Concrete Mix Design Optimization Versus Adjustment

Concrete industry veteran, Jack Holley, has 45 years of experience in concrete technology, quality assurance, and new product development. He is an expert in concrete mix optimization, plant processes, performance concretes, and quality control (QC) and quality assurance (QA) systems.

In a recent webinar with CarbonCure, Jack shared some valuable insights about his experience with concrete mix design best practices for mix optimization. 

“The ideal mix optimization process should begin with a full review of the QC/QA staffing, functions, and responsibilities in the mix design process,” said Jack. “Then look at mix designs and average strengths.”

This blog post will break down a typical mix optimization process into three phases, then highlight the key differences between concrete mix optimization and mix adjustment.

Concrete mix design

A Typical Mix Optimization Process

Jack described a three-phase mix optimization process organizations should follow:

Phase One: Yield Checks, Testing, and Adjustments

  • Establish water demands (you will not get accurate yields if you don't have accurate water demands).
  • Start testing admixture dosage rates and SCM replacements at different percentages. Push the envelope on SCM replacements so everybody knows where the strength gain and workability limits are.
  • Review and test aggregate combinations and their effects.

This phase can take some time. It’s best to map it all out so it's set up correctly from the get-go.

Phase Two: Optimization

  • Admixture and SCM optimization: Test different types, amounts, dosage rates, cementitious content, and temperatures.
  • Develop tables from phase one and use them as a tool to maximize replacements without affecting expected performance.
  • Start reviewing test results in a timely manner and track key performance indicators (KPIs).
  • Set KPI targets with timelines.

After phase two, the mixes are optimized and ready to implement in the plant.

Phase Three: Analysis

  • Begin the statistical analysis.
  • Control production to ensure results are the same as lab test results.
  • Involve operations so everyone is up-to-speed and watching out for things like moisture control and so on.
  • If you have probes installed, ensure they are correctly calibrated.

The goal in phase three is to lower the standard deviation, as “when you do that, you can follow up by lowering your required strengths or your FCR and gain even more benefits,” Jack said.

Concrete mix design in practice

Mix Optimization versus Mix Adjustment

“People hear the term ‘mix optimization’ and run for cover,” Jack said. “That’s because many people have had bad experiences with it.”

There is a large difference between typical mix optimizations and the mix adjustments that are conducted with CarbonCure. The CarbonCure mix adjustment simply reduces the total cementitious content, based on the strength improvements gained from the addition of CO2 to the mix, while maintaining the mix’s strength and volume.

Jack recommends the use of sand to maintain the absolute volume of the mix. This procedure, sand-for-cementitious volume, maintains the mortar fraction and the original workability of the mix.

Mix optimization, on the other hand, is establishing the optimum amounts of supplementary cementitious materials (SCMs), admixtures, and aggregate proportions to achieve the highest levels of cementitious efficiency (the psi per pound or MPa per kilogram), while maintaining the required performance of the mix for a specific application.

Unlike adjusting mixes to account for the added CO2, a significant amount of testing and evaluation is required for mix optimization, along with field performance monitoring of the optimized mixes to ensure that the features and benefits are delivered according to the customer’s expectations.

For more design tips from Jack, watch the full webinar on demand. If you’d like to learn about how you can adjust your mix design with CarbonCure, stay tuned for the second part of this blog or feel free get in touch with a CarbonCure representative.

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