ASTM C172 - Sampling Fresh Concrete and ACI Oral Exam Questions

Sampling Fresh Concrete

This method of testing can be used on any type of concrete–lightweight, regular weight or heavy weight. Sieve aggregate that would be retained on a 1-1/2" sieve over a 1" sieve.

• Sampling, according to ASTM standards, must be done in intervals from the middle portion of the batch of concrete.
• If sampling from a stationary mixer, revolving drum mixer (i.e. mixer truck) or agitator, or a continuous mixer, then two or more samples must be taken at regularly spaced intervals from the middle of the batch and mixed together for your official sample.
• If sampling from a paving mixer, then sampling must be taken from at least five different portions of the pile after the contents have been completely discharged, then combined and mixed.

NOTE: No sampling can be taken from the first 10 percent or last 10 percent of the load.

Important Sampling Guidelines

1. Sample must be taken by either passing a receptacle completely through the discharge stream, or by completely diverting the discharge stream into a sample container.
2. The receptacle or sample container must be dampened.
3. 15 min. time limit between grabbing the first and final portions of samples for your composite sample.
4. Composite sample must be at least 1 cubic foot of concrete.
5. Start slump, temperature and air content tests within 5 min. after obtaining your final portion of the composite sample.
6. Start molding strength specimens within 15 minutes after obtaining the composite sample.
7. When sampling from a revolving drum truck mixer, sample only after all water and admixtures have been added to the batch and uniformly mixed in.

Oral Recitation - ACI Oral Exam Questions for Practical Test

The information below is taken from the "Alternate Oral Exam" from the ACI Technician Workbook CP-1.

For the performance part of the ACI test, you will be required to recite the answers to the following questions the instructor will ask you. The questions are listed in bold, and your oral responses are below each question. You should memorize the answers.

What must occur before a sample of concrete can be taken from a revolving drum mixer?

• All Of The Water Must Be Added To The Mixer

Describe the different procedures that can be used to collect a sample of concrete from a stationary or revolving drum mixer.

• Repeatedly Pass A Receptacle Through The Entire Discharge Stream
• Completely Divert The Discharge Stream Into A Sample Container

What are the required steps for collecting a sample of concrete from a stationary or revolving drum mixer?

• Collect Two Or More Portions Of Concrete
• Take The Portions At Regularly Spaced Intervals
• Take The Portions From The Middle Of The Batch

Describe the procedure for collecting a sample of pavement concrete deposited on grade.

• Discharge The Contents Of The Paving Mixer
• Obtain Samples From At Least Five Different Portions Of The Pile

What is the minimum size for a sample when strength test specimens are to be molded?

• 28 L (1 Cu.ft.)

What is the maximum allowable time between obtaining the first and final portions of the composite sample?

• 15 Min.

What is required after the concrete sample has been transported to the place where tests are to be performed?

• Combine And Remix The Sample With A Shovel

What should be done if the concrete contains aggregate larger than is appropriate for the size of the molds or equipment being used?

• Wet Sieving

Start the tests for slump, temperature, and air content within how many minutes after obtaining the final portion of the composite sample?

• 5 Min.

Start molding specimens for strength tests within how many minutes after fabricating the composite sample?

• 15 Min.

From what must the concrete sample be protected?

• Sun, wind, and rapid evaporation
• Contamination