Concrete will never meet its end result if the materials are not batched correctly. Whether you are a contractor operating your own batch plant, or you are examining a ready-mix concrete batch plant, here are some key things to look for:

Material Inspection and Arrangement

When materials arrive on site, it is a good idea to complete an inspection of the coarse aggregates and fine aggregates before they are placed in their specific storage areas. Measuring the coarse aggregate and fine aggregate by sieve analysis and checking for possible contamination within each are two areas where you should focus your attention. Even leaves, "particularly pine needles, can delay the setting of cement, and cause local weaknesses" (Murdock, 129). Thorough inspection of your concrete can help alleviate potential problems before they occur and ensure the success of your mixture.

Visible clay lumps found in the aggregate should be removed. A more detailed study can be done on the aggregate by placing a sample of aggregate into water and shaking it up. As the water settles, you should be able to "differentiate between clean and dirty samples by the muddiness of the water immediately after the larger particles have settled" (Murdock, 126).

The contamination of fine aggregates can be verified by rubbing a handful in your palm. If any marks are left on your palm, then some possible contamination exists and more detailed tests should be completed.

It is very important to minimize segregation and to ensure that undersized materials do not contaminate your mix. The amount of undersized materials in your concrete can be controlled by rescreening it as it is charged into the batcher supply bins. "Undersize materials in the smaller coarse aggregates fractions can be consistently reduced to as low as two percent by rescreening" (Beavers, 304R-3). Excessive quantities of sand (minus No. 200 sieve) must also be controlled. Failure to do so will increase the water requirement, the slump loss, and drying shrinkage, which will result in decreased strength. ASTM C33 requires that the fineness modulus of the fine aggregate be maintained within 0.20 of the design value.

All materials delivered must be adequately stockpiled, stored, and separated with partitions by size. Stockpiles should be built up in horizontal layers (i.e. no end dumping from vehicles) and no vehicles should operate on the stockpiles. This can break the aggregate and frequently cause contamination from the tracks. Adequate site drainage and wind protection is also encouraged.

Aggregates are commonly dewatered so that the temperature of the aggregates can be cooled and the absorption can be reduced. Failure to dewater can lead to an increase in the water-cement ratio of the mix and decrease the strength and workability of the concrete. "Experience has shown that a free moisture content as high as 6 percent and occasionally as high as 8 percent can be stable in fine aggregate" (Beavers, 304R-6). The moisture content of the aggregate does not cause as much concern when compared to the moisture content of the fine aggregate. By forming two stockpiles of fine aggregate, one for delivery and the other for mixing, this problem will be eliminated which will reasonably improve your chances of maintaining a uniform water-cement ratio during mixing.

Weighing Materials

There are two ways of batching of aggregates for concrete mixes: by volume or by weight. The latter method is the most common since it "eliminates errors due to variations contained in a specific volume" (Murdock, 130). Regular inspection, maintenance, and calibration of the equipment must be done in order to provide a consistent batch of aggregates between mixes. On smaller sites, a loading hopper is commonly utilized; on larger sites, the whole weighing process is typically mechanized.

Cement is batched by the bag (typically 94 lb/bag) at small plants and is typically batched from silos on large construction sites or batch plants. The latter is the most preferred means since it is cheaper than bagged cement, less labor intensive, provides greater protection from contamination (due to its storage in a silo), ensures that the maximum capacity of the mixer is always achieved, and can adapt to changes in proportioning much more easily.

Regular inspection, maintenance, and calibration must be performed on the weighing equipment in order to provide uniform flow. On damp days, it is essential that the weigh hopper is cleared of any cement build-up which could lower your end strength expectations.

The American Concrete Institute has set proper tolerances. These are shown below:
 

Ingredient	Batch weights greater than 30 percent of scale capacity		Batch weights less than 30 percent of scale capacity
	Individual Batching	Cumulative Batching	Individual Batching	Cumulative Batching
Cement and other cementitious materials	+/- 1 percent of +/- 0.3 percent of scale capacity whichever is greater		Not less than required weight or 4 percent more than required weight
Water (by volume or weight), percent	+/- 1 percent	Not recommended	+/- 1 percent	Not recommended
Aggregates, percent	+/- 2 percent	+/- 1 percent	+/- 2 percent	+/- 0.3 percent of scale capacity or +/- 3 percent of required cumulative weight whichever is less
Admixtures (by volume or weight), percent	+/- 3 percent	Not recommended	+/- 3 percent	Not recommended

Adhering to these tolerances and properly sequencing and blending the ingredients of the mix, will provide you with a uniform and homogeneous product.

Water Supply

As a general rule, any potable water is considered acceptable. Some materials found in water that can cause significant problems are silt, clay, organic matter and organic acids, alkalis and other salts. Water plays a significant role in the mixing and curing of concrete, therefore, its source of supply and the amount needed to create the appropriate mix must be carefully factored into the process.

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