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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/4915

Title: Mechanical properties of lightweight aggregate concrete--effect of lightweight aggregate on the concrete
Other Titles: Qing gu liao hun ning tu li xue xing neng--qing gu liao dui hun ning tu de ying xiang
輕骨料混凝土力學性能--輕骨料對混凝土的影響
Authors: Cui, Hong Zhi (崔宏志)
Department: Dept. of Building and Construction
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Aggregates (Building materials)
Lightweight concrete
Notes: CityU Call Number: TA439.C85 2007
Includes bibliographical references (leaves 193-206)
Thesis (Ph.D.)--City University of Hong Kong, 2007
xxiii, 245 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: The physical and mechanical properties of manufactured lightweight aggregate (LWA) may vary with different raw materials and from place to place. Compared with normal weight concrete (NWC) of the same mix proportions, the use of LWA from different sources may lead to significant differences in the properties of the resulting lightweight aggregate concrete (LWAC). This study aims to 1) investigate the effects of LWA properties on the properties of the resulting LWAC; 2) obtain the stress-strain curves of different types of LWAC through experimentation and then fit the curves with mathematical equations; and 3) develop four models that are based on LWA properties to predict the stress-strain relationship, compressive strength, elastic modulus, and peak strain of the LWAC. For this research, five types of LWA (LT, LS, DG, YC, and SH) were used to set up the prediction models, and another type of LWA (MOD) was used to validate them. Based on the above objectives, the achievements of this research can be divided into four main parts. In the first part (Chapter 5), the effects of LWA properties on the mechanical properties of LWAC are studied. Through experimental study, it is found that 1) an increase in the volume content of LWA leads to a decrease in the compressive strength and elastic modulus of the LWAC; 2) a higher particle density of LWA generally increases the compressive strength, strain, and elastic modulus of the resulting LWAC, but it is not the case that a higher LWA particle density always leads to higher mechanical LWAC properties; and 3) LWA shape can affect the compressive strength and peak strain of LWAC. Where the particle density is similar, the shape characteristics of LWA have more influence on the properties of the resulting LWAC than does LWA tube crushing strength. Because of the influence of LWA shape, it is not certain that using LWA with a higher tube crushing strength will lead to higher compressive strength and modulus of elasticity in the resulting LWAC. Therefore, in the prediction models for LWAC properties, LWA volume content (aV), strength (TR), and shape (sI) must be taken into account. Also, the stress-displacement (20 and 40 mm) curves from the tests of LWA tube crushing strength show that the standard (GB/T 17431.2-1998) tube crushing strength method is not appropriate to assess LWA strength. A higher concentration of modifier is more effective to reduce the water absorption of LWA. The modification of the aggregate does not appear to affect the permeability of the LWAC. The test results show that surface modification effectively reduces the water absorption of LWA, but does not have any obvious adverse effect on the mechanical properties and durability of the resulting LWAC, and that there is an optimal value of modifier concentration. In the second part of the research (Chapter 6), the stress-strain relationships of LWAC under uniaxial compression are studied. Through analysis of the experimental results, some valuable observations are made. First, a comparison between the experimental curves of the stress-strain relationship of LWAC and NWC shows the former to be much more brittle than the latter. Therefore, the quadratic equation is better than the cubic equation to represent the stress-strain relationship of LWAC under uniaxial compression. Second, it is difficult to model stress-strain curves using linear and quadratic equations, because the model curves may not match well at the intersections. The fitting of the entire stress-strain curve of concrete is best achieved by using the single quadratic equation bxaxy+=2, although the chance of error with this method is greater. Third, experimental study of the pre- and peak stress-strain relationships of LWAC shows that the existing model of the NWC stress-strain relationship,xxy22+−=, can be modified as xxyϕψ22+−= to model LWAC through the