engineering. The hydrometer analysis is the most widely used technique for analyzing the particle size distribution of the fine grained fraction of soil. The various hydrometer test methods (internationally) generally vary mainly in the use of the prescribed dispersing agent. In addition, certain methods vary with the use of the prescribed hydrometer. The purpose of this study was to establish the optimum concentration and volume of the three dispersing agents (calgon, sodium pyrophosphate decahydrate and sodium tetra pyrophosphate) using the ASTM Hydrometer 152H: E100 (instead of the Bouyoucos hydrometer 152H) for three soil classes, selected for their varying activity. The results indicated that the ASTM hydrometer 152H: E100 generally yielded lower results than hydrometer 152H in terms of percentage fines, with the differences varying from 38% to -65% for all the soils and the dispersing agents

Effect of Sodium Carbonate Concentration in Calgon on Hydrometer Analysis Results

Calgon (a combination of sodium hexametaphosphate and sodium carbonate) has proved to be the most effective dispersing agent in determining the grain size distribution of fine-grained soils by means of the hydrometer analysis. Previous research on the effect of the sodium hexametaphosphate content of dispersing agents on the clay contents showed that the addition of sodium carbonate to sodium hexametaphosphate increases its dispersing effectiveness. Hence, Calgon 35:7 was used /recommended by many researchers/methods and proved to be the most effective dispersing agent. Although previous work focusing on the effect of varying the concentration of sodium hexametaphosphate in Calgon has been reported, the effect of the concentration of sodium carbonate in Calgon has not been assessed and reported. For this reason, in this investigation a series of hydrometer test analyses were conducted using the 152H and ASTM 152H: E100 hydrometers with Calgon in ratios of 35:0, 35:20 and 35:30.  It was observed that with any increase in sodium carbonate content beyond 7 grams, the percentage clay content actually decreased tremendously in the case of hydrometer 152H. However, for the other hydrometer, Calgon (35:0) proved to be most effective combination. Thus, the increase in the sodium carbonate content in Calgon, beyond 7 g/ litre, is not recommended

The effect of type, concentration and volume of dispersing agent on the magnitude of the clay content determined by the hydrometer analysis

Abstract: Knowledge of the physical properties of soils, including the clay content, is of utmost importance in the field of geotechnical engineering. The hydrometer analysis is the most widely used technique for the analysis of the particle size distribution of the fine-grained fraction of a soil, calculated using sedimentation principles. The hydrometer analysis utilises a dispersing agent, Calgon 33:7 (comprising 33 grams of sodium hexametaphosphate and 7 grams of sodium carbonate when mixed in 1 litre of water) is universally considered as the most effective dispersing agent. In this investigation, hydrometer analyses were conducted (according to the TMH1 1986 method) on two soils (alluvium and black clay), using five dispersing agents. The results show that the clay size fraction can vary significantly (from 1% to 32%) for the two soils, depending upon the dispersing agent used. From these initial results, the two most effective dispersing agents (Calgon and sodium pyrophosphate decahydrate - NaPP) were investigated further to establish the optimum concentration and volume. Calgon proved to be the most effective in the alluvial soil, increasing the clay content by 38%. The NaPP was most effective in the relatively active black soil, increasing the clay content by 25%