Guidelines applied to manufactured concrete product (MCP) operations can help ensure consistent quality of output. Following are six steps recommended
GARRY CULTON, CCCM, CDT
Guidelines applied to manufactured concrete product (MCP) operations can help ensure consistent quality of output. Following are six steps recommended as best practices to be incorporated in quality assurance programs.
STEP 1 ? QUALITY PROGRAM
The first step in manufacturing any product consistently is having a documented quality assurance program. All company employees should be apprised of the QA program and the potential positive impact of their jobs on the bottom line. The program is best viewed as a means to meet high industry standards as well as customers’ Û and their customers’ Û expectations of top-notch quality.
STEP 2 ? RAW MATERIALS
As the adage Îgarbage in = garbage outÌ asserts, consistent quality in the materials used to manufacture concrete products is vitally important. While materials must meet relevant ASTM standards, also crucial is how they perform in a particular manufacturing environment. Accordingly, the material supplier should be reliable and well informed regarding characteristics required for a specific operation. Moreover, monitoring material quality on a regular basis facilitates design adjustments to prevent output of compromised product.
Cement as a binding agent is a primary ingredient of concrete. Section 4 of ASTM C90 and C1372 specifies that cementitious materials should meet minimum requirements of ASTM C1157, C595, C150, or C150 modified. For quality purposes, such substances should be consistent in color and performance over time. The cementitious content of masonry mix designs varies by region, materials, equipment, and product type. Yet, for typical products, cement content (in percent by total aggregate weight) generally falls within the following ranges:
- Standard gray concrete masonry unit (CMU) Û 9-10 percent
- Architectural CMU (often split-face) Û 11-12 percent
- Segmental retaining wall (SRW) units Û 11-14 percent (13-14 percent, freeze/ thaw applications)
- Other concrete landscaping products Û varies with performance requirements
Aggregates should meet ASTM C33 (normal weight) or ASTM C331 (lightweight) requirements; or, they must be proven to provide minimum performance and desired characteristics in the final product. Several types of aggregate available regionally supply varying degrees of strength, hardness, density, absorption, gradation, particle shape, and color. Special aggregate-handling procedures help maintain consistent product characteristics.
As compared to ready mixed, MCP mixtures typically use a much finer aggregate blend, so the cement paste covers more surface area and, thus, plays an even greater role in concrete performance. Many producers have the capability to blend three to five aggregates: the more aggregates blended, the less a single modified aggregate will influence concrete properties. Individual and composite gradations should be monitored on a regular basis to maintain consistency in production and performance. Especially with lightweight blends, aggregate moisture should be monitored as well.
Water should be potable grade and accurately dispensed into the mixing vessel. Although MCP mixtures appear dry, the water/cement ratios are comparable to typical ready mixed designs (0.35-0.50).
Pigments increasingly have been introduced in concrete masonry mixes over the years, and in some areas, output of colored units exceeds that of standard gray product. Of three pigment types Û raw powder, liquid, or granulated Û the latter is most commonly used for manufactured concrete products, often as part of an automated color-dispensing system. Minimum requirements for pigments can be found in the ASTM C979 standard.
Color quality is not determined exclusively by the quality of a pigment, i.e., its tinting strength and particle-size distribution; additional factors influencing the product’s final color include loading rate (percent by cementitious weight), water/cementitious ratio, aggregates, curing, and degree of potential efflorescence. Consistency of all such variables is key to maintaining color homogeneity.
Admixtures should be obtained from a reliable provider that guarantees consistent quality and, preferably, supplies and maintains dispensing equipment. While admixtures may not be essential to manufacture quality concrete products, they generally provide benefits to both the producer and end user.
Admixtures formulated for MCP mixes are not required to meet ASTM C494, a specification for wet-cast or zero-slump concrete. Plasticizing admixtures designed for dry-cast mixes may enhance production output; optimize cement content, strength, density, and durability; and, minimize machine/mold wear. Water-repellent/efflorescence-controlling admixtures are used to reduce absorption rates, improve color vibrancy, and reduce efflorescence potential. Both can help achieve desired surface texture as well.
