Odd E. Gjørv

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.

Development of microstructures in plain cement pastes hydrated at different temperatures

Abstract Various methods have shown indirectly that insufficient time for diffusion of the hydration products and the large pores that form as a result are responsible for the reduction in strength of concretes cured at elevated temperatures. backscattered electron imaging provides a direct means of examining the uniformity of diffusion. This paper describes an examination of the developing microstructure of cement pastes hydrated at 5–50°C. In accordance with the indirect evidence developed previously, the investigation shows that low curing temperatures result in a uniform distribution of hydration products, while elevated temperatures result in a coarsened pore structure. Specimens cured under variable-temperature regimes show some features typical of both initial and final curing temperatures. Compressive strengths of companion mortar specimens are consistent with the observed pore structure of the pastes.

Properties of Portland cement concrete containing fly ash and condensed silica-fume

Abstract Normal pozzolan additives, due to their low surface area and reactivity are not able to improve early strengths and durability of concrete. The problem can be solved by using a mixture of normal and highly reactive pozzolans, such as condensed silica-fume. Results of an investigation are reported here in which 30 percent portland cement in concrete was replaced by an equal volume of fly ash, condensed silica-fume, or a 50:50 mixture of the two. Sand-to-gravel proportions were adjusted to obtain workable concretes having the same water-cement ratio. As compared to the control concrete, the 7 and 28 days compressive strengths of the fly ash concretes were significantly lower, however, in the case of mixed-pozzolan addition, the 7-days strength was similar and the 28-days strength was higher. The differences in the pozzolanic activity of the additivies were confirmed by a parallel investigation involving determination of free lime and pore-size distribution of the cement pastes.

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.

Pore structure of plain cement pastes hydrated at different temperatures

Abstract Concrete outside the laboratory cures at temperatures other than 20°C. This paper describes an investigation of the pore structure of plain cement pastes hydrated at 5°, 20°, and 50°C to reflect a range of temperatures encountered in practice. Parallel specimens of 0.50 water/cement ratio pastes were examined using mercury intrusion porosimetry and backscattered electron image analysis. Increases in curing temperature resulted in increased porosity, particularly for pores of radius 200–1000 Ȧ as measured by mercury intrusion, or 2500–12,500 Ȧ as measured in the backscattered electron images. The difference between the two results indicates the magnitude of the “ink bottle effect” inherent in the mercury intrusion technique. However, both methods suggest that elevated curing temperatures could have a deleterious effect on the durability of plain cement concretes.

An electrochemical method for accelerated testing of chloride diffusivity in concrete

Abstract In the present paper an electrochemical method for accelerated testing of chloride diffusivity in concrete is presented. The method is based on a theoretical relationship between chloride diffusivity and observed steady-state rate of chloride migration through the concrete. The concrentration of the chloride source solution has a significant influence on the rate of chloride migration and, therefore, a correction factor for ionic interaction in the relationship is introduced. It is shown that the relationship can be used for calculation of chloride diffusivity under various testing conditions. Some experimental results are also presented.

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.

Resistance to chloride intrusion of concrete cured at different temperatures

This paper desribes a preliminary investigation of the ability of concrete to protect against the corrosion of reinforcing steel. Two types of tests measured the rate of chloride diffusion. An accelerated corrosion test compared the ability of the concrete to protect against the corrosion of reinforcing steel. Both test methods are described. The results of both tests indicate that at a given water-cement ratio, elevated curing temperatures reduce the ability of portland cement concrete to protect against chloride diffusion and the consequent depassivation of reinforcement. This effect is more pronounced at lower water-cement ratios. These findings should be taken into account in the construction of concrete structures for which durability is a concern.

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.

Performance of concrete under different curing conditions

The effect of curing conditions on strength and permeability of concrete was studied. Test results showed that after 3 and 7 days moist curing only the concretes with w/c ratios equal to or less than 0.4 were accepted, while after 28 days of moist curing however, even the concrete with w/c of 0.6 could be accepted. Silica fume has a significant effect on the resistance to water penetration. For the concretes both with and without silica fume and with w/c + s of 0.5, the 28-day compressive strengths of 3 and 7 days moist curing were higher than those of 28 days moist curing, and the silica fume concrete seemed to be less sensitive to early drying. The curing temperatures did not affect the water penetration of concrete, but affected the chloride penetration and compressive strength of concrete significantly.