Workability and stability of lightweight aggregate concrete from rheology perspective

WORKABILITY AND STABILITY OF LIGHTWEIGHT AGGREGATE CONCRETE FROM RHEOLOGY PERSPECTIVE CHIA KOK SENG (B.Eng.(Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2006 SUMMARY This thesis describes an experimental study on workability and stability of fresh lightweight aggregate concrete (LWAC) from rheology perspective It involves using rheological parameters of Bingham model, which are yield stress and plastic viscosity, to evaluate the workability, and stability of concrete under vibration In general, a lower yield stress and plastic viscosity improves the flowability but increases the segregation potential of fresh concrete Hence, there is a need to provide information to address this dilemma in design of concrete mixtures The rheological parameters of the concrete in this study are modified using a superplasticizer (SP) and an air entraining agent, and measured by a coaxial-cylinders rheometer Information on the behaviour of the fresh LWAC, with and without air entrainment, is presented and discussed Empirical relationships between the rheological parameters and the slump are proposed based on the experimental results The results indicated that the increase in the SP content reduced the yield stress without a significant effect on the plastic viscosity The yield stress and plastic viscosity were reduced with air entrainment As the entrained air content increased, the plastic viscosity of the concrete decreased, however, the yield stress remained relatively unchanged The air entrained concrete had higher yield stress and lower plastic viscosity compared with the non-air entrained concrete at similar slump Thus, a higher shear stress is required to initiate flow in the former but its flow rate would be higher than the latter The slump of the concrete increased as the yield stress decreased The slump of the non-air entrained concrete did not appear to have any correlation with the plastic viscosity, while the slump of the air entrained concrete increased as the plastic -i- viscosity decreased The slump of the concrete increased significantly with the incorporation of entrained air When fresh LWAC experienced vibration, the stability decreased with decrease in its yield stress or plastic viscosity The LWAC with denser LWA had better stability due to a smaller density difference between the LWA and the mortar matrix During vibration, there was a minimum amplitude above which the concrete could be fluidised, and relative movement between coarse aggregate and mortar matrix might occur, leading to segregation When the LWAC was fluidised, the air entrained concrete had better stability than the corresponding non-air entrained concrete However, the stability of air entrained concrete decreased as entrained air content increased The concrete had more segregation when the vibratory frequency, amplitude, and acceleration increased For a given vibratory acceleration, a combination of higher amplitude and lower frequency led to more segregation in the concrete with low yield stresses Keywords: air entrainment; lightweight aggregate concrete; plastic viscosity; rheology; segregation; slump; stability; superplasticizer; vibration; workability; yield stress - ii - ACKNOWLEDGEMENTS The author wishes to express his sincere thanks and appreciation to his supervisor, Associate Professor Zhang Min Hong, for her invaluable guidance, constructive and interesting discussions, patience, and full support throughout this research Her commitment towards academic professionalism has inspired the author to strive for excellence Gratification is also extended to all the technologists of the Structural and Concrete Laboratory for their invaluable assistance in ensuring the successful completion of all laboratory experimental works, especially to Sit Beng Chiat, Ang Beng Oon, Tan Annie and Yip Kwok Keong Special thanks to all the past undergraduate students who had contributed towards the experimental work in this study They are Benjamin Chua Chuen Hua, Gerald Wu Sher-Min, Sun Dao Jun, Kho Chen Chung, Daniel Chong Chee Siong, and Edmund Gerard Yong Wee Soon Acknowledgments are also due to those who have in one way or another contributed to this research and to the authors of various papers and materials quoted in the references This study is especially dedicated to my beautiful wife, and beloved family for their moral support and encouragement throughout my education in the university Finally, the author gratefully acknowledges the National University