Min-Hong Zhang

MICROSTRUCTURE OF THE INTERFACIAL ZONE BETWEEN LIGHTWEIGHT AGGREGATE AND CEMENT PASTE

Abstract Over recent years much attention has been given to the influence of the interfacial zone between aggregate and cement paste on the various properties of concrete. Most of the research work on this area has been carried out using normal weight aggregate. In the present paper experimental work on the interfacial zone between different types of lightweight aggregates and cement paste is presented. For high strength lightweight aggregate with a dense outer layer the nature of the interfacial zone between aggregate and cement paste is similar to that for normal weight aggregate. For lightweight aggregate with a weaker and more porous outer layer and for aggregate without any outer layer, the interfacial zone is more dense and homogeneous. Also for such aggregates the bond appears to be better due to an improved mechanical interlocking between the aggregate and the cement paste. The nature of the interfacial zone appears to depend on the microstructural characteristics of the aggregate.

MECHANICAL PROPERTIES OF HIGH-STRENGTH LIGHTWEIGHT CONCRETE

Information is presented on the mechanical properties of high-strength lightweight concrete up to 100 MPa with a corresponding density of 1865 kg/ sq m. Five different types of lightweight aggregates were investigatged and the strength of the aggregate appears to be the primary factor controlling the strength of the high strength lightweight concrete. The tensile/compressive strength ratio appears to be lower for high strength lightweight concrete than that for high strength normal weight concrete, and the elastic modulus, which varied from 17.8 to 25.9 GPa, is much lower than that of normal weight concrete. For the high strength lightweight concrete, the shape of the ascending part of the stress-strain curve was more linear than that of lightweight concrete with low to medium strength.

Water permeability and chloride penetrability of high-strength lightweight aggregate concrete

Abstract This paper presents an experimental study on the water permeability and chloride penetrability of high-strength lightweight concrete (LWC) in comparison to that of high-strength normal-weight concrete (NWC) with or without silica fume. The results were also compared with those of the concrete at a normal-strength level of about 30–40 MPa. In order to compare the water permeability and chloride-ion penetrability, LWC and NWC had the same proportion by volume. The only difference between them was the coarse aggregate used. The results indicated that at the strength level of about 30–40 MPa, the water permeability of the LWC was lower than that of the corresponding NWC. However, the water permeability of the high-strength LWC and NWC was of the same order. The resistance of the LWC to the chloride penetration was similar to that of the corresponding NWC both in the normal-strength and high-strength levels. As the compressive strength of the LWC was lower than that of the corresponding NWC, the results indicated that for a given 28-day strength, the LWC would probably have high resistance to water and chloride-ion penetration than the NWC. The results indicated that the resistance to the chloride penetration does not seem to be correlated to the water permeability of the concrete. There appears to be, however, a direct relationship between the rapid chloride penetrability determined by ASTM C1202 and the observed chloride penetration depth determined by the immersion and salt ponding tests.

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

Abstract This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement. The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.

Permeability of High-Strength Lightweight Concrete

Information on the resistance of high strength lightweight concrete 50 to 100 MPa to water penetration and accelerated chloride penetration is presented. Testing techniques are also discussed. The permeability of high strength-lightweight concrete appears to be very low, but it may be higher than that of normal weight concrete at a similar strength level. The peremeability of high strength lightweight concrete appears to be more dependent on the porosity of the mortar matrix than the porosity of the lightweight aggregate. There appears to be an optimum cement content for permeability. A too high cement content may increase the permeability. No direct relationship between water permeability and electrical conductivity was observed, but a direct relationship between water permeability and accelerated rate of chloride penetration was observed. Hence, accelerated testing of chloride penetration appears to be a more valuable way to test the permeability than testing the electrical conductivity.

Penetration of cement paste into lightweight aggregate

Abstract Mix proportioning of lightweight concrete is generally less accurate than that of normal weight concrete. From time to time it has been speculated whether a part of the cement paste from the fresh mix also penetrates into the aggregate. This paper presents an experimental investigation on penetration of cement paste into the lightweight aggregate. During mixing of lightweigth aggregate concrete the cement paste will penetrate most of the open pores in a surface layer of the aggregate. However, a deeper penetration of silica fume particles from the cement paste was not observed. The amount of paste penetration depends on the microstructure of the surface layer of the aggregate, the particle size distribution of the cement and silica fume, and the viscosity of the paste. Since the microstructure of the surface layer may vary from one type of aggregate to another and also within the same aggregate, it may be diffcult to quantify the amount of cement paste penetrating into the aggregate. In addition to water absorption, penetration of cement paste further adds up to the unaccuracy in the mix design of lightweight aggregate concrete.

