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Krympreducerares inverkan på cementbaserade materials krympning
CBI - Cement- och betonginstitutet.
2002 (Swedish)Report (Refereed)
Abstract [sv]

Shrinkage of concrete is a well-known fact in concrete technology. The most important reason for shrinkage is water movements in the concrete, which leads to contraction of the pores in the cement paste and the whole concrete shrinks. A great number of phenomena affect the shrinkage of concrete, such as the concrete composition, type of construction, humidity and temperature. Furthermore, different types of shrinkage and different shrinkage mechanisms exist, which appear at different relative humidities. Depending on e.g. the concrete composition, water to cement ratio and relative humidity, different types of mechanisms are dominating. The different shrinkage mechanisms result altogether in the free shrinkage of the concrete. _x000D_ _x000D_ A literature review was made to give a theoretical background of what kind of shrinkage mechanisms exist, how the shrinkage can be reduced and practical cases where the shrinkage has been reduced. _x000D_ _x000D_ Shrinkage reducing admixtures were started to develop in the beginning of the 1980's and they are different types of alcohols. They are considered to act mainly by reducing the surface tension of the pore water, whereby the shrinkage tensions in the concrete are reduced. This gives positive effects regardless of aggregate content, reinforcement and type of construction. Data exist in the literature where the shrinkage at 28 days has been reduced by 50-80 % and the final shrinkage by 25-50 %. _x000D_ _x000D_ In the laboratory experiments mortar prisms were made for the shrinkage measurements and for the determination of the compressive and tensile strengths. The experiments were planned so that the water content was constant while the cement and aggregate contents were changed. The cements used were the Swedish standard cements, Std P Slite designed for house building purposes and Degerhamn standard Portland cement designed for civil engineering structures. The shrinkage-reducing admixture was a polyalifatic alcohol. _x000D_ _x000D_ The microstructure was analysed by scanning electron microscope on both polished and cracked surfaces and by polarisation microscope on thin slices. The mineral composition was analysed by energy dispersive x-ray spectroscopy and x-ray diffraction. The hydration development was measured by isotherm calorimetry. _x000D_ _x000D_ The mortars with the lowest water to cement ratios and the highest cement contents shrinked most both with and without shrinkage reducing admixture and independent of type of cement. The shrinkage amount was, however, larger with the more finely ground cement, Std P Slite. _x000D_ _x000D_ With shrinkage-reducing admixture the final shrinkage (at about 650 days) could be reduced by about 19-37 % for the used mixtures. The shrinkage reduction is strongly dependent on the mix design and type of cement. The shrinkage was highest with the highest cement contents and with the more finely ground cement Std P Slite, for which the shrinkage reduction also was highest. _x000D_ _x000D_ The specimens containing shrinkage-reducing admixture lost more weight at drying and the weight was also more increased at rewetting. Regardless of this fact the shrinkage was reduced, which probably is an effect of the reduced surface tension and that the water thereby is kept in the smaller pores without vaporising and more water vaporises from the coarser capillaries. The greater water loss with shrinkage-reducing admixture indicates that the capillary system has become coarser. The rewetting experiments where the specimens with shrinkage-reducing admixture adsorb more water without giving larger expansion indicate that the change of the structure is permanent. _x000D_ _x000D_ The water loss was greater at higher water to cement ratios, when the content of the capillary water was greater. This indicates that water loss from finer pores (gel water) gives greater shrinkage than water loss from coarser capillary pores. This can partly be due to the fact that the mortar with the lower water to cement ratio contains less aggregate, which means that the stone skeleton becomes weaker and that the free shrinkage thereby becomes greater. _x000D_ _x000D_ The shrinkage-reducing admixture retarded the cement reactions and the retardation became stronger at lower water to cement ratios than at higher. The change that can be seen in the microstructure was a consequence of this, e.g. the number of larger calcium hydroxide crystals was increased, which probably contributed to the lower shrinkage (they provide more restraint to shrinkage in the material). The chemical composition of the C-S-H-gel is, however, almost unaffected by the shrinkage-reducing admixture. _x000D_

Place, publisher, year, edition, pages
Cement och Betong Institutet , 2002. , 82 p.
Series
CBI rapport, ISSN 0346-8240 ; 2002:1
Keyword [sv]
krympreducerare, uttorkningskrympning, viktförändring, värmeutveckling, mikrostruktur, hållfasthet
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:ri:diva-2860OAI: oai:DiVA.org:ri-2860DiVA: diva2:960465
Note
272 Går att beställa: kontakta eva.lundgren@cbi.se " (300kr)The effect of shrinkage-reducing admixtures on the shrinkage of cement based materials"Available from: 2016-09-07 Created: 2016-09-07Bibliographically approved

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