Pump lines, chute sampling, and composite sample integrity
ASTM C172 Composite Sample Requirements and Timing
A composite sample must be assembled from two or more portions obtained at regularly spaced intervals during discharge of the middle portion of the load. The entire composite must be collected within 15 minutes. Sampling from the first or last 10 percent of discharge is prohibited because those fractions do not represent the batch. The minimum sample volume is 1 cubic foot (28 liters), which provides enough material for slump, air, unit weight, and at least two cylinder pairs.
Composite sample requirements per ASTM C172: Minimum volume: 1 ft^3 (28 L). Assembly time window: 15 minutes maximum. Portions: 2 or more, equally spaced through middle 80% of load. Prohibited zone: first and last 10% of drum discharge. Sample represents: a single load at a specific time and location.
ASTM C172 Sections 6-8; ACI 301 Section 1.6.7.1; ACI 305R (hot weather) for modified sampling timing.
Taking a single grab sample from one discharge position and calling it a composite sample. A single portion is not a composite sample per ASTM C172. The distinction matters: disputes over rejected loads often center on whether sampling was procedurally correct, and a single-grab sample can invalidate the test result.
Pump Lines, Chute Sampling, and Composite Sample Integrity in Difficult Conditions
In the field, you rarely get the luxury of a perfect sampling environment. ASTM C172 is very clear about obtaining a composite sample from the middle portion of the load, but when you're dealing with a 100-foot pump line or a steep, narrow chute, the mechanics of that collection change. The core principle remains the same: the sample must represent the concrete as it is being placed into the forms. If the concrete is being pumped, that means sampling at the discharge end of the hose, not at the truck hopper. Sampling at the truck when the concrete still has to travel through 200 feet of line is a recipe for inaccurate results, as the pressure and friction within the pump line can significantly alter air content and slump.
What this means in the field is that you need to coordinate with the pump operator and the placement crew well before the first truck arrives. If you're sampling from a pump discharge, you need a safe area to collect the concrete where you won't be in the way of the pour but can still get a full cross-section of the stream. Never try to catch a sample by sticking a bucket into a high-pressure stream; you'll only catch the paste and fine aggregate, leaving the heavy stone behind. This 'scavenging' leads to false high air readings and inflated slump results. Instead, have the operator divert the full flow into your collection container for a brief moment.
One thing new inspectors often overlook is the 15-minute clock. ASTM C172 dictates that you must start your tests for slump, temperature, and air content within 5 minutes of obtaining the final portion of the composite sample, and you must start molding cylinders within 15 minutes. In difficult conditions-like a high-rise deck pour in July-those minutes disappear fast. If you're sampling at the end of a long line on a hot day, the concrete is already starting to lose moisture and gain heat. Your analysis must account for the transit time. If you see the crew struggling to manage the hose, or if the pump is plugging, the integrity of your sample is at risk. If you can't start your tests within that 5-minute window because of logistical delays, that sample is technically invalid and should not be used for official record cylinders.
When sampling from a discharge stream: (1) Ensure the area is safe and you have a stable footing. (2) Pass the collection container through the entire discharge stream or divert the entire stream into the container. (3) Collect two or more portions at regularly spaced intervals during discharge of the middle portion of the load. (4) Transport the portions to the testing station and combine them into a single composite sample. (5) Thoroughly remix the composite sample with a shovel to ensure uniformity before beginning any tests. (6) Protect the sample from sun, wind, and contamination during the brief period between collection and testing.
ASTM C172 – Standard Practice for Sampling Freshly Mixed Concrete; Section 5.2.1 covers sampling from stationary mixers; Section 5.2.2 covers sampling from paving mixers; Section 5.2.3 specifically addresses the requirements for drum mixers, emphasizing the exclusion of the first and last portions of the batch.
Containers, Transport, and Re-Tempering Limits
Immediately after sampling, the concrete must be protected from contamination, evaporation, and temperature extremes. Transport containers must be non-absorptive and non-reactive with cement. The sample must be tested within 5 minutes of obtaining the final portion, or within 5 minutes of delivery to the test location for mixes requiring transport. Re-tempering (adding water at the point of use) is not permitted without written approval from the concrete producer and engineer; unauthorized water addition invalidates the sample.
In hot, sunny conditions the sample will lose workability rapidly. Cover the sample with damp burlap or a tight-fitting lid during the 5-minute transport window. If testing must be delayed beyond 5 minutes due to equipment issues, document the exact time elapsed and notify the EOR. Many jurisdictions require the test to be voided and a new sample obtained.
ASTM C172 Section 9; ASTM C94 Section 11.8 (re-tempering limits); ACI 305R Section 6.4.3 (hot weather adjustments).
Using a wet but dirty sample bucket without rinsing. Residual admixture or previous concrete chemistry in the container can alter air content or set time readings. Always rinse the container with clean water and drain before use.