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. concreteQC 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.
After creating the three trial mixes, test batches are run
and entered as concrete tests into concreteQC. These tests are associated with
their corresponding trial mixes for evaluation purposes. Once all the concrete tests have been completed,
the Trials Tab in the concreteQC Mix Design form can be used for evaluation. The
associated trials are checked for evaluation, and a listing of results
associated with each mix test assigned to the trials will load into the
evaluation section. In the example below, a custom report title was entered
that will appear on report that can be printed or exported as a pdf document.
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.
Michael
Rodriguez
StonemontSolutions, Inc.