Other cementitious/pozzolanic materials, such as slag, fly ash, or silica fume, can be used as a partial cement replacement (typically 10-15 percent range). Of these, fly ash is the most popular cement substitute. Concrete properties significantly affected include early strength and color consistency over time. Relevant standards include ASTM C618 for fly ash, ASTM C989 for slag, and ASTM C1240 for silica fume.
STEP 3 ? MIX DESIGN OPTIMIZATION
While long-standing operations typically have established concrete mix designs, evaluation may be appropriate for mix optimization. Most MCP producers batch materials on the basis of weight formulations, rather than by volume.
Checking cementitious content to determine if it falls in the ranges cited above is a first step. Many producers have successfully used supplemental cementitious materials, such as fly ash, slag, or silica fume, to enhance mix designs Û again, taking into account consistency over time, especially with color products. Plasticizing admixtures may provide greater strength and density or reduce the cement required. (Test samples are typically provided by the supplier.)
More cement paste is needed in an MCP mix to cover the greater surface area resulting from finer aggregate constituents. Overall aggregate gradation is a key factor, since a better fit of aggregates requires less cement paste to fill the gaps. Typical best-fit curves, used to analyze and optimize aggregate gradation, are designed for best granular flowability, compaction, density, and performance. Such curves are tailored to specific products (See Figure 1).
For complete mix optimization, trial mixtures are recommended to demonstrate best production, appearance, performance, and profitability. When comparing mix designs, wet densities should be considered along with surface appearance and cycle times. Many vendors offer technical consulting services as well.
STEP 4 – PRODUCTION
First, all batching equipment should be checked on a regular basis to assure accuracy. Secondly, monitoring the moisture in aggregates is indispensable: If not adjusted, significant water contributed by wet aggregates will increase the water/cement ratio, lighten color, intensify chipping and efflorescence potential, as well as reduce concrete strength, durability and density. By contrast, excessively dry aggregates can negatively impact manufactured concrete products by absorbing moisture needed for cement hydration and/or workability.
Accordingly, consideration should be given to wet aggregates’ free-moisture, i.e., water above and beyond what the aggregate can absorb. For example, if an aggregate demonstrating 3 percent absorption rate is found to have 8 percent total moisture, it is contributing 5 percent free moisture. Since a 5,000-lb. weight of this aggregate would yield 250 lb. of mix water, batching the correct amount would require 5,263 lb.; then, 263 lb. of free-moisture would be included as mix water.
As production equipment brands vary considerably, most manufacturers offer training programs for operators in the efficient use of their machines. Further enhancing production are plasticizing admixtures that can reduce cycle times, minimize mold/machine wear, and improve product appearance plus performance characteristics. Analysis of cycle times as well as concrete density, strength, and surface appearance is vital in evaluating plasticizers.
Also critical is thorough mixing of concrete with proper sequencing of raw materials. Generally, the following procedure assures homogenous mixes for MCP applications:
- Aggregates and partial mix water, blend 30-45 seconds; for lightweight, 75-90 seconds pre-wet mixing;
- Add cement, mix minimum 45-60 seconds;
- Final water and admixtures, mix minimum 60-75 seconds after addition of all ingredients;
- Total mix time five to six minutes, depending on equipment mixing efficiencies
As equipment, including paddles and blades, becomes worn, mixing action will be less efficient and may require longer cycles to achieve homogeneity. On-line inspection targeting visual appearance, ease of feed/finish times, and wet density evaluation should be performed daily and whenever a change in product or process has been implemented.
An accurate check on density (unit weight) is obtained by weighing units on a pallet, subtracting the pallet weight, and dividing by the number of specimens. An alternate method is to scrape one unit into a container and record the net weight. For that purpose, a thin sheet of Plexiglas or other rigid material placed between units makes a clean swipe off the pallet.
STEP 5 – CURING
Lack of proper curing can lead to as much as a 50 percent loss in concrete strength. The key to concrete curing is preventing moisture loss, thereby facilitating cement hydration. Higher temperatures will accelerate set time and strength gain. Figure 2 illustrates the cement hydration process.
