Early-age Strength Development Research Articles

Effect of Early-Age Temperature Rise on the Stability of Rapid-Hardening Cement Fiber Composites

The fiber composites reported herein comprise a particular class of multiphase materials in which the major volumetric phase, the binder, is a rapid-hardening cement-based material and the minor volumetric phase, the reinforcement, is a fibrous steel material. A further feature of these composites was that the proportion of fiber to binder was significantly high, up to 20% by volume. The work was limited to commercially available steel fibers and cements; the latter were a magnesium phosphate and an accelerated calcium aluminate. The objectives of this work were (1) to carry out a brief review on rapid-hardening cements; (2) to identify the influence of early-age temperature rise on the stability of cement grouts; and (3) to investigate their early-age strength development. The work established that both cements could be used as binders in fiber composites.

Characterization of Cracking Restraint at Saw-Cut Joints Using the German Cracking Frame

Cracking of concrete slabs at saw-cut notches under a given combination of weather and slab conditions is a major concern in the early stages of construction. Several factors affect crack initiation such as material strength parameters, the method and quality of curing, slab–subbase stiffness, and concrete shrinkage and creep. Early-age creep and shrinkage also appear to be key factors in stress and crack development at the tip of the saw-cut notch. These two factors were analyzed and characterized by experiments that provide some insight relative to how shrinkage and creep strain affect crack development. These results can be useful in evaluating the behavior of concrete and the development of cracking at saw-cut notches. The German cracking frame was used to provide restraint and the means to initiate cracking at an artificial notch during early-age strength development in the concrete while the concrete was undergoing creep and shrinkage strains. The cracking frame tests were conducted under constant temperature and relative humidity conditions in the laboratory. A fracture mechanics approach was followed to determine concrete strength at the saw-cut notch. It can be concluded that the development of creep strain in the concrete affects the cracking tendency of the concrete. The data seemed to suggest that a greater cracking tendency at the saw-cut notches did not result until after the incremental creep strain in the concrete had diminished sufficiently. The role of creep and shrinkage under field conditions relative to the control of cracking due to saw cutting in newly constructed concrete slabs is also considered indirectly.

In-situ strength testing of high strength concrete

High strength concrete is being used increasingly in major civil engineering and building projects. Measurement of the in-situ strength is often required as a means of determining early age strength development and form work stripping times, or, for verifying structural performance. A number of techniques for the estimation of the in-situ strength of high strength concrete have been examined. The use of temperature matched cured test cubes was found to give the closest correlation with in-situ strength although ultrasonic pulse velocity measurements and pull-out tests were useful techniques. One aspect of testing high strength concrete that emerged as significant, was the need to base calibration or correlation curves on specimens sealed against moisture ingress or moisture loss. rather than on water-cured specimens. Two high strength concrete mixes incorporating different aggregate types (with mean 28 day cube strengths of 90 MPa and 100 MPa) were investigated in large-scale test items. A comparison of maturi measurements, UPV and pull-out tests is given together with discussion of the nature of in-situ strength development in high strength concrete.

Monitoring concrete early age strength development: 40955 Bungey, J.H. Proceedings of the 12th World Conference on Non-Destructive Testing, Amsterdam (Netherlands), 23–28 Apr. 1989, Vol. 2, pp. 1243–1248. Edited by J. Boogaard and G.M. van Dijk, Elsevier, 1989

Monitoring concrete early age strength development: 40955 Bungey, J.H. Proceedings of the 12th World Conference on Non-Destructive Testing, Amsterdam (Netherlands), 23–28 Apr. 1989, Vol. 2, pp. 1243–1248. Edited by J. Boogaard and G.M. van Dijk, Elsevier, 1989

ＧＦＲＣセメントを用いたガラス繊維補強コンクリートに対する蒸気養生条件の影響

This report presents the results of the investigation on the influence of steam curing on GFRC using GFRC cement of low alkaline type. The glass fiber content and condition of steam curing were varied. Prism specimens were tested for compressive strength, flexural strength, shear strength, drying shrinkage and load-deflection curve.These investigations provided the following results; The strength of GFRC using GFRC cement became higher with the increase in the presteaming period and in the wet curing period after steam curing. Not only the early-age strength development but also the later strength development for steam-cured GFRC using GFRC cement were improved compared with GFRC using ordinary portland cement. The adoption of steam curing was effective in decreasing the drying shrinkage of GFRC. The toughness of GFRC using ordinary portland cement was not so good after the curing period. Especially, the wet-cured GFRC gave the lowest value. It was considered that the alkali resistant properties of GFRC could be improved by the use of GFRC cement.

Fly ash in concrete — a microstructure study

The paper presents preliminary details of a laboratory study aimed at identifying the mechanism or mechanisms whereby the incorporation of fly ash influences the strength of lean concrete mixes suitable for use in the construction of concrete gravity dams. It involved a scanning electron microsope study and compressive strength evaluation of the paste fraction and covered a range of mixes containing various amounts of a single supply of fly ash. Test specimens were cured at 20°C and at 40°C, data being obtained at intervals of seven days up to the age of three months for the main part of the investigation. In addition, some autoclave tests were undertaken and, for specimens cured at 20°C, strength tests were also carried out at ages up to twelve months. Although it is not possible to propose a firm hypothesis, the study suggests that the early age strength development is largely influenced by the physical presence of the ash particles rather than by a compositional change of the particles as a result of chemical reaction. However, the continuing increase in strength at greater ages is believed to be pozzolanic in nature and the need for further work is emphasised.