Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach

Accumulations of worn‐out automobile tires create fire and health hazards. As a possible solution to the problem of scrap‐tire disposal, an experimental study was conducted to examine the potential of using tire chips and crumb rubber as aggregate in portland‐cement concrete. This paper examines strength and toughness properties of concrete in which different amounts of rubber‐tire particles of several sizes were used as aggregate. The concrete mixtures exhibited lower compressive and splitting‐tensile strength than did normal concrete. However, these mixtures did not demonstrate brittle failure, but rather a ductile, plastic failure, and had the ability to absorb a large amount of plastic energy under compressive and tensile loads. A mathematical model is used to describe the effects of rubber aggregate on the compressive and tensile strength reduction of concrete.

Rubber-Tire Particles as Concrete Aggregate

In fresh concrete a watercement (W:C) ratio gradient develops around the aggregate particles during casting, resulting in a different microstructure of the surrounding hydrated cement paste. This zone around the aggregate is called the interfacial transition zone (ITZ). This review describes the formation mechanisms and the microstructure of the ITZ. The higher W:C implies a diffusion process during hydration, and this zone may be consequently described as a heterogeneous area with a porosity gradient and a complementary gradient of anhydrous and hydrated phases. By using very fine and well-dispersed mineral additions, the initial W:C gradient around the aggregates is lowered and the ITZ is densified. The microstructure of the ITZ may be improved in the vacinity of calcereous aggregate, which reacts with calcium aluminates of Portland cement paste, forming calcium carboaluminates. The overall engineering properties of concrete in relation to the ITZ are beyond the scope of this review. Nevertheless, the local properties of the interfacial zone are reviewed: the mechanical and the transport characteristics of the ITZ are discussed in relation to the porosity and connectivity of pores.

Interfacial Transition Zone in Concrete

With a rising tide of adoption of recycled aggregate (RA) for construction, investigation on ways to improve the quality of RA has been overwhelming. The adoption of RA brings benefits including savings in the limited landfill spaces and the use of natural resources. However, the poorer quality of RA often limits its utilization to low grade applications such as sub-grade activities, filling materials and low grade concrete. The major reason that affects the quality of RA is the large amount of cement mortar remains on the surface of the aggregate, resulting in higher porosity, water absorption rates and thus a weaker interfacial zone between new cement mortar and aggregates, which weakens the strength and mechanical performance of concrete made from RA. This paper attempts to study three pre-soaking treatment approaches; namely ReMortarHCl, ReMortarH2SO4 and ReMortarH3PO4 in reducing the mortar attached to RA. The results show that the behaviour of RA has improved with reduction in water absorption, without simultaneous exceeding the limits of chloride and sulphate compositions after the treatment. This work has also compared the compressive strength, flexural strength and modulus of elasticity of concrete made from the approaches, which shows marked improvements in quality when compared with those using traditional approaches.

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Removal of cement mortar remains from recycled aggregate using pre-soaking approaches

Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.

High strength concrete containing natural pozzolan and silica fume

The behaviour of ultrasonic pulses in concrete

This is a review paper comparing critically eight standards. Methods for the determination of longitudinal pulse velocity and assessment of concrete properties by ultrasonic pulse velocity, as recommended by standards of the UK, USA, Germany, Russia, Slovakia, Hungary, and RILEM, are evaluated. It is shown that, despite the common basis of the measurement of ultrasonic longitudinal wave velocity, there are differences among the procedures as recommended by different nations. For instance, the most frequent use of pulse velocity, the assessment of concrete strength, is discussed only briefly in ASTM and in DIN; the other standards, especially the Russian and the Slovak standards, provide much more detail and description. The assessments of other concrete properties are also compared: dynamic elastic constants, defects inside concrete, concrete uniformity, and changes in concrete properties with time. The inherent uncertainty in the various assessments is so high that the assessments are not suitable for many practical purposes. The common weakness of the analyzed standards is that they do not warn the user strongly enough about the uncertainties. For instance, the assessment procedures could be rated according to their reliability.

Ultrasonic pulse velocity test of concrete properties as specified in various standards

The first rarely mentioned fundamental assumptions for the strength versus water-cement ratio relationship are discussed, namely, that: (a) the strength of structural concrete is controlled by the strength of the cement paste in it; (b) the strength of a cement paste depends strongly on the porosity in it; and (c) the porosity (capillary) is a function of the water-cement ratio. This is the foundation of the relationship between concrete strength and water-cement ratio. Numerous empirical formulas, so-called strength formulas, have been developed for this relationship; the Abrams' formula for instance. These formulas estimate the concrete strength from the water-cement ratio only, and they are usually simple but have restricted limits of validity. For improvement, a new type of strength formulas is offered in this paper, formulas that have a second independent variable beside the water-cement ratio, such as the cement content, or water content, or paste content, etc. Such augmentation (a) improves the accuracy of the strength estimation; (b) shows that if two comparable concretes have the same water-cement ratio, the concrete having the higher cement content has the lower strength; and (c) shows that the magnitude of the changes in concrete strength depends on how the water-cement ratio is changed, by changing the cement content or the water content. Experimental data support these expectations. A formula is also presented for the effect of the other kind of porosity, the air content on the concrete strength. The combination of this with any good strength formula gives good fit with experimental results both of air-entrained and nonair-entrained concretes.

Contribution to the Concrete Strength versus Water-Cement Ratio Relationship

Since the stress-strain curve of a concrete is not linear, at least under the first loading, one would expect that the presence of stresses would reduce the velocity of ultrasonic pulses in the concrete. The experimental investigation of this expectation is the topic of this paper. Mature concrete cylinders of different compositions were subjected to various types of compression loading (gradually increasing load, repeated loading, etc.), and pulse velocities were measured in various manners (with different frequencies and various paths) both in the loaded and unloaded states. The experimental results clearly indicate that the pulse velocity in concrete is independent of the stress level to a surprisingly large extent, that is, up to about 70% of the compressive strength. This means, in practical terms, that stresses prevailing in the concrete of a structure do not have to be taken into account when pulse velocity data are used for the evaluation of the quality of concrete.

Sandor Popovics

Papers

The behaviour of ultrasonic pulses in concrete

Ultrasonic pulse velocity test of concrete properties as specified in various standards

Contribution to the Concrete Strength versus Water-Cement Ratio Relationship

Effect of stresses on the ultrasonic pulse velocity in concrete

Portland cement-fly ash-silica fume systems in concrete