The Trial and Error Method is a way of designing a concrete mix by determining the proportion of fine and coarse aggregates through trial and error. The objective is to find the combination that results in the maximum weight of the combined aggregate, as this will have the maximum solid matter and the least voids and surface area. Since the density of fine and coarse aggregates is nearly the same, the trial mixture that gives the maximum weight will have the desired properties.
Another method of determining the ideal mix is by conducting trial mixes with varying proportions of fine and coarse aggregates. The quantity of cement needed to achieve a certain water-cement ratio is measured for each mixture, and the percentage of sand that yields the optimum percentage is determined. It is important to note that using more sand than the optimum percentage will require more cement to achieve the same consistency, while using less sand will result in a harsh mix. The optimum percentage of sand is lower for lower water-cement ratios.
To determine the mix proportions of a cement concrete mix using the trial and error method, there are several steps that need to be followed. Firstly, the water-cement ratio should be selected based on the target average compressive strength of the concrete and the climatic conditions.
Next, the workability of the mix should be determined in terms of slump for the selected water-cement ratio. This will help to ensure that the concrete is easy to handle and place during the job.
The maximum size of coarse aggregate that is desired or available should also be determined. This will help to ensure that the mix has the appropriate texture and strength.
The fine and coarse aggregates should be mixed in a way that either meets the grading requirements of Fuller’s formula, has an economical value of fineness modulus for combined aggregate, or has a weight per litre of mixed aggregates that is the maximum or the sand corresponds to the optimum percentage.
After the aggregates have been mixed, the quantity of cement required per unit volume of coarse aggregates should be determined through actual trials. This will help to ensure that the desired slump is achieved.
Finally, the actual proportions of cement, fine aggregate, coarse aggregate, and water should be determined based on the requirements of strength, workability, durability, and economy. By following these steps, it is possible to achieve a well-designed and effective concrete mix using the trial and error method.