Aggregates should meet the requirements of ASTM C33 (normal weight), ASTM C331 (lightweight), or be proven to provide minimum performance and desired characteristics in the final product. Often, a producer can choose from several types of aggregate available regionally. These aggregates can vary regarding strength, hardness, density, absorption, gradation, particle shape, and color.
While purchasing the proper aggregate is important, producers should also review any special procedures for handling them to maintain consistency. Stockpiles, trucking, and even conveying materials can cause degradation. Avoiding processes that create an extra minus 200 mesh material is extremely important.
MCP mix designs often use a much finer aggregate blend than ready-mixed concrete. With the additional surface area, the cement paste is leaner. So, gradation changes can significantly affect final product appearance and performance.
Many producers opt to combine three to five different aggregates in their mix designs. The more aggregates blended, the less significant individual aggregate changes will influence performance. Make sure there's a procedure that monitors the individual and composite gradations regularly to maintain consistency in production and performance. Also, review how the operation monitors aggregate moisture, especially with lightweight aggregate.
Water should be potable (drinkable) and accurately dispensed into the mixing vessel. Even though these concrete mixtures appear very dry, the water/cement ratios are similar to typical ready-mix designs (0.35-0.50).
Pigment use in concrete masonry has increased over the years. Producing colored units has exceeded standard gray units in some markets. Pigments come in three forms: raw powders, granulated, and liquid. The most common for the MCP market is granulated. Many large producers now use automated systems. Refer to ASTM C979 for the minimum requirements.
While the quality of pigment, such as tinting strength and particle size distribution, is extremely important, several other factors will also affect the final color of the product. These include loading rate (percent by cementitious weight), water/cement ratio, aggregates, curing, and the degree of potential efflorescence on the concrete surface. The main issue with maintaining homogeneous color is the overall consistency of all of the above.
Admixtures should be supplied by a reliable provider that can guarantee consistency over time and preferably supply and maintain dispensing equipment. While quality concrete products can be manufactured without admixtures, they generally provide benefits to both the producer and end-user.
Admixtures formulated for MCP mixtures are not required to meet ASTM C494 (wet-cast or slump concrete mixtures). As a result, many plasticizing admixtures are designed specifically for dry-cast concrete mixtures. These additives can enhance machine output; improve cement efficiency; increase strength, density, and durability; and lower machine/mold wear. Water-repellent/efflorescence-controlling admixtures reduce absorption rate, improve color vibrancy, and lower efflorescence potential. Both types can help achieve desired surface texture.
Other cementitious/pozzolanic materials, such as slag, fly ash, and silica fume, may replace 10%-20% of the cement. Fly ash is the most popular cement substitute, with early strength and color consistency over time being concerns. The ASTM standard for fly ash is ASTM C618, ASTM C989 for slag, and ASTM C1240 for silica fume.