Concrete Mix Design by Trial and Error by urd17793


									Revised 2007, WKS                                                          Datasheet No. 7.3


Concrete Mix Design by Trial and Error


        The purpose of this laboratory procedure is to proportion a concrete mix of
        specified water/cement ratio and slump by a trial and error procedure. The student
        will also have an opportunity to practice performing slump tests, fresh density tests
        and casting cylinders for testing. This lab also demonstrates the concept of
        repeatability in measurements by requiring two cylinders per batch be cast.


    1. Each group will be assigned a water-cement ratio:

           Group:      1      2       3        4        5        6        7           8

              w/c:    0.40   0.43    0.45     0.47     0.50     0.53     0.55       0.57

    2. In this procedure aggregates on the dry side of SSD will be used, since it is much
       easier to add water to achieve the SSD state than it is to make adjustments to the
       cement. Obtain about 12.5 kg of each aggregate and, after mixing separately, take a
       500 g moisture sample, weigh to 0.1 g and dry over a flame. Calculate the moisture
       correction factors Cc and Cf and the equivalent SSD mass of the aggregate as
       outlined in Test Procedures and Formulae.

    3. Add sufficient water to both the coarse and fine aggregate to bring them to the
       SSD condition and mix thoroughly. Weigh the containers with the aggregate in them
       and record this weight.

    4. Weigh out 4.5 kg of cement and enough water to yield the water-cement ratio
       assigned to your group.

    5. After moistening the mixing pan and utensils, place the cement and water in the pan
       and mix thoroughly to a smooth paste.

    6. From the measured stock of SSD aggregate add CA and FA to achieve a slump of 70
       ± 10 mm and an acceptable ratio of CA to FA.

  7. Slump testing should be started while the mix is on the wet side so that the desired
     slump point will not be passed. The optimum ratio of CA to FA is not sharply
     defined. In general the correct amount of sand to be used is that amount which will
     make enough mortar to fill the spaces between the CA and leave some excess for
     workability. This condition can best be detected by troweling. If a smooth, thin
     mortar surface can be obtained with three or four trowel strokes, then proportions
     are approximately correct. The addition of CA will have a much smaller effect on
     the slump than the addition of FA. It is better to keep the mix slightly over-sanded
     as the required slump is approached.

  8. When the mix is satisfactory, weigh the amounts of the SSD CA and FA (in their
     containers) remaining. Record the slump, perform a fresh density determination and
     cast two cylinders for testing during the next laboratory session.


  1. The batch proportions are calculated by subtracting the FA and CA left over from
     the starting masses.

  2. By referring to the Test Procedures and Formulae calculate:

     i) the fresh density,
     ii) the proportions per m3 of the mix,
     iii) the hardened density and
     iv) the compressive strength.


  1. The report should include a title page, the completed Report Form, two graphs as
     described below and the completed data sheet 7.3. The proportions per m3 of the
     mix should be reported. The masses should be based on SSD aggregates and the
     water to cement ratio should be calculated therefrom and be reported.
  2. The above values should be available during (brought to) the subsequent laboratory
     session as each group must record its results on the data collection sheet provided
     during the lab session in which the cylinders are tested. The values entered will be
     evaluated as part of the mark for the report. A summary of the class results in this
     same format will be posted on the instructor’s website to enable the performance of
     the required analysis in the report and the production of the following graphs:
     a)    compressive strength on the Y–axis and water-cement ratio on the X–axis.

     b)    hardened density on the Y–axis and water-cement ratio on the X–axis.


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