Mixing concrete thoroughly is critical to casting a quality product. It starts with sequencing raw materials properly into the batching process. The following guidelines typically produce homogenous concrete mixtures for MCP production.
- Blend aggregates and partial mix water (mix 30-45 seconds, lightweight 75-90 seconds pre-wet mixing).
- Add cement and mix at least 45 seconds.
- Add final water and admixtures (mix a minimum of 60-75 seconds after adding all ingredients).
- Total mix time: five to six minutes optimum, depending on the equipment's mixing efficiencies.
As mixing equipment (paddles and blades) become worn, the mixing will be less efficient and may require longer cycles to achieve homogeneity.
Every program should include an online inspection process. This includes appearance, ease of feed/finish times, and wet density evaluation. Checks should be daily and whenever there's a product change.
The most accurate check on density (unit weight) is to weigh the units on the pallet, subtract the actual pallet weight, and divide by the number of specimens. Another method is to scrape off one unit into a container and record the net weight. You can use a thin sheet of Plexiglas or other rigid material to place in between units, making a clean swipe off the pallet.Step 5 - Curing Checks
Lack of curing in concrete can lead to loss of up to 50% of the strength of the same mixture properly cured. Proper curing prevents moisture loss in concrete so that cement can hydrate. Increased temperature will accelerate set time and strength gain. The cement hydration process is shown in Figure 2.
Steam curing is commonly used, as it provides moisture and high temperatures. Products should be allowed to set for two to three hours before being exposed to steaming. Premature steaming may lead to case-hardening. A maximum rate of 60° F per hour of temperature rise (or drop) in the concrete is a good rule.
Shut the steam off at equilibrium, when concrete will gain no further weight or when the inside concrete temperature is equal to the ambient air temperature of the kiln. While equilibrium typically occurs at 130-180° F, the time will vary with kiln size, type of block, rate of temperature gain, insulation, and starting temperature.
After shutting off the steam, the concrete should go through a four- to six-hour soaking period before ramping down the temperature. Pulling product from the kiln can be done as early as handling can be performed without damaging the product. You can use temperature and humidity probes to evaluate kiln efficiencies as shown in Figure 3 and Figure 4.
Carbon dioxide in the curing process is sometimes used in MCP curing to control efflorescence below the concrete surface. This process seems to be very sensitive and takes a great deal of adjustment to dial in perfection. Variances in vapor pressure, circulation, product density, or absorption rate will generate different results.