In Trial and Error Method of concrete mix design, the proportion of fine aggregate to coarse aggregate is find out by trial, which gives the maximum weight of the combined aggregate. The density of particle of fine aggregate and coarse aggregate is nearly the same, the mixture which will give the maximum weight will have the maximum solid matter therefore the least voids and surface area. In another method of trial mix fine aggregate and coarse aggregate are combined in several proportions. The quantity of cement per unit volume of concrete is found to give a certain water-cement ratio for each mixture. The percentage of sand is known as optimum percentage.
Cement required will be more for the same consistency if sand used is more than optimum. The mix will become harsh for low quantity of sand. Optimum percentage of sand is lower for lower water-cement ratio.
Steps to determine the mix proportions of a cement concrete mix by Trial and Error Method.
(a) Select the water-cement ratio corresponding to the designed target of average compressive strength of concrete and for the required climatic condition.
(b) Find out the workability in terms of slump for selected water-cement ratio for a particular job.
(c) Find out the maximum size of coarse aggregate, which is desired to use, or which is available.
(d) Mix fine aggregate and coarse aggregate in such a manner that either the mixed aggregate has a grading corresponding to Fuller’s formula or mix aggregate has a fineness modulus as economical value of fineness modulus for combined aggregate or mix the aggregate so that the weight per litre of the mixed aggregates is the maximum or the sand corresponds to the optimum percentage.
(e) Determine the quantity of cement required per unit volume of coarse aggregates to get desired slump by actual trials.
(f) Find out the actual proportions of cement, fine aggregate, coarse aggregate and water to meet the requirements of strength, workability, durability and economy.