3.3 Composite Sample Timing, Remixing, and Protection

The 15-Minute Composite Window

C172 timing begins the instant the first portion of the composite sample is obtained. The elapsed time between the first and final portions must not exceed 15 minutes. This limit prevents the composite from becoming a blend of concrete taken at materially different ages. Fresh concrete keeps hydrating, loses or gains moisture, changes temperature, and stiffens as time passes, so portions taken 25 minutes apart would not represent the same concrete.

In practice, the 15-minute window rarely feels tight when sampling a single truck, because a few portions are collected during one discharge. It becomes important when discharge is interrupted, when the technician is also setting up equipment, or when sampling spans a slow pour. The rule is a hard ceiling: if more than 15 minutes pass between the first and last portion, the composite is no longer valid and the technician should start over.

The 5-Minute Test-Start Window

Once the final portion is obtained, the clock tightens for the fresh-property tests. Slump, temperature, and air-content testing must be started within 5 minutes after obtaining the final portion of the composite sample. Strength-specimen molding has its own start requirement tied to fabricating the composite (covered in the next section), but the habit is the same: do not let the sample wait. Waiting to wash tools, hunt for a thermometer, or assemble an air meter can consume the 5-minute window and let the sample age before the test even begins.

Transport, Remix, and Protect

After the portions are carried to the testing location, the technician combines and remixes the sample with a shovel only enough to make it uniform. The goal is uniformity, not retempering. Adding water, overworking the concrete, letting it segregate, or shuttling it between containers all change the very properties the downstream tests are meant to measure. Remixing corrects the portion-to-portion variation that exists right after collection; it is not permission to adjust the mix.

Sample protection is mandatory, not optional. C172 requires that the sample be protected from sun, wind, rapid evaporation, and contamination. Direct sun raises temperature and dries the surface; wind strips moisture from a shallow pan quickly; rain adds water; and dust, soil, form oil, or wash water contaminate the concrete. Use clean equipment, shield the sample when conditions are harsh, and keep the container close to the test area so transport time stays short.

A practical exam habit is to stage equipment before sampling: the slump cone damp and ready, the thermometer verified, the air meter assembled and clean, and the cylinder molds positioned where they can be filled on time. C172 does not stand alone; it launches a timed sequence that must support every later method.

Verbal sequence under exam pressure:

• Keep the first-to-final portion interval within 15 minutes.
• Combine the portions in a clean receptacle and remix with a shovel to uniformity.
• Protect the sample from sun, wind, evaporation, and contamination.
• Start slump, temperature, and air-content tests within 5 minutes of the final portion.

Why the Clocks Are So Tight

The 15-minute and 5-minute limits are not arbitrary; they exist because fresh concrete changes measurably on that timescale. Slump loss begins almost immediately as cement hydrates and aggregate absorbs water, so a slump measured 15 minutes late can read an inch or more lower than the true value at the point of sampling. Air content can change as the concrete is handled, jostled, and allowed to sit, and entrained-air bubbles can coalesce or escape. Temperature drifts toward ambient. Each delay therefore moves the measured value away from the property the concrete actually had when it left the truck.

Because the tests evaluate the concrete as delivered, the standard pins the measurement to a narrow window after sampling. The 5-minute rule protects the as-sampled condition. A technician who starts slump at minute 9 has reported a property the concrete no longer has.

Two Clocks, Not One

A frequent point of confusion is treating C172 as having a single time limit. There are two distinct clocks that start at different events:

• Clock 1 (15 minutes): runs from the first portion to the final portion of the composite. It governs collection.
• Clock 2 (5 minutes): runs from the final portion to the start of slump, temperature, and air tests. It governs the fresh-property tests.
• A third limit (15 minutes from fabricating the composite to the start of molding strength specimens) is covered with sample size in the next section.

Keeping the three straight is a classic written-exam discriminator. Identify which event just happened (last portion obtained? composite fabricated?) and apply the matching clock. The 5-minute window is only for slump, temperature, and air; do not apply it to cylinder molding.