International Concrete Abstracts Portal

The acid resistance of six different sets of concrete materials was measured using hydrochloric acid in a test method developed at the University of Cape Town. The concrete mixtures included a standard mix used for the preparation of sewer pipes by the roller suspension method, and five modifications of the standard mix. Four of the test mixtures were modified by partial replacement of normal portland cement with a mineral admixture, namely slag, fly ash, condensed silica fume or meta-kaolin. In the fifth test mixture, normal portland cement was replaced with a calcium aluminate cement. Silica fume concrete showed better acid resistance compared to the standard concrete. In general, at 28 days the physical properties of concrete with fine mineral admixtures (condensed silica fume, meta-kaolin) were superior to the other concretes. However, the acid resistance of the meta-kaolin concrete was not improved despite its superior quality. Improvement in the acid resistance of the concrete with condensed silica fume indicates that the concrete is improved both chemically and physically by the addition of silica fume.

This paper describes a study on the long-term strength development of concrete cured under high-temperature conditions at an early age. The concrete specimens were made with normal portland cement, high-early-strength portland cement, and low-heat portland cement, and were cured under 26 different temperature conditions. The temperature conditions were set so as to give systematic variations in the maximum temperature and the initial curing time. The strength development of concrete was examined over a period from 1 to 365 days. It was clarified that a higher maximum temperature improved the strength development of concrete at an early age, but inhibited the strength development of concrete at later ages. A shorter initial curing time inhibited the strength development of concrete at later ages. A time dependence of the effect of curing temperature on the strength development of concrete was observed. For concrete made with normal portland cement, a higher temperature during the period from 0 to 12 hours after mixing results in lower strength development after 3 days. For concrete made with high-early -strength portland cement, a higher temperature during the period from 0 to 3 hours after mixing results in lower strength development after 1 day. For concrete made with low-heat portland cement, a higher temperature during the period from 0 to 12 hours after mixing results in lower strength development after 7 days, but a higher temperature after 72 hours results in greater strength development at later ages.

In many situations concrete structures may be exposed to temperatures higher than those assumed as normal environmental conditions. This can be produced by accidental causes, such as fires. In other cases, high temperatures can be generated by the characteristics of the structural application, as in walls, pipes or vessels in some specific industries (nuclear, chemical processing, metallurgy). Most of the knowledge about concrete behavior is based on experimental results obtained on specimens at normal temperatures, which are not representative of the material properties under extreme conditions. In this paper, an extensive analysis of the physical and mechanical properties of normal and high-strength concretes exposed to temperatures up to 700 degrees C is presented. Ultrasonic pulse velocity, static and dynamic modulus of elasticity, and strength and deformations (axial and transversal) under compressive loading were measured. In addition, flexural and splitting tensile strength and the fracture energy were measured. Also, other tests for determining the water permeability, water penetration and absorption were performed on concrete slices with the aim of analyzing the differences in the physical properties of the cover and bulk concrete.

The research dealt with the durability of multilayer sandwich panels cast in situ with shotcrete technology. The research analysed the behaviour of different shotcrete mixtures towards actions caused by the climatic variations changes. A standard shotcrete mixture (traditional), normally used by panel producers, and three differently modified and optimized shotcretes (with silica fume, expansive agent, plastic fibres) were compared. Two accelerated tests were carried out: an accelerated cracking test for the short term degradation and an accelerated aging test for the long term degradation. The test procedures and the climatic chamber used in the aging test are illustrated. This report present the results of investigation on the superficial cracking density of panel specimens. The data obtained obtained at the end of the accelerated tests (total length of cracks per area unit-cm/m2) were compared. The results point out a cracking density of the optimized and modified shotcretes lower than that of the standard shotcrete. In particular the study proved the efficiency of plastic fibre-reinforced shotcrete towards the climatic variations changes both at short and long term.

Controlling and mitigating cracks in concrete is one of the most serious and inherent problems. In this research, improvement of crack resistance against thermal stress and shrinkage of the mass concrete at an early age were investigated using the hybrid fiber reinforced concrete. The fibers used were steel and polypropylene fibers with the lengths of 6,12, and 30mm. The physical properties as well as crack resistance capabilities of the hybrid fiber-reinforced concrete were evaluated. As for the evaluation of crack resistant property, strain energy release rate, calculated by the fracture mechanics, has been proposed as a result of this research.