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Effects of prolonged mixing of concrete

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Mixing of concrete for an extended period can adversely affect the properties of a concrete mix. Tests have shown that mixing air-entrained concrete for a period longer than 180 minutes can significantly reduce the content of entrained air. Air-entrained admixture used to prevent the damage of concrete due to freeze-thaw cycles in cold weather. The reduction of the content of air-entrained admixture will reduce the ability of concrete to resist the damage caused by freeze-thaw cycles. 
Figure 1


The temperature of the concrete mixture will increase with the increase of mixing time. The friction effects due to mixing will result in an increase in concrete mix temperature. The workability of the concrete mix will also be reduced due to the prolonged mixture of concrete. The loss of workability occurs due to water evaporation from concrete. Moreover, higher mixing speed will result in a higher reduction of concrete workability. Reducing the concrete workability will result in difficulties in placing and compacting concrete. This may cause concrete honeycomb and increasing concrete porosity, which means reducing concrete durability. Tests have shown that prolonged mixing did not affect concrete compressive strength. However, the experiment showed that mixing concrete for a period of time more than 180 minutes at a speed of 15 rpm resulted in reducing concrete modulus of elasticity and modulus of rapture. The reduction of modulus of elasticity and modulus of rupture attributed to the reduced workability of concrete mix. Reducing the workability of a concrete mixture will result in more voids in hardened concrete. Adding water to concrete is know as re-tamping. Water added to restore concrete workability, but it will reduce the concrete strength and result in higher shrinkage of concrete. It is not recommended to add water to concrete because it will reduce concrete durability and strength. Given this, many specifications and codes specifying a maximum period of 90 minutes from the time of mixing concrete to placing. If concrete exceeds this period, it should not be used. 

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  1. Aqua Blu ServicesMarch 25, 2021 at 11:57 PM

    The information you have published here is really awesome, as it contains some great knowledge about Mixing of concrete which is very essential for me. Thanks for posting it. Residential concrete contractors

    ReplyDelete

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My name is Mohammad Hamdan, and I am a Civil/bridge Engineer with extensive professional background and experience in the field of construction. I have 10+ years of experience in the construction field. During my professional career, my emphasis has been placed on structural projects, mainly bridges, infrastructure and roads. Reading and practicing the design of structures is my passion.i was lucky to be a part of a big team that executed major highway interchanges. I believe in spreading the knowledge, thus, i compose these courses to share my experience and knowledge. My course focuses on the practical side of construction field. On other word, i am sharing some of what new engineers need to know about construction field. Beside working on construction field, i love writing some articles on my website ( curious civil engineer) Moreover, i wrote some engineering papers, you can check them  by searching my name on Google (Mohammad mamon Hamdan)