Effect of silica fume on pore structure and chloride diffusivity of low parosity cement pastes

Abstract This paper presents the results of an experimental investigation on the effect of silica fume on pore structure and diffusivity of low porosity cement pastes. For cement pastes with w/c ratios in the range of 0.20 to 0.30, a 10% replacement of the cement with silica fume only reduced the total porosity to a small extent. However, a refinement of the pore size distribution took place in such a way that the content of larger pores was reduced for decreasing w/c ratio. For pure portland cement pastes the effect of a certain constant change of w/c ratio on the chloride diffusivity was substantially higher at high w/c ratios than at low w/c ratios. A 10% replacement with silica fume reduced the chloride diffusivity for all w/c ratios so much that the effect of w/c ratio became less important.

Effect of silica fume on cement hydration in low porosity cement pastes

This paper presents some experimental test results on how silica fume affects the cement hydration in low porosity cement pastes. For cement pastes of low w/c ratios (less than and equal to 0.4) only a moderate increase in non-evaporable water content during a period of up to 550 days was observed. In the presence of silica fume, however, the non- evaporable water content decreased from 90 to 550 days, which is assumed to be due to a release of water during a polymerization of the silicates. Thus, the non- evaporable water content does not appear to be a valid measure for monitoring the hydration for cement paste containing pozzolanic materials such as silica fume, especially over a long period of time. The pozzolanic reaction started even before one day and continued up to 550 days. The reaction kinetics appears to be very much influenced by the level of w/c ratio. For decreasing w/c ratios from 0.4 to 0.3 and 0.2 a Ca(OH)2 content of approximately 16, 12, and 8% was observed, respectively. At 16% replacement, approximately all of the Ca(OH)2 was consumed regardless of w/c ratio. Analysis of the pore solution composition showed that even for such a low w/c ratio as 0.2, a 16% replacement of silica fume did not reduce the pH level to more than 12.7, which is slightly more than that of a saturated Ca(OH)2 solution. (A)

Effects of the Parent Concrete Properties and Crushing Procedure on the Properties of Coarse Recycled Concrete Aggregates

AbstractFine tuning of concrete recycling processes requires a comprehensive understanding of the overlapping interactions and effects of the various influencing parameters. The most important parameters associated with recycled concrete aggregate (RCA) production that may affect quality and yield include such properties of the parent concrete as the composition, strength and aggregate grading, type of crushers used, number of crushing stages, the size of the RCA particles, and the size reduction sequence. The effects of such parameters overlap and interact with one another and render it impossible for firm conclusions to be drawn on the effects of one parameter in isolation as evidenced by results reported in available literature. The present paper reports on the findings of an experimental study on the concomitant effects of the strength of the parent concrete, size of the natural aggregates used in the parent concrete, and the number of crushing stages on the properties of coarse recycled concrete aggr...

HYDRATION IN HIGH-VOLUME FLY ASH CONCRETE BINDERS

This paper presents a summary of work in progress on an examination of the hydration chemistry and microstructure of a paste prepared incorporating 58 percent of a typical American Society for Testing and Materials (ASTM) Class F fly ash and a portland cement from United States sources and a paste with the portland cement only. Thermal analysis, x-ray diffraction, pore fluid extraction, and scanning electron microscope have been employed to study cement and cement-fly ash pastes cured up to 180 days. High levels of nonevaporable water and removal of alkali ions from pore solutions in pastes cured for 7 to 14 days were found. Etching of fly ash particles and extensive deposition of reaction products at ash/matrix boundaries were evident in scanning electromicrographs. Together, these observations clearly demonstrate extensive participation by the fly ash in hydration and cementation reactions. However, despite the extensive reactivity, up to 180 days, many fly ash particles remain as intact pseudomorphs embedded in the hydrate mass. A model based on siloxane and silaloxane hydrolysis, alkali ion exchange, and precipitation of calcium silicates, aluminates, and aluminosilicates, is proposed to explain the observed processes.