By Kayla Hanson, P.E.
Editor’s Note: This is the second article in a year-long series that focuses on the details and more technical aspects of one common thing precast concrete producers do on a daily basis.
Collecting fresh concrete samples is a routine, yet critical, procedure performed daily at precast plants. The concrete sample is used to test the concrete’s slump (or in the case of self-consolidating concrete, the slump flow and visual stability index), temperature, density and air content. The sample is also used to prepare specimens for testing the concrete’s compressive strength. Although we place significant emphasis on the test procedures and results, the validity of the tests and the accuracy of the results depend on the collection of a truly representative sample of concrete.
A representative sample is a portion of a fresh concrete batch that accurately reflects the characteristics of the batch as a whole. If the sample is not truly representative, then test results will not accurately reflect the characteristics of the product made with that concrete, even if testing is performed in strict accordance with applicable standards. If anomalies in test results point to having not used a truly representative sample, it is impossible to go back and obtain a new sample. The importance of obtaining a sample properly cannot be overstated.
There are various standards that outline how to obtain a representative sample when using various types of mixing and conveyance equipment (see sidebar). However, ASTM C172, “Standard Practice for Sampling Freshly Mixed Concrete,” is the sampling procedure referenced in the ASTM fresh concrete testing methods and ASTM methods for creating compressive strength specimens that are cited in the NPCA Quality Control Manual for Precast Concrete Plants.
When following ASTM C172 procedures, be sure to use best practices throughout the collection process to help ensure the composite sample is truly representative of the nature and condition of the concrete batch. Sampling should be performed when the fresh concrete is delivered from the mixer to the conveyance equipment used to deposit the concrete into the forms. Note that some specifications may require the sample to be obtained at other points.
Take care to protect the sample from the elements like sun, wind, rain, snow, temperature fluctuations, sources of rapid evaporation or unintended added moisture and sources of contamination.
Collecting the sample, conducting the fresh concrete tests and fabricating the compressive strength specimens are time-sensitive procedures. In addition to preparing the fresh concrete sampling equipment, also be sure to have the fresh concrete testing and compressive strength cylinder fabrication equipment ready before collecting the concrete sample.
A representative sample will consist of two or more portions of fresh concrete combined into a single composite sample. The elapsed time between collecting the first and final portions of the composite sample must not exceed 15 minutes. Here’s why it’s important to conduct the procedure this way and what could happen if the protocols aren’t followed:
- Allowing more than 15 minutes to elapse while collecting portions of fresh concrete for the composite sample could result in a nonrepresentative sample. The longer the fresh concrete sits idle – whether in the mixer, chute or sample collection container – the higher the likelihood for segregation, which would result in a nonhomogeneous sample.
- If more than 15 minutes have elapsed, the sample must be discarded and may not be used for any testing or compressive strength specimen fabrication.
Whether the sample is collected from a stationary mixer, revolving drum truck mixer or continuous mixer, the total volume of the composite sample must be at least one cubic foot. Here’s why:
- The required sample size is dependent upon the maximum aggregate size used in the mix and increases as aggregate size increases. This ensures the sample will contain a sufficient volumetric ratio of paste-to-aggregate and helps provide an accurate representation of the whole batch.
- Using a composite sample smaller than one cubic foot may not provide a sufficient volume of concrete to perform all the fresh concrete tests and fabricate the required number of compressive strength cylinders.
Collecting the composite sample
Whether sampling from a stationary mixer, revolving drum truck mixer or continuous mixer, collect portions of fresh concrete for the composite sample only after all raw materials (including water, admixtures and fibers) have been added to the mixer and the mixing cycle has completed.
- Collecting portions of concrete from the mixer after all the raw materials have been added but before the mixer has completely mixed the concrete will result in a nonrepresentative composite sample.
Collect two or more portions of fresh concrete obtained at regularly spaced intervals from the middle portion of the batch. When sampling from a continuous mixer, and after any mix proportioning adjustments have been made, allow at least five cubic feet of concrete to discharge from the mixer prior to collecting portions of fresh concrete for the composite sample.
- It is recommended that composite samples should not be collected from the first 10% or last 10% of the batch in order to focus on the bulk of the batch. However, it can be challenging to determine how much of the batch has been discharged, so this is usually a guideline rather than a strict requirement.
Obtain each portion of concrete by passing the sample collection container completely through the concrete discharge stream or by completely diverting it into the sample container. The sample container must be a receptacle with a nonabsorbent surface and must be a suitable size to contain the sample and allow for hand mixing.
- Collecting the portions of fresh concrete in a fluid motion helps ensure the concrete is homogeneous and representative of the entire batch. If the discharge stream is started and stopped while collecting the sample, the individual portions of concrete collected for the composite sample may have paste-to-aggregate ratios slightly different than that of the entire batch.
- Be sure the concrete flow is not restricted when dispensing from the mixer, chute or transportation equipment. This could result in segregation and may alter the homogeneity of the concrete that is collected.
- When sampling from a revolving drum truck mixer, regulate the concrete flow rate by adjusting the revolution rate of the drum rather than by adjusting the gate opening.
Transport the sample collection container with the composite sample to the QC lab, wherever the fresh concrete tests will be performed and/or where the compressive strength specimens will be cast.
Combine and mix the portions of concrete in the sample collection container with a shovel or scoop to create a composite sample. Mix the concrete only for the minimum time necessary to ensure a homogeneous mix.
- Continuing to mix the composite sample after the concrete portions are sufficiently combined into a homogeneous sample could lead to segregation of aggregate sizes or separation of paste from the aggregate.
Conducting fresh concrete tests and fabricating compressive strength specimens
Begin slump, temperature and air content tests within five minutes after obtaining the final portion of the composite sample. Begin fabricating compressive strength specimens within 15 minutes of collecting the final portion of the composite sample. When sampling from a continuous mixer, wait at least two minutes but not more than five minutes before beginning fresh concrete tests or fabricating compressive strength specimens.
- Fresh concrete tests must be conducted efficiently. The test procedures must be performed only on freshly mixed concrete. A mix’s characteristics and, in turn, fresh concrete test results can change rapidly as time progresses after discharging from the mixer as cement hydration begins.
- Enforcing timeframes for starting and completing fresh concrete tests and compressive strength specimen fabrication also helps reduce variability in the test results.
- The two- to five-minute waiting period between collecting a composite sample from a continuous mixer and beginning fresh concrete tests is required because the mix water is added only seconds prior to discharging the concrete from a continuous mixer.
- Regardless of mixer type, slightly more time is allowed after collecting a sample and beginning compressive strength specimen fabrication. The results of fresh concrete tests are more sensitive to the elapsed time between collecting the sample and beginning the tests. Allowing up to 15 minutes between collecting the composite sample and starting to fabricate the test specimens is unlikely to have a measurable impact on the compressive strength results.
- Be mindful of time limits for completing the fresh concrete tests and finishing fabrication of compressive strength specimens as outlined in the applicable ASTM test methods.
Best practices in your plant
Collecting a sample properly is as important to slump, slump flow, visual stability index, temperature, density and air content test results as conducting the tests in accordance with the ASTM test methods.
A truly representative fresh concrete sample will provide an accurate example of the whole concrete batch, but a nonrepresentative sample could render the test results invalid and even cause an entire batch to be rejected.
Take this opportunity to review the sampling procedures that apply to each mixer type outlined in ASTM C172, and consider reviewing the sampling procedures and best practices at your next QC meeting or toolbox talk.
Kayla Hanson, P.E. is NPCA’s director of technical services.