of Singapore for the opportunity and the award of the Research Scholarship to purse this study January, 2006 Chia Kok Seng - iii - TABLE OF CONTENTS SUMMARY I ACKNOWLEDGEMENTS III TABLE OF CONTENTS IV LIST OF TABLES VII LIST OF FIGURES IX LIST OF NOTATIONS XV INTRODUCTION 1.1 1.2 BACKGROUND REVIEW OBJECTIVE 11 LITERATURE REVIEW 13 2.1 RHEOLOGICAL MODELS AND PROPERTIES .13 2.2 RHEOLOGY OF FRESH CONCRETE 17 2.2.1 Effect of superplasticizer 20 2.2.2 Effect of air entraining admixture 24 2.3 COAXIAL-CYLINDERS RHEOMETER – THE BML VISCOMETER 28 2.3.1 Principles of measurement in BML viscometer 31 2.3.2 Limitations in measurement of rheological parameters of fresh concrete 35 2.4 2.5 VIBRATION OF FRESH CONCRETE 46 2.6 SLUMP OF FRESH CONCRETE 39 WATER ABSORPTION OF LIGHTWEIGHT AGGREGATES 55 EXPERIMENTAL DETAILS 58 3.1 INTRODUCTION .58 3.2 MATERIALS 58 3.3 MIXTURE PROPORTION AND PREPARATION OF CONCRETE .61 3.4 TEST METHODS .63 3.4.1 Yield stress and plastic viscosity 63 3.4.2 Segregation 67 3.5 METHODOLOGY 74 - iv - EFFECT OF RHEOLOGICAL PARAMETERS ON WORKABILITY OF LWAC 76 4.1 INTRODUCTION .76 4.2 REPEATABILITY OF TEST RESULTS .77 4.3 INFLUENCE OF A NAPHTHALENE-BASED SUPERPLASTICIZER 83 4.4 INFLUENCE OF AIR ENTRAINING ADMIXTURE 90 4.4.1 Effect of air entrainment in concrete 93 4.4.2 Effect of increasing air entrainment in air entrained concrete 95 4.5 COMPARISON ON WORKABILITY OF NON-AIR AND AIR ENTRAINED LWAC 96 4.6 RELATIONSHIP BETWEEN RHEOLOGICAL PARAMETERS AND SLUMP 99 4.6.1 Effect of yield stress and plastic viscosity on slump of non-air entrained concrete 99 4.6.2 Increase in slump of air entrained concrete at similar yield stress 101 4.6.3 Empirical relationships between slump, density and rheological parameters 105 4.7 SUMMARY AND CONCLUSIONS 110 MASS DEVIATION INDEX – AN INDICATOR OF SEGREGATION 114 5.1 EVALUATION OF MASS DEVIATION INDEX 114 5.2 EFFECT OF MASS DEVIATION INDEX ON PROPERTIES OF HARDENED LWAC 122 EFFECT OF RHEOLOGICAL PARAMETERS ON STABILITY OF LWAC 127 6.1 INTRODUCTION 127 6.2 EFFECT OF LWA DENSITY AND W/C ON STABILITY OF LWAC 127 6.3 EFFECT OF INCREASING AIR ENTRAINMENT ON STABILITY OF AIR ENTRAINED LWAC 133 6.4 COMPARISON OF STABILITY OF NON-AIR AND AIR ENTRAINED LWAC WITH SIMILAR YIELD STRESS 135 6.4.1 Stability of the concretes at high yield stress of 650 Pa and low yield stresses of 200 and 350 Pa 136 6.4.2 6.5 Effect of yield stress on fluidisation of fresh concrete under vibration 139 COMPARISON OF STABILITY OF NON-AIR AND AIR ENTRAINED LWAC WITH SIMILAR SLUMP 143 6.6 SUMMARY AND CONCLUSIONS 147 -v- EFFECT OF VIBRATORY PARAMETERS ON STABILITY OF LWAC 150 7.1 7.2 EXPERIMENTAL RESULTS 150 7.3 EFFECT OF FREQUENCY AND AMPLITUDE ON STABILITY OF CONCRETE .155 7.4 EFFECT OF VIBRATORY ACCELERATION ON STABILITY OF CONCRETE 162 7.5 INTRODUCTION 150 SUMMARY AND CONCLUSIONS 168 SUMMARY AND CONCLUSIONS 170 8.1 SUMMARY AND CONCLUSIONS OF RESULTS 170 8.2 RECOMMENDATIONS ON THE USE OF ADMIXTURES IN CONCRETE 176 8.3 RECOMMENDATIONS FOR FURTHER RESEARCH 177 REFERENCES .179 PUBLICATION AND DISSEMINATION OF RESULTS 190 - vi - LIST OF TABLES Table 3.1 – Chemical composition and physical properties of cement used .59 Table 3.2 – Physical properties of lightweight aggregates 60 Table 3.3 – Water absorption (%) of oven-dried LWA .60 Table 3.4 – Sieve analysis (cumulative retained) of coarse LWA and normalweight sand 61 Table 3.5 – Mixture proportion of concrete .62 Table 3.6 – Parameter set-up for the BML rheometer 65 Table 3.7 – Vibratory acceleration in terms of gravitational acceleration (g) .68 Table 4.1 – Properties of non-air entrained concrete with a w/c of 0.35 (Series I) .80 Table 4.2 – Properties of non-air entrained concrete with a w/c of 0.35 (Series II) 81 Table 4.3 – Properties of non-air entrained concrete with a w/c of 0.45 (Series I) .85 Table 4.4 – Properties of air entrained concrete with F6.5 aggregate and a w/c of 0.35 in Series I .91 Table 4.5 – Properties of air entrained concrete with F6.5 aggregate and a w/c of 0.35 in Series II 97 Table 4.6 – Properties of non-air and air entrained concrete in Series I having similar yield stress of about 650 Pa 101 Table 5.1 – Properties and test results of non-air entrained concrete to determine the significance of Mass Deviation Index (MI) relative to density, compressive strength and elastic modulus 117 Table 5.2 – Properties and test results of non-air entrained concrete to determine the significance of Mass Deviation Index (MI) relative to density and compressive strength .118 Table 5.3 – Properties and test results of air entrained concrete