Step 3 - Mix Design Optimization
Unless the plant is a start-up operation, most producers have been using the same concrete mix design for many reasons. While these are proven and safe, it's important to annually evaluate these recipes for further optimization.
First, look at the cement content to see if it is near the ranges matching those in Step 2. Many producers opt to over-compensate with cement to gain early strength.
Second, many producers have had success using supplemental cementitious materials such as fly ash, slag, or silica fume. These materials may enhance mix designs by helping in final product appearance, and/or lowering unit costs.
Again, since final appearance consistency is becoming a key selling point (especially with color products), determine which recipes have yielded the least problems.
Periodically recheck how the plasticizing admixtures are working with current materials. The proper admixture may provide additional strength, density, or potential cement reduction. Most admix suppliers are happy to provide test samples. When evaluating various plasticizers, it is important to analyze cycle time, density, strength, and appearance of each unit type.
As mentioned, it takes more cement paste to cover the higher surface area. The overall aggregate gradation is very important, and the better fit of aggregates, the less cement paste is needed to fill the gaps.
Typically, you can use best-fit curves to analyze and optimize your aggregate gradation such as the one shown in Figure 1. These curves were designed for best granular flowability, compaction, density, and performance. Different curves have been developed for various products.
For complete mix optimization, experts recommend trial mixtures to demonstrate best production, appearance, performance, and profitability. When comparing mix designs, consider wet densities, along with surface appearance and cycle times. Many consultants and testing laboratories offer these technical services.Step 4 - Production Checks
The slow winter period is a great time to check all batching equipment to assure accuracy. One area that always seems to be difficult to monitor is the moisture in the aggregate.
If not adjusted properly, significant water contributed by wet aggregate will increase the designed water/cement ratio; lighten color; increase chipping and efflorescence potential; and lower strengths, durability, and density of the product. But aggregates that are too dry can also negatively impact MCPs by absorbing moisture needed for cement hydration and/or workability.
It is important to account for the free-moisture that wet aggregate may contribute. Free moisture is moisture beyond what the aggregate can absorb. For example, if an aggregate has 8% total moisture and has a 3% absorption rate, it is contributing 5% free moisture. (If 5000 pounds of this aggregate were weighed, 250 pounds should be considered mix water.) To batch the correct amount of aggregate with this moisture, 5263 pounds should be weighed. The 263 pounds of free-moisture would be included as mix water.
Since production equipment types/brands vary considerably, most manufacturers offer training programs for operators.