addition of the modification factors ψ and ϕ, with ϕ, ψ = 1 for NWC and ϕ, ψ < 1 for LWAC. Using the single-curve fitting method for the stress-strain curve model for LWAC, the reduction factors ϕ and ψ are a function of LWA volume content (Va), LWA crushing strength (RT), and LWA shape index (IS), that is, ),,(),(STaIRVf=ψϕ. A study of the influence of LWA properties on the shape of the stress-strain curve shows that the brittleness of LWAC increases as the LWA volume content increases; that the greater the strength of the LWA, the greater the plasticity of the resulting LWAC; and that the brittleness of LWAC increases as the shape index of the LWA increases. Fourth, for NWC and LWAC of the same aggregate volume content, the fracture energy GF of the NWC is greater than that of the LWAC. Therefore, LWAC is more brittle than NWC with the same aggregate content. For LWACs that use different LWAs, however, the values of the GF show no great difference. Therefore, the effect of LWA volume content on the fracture energy of the resulting LWAC is not obvious. In the third part (Chapters 7 and 8), prediction models that are based on the properties of LWA are developed for the uniaxial stress-strain relationship (bxaxy+=2), compressive strength (28R), modulus of elasticity (E), and peak strain (cε) of LWAC. For convenience of expression, these four models are named the stress-strain model, the strength model, the elastic modulus model, and the strain model. In Chapter 7, to choose a better stress-strain model for LWAC, four models that are based on different variables (that is, influence factors) are developed using the multiple linear regression method. These are ),,,(,TsaaRIVfbaρ=, ),,,,(,TsamaRIRVfbaρ=, ),,,,,(,TsaccaRIfEVfbaρ=, and ),,,,(,TsacaRIfVfbaρ=. Through comparison, it is concluded that LWA properties are more critical to the stress-strain behaviors of LWAC than are mortar properties. Using the results of the analysis and model comparison, the constitutive relationships of NWC and LWAC can be expressed as 22xxyψϕ−=, and the prediction models for the modification factors ϕ and ψ can be expressed as 961.00112.0236.00001967.000292.0+−−+−=TsaaRIVρϕ and 85.00186.0443.0000374.0005507.0+−−+−=TsaaRIVρψ, respectively. In Chapter 8, the prediction models for the compressive strength (28R), modulus of elasticity (E), and peak strain (cε) of LWAC are developed. Multiple linear regression is the mathematical method for all of the modeling. Three models to predict the 28-day compressive strength of LWAC are considered: (i)),,,,(128sTaamIRVRfRρ=, (ii) ),,,(228sTaaIRVfRρ=, and (iii) ),,(328sTaIRVfR=. LWAC strength can be expressed by the equation sTaaIRVR705.28308.1008382.039.028+++−=ρ, which achieves the best prediction discrepancy of 13%, compared with the experimental results. Three models to predict the elastic modulus of LWAC are considered: (i) ),,,,(1sTaamIRVRfEρ=, (ii) ),,,(2sTaaIRVfEρ=, and (iii) ),,(3saaIVfEρ=. The elastic modulus of LWAC can be expressed by the equation Eρ, which achieves the best prediction discrepancy of 11.88%, compared with the experimental results. Three models to predict the peak strain of LWAC are considered: (i) ),,,,(1sTaamcIRVRfρε=, (ii) ),,,(2sTaacIRVfρε=, and (iii) ),(3sTcIRf=ε. The peak strain of LWAC can be expressed by the equation 687.1258268.422849.117++=sTcIRε, which achieves the best prediction discrepancy of 14.88%, compared with the experimental results. In the fourth part (Chapter 9), three studies are carried out. The first is a validity study of the prediction models (the stress-strain, strength, elastic modulus, and strain models), which shows the difference between the experimental results and the predicted results to be less than 5%. Thus, the precision of the prediction models demonstrates the effectiveness of the method and the potential application of the models to the analysis of LWAC in practice. The second study involves experiments to compare the mechanical properties of NWC and LWAC beams. The results show the mechanical properties and deformation of LWAC and NWC beams with the same steel bar arrangement to be similar. It is therefore concluded that LWAC beams can be used in place of NWC beams that have the same steel bar arrangement with no appreciable loss of performance. In the final chapter, the research conclusions are summarized, and potential applications, limitations of the research, and future directions for research are discussed.
Online Catalog Link: http://lib.cityu.edu.hk/record=b2217841
Appears in Collections:BC - Doctor of Philosophy

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