to determine the significance of Mass Deviation Index (MI) relative to density and compressive strength .119 Table 5.4 – Distribution profile of coarse aggregate mass for concrete and corresponding Mass Deviation Index (MI) 121 Table 6.1 – Distribution profile of coarse aggregate mass for concrete with F5 aggregate in Series I and corresponding Mass Deviation Index (MI) 129 - vii - Table 6.2 – Distribution profile of coarse aggregate mass for concrete with F6.5 aggregate in Series I and corresponding Mass Deviation Index (MI) 129 Table 6.3 – Distribution profile of coarse aggregate mass for concrete with F8 aggregate in Series I and corresponding Mass Deviation Index (MI) 130 Table 6.4 – Distribution profile of coarse aggregate mass for air entrained concrete with F6.5 aggregate in Series I and corresponding Mass Deviation Index (MI) 130 Table 6.5 – Properties of non-air and air entrained concrete with F6.5 aggregate in Series I and II grouped according to similar yield stress .136 Table 6.6 – Properties of non-air and air entrained concretes with F6.5 aggregate and slumps greater than 120 mm in Series I and II 145 Table 7.1 – Properties of concrete and Mass Deviation Index (MI) in Series II .152 Table 7.2 – Distribution profile of coarse aggregate mass and corresponding Mass Deviation Index (MI) of non-air entrained concrete in Series II 153 Table 7.3 – Distribution profile of coarse aggregate mass and corresponding Mass Deviation Index (MI) of air entrained concrete in Series II 154 - viii - LIST OF FIGURES & Fig.1.1 – The Bingham model is given by τ = τ0 + ηp γ , where τ is shear stress, τ0 is & yield stress, ηp is plastic viscosity and γ is shear rate .6 Fig.1.2 – Different processing operations in different ranges of shear rate (Reed, 1995) Fig.1.3 – Effect of shear rate upon the results of single-point tests .7 Fig.2.1 – The apparent viscosity of a Bingham material is higher for higher yield stress (a) and decreases with increasing shear rate (b) 15 Fig.2.2 – Various rheological models showing variation of shear stress with shear rate (Reed, 1995) .16 Fig.2.3 – Shear stress decreases with shear flow at constant shear rate, which indicates thixotropic behaviour (Reed, 1995) 17 Fig.2.4 – Bingham model: τ = τ0 + ηp γ (A and B represent two experimental points & needed to fix the line) .20 Fig.2.5 – Effect of superplasticizers on g-value and h-value (Tattersall, 1991) 24 Fig.2.6 – Structure of air-entrained cement paste (Kreijger, 1980) .26 Fig.2.7 – Effect of increasing superplasticizer dosage (a) and air content (b) 27 Fig.2.8 – The ConTec BML Viscometer and the measuring system 29 Fig.2.9 – Principle of the coaxial cylinders viscometer (Tattersall, 1991) 29 Fig.2.10 – The assembly of the inner cylinder unit and the top ring 30 Fig.2.11 – Top view (left) and cross section (right) of the viscometer cylinders 31 Fig.2.12 – Inner and outer cylinder showing the ribs to prevent slippage 31 Fig.2.13 – A typical chart of torque-rotational speed in BML viscometer software 33 Fig.2.14 – Bridging of coarse aggregates during shearing of fresh concrete in a coaxial-cylinders rheometer with rotating outer cylinder .39 Fig.2.15 – Comparison of equations relating yield stress and slump where the yield stress of the first equations is measured from the parallel-plates BTRHEOM rheometer while the last one is from a coaxial-cylinders rheometer (ACI 236A, 2005) 43 - ix - amplitude led to more segregation and this contradicts the report on vibration by ACI 309 (1993) Further research is needed for verification 8.2 Recommendations on the use of admixtures in concrete From the results, it is recognized that the incorporation of either SP or AEA leads to slump increase and workability improvement of the fresh LWAC However, it was noted that these chemical admixtures improve the workability differently from rheology perspective The use of SP results in the decrease of yield stress of the concrete whereas its plastic viscosity is relatively unaffected This implies that shear stress required to initiate flow of the superplasticized concrete is reduced while its flow rate remains relatively unchanged Comparatively, the use of a small amount of AEA reduced both the yield stress and plastic viscosity of the air entrained LWAC, as presented in this study Hence, the air entrained concrete would require a lower shear stress to initiate the flow and its flow rate would also be higher When the concrete was not fluidised during vibration, it was observed that at similar yield stress the non-air entrained concrete had better stability