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Shrinkage of LWAC - Effect of aggregate absorption
Abstract
The paper deals with the lightweight aggregate concrete, especially with the influence of moisture in the inner structure of the porous aggregate on the rheology properties. Above all, it tackles the issue of shrinkage measuring in the early stage of concrete setting and hardening. Three different mixtures of concrete were made in the experimental part for the purpose of testing. Differences between the mixtures were only in the degrees of saturation of the used porous aggregate. The other ingredients were the same in all mixtures. The results of measurement are presented in the paper.
... These microcracking greatly affects the transport properties of the cement-based material [26][27][28]. Nevertheless, the evaluation of microcrack formation and their dependence on the size and volume fraction of aggregate is not yet fully understood [20,29]. The measurement of shrinkage and thermal dilation, as well as the resulting stress in the case of restraint, may be performed with so-called temperature-stress testing machines [2]. ...
Cement-based composites belong among the basic building materials used in civil engineering. Their properties are given not only by their composition but also by their behaviour after mixing, as well as by the methods of curing. Monitoring the processes and phenomena during the early stages of setting is vital for determining the resulting properties and durability. The acoustic emission method is a unique non-destructive method that can detect structural changes as a cement-based composite is setting. It can also detect the onset and growth of cracks during the service life of a cement-based composite since the moment it has been mixed. The paper discusses the use of the acoustic emission method with a focus on the early stage of the lifespan of a cement-based composite including the measures necessary for its use and description of the parameters of acoustic emission signals.
We dealt with a process of design and development of new building materials based on the secondary raw materials. The secondary raw materials can, due to their variable properties, unpredictably influence physico mechanical characteristic of the resulting material. In order to eliminate these risks, many preventive methods are applied. The best-known method is FMEA (Failure Mode and Effects Analysis). The work was performed in two parts. The first part deals with mathematical models of tensile cohesion in dependence on percentage substitution of fly ash for milled limestone using fuzzy logic theory. In the second part the possibilities of an extension of FMEA methodology by using fuzzy inference systems (FIS) is shown. Practical applications show that using FIS for modelling of physico-mechanical properties of materials with secondary raw substances and for quantification of risks is acceptable.
An experimental program was carried out to study the effect of water absorption of lightweight aggregate on the pore size distribution at the interfacial zone of lightweight aggregate concrete. Properties of lightweight expanded clay aggregates with absorption of 8.9%, 9.4% and 11% were compared. In order to do the study, three lightweight concrete mixes of water to cement ratio of 0.4, 0.44 and 0.48 were batched. The pore area percentage and pore size distribution in the cement paste and at the interfacial zone of the concretes cured for 28 days were examined using a high resolution optical microscope and image analysis software. The findings indicated that the pore area percentage at the interfacial zone was much higher than that at the hardened cement paste. The effect of W/C ratio of concrete on the pore area percentage at the interfacial zone was not significant. The aggregate of larger water absorption produced greater pore area percentage at the interfacial zone of concrete. For aggregates with absorption ranging from 8.9% to 11%, the pore area percentage ranged between 14.4% and 21.7%.
Because there are many coatings on the surface of coarse aggregate particles that reduce the concrete strengths through the
reduction of the bond between cement paste and aggregate, a laboratory work is presented whose objective was to find such
aggregate coating that improves the concrete strength. To achieve this, coarse aggregate particles were coated with a thin
epoxy layer, with or without an adhesive promoter, in which either fine sand particles, or unhydrated Portland cement particles
were embedded to make the surface rougher, or chemically more reactive, or both. After the hardening of the epoxy, concrete
was made with the coated aggregate and concrete strengths were tested. The test results obtained show that the attempts to
increase strength were unsuccessful because even the most effective coating, that with unhydrated Portland cement, produced
only small increases in strength.
Etant donné que de nombreux revêtements appliqués sur la surface des particules de gros granulats diminuent la résistance
du béton de par la réduction de l'adhérence pâte de ciment/granulat, on présente un travail de laboratoire qui avait pour
objectif de trouver un revêtement de granulat qui améliore la résistance du béton. Dans ce but, on a recouvert des particules
de gros granulats d'une mince couche de revêtement époxyde, avec ou sans agent d'adhérence, dans laquelle de fines particules
de sable ou des particules de ciment Portland non-hydratées étaient enrobées afin de rendre la surface plus rugueuse ou chimiquement
plus réactive, ou les deux. Après séchage de l'époxy, le béton était confectionné avec le granulat revêtu, et la résistance
des bétons essayée.
Les résultats obtenus montrent que les tentatives d'accroître la résistance ont été vaines parce que, même le revêtement le
plus efficace—celui comportant du ciment Portland non-hydraté n'a produit qu'une faible amélioration de la résistance.