Steam curing is common, since it provides moisture and high temperatures. Product should set for two to three hours prior to steaming, as premature steam curing may lead to case hardening. A good rule of thumb is a maximum rate of 60_F per hour temperature rise (or drop) in the concrete. Steam should be shut off at equilibrium, when concrete will gain no further weight or internal concrete temperature equals the kiln ambient air temperature. While equilibrium typically occurs at 130_-180_F, time will vary with kiln size, type of block, rate of temperature change, insulation, and starting temperature.
After the steam is shut off, concrete is exposed to soaking Û typically for a period of four to six hours Û prior to reducing the temperature. Product can be pulled from the kiln as soon as handling is possible without damage to its appearance. Temperature and humidity probes can be used to evaluate kiln efficiencies, as shown in Figures 3 and 4.
Using carbon dioxide in the curing process for control of efflorescence below the concrete surface has been attempted in MCP operations. The process has proved highly sensitive, requiring considerable adjustments to obtain desired results. Variances in vapor pressure, circulation, and product density or absorption rate will yield different outcomes. Consequently, some producers using CO2 have eventually adopted or reverted to steam curing.
STEP 6 – QUALITY CONTROL AND CONTINUOUS IMPROVEMENT
A successful quality program requires companywide support and participation. Employees at all levels need to understand their role in contributing to quality production. Quality control may be defined as ensuring compliance with design specifications, whereas quality assurance may be said to maintain set standards throughout the entire process. A producer may choose to meet internal QC requirements exceeding those of ASTM. In that case, a product borderline by internal QC standards will nevertheless be ASTM compliant.
CMUs should meet the current minimum requirements of ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units, which is suitable for loadbearing or non-loadbearing units. Upon delivery to a customer, the units should have at least 1,900-psi net compressive strength, determined as the average of three units, with no individual break less than 1,700 psi. Absorption rates should register at less than 13 lb./ft.3 for normal-weight block (125 lb./ft.3 or higher); 15 lb./ft.3 for medium-weight block (105-125 lb./ft.3); or, 18 lb./ft.3 for lightweight block (less than 105 lb./ft.3).
ASTM C140 covers sampling and testing methods for dimensions, compressive strength, absorption, weight (density), and moisture content of the units. Dimensional standards and a linear shrinkage requirement of 0.065 percent or less are also cited in ASTM C426. Situations may arise that demand specifications more stringent than those of the corresponding ASTM standards.
SRWs should meet the current minimum requirements of ASTM C1372, Standard Specification for Segmental Retaining Wall Units. While water absorption requirements are the same as for CMU, the minimum strength requirement is higher: 3,000 psi, calculated as the average of three units with no individual break less than 2,500 psi.
For areas subject to repeated freezing and thawing under saturated conditions, durability must be demonstrated by proven field performance or testing. ASTM C1262 is the applicable standard if a test is specified. As such, the weight loss of five specimens (also cut coupons) must not exceed 1 percent over 100 cycles (or 1.5 percent over 150 cycles in four of five specimens). Some states may require testing in a saline solution rather than water Û a significantly more severe standard.
Durability testing under freeze/thaw conditions is being investigated for its high degree of variability. Evidence indicates that cement content, aggregate durability, absorption rate, and density have the greatest impact on freeze/thaw performance. While aggregates can be analyzed for soundness under ASTM C88, no guarantee is provided that the SRW will meet specifications. The best practice seems to involve increasing cement content (13-14 percent minimum), mixing a little wetter and longer than normal, and increasing compaction (for increased density and lower absorption). Some admixtures also have been shown to improve freeze/thaw resistance. Again, occasionally, SRW specifications may call for more stringent requirements than the corresponding ASTM standards.
Quality assurance should entail a process of continuous improvement. Highly recommended is a documented QA/QC program, including thoroughly maintained records. Regular meetings with all levels of operation are helpful in showing progress (or regression) and discussing plans for improvement.
Cemex MCP Technical Service Manager Garry Culton is a 24-year veteran in the concrete industry. He holds a BS Degree in Industrial Management and an AS in Industrial Engineering with a Quality major. Specializing in the area of manufactured concrete products for the past several years, Culton is a Certified Consultant of Concrete Masonry (National Concrete Masonry Association) and Construction Documents Technologist (Construction Specifications Institute). He serves on several NCMA and Interlocking Concrete Pavement Institute technical committees.
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