than the air entrained concrete On the other hand, when the concrete was fluidised during vibration, the non-air entrained concrete had lower stability than the air entrained concrete, even though the former had higher plastic viscosity This implied that it is not necessary for a concrete with lower plastic viscosity to be less stable In design of LWAC mixtures with a specified slump, air entrainment is recommended for improvement of both the workability and the stability of fresh concrete even when the concrete is not subjected to repeated freezing and thawing cycles, so long as fluidisation of the concrete occurs during vibration With small amount of entrained air, mechanical properties of hardened concrete would not be - 176 - affected significantly In this study, an increase of total air content by about 1.5% (from 4.5 to 6%) from non-air to air entrained concrete had increased the slump significantly while the 28-day strength compressive was relatively unaffected This is due to a drastic difference in the air void structure between the non-air and air entrained concrete, as shown in Fig.4.15 Therefore, it would be worthwhile to consider using air entrainment as an alternative mean to improve workability However, for concrete with a given maximum aggregate size, the amount of air entrainment should be limited with considerations of stability and mechanical properties For production of LWAC, the LWA is often selected based on its availability and particle density to achieve specified strength and unit weight of concrete If the difference between the particle density of LWA and the density of mortar matrix is relatively large, caution should be exercised to avoid overdoses of superplasticizers and air entraining admixtures in order to reduce potential segregation of fresh concrete 8.3 Recommendations for further research The following are some suggestions for future research to attain more insight on the behaviour of the fresh LWAC: The influence of mortar rheology on migration of the LWA, with emphasis on the particle size and density, will provide better understanding for some of the current results The results from past research and current study indicate that the yield stressslump relationship is affected by the binder content, which affects the average spacing of the aggregates Instead of varying the binder content, the average - 177 - spacing of the aggregate can also be changed by variation of the entrained air content Due to the limited data in the current study, it is not possible to establish if the change in entrained air content affects the yield stress-slump relationship Therefore, concrete in series of or different entrained air contents can be tested and relate to the slump Each of these series should consist of concrete with different yield stresses From the current study, it was found that the air entrained LWAC compared with that of the non-air entrained LWAC at similar slump had higher yield stress but lower plastic viscosity This implied that a higher shear stress will be required to initiate flow in the air entrained LWAC The air entrained LWAC is expected to have a higher flow rate when flow occurs In order to verify if the suggested effect for the LWAC is also the same for NWAC, it will be necessary to compare the rheological parameters between the non-air and air entrained NWAC at similar slump For the concrete with higher yield stresses in this study, it appears that a combination of higher frequency and lower amplitude led to more segregation and this contradicts the report on vibration by ACI 309 (1993) Further research is needed for verification Much research has been done on self-compacting concrete using rock-based normal weight aggregates Information from the current study may be used as a platform for extension to rheology of self-compacting LWAC, including the use of silica fume to improve the level of compressive strength - 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190 - ... study on workability and stability of fresh lightweight aggregate concrete (LWAC) from rheology perspective It involves using rheological parameters of Bingham model, which are yield stress and plastic... particles (Du and Folliard, 2005) and aggregate in normal-weight aggregate concrete In lightweight aggregate concrete (LWAC), however, the floatation forces of the air bubbles and lightweight aggregates... 150 7.3 EFFECT OF FREQUENCY AND AMPLITUDE ON STABILITY OF CONCRETE .155 7.4 EFFECT OF VIBRATORY ACCELERATION ON STABILITY OF CONCRETE 162 7.5 INTRODUCTION 150 SUMMARY AND CONCLUSIONS