Most concrete producers have hundreds of mix designs to choose from. They cover a multitude of design characteristics to meet the needs of their customers. Despite this wealth of variation in mix designs, project specifications can prescribe a mix that doesn’t match anything that is already at the plant. It is often common practice to pick any mix that meets the specifications and has enough historical data to submit to the project.
Fortunately many project specifications reference national standards for the design of submitted concrete mixes that provides producers with alternatives to the above approach. One approach that can be especially helpful is to evaluate strength relative to the water/cement (w/c) ratio. StonemontQC includes the ability to quickly evaluate multiple trial batches to evaluate strength as a function of water/cement ratio. In order to use this tool, trial batches must be completed ahead of time. Each trial batch represents a significantly different w/c ratio and the spread of different water cement ratios should encompass the w/c ratio used in the new mix design being developed.
It is typical to start with a basic concrete mix design that is similar to the new mix that is being created. Having the same nominal maximum size of aggregate, coarse aggregate fraction, percentages of mineral admixtures, and chemical admixtures are common similarities that should be maintained for each trial batch. Basing trial batches on mixes that have plenty of historical data is especially helpful.
For this example, a standard 1” 4000 PSI mix was chosen as the starting point. Three trial mixes were created for testing. The first batch was the low-point for the w/c ratio curve having the lowest cement content and corresponding strength.
The second batch was the mid-point for the w/c ratio curve.
The third batch was the highest point for the w/c ratio curve, having the highest strength out of all the batches.
A summary evaluation chart is available by selecting the Chart Tab. The Chart Tab has several options; entering a w/c ratio and pressing the calculate button graphs strength vs. w/c ratio using the trial batch data, and gives the corresponding strength based on the best-fit line; entering strength and pressing the calculate button graphs strength vs. w/c ratio using the trial batch data, and gives a corresponding w/c ratio based on the best-fit line. The user can decide which trial mixes to include in the evaluation and reload the test results for those trial batches right from this tab.
Here’s an example of how this might work. The project specifications call out for a .50 w/c, 3000 PSI mix. Plenty of data exists for a 4000 PSI mix but it doesn’t meet the w/c ratio requirement. After running an evaluation on the 4000 PSI mix, an over design strength of 4600 PSI is determined.
Using trial data, a plot of the equivalent strength for a .50 w/c ratio mix is used to find out that the equivalent strength will be about 5100 PSI. Since there is already an established over design amount of 600 PSI on a mix that is similar and is within 1000 PSI of design strength of the new mix, the established overdesign amount can be applied to the new mix instead of the 1200 PSI needed if there was no historical data. The new mix will have a design strength of 4500 PSI with a .50 w/c ratio. Between the w/c ratio chart, which can be printed out as a report, and the evaluation on a similar mix, there is enough validation to submit the new mix design. Better yet, the mix has an appropriate design strength that can be charged an appropriate amount rather than creating .50 w/c ratio mix and calling it a 3000 PSI design. This approach should provide plenty of justification when it is questioned why a 4500 PSI mix was submitted rather than a 3000 PSI mix. It can be clearly shown that it was the .50 w/c ratio requirements in the specifications. This puts the responsibility for cost back on the project requirements and not on the producer.
Before using this method, be sure that the specification requirements allow for this procedure and as always trial batches are recommended. Note that this same tool can be used to evaluate strength vs % efficiency and strength vs % replacement of additives, which will be discussed in future posts. For more information on how easily you can evaluate water/ cement ratios using concreteQC contact Stonemont Solutions, Inc.