International Concrete Abstracts Portal

In Japan, it has been predicted that enormous quantities of demolished concrete will be produced in the future. Therefore, a great deal of research is being conducted to find ways to recycle this demolished concrete as concrete aggregate. However, because the characteristics of the original concrete are not well known in many cases, the results are also not clear. Moreover, the recycling of the cement in concrete is also necessary from the standpoint of resolving global environmental problems and achieving sustainable development . This paper describes the properties of the concrete made with recycled aggregates from the original concrete of known quality and the recycling of the cement in concrete as the approach that should be taken in the 2 1 st century.

The paper presents a critical review of the relationship between ettringite formation and sulfate attack. Ettringite formation is associated with expansion. However, not necessarily any ettringite-related expansion is related to sulfate attack. Early ettringite formation (EEF) which occurs immediately (within hours) in a plastic fresh mixture does not produce any damaging expansion and is associated with the regulation of setting time of portland cement paste. Expansion after the hardening of cement paste can be advantageously used for development of chemical prestress in expansive cements. Delayed ettringite formation (DEF) occurs at late ages and the related heterogeneous expansion in a very rigid hardened concrete can produce cracking and spalling. Two different types of DEF are examined depending on the sulfate source: DEF caused by external sulfate attack (EM) or internal sulfate attack (ISA). ESA, related to the interaction of environmental sulfate can be precluded by the use of impermeable concrete. with the cement matrix, On the other hand, ISA occurs in a sulfate-free environment due to the interaction of internal sulfate (from cement or gypsum contamined aggregate) with calcium-aluminate hydrates of the cement paste. Two different mechanisms of DEF caused by ISA are examined. The first one is based on the thermal decomposition of ettringite in high-temperature cured concrete elements and the subsequent re-formation of ettringite at ambient temperature in a saturated atmosphere. According to the second mechanism ISA is based on a chain of three essential events (microcracking, late sulfate release, and exposure to water) and DEF could occur even at room temperature.

High dosages of lignosulphonates for super-plasticizing are prohibited by the simultaneous setting retardation. However, the set retardation of lignosulphonates can be counteracted by calcium nitrate without destroying the rheology. Thus, such combinations can be cost-effective alternatives to super-plasticizers. The effect of different calcium nitrate dosages on 5 different lignosulphonates is documented by Fann viscosity, flow resistance and setting time on two different cement pastes. It has also been shown that calcium nitrate is capable of counteracting the effect of strong setting retarders like citric acid etc. Another application can thus be over-retardation of concrete for long transport from the concrete ready mix plant, followed by activation by adding calcium nitrate to the revolving drum of the concrete truck when arriving at, or being close to, the construction site.

The use of high-strength concrete in precast factories is a common solution to attain larger spans and columns with a higher load bearing capacity. Taking into account the cost for transform the mold turnover in the precast line, it is economical to alter the concrete composition and the amount and size of reinforcement to attain the higher requirements. In collaboration with a precast factory, a test program was carried out to reveal the influence of concrete composition and curing temperature on material properties. The goal of the project was to predict the strength evolution of the material, using conventional maturity functions. Therefore, different concrete compositions were used, varying from concretes normally used on construction site, to high-strength concrete. Different curing temperatures were considered. The paper presents the evolution of the compressive strength in time, taking into account different parameters as well as the evaluation of existing maturity functions. Among others, the maturity functions of Plowman, of Kee, of Freiesleben -Hansen and Pedersen and of Carino are investigated. Especially the model, proposed by Carino predicted the strength evolution adequately.

One of the main problems of sustainable building is that the existing systems don’t lead to clean and direct reusable secondary building materials after demolition. In constructions in the Netherlands many different building materials are being used. When a building has reached its end of life, it will be demolished and it becomes demolition waste. Because a lot of different building materials will be mixed together during the demolition process, much effort must be taken before the Demolition and Construction Waste can be re-used. To solve this problem, two steps need to be taken. Firstly a building should be designed for recycling and secondly all buildings should be dismantled into elements or reduce to in clean secondary materials. The approach here is twofold. Firstly, research into the demolition/dismantling-process in order to find the bottlenecks in closing of the material cycle (at either element or material level). Secondly, as a spin-off of solving these problems, recommendations for future building-systems: design for recycling. This paper describes how certain demolition and dismantling techniques can be used in achieving the goals of sustainable design and construction.