3.3 Composite Sample Timing, Remixing, and Protection

Timing the Composite Sample

C172 timing starts when the first portion of the composite sample is obtained. The elapsed time between the first and final portions must not exceed 15 minutes. That limit prevents a sample from becoming a blend of concrete taken at materially different ages. Fresh concrete continues hydrating, losing moisture, gaining or losing temperature, and changing workability as time passes.

After the final portion is obtained, the clock becomes even tighter for several fresh-property tests. Slump, temperature, and air-content testing must be started within 5 minutes after the final portion of the composite sample is obtained. Strength specimens have their own start requirement tied to fabricating the composite sample, but the central habit is the same: do not let the sample wait.

Remixing is required because the separate portions may not be uniform after collection. The goal is not to retemper the concrete or overwork it. Remix with a shovel the minimum amount needed to make the composite sample uniform. Overmixing, adding water, letting material segregate, or repeatedly moving the sample between containers can change the properties that the later tests are supposed to measure.

Sample protection is not optional. Direct sun can raise temperature and dry the surface. Wind can remove moisture quickly from a shallow pan. Rain can add water. Dust, soil, form oil, or wash water can contaminate the sample. A technician should use clean equipment, shield the sample when needed, and keep the container close enough to the test area that unnecessary transport time is avoided.

A practical exam habit is to stage equipment before sampling. The slump cone should be damp and ready, the thermometer should be available and verified, the air meter should be assembled and clean, and cylinder molds should be placed where they can be filled on time. C172 does not stand alone; it starts a timed sequence that must support all later methods.

Use this verbal sequence under exam pressure:

• Collect the required portions and keep the first-to-final interval within 15 minutes.

• Combine the portions in a clean receptacle.

• Remix the composite sample enough to make it uniform.

• Protect the sample from evaporation, temperature change, and contamination.

• Start slump, temperature, and air-content tests within 5 minutes after the final portion.