Mixing of high performance concrete: effect of concrete composition and mixing intensity on mixing time (original) (raw)
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The development of non-proprietary Ultra-High Performance Concrete (UHPC) is one way to reduce the initial cost of construction. However, workability is a major issue for which such mixtures are not practical in field conditions. Ultra-high performance cannot be achieved in field conditions if the concrete is not placed, finished, and compacted properly during placement. In this research, six UHPC mixtures were developed (three with steel fibers and three without fibers) using materials which are readily available on the local marketplace with water-to-cementitious materials ratios ranging between 0.17 to 0.30. The workability was determined using standard ASTM flow table apparatus, and specimens were prepared to determine compressive strength, splitting tensile strength, and permeable porosity. Flow table test exhibited flow values greater than 250 mm. Such high workability of the mixtures was achieved by optimizing the silica fume content and water reducing admixture dosage. These...
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Mixing concrete is not yet a fully understood issue, with many parameters having an influence on the resulting fresh and hardened concrete properties. Even for the same composition, a somewhat different microstructure can be obtained by changing the mixing procedure and the mixer type. A mixing procedure can differ in mixing time, mixing speed, air pressure in the mixing pan, addition time of the superplasticizer, temperature, etc. The concrete industry shows a great interest in controlling these influences in order to produce a concrete of which the mechanical, rheological and durability properties are well known. In this overview, different concrete mixers, mixing times, mixing speeds, different addition times of the superplasticizer and a different air pressure in the mixing pan will be examined. A review of existing literature is presented, as well as some new experimental results.
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Effect of Mixing Time on Some Hardened Concrete Properties
Civil Engineering and Architecture, 2023
The mixing time of ready-mixed concrete may be prolonged for many reasons, which may affect the resulting properties in its fresh and hardened states. The effects of prolonged mixing time on the slump, compressive strength, porosity, and permeability of concrete were investigated in this paper. All mixtures were prepared with the proportion 1:1.5:2 by weight for cement, fine aggregate, and coarse aggregate, respectively. The investigated variable was the effect of water-to-cement ratios, which were 0.40, 0.45, 0.55, and 0.65. The materials were continuously mixed up for 15, 60, 120, and 240 minutes without any additional mixing water. To reduce the evaporation of the mixing water, the drum opening was tightly covered with a polyethylene perforated film. The results showed an increasing trend in the coefficient of permeability values with mixing time for all mixtures except for the mixture with a water-to-cement ratio of 0.65 which showed a reduction in value. A different trend of results for the coefficient of permeability in comparison with that for porosity was observed. The prolonged mixing time (longer than 90 minutes discharged limit specified by ASTM) does not cause a negative effect on compressive strength as long as it has workability that facilitates the casting process, as the mixtures with higher water-to-cement ratios showed an increasing trend in compressive strength.