Effect of Low Mixing Speed on the Properties of Prolonged Mixed Concrete (original) (raw)
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Influence of mixing procedure and mixer type on fresh and hardened properties of concrete: a review
Materials and Structures, 2012
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.
Materials and Structures, 2010
At present the use of ultra high performance concrete (UHPC) is limited to a small number of special applications. This is essentially due to the time needed to blend the UHPC mix which is considerably longer than required for ordinary concrete. The present investigations aimed at the determination of the composition features which result in the longer mixing time of UHPC. The individual components of the concrete mix (silica fume, quartz flour, water, superplasticizer and coarse aggregate) were systematically varied in order to investigate their effect on the mixing time needed to achieve optimal flow properties. Besides concrete composition, the effect of mixing tool speed was considered. The shortest necessary mixing time (stabilisation time) was calculated from the evolution of the power applied to the tool during mixing. It was confirmed that high w/c values resulted in short stabilisation times. In addition, the contents of silica fume and quartz flour as well as the type of cement and superplasticizer affected the stabilisation time significantly. It was possible to describe the effect of the individual variables in terms of the relative solid concentration /// max which depends on concrete composition. This is the ratio of the actual volumetric concentration of solids in the concrete mix / to the maximum possible concentration of solids / max calculated from the volumes of particles (particle size distribution). The relative solid concentration is the basis of a model for the calculation of the mixing time of UHPC of a given composition and for given tool speed. High tool speeds and low actual concentrations of solids shorten the stabilisation time.
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.
The Feasibility of Re Mixing Concrete
Delays may occur after the completion of the mixing process for several reasons beyond the existing ability on the site. This study was conducted to determine the feasibility of re-mix concrete (mechanical mixing), after leaving it in the open air. The concrete mixtures were left after completion of mixing process in room temperature (25 o C) for different periods of times 0, 15, 30, 45, 60 and 75 minutes then returned the mixing process then, placed in its moulds. Results were compared to those samples cast within the mould immediately after mixing. A number of 144 standard cubes were cast to measure the concrete compressive strengths at 7, 28 days. To improve the properties of concrete after re-mixing different additives were used. The used additives were water or cement mortar or workability additive compared to mixing without any additions to improve the mixing process. The most important results of the study confirm that the compressive strength of the samples increases when the mix left then re-mixed after 15 minutes. The mix could be also re-mixed after leaving in the fresh air for periods of up to 75 minutes. The results also showed that the values of compressive strength were lower than that recorded for control, but it could be used with loss of strength by about 18% of the maximum compressive strength. For re-mixing concrete it was preferred to use cement mortar to provide best results.
Influence of Chemical Admixtures on Fresh and Hardened Properties of Prolonged Mixed Concrete
Advances in Materials Science and Engineering
Effects of different chemical admixtures on fresh and hardened properties of prolonged mixed concrete and their cost-effectiveness were investigated. Influence of sand to aggregate volume ratio, cement content, and use of chilled mixing water on the properties of prolonged mixed concrete was studied as well. Different concrete mixtures were prepared using five different types of chemical admixture (one water reducer based on lignosulfonate and four superplasticizers based on sulfonated naphthalene polymer, polycarboxylic ether, second-generation polycarboxylic ether polymer, and organic polymer), varying s/a ratio (0.40 and 0.45) and cement content (340 kg/m3 and 380 kg/m3) and using chilled mixing water. Slump tests were performed at 15-minute intervals to assess the fresh performance of each prolonged mixed concrete mixture. 100 mm by 200 mm cylindrical concrete specimens were prepared and tested for compressive strength, Young’s modulus, splitting tensile strength, and ultrasonic...
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2020
Fresh properties of concrete are vitally important to make the well compaction in order to achieve the designed target of strength, serviceability and durability of the concrete structures. Fresh property of cement mortar (mortar) is generally judged by the workability and the air content. The workability refers to the compacting condition of the concrete which is directly related to the density and strength. The air content is required to maintain the durability of the structures especially minimizing the effect of freezing and thawing action. The workability, density and air content of the mortar are directly proportional to those of mortar for the given concrete mix. These parameters greatly depend upon the mixing speed and time of the mixer. This paper reports about the workability, densityand air content of the mortar mixed in the mortar mixer with varying of mixing speed and time. The workability of mortar was measured by the table flow test. It was concluded that there should...
