concrete delivery |

Concrete Mix Designs

For work requiring more than one cubic yard of material, concrete is usually ordered from a ready mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mix-ing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.

Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.

Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.

Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.

Sampling of Concrete

Critical decisions, often involving very high potential costs, are made on the basis of concrete test results. Correct sampling is paramount to the validity of these test results but is an aspect of testing that is frequently overlooked and often carried out by untrained people. It is therefore essential that the sampling is done correctly and is representative of the concrete delivered.

After the truck-mixer has re-mixed its delivery on site allow at least the first one-third of a m3 of concrete to be discharged prior to taking any samples. Take at least 4 incremental samples from the remainder of the load avoiding sampling the last cubic meter of concrete. Thoroughly re-mix this composite sample either on a mixing tray or in the sampling bucket and proceed with the required testing.

Describe the recommended sampling methods for ready mixed concrete in British code. Using a standard scoop, this can collect about 5kg of normal weight concrete. Each load of concrete to be tested should be nominally divided into a number of scoopfuls.

The Standard method: To ensure that the concrete is representative of the whole load is standard sample consists of scoopfuls taken from at least four different parts of the load and collected in buckets. The scoopfuls should be taken at equally spaced intervals; the scoop being passed through the whole width and thickness of the stream in a single movement. The first and the last 1/6th

portion of the discharge should be disregarded as unrepresentative. This is then thoroughly re-mixed on a non-absorbent surface before carrying out any individual test. This operation is necessary to even out any variation between individual scoopfuls and to counteract any segregation that may have occurred in transporting the sample from the sampling point to the testing area.

The Alter native Method: An alternative method of sampling concrete for slump testing from

a truck-mixer before the majority of the load has been discharged is permitted. This enables the concrete to be tested before being placed. When this alternate method is used, an initial discharge of at least 0.3 m3 is made before a sample of six scoopfuls is collected from the

moving stream; The sample is then r e-mixed on a non-absorbent surface and split into two equal parts. Each part is then tested or slump, with the average of the two tests recorded as the test result. This method of sampling is only applicable to the slump test. Concrete sampled by this method must not be used to make cubes for compliance testing, as it will produce erroneous results.