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  • 1.  Workability of concrete and compressive test

    Posted 06-19-2017 02:23 PM
    Edited by Veronique Nguyen 06-19-2017 02:23 PM
    Hey ASCE Collaborate,
    I am in my 1st year of studies and I am having trouble understanding workability of concrete and compressive tests, where we take concrete cubes in the size of 150mm×150mm×150mm. Why are these the only dimensions taken?


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    Divyansh Thakkar S.M.ASCE
    Student
    Bhopal MP
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  • 2.  RE: Workability of concrete and compressive test

    Posted 06-20-2017 12:56 PM
    ​Workability of concrete isn't considered a measurable quantity typically. It's more of a subjective property of concrete especially in the field where workers are placing it. Other formal tests tend to correlate to workable concrete.

    For example, slump. This is a formal test. For US codes this would be ASTM C143. High slump is wet flowing concrete, low slump is dry concrete that doesn't flow. High water content makes it more workable but also lowers the strength given the same amount of cement in each batch.

    Cube size used to measure strength is usually based on some kind of standard. In the US it's frequently ASTM C39 which uses round cylinders typically of a specified size.

    A 150mm cube may align with Eurocode, for example, or it may simply be what works best for the testing facility where you are learning.

    If you mix two batches of concrete with the same amount of cement and aggregate, but make one wet so it pours into the mold and make one dry so it must be packed into the mold, the dry one will be much stronger when tested. The wet one will be much more workable and easier to mold.

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    Brett King P.E., M.ASCE
    Senior Structural Engineer
    GHD Inc.
    Lake Oswego OR
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    Original Message


  • 3.  RE: Workability of concrete and compressive test

    Posted 06-20-2017 12:56 PM
    Hi Divyansh Thakkar.

    Based on my own understanding, 150 x 150 x 150 cube size is not the only size available. There are also cube sizes of 50 x 50 x 50, 100 x 100 x 100mm and cylinder sizes of 150 x 300mm, 100 x 200mm. According to the research carried out on the 'Effect of Cube size on the Compressive Strength of Concrete by 'Misba Gul'', the cube size of 150 x 150 x 150mm represented the sizes that are most commonly used locally and universally in the construction research. It is true that the use of 100 x 100 x100mm cube size has greater advantages in the sense that, it is easier to handle, it saves materials, it needs smaller space for curing and storage, as well as lesser labor and time, but the Engineers and Architects are still reluctant in using 100mm cubes for determining the compressive strength of concrete because of the perceived greater variability in their compressive strengths over that of the 150mm cubes. In the research results, it was observed that the compressive strength of concrete on 100mm cubes was between 5% to  6% higher than that of 150mm cubes at the age of 28days. Which means that, if you happen to use 100mm cubes, the values obtained should be reduced by 5% to 6% to achieve the Actual Characteristic Strength. Of course, this may look tedious. There is also lack of compliance criteria for the 100mm cubes as the general specification provides acceptance criteria based on 150mm cube strengths only. This is why, in this situation, 150 x 150 x 150mm cube is always used.

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    Anthony Ekwuno PMP, CT, Aff.M.ASCE
    Professional Engineering Technologist
    FT Consultants
    Pretoria
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    Original Message


  • 4.  RE: Workability of concrete and compressive test

    Posted 06-20-2017 12:57 PM
    The size of the concrete samples has a significant effect on its compressive strength. Smaller concrete samples have less interior defaults or small cracks than larger concrete samples. When the cube is loaded, these already existing micro cracks start to widen and propagate causing failure eventually. the bigger the samples are, the higher failure potential is. Thus, smaller samples have higher strength.

    For standard testing we need to reduce the variables as much as possible to be able to compare different concrete mixes, which is why a unified sample size and dimensions are used for the testing.

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    Mohamed Gallow S.M.ASCE
    Birmingham AL
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    Original Message


  • 5.  RE: Workability of concrete and compressive test

    Posted 06-21-2017 05:04 PM

    My recollection .

     

    The increase in compressive strength for smaller "samples" is mostly due to confinement stresses induced by the friction of the "sample" with reaction plates.  Confinement stresses act in same fashion as a composite column (steel HHS/pipe filled with concrete where the confinement increases concrete strength) or the triaxial stresses described in geotechnical engineering.  The 6" x 12"cylinder, typical in the US, have a 2:1 height to base ratio to minimize this "triaxial" effect vs a cube that is 1:1.

     

    The effect of workability on compressive strength.   The least expensive way to improve workability is to add water and the addition of water reduces compressive strength.

     

    Rule of Thumb: The addition of a gallon (3.78 lts) of water to a cu.yd (0.75 m3) of 3000 psi (21MPa) concrete increases your slump by 1" (25.4 mm) but it is equivalent to subtracting 25 pounds (10kg) of cement, i.e. your compressive strength is reduced by 250 psi (1.75 MPa). 

     

    Yet, you can improve workability by adding air (entrained), superplasticizer (water reducers), etc. 

     

    GAE




    Original Message


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