Innovative Systems Design and Engineering, 2011
In situations like delivery of concrete from central mixing plant, in road construction, in constructing lengthy tunnels, in transportation of concrete by manual labor, in hilly terrain long hauling of concrete is required. Loss of workability and undue stiffening of concrete may take place at the time of placing on actual work site 1. In such situations engineers at site, many a time reject the concrete partially set and unduly stiffened due to the time elapsed between mixing and placing. Mixed concrete is a costly material and it cannot be wasted without any regard to cost. It is required to see whether such a stiffened concrete could be used on work without undue harm with use of retarding admixtures. The process of remixing of concrete, if necessary, with addition of just the required quantity of water is known as "retempering" of concrete 1. Sometimes, a small quantity of extra cement is also added while retempering. In the site sometimes the concrete has to wait for some time to enter in the formwork after it is mixed. This may be due to some break down in the conveyance or quarrel between the labors. In such situations the concrete looses its plasticity. But since the quantity is enormous 2 , such concrete cannot be wasted. In such situations addition of small quantity of cement and water along with retarding admixture can bring back the plasticity to concrete. Thus retempering becomes important in such odd situations 2. In this paper an attempt is made to study the strength characteristics of concrete containing retarding admixture at retempering time of 15min upto 90 min. The tests are conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength for different retempering times.
Effect of retarding superplasticizers on the properties of cement paste, mortar and concrete
Asian Journal of Civil Engineering, 2019
Chemical admixtures are becoming increasingly popular as these could improve several properties of concrete, both in plastic and hardened condition. However, improper use of admixture may lead to the inferior performance of produced concrete. Effects of admixtures dosage on the properties of cement paste, mortar and concrete were studied in this research. The most recently developed polycarboxylic ether (PCE)-based admixtures from different manufacturers were used. The trial dosages were fixed considering their properties and suppliers' specifications. Firstly, the effect of retarding on cement setting times was evaluated using cement paste and then flow test was carried out on cement mortars to examine change in workability, i.e., superplasticizing effect with this. Compressive and tensile strength test on mortar cube and briquette samples was conducted to evaluate variation in strength with different dosage of admixture while keeping other materials' proportions same. Finally, the dosage effect on workability (slump retention) of fresh concrete was studied for every 15-min interval up to 1.5 h. Effect of slump retention using admixture on compressive strength was also evaluated. Experimental results show that a small variation in admixture dosage changes these properties significantly. It was concluded that properties of cement paste, mortar and concrete could be improved with an optimum dosage for each particular type of admixture; however, higher or lower dosage than this may put adverse effect. Within the usable time, there might be other chemical effects that enhance/ decrease the concrete performance while keeping all other quantity and parameters the same for all these concrete mixes.
Construction and Building Materials
INVESTIGATION OF SETTING TIME AND COMPRESSIVE STRENGTH OF READY-MIXED CONCRETE BLENDED WITH RETURNED FRESH CONCRETE The Center for Sustainable Transportation Infrastructure (CSTI), in its 2012 study, estimated that out of all the concrete leaving the plant, between 2% and 7% of concrete returns to the plant unused as a returned fresh concrete (RFC). Disposal of both the truck wash water and RFC is a growing concern for the industry. Most industry personnel contacted during the investigation, agree that reusing is superior to recycling of this substantial RFC economically and environmentally. This study will determine if the reuse of RFC in subsequent batches compromises the quality of newly blended concrete. The effect of RFC on fresh and hardened characteristics of subsequent batches was studied. This research will be performed in a laboratory where setting time and compressive strength will be tested for both the control and blends of varying proportion and age of plain or retarded RFC with subsequent fresh batches. This study will discover the C1798/C1798M-16 (Standard Procedure for reusing returned fresh ready-mixed concrete) recommendation. In this procedure, it is stated RFC up to 8-hours old at 100F, treated with hydration stabilizing admixture, can be blended in up to 50% proportion with a new batch of RMC without adversely affecting the fresh and hardened characteristics of the blend. The reuse of RFC has been neither explicitly banned nor allowed by end users due to the uncertainties of the effects of the RFC on the properties and characteristics of the blend. This standard procedure has not encouraged the end users to reuse of RFC, despite its liberal allowance of reuse of RFC. Not only that, the prohibitive practices of the states of California and Iowa iv experience in the reuse of RFC and the limitations set by ASTM C94 reinforces the negative perception surrounding the reuse of RFC in concrete blends. A commonly used, Clark County qualified mix design No. 101, was batched both indoors and outdoors. Each batch was tested shortly after batching as a control sample. The concrete was then held for 1hr, 2hrs, 3hrs or 4hrs to simulate RFC. The simulated RFC was then mixed with newly batched concrete in various proportions. Both the control and blends were tested for slump, air entrapped, unit weight, setting time, and compressive strength as per ASTM standard and specifications. Thirty samples blended with indoor batched RFC and other 40 samples blended with outdoor batched RFC were tested in this investigation. The test results of this investigation showed that, for the mix design 101, retarded RFC up to three-hour-old and plain RFC up to twohour-old can be used in 30% and 20% proportions respectively without affecting the fresh and hardened characteristics of subsequently blended concrete. v