MC Magazine

How to Minimize Botched Batches

By Greg Stutz

Greg Stutz is NPCA’s vice president of Technical Services.

We all have mix designs. Some may say, “The measuring and batching of materials that go into a concrete mix is not rocket science” or “There is no mystery to ‘making mud.’” However, when you consider that the desired outcome is to produce concrete of a uniform quality and consistency, it’s not all that easy. Many variables can affect the quality of the mix.

Mix design parameters
The proper combination of aggregates, water, cementitious materials (cement, pozzolans, fly ash, etc.) and admixtures is crucial. The ultimate mix design goal is to provide an economical and practical combination of these materials that satisfy the performance requirements of an intended use. A properly proportioned concrete mix should satisfy the following:

• Workability
• Durability, strength and uniform appearance
• Economy
• Exposure conditions
• Shape and size of the member

What could go wrong?
Water quality
If you can drink it, you can use it to make concrete. Some areas of the country have to rely on non-municipal water sources. It is prudent to have these sources tested for chemicals and suspended solids. Impurities in mixing water can cause increased setting times, reduced strengths, efflorescence, staining, corrosion of reinforcement and reduced durability. Therefore, it is necessary to check for chloride content, sulfates, alkalis, organic impurities and suspended solids. Increased government regulations are restricting the disposal of wash water or runoff. Therefore, some producers are recycling this water to minimize the effects to the environment and to comply with local regulations. The suspended solids in this water are damaging.

Aggregates
Fine and course aggregates are usually 60 percent to 70 percent of the concrete volume (70 percent to 85 percent by weight). Aggregates are usually washed and graded at the pit or plant. Variation in the type, quality, cleanliness, grading and moisture content is expected. Aggregates must be clean, hard, strong, durable and free of absorbed chemicals, coatings of clay and other fine materials that can affect hydration and bond with the cement paste.

All aggregates need to meet the requirements of ASTM C33. During extremely dry periods, aggregate piles stored outside may lose some of their moisture content. This may pose a problem when mix water is added during the mixing process since the water will be absorbed into the aggregate, ultimately affecting the slumps/strengths. Therefore, it is advisable to sprinkle the aggregate piles with water as needed.

Admixtures
Admixtures exist for air entrainment, water reducing, retarding the cure cycle, accelerating the cure cycle, increasing the flow of the mix (superplasticizers) and coloration. Each admixture should be used according to the manufacturers’ specifications. These additives are usually dispensed via volumetric dispensing systems or in prepackaged fixed amounts. Admixtures are required to be batched within plus or minus 3 percent of the desired amount required per weight of cement. The admixture weight should be considered part of the mixing water.

Cementitious materials
The weighing tolerance for cement is plus or minus 1 percent of the required cumulative weight. Many producers use fly ash as an additive to the mix design in order to reduce cement consumption and to be more cost effective. It is recommended that a separate silo be kept for each material so that the two cannot be mixed. There is always the possibility that the material can be placed in the wrong silo. This will ultimately affect the mix properties. Clumping of cementitious materials is usually caused by moisture getting into the storage compartment of the silo or somewhere within the raw material manufacturing process.

Batching
The order in which the materials are combined in the mixer can affect the mix properties. Usually, some portions of the mix water are added and then the admixtures, followed by the aggregates and the remaining water. Each mix design should include a mix sequence and associated proportion for adding the materials. Time also plays a role in the mixing operation. If the concrete has been adequately mixed, samples taken from different portions of the batch will essentially have the same unit weight, air content, slump and visible course-aggregate content.

Weather extremes can also affect batching operations. This can be monitored through the use of temperature sensors on the forms or in the mix itself.

Whether you use stationary mixing equipment or ready-mixed concrete, the measured mix properties and associated test results should be the same. Adjustments can be made to the mix output, but only within tolerances as called for in ASTM C94. NPCA’s Quality Control Manual for Precast Plants provides additional suggestions and is a wonderful tool to assure a consistent mixing operation.

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