Shrinkage Of High-strength Concrete Research Articles

Influence of ultra-fine fly ash on hydration shrinkage of cement paste

Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data analysis in a series of experimental studies, the influence of ultra-fine fly ash on the hydration shrinkage of composite cementitious materials was investigated. It is found that ultra-fine fly ash can reduce the hydration shrinkage of cement paste effectively, and the more the ultra-fine fly ash, the less the hydration shrinkage. Compared with cement paste without the ultra-fine fly ash, the shrinkage ratio of cement paste reduces from 23.4% to 39.7% when the ultra-fine fly ash replaces cement from 20% to 50%. Moreover, the microscopic mechanism of the ultra-fine fly ash restraining the hydration shrinkage was also studied by scanning electron microscopy, X-ray diffraction and hydrated equations. The results show that the hydration shrinkage can be restrained to a certain degree because the ultra-fine fly ash does not participate in the hydration at the early stage and the secondary hydration products are different at the later stage.

Pre-Soaked Lightweight Aggregates as Additives for Internal Curing of High-Strength Concretes

Abstract Self-desiccation and autogenous shrinkage of high-strength concretes is one of their drawbacks which cannot be readily accommodated by conventional curing. The concept of internal curing using lightweight soaked aggregates, to provide internal reservoirs of water which enable uniform curing of the whole cross-section, has been advanced by several groups. The present study is intended to develop this approach further, by optimizing the porosity and size of the aggregates to enable successful internal curing with only a small amount of aggregates, which could be viewed as additives rather than bulk replacement of conventional aggregates. The approach taken was to increase the porosity of the aggregates and reduce their size, to obtain a system with numerous internal reservoirs of sufficiently small spacing to allow water to be readily discharged from the aggregate and transported over the whole range of the matrix. It was shown that aggregates with porosities of about 50% by volume, a size of a few millimeters, and contents of less than 50 kg/m3 could provide full elimination of autogenous shrinkage in concretes having w/cm as low as 0.25, with only a small affect on strength. The parameters controlling the efficiency of the aggregates were assessed, indicating that their pore structure is the most important one, and that water from within the aggregates can be readily transported into the matrix to a distance of few millimeters.

Long-Term Performance of High Strength Concrete

This paper describes an experimental investigation of how time-dependent deformations of high strength concretes are affected by maximum size of coarse aggregate, curing time, and relatively low sustained stress level. A set of high strength concrete mixes, mainly containing two different maximum sizes of coarse aggregate, have been used to investigate drying shrinkage and creep strain of high strength concrete for 7 and 28-day moist cured cylinder specimens. Based upon one-year experimental results, drying shrinkage of high strength concrete was significantly affected by the maximum size of coarse aggregate at early age, and become gradually decreased at late age. The larger the maximum size of coarse aggregate in high strength concrete shows the lower the creep strain. The prediction equations for drying shrinkage and creep coefficient were developed on the basis of the experimental results, and compared with existing prediction models.

高強度コンクリートの収縮に関する基礎的研究

In this study, we examined the fundamental property of the shrinkage of high strengthconcrete over 600kgf/cm^2 by experiment. And we tried to suggest a formura estimating.the shrinkage of high strength concrete. The results are as follows. (1)The ratio of the shrinkage due to drying to the shrinkage due to hydration gets higher as concrete strength increase. (2)The shrinkage of high strength concrete due to hydration is influenced by cement contents and admixture, and the one due tp drying is influenced by water cement ratio,, cement contents and volume ratio of agregate.

Practical prediction of creep and shrinkage of high strength concrete

Model for practical prediction of creep and shrinkage of normal strength concrete, developed previously, is extended to high strength concrete. It is found that only a minor adjustment for the concrete strength effect is needed in the formulas for drying creep. The formulas for basic creep and shrinkage need no adjustment. The prediction model is compared with test data for creep and shrinkage obtained recently by Ngab, Nilson and Slate, and by Collepardi, Corradi and Valente, and a satisfactory agreement is demonstrated. The coefficient of variation of the deviations from test data is not larger than that for the normal cient of variation of the deviations from test data is not larger than that for the normal strength range. However, the existing data are rather limited and further testing is desirable.