Quality Control Testing
Freshly mixed concrete is often quality control tested for compliance to specifications, set by the buyer or customer for performance evaluation. The following is a brief summary of these tests, the tools needed to perform these tests, and what ASTM standards govern procedures to perform each of the tests mentioned.
It should be noted that anyone administering these tests must be ACI Concrete Field Testing Technician - Grade 1 certified for any of these test results to be considered as valid.
Basic Field Testing - The following series of tests should all be performed on the same sample anytime a concrete test is desired, to give a well-rounded picture of the data.
Sampling - ASTM C172: It all starts with sampling. The importance of obtaining a truly representative sample of freshly mixed concrete for the purpose of quality control testing cannot be over emphasized. If the sample is not a true representation, then the test results will be compromised. The sample must be taken from the middle portion of the concrete batch discharge. All tests intended to be drawn from the sample must be completed within 15 minutes from the time of collection. It is very important that the sample be protected from direct sunlight, wind, and other sources of evaporation during this period. The sample must be thoroughly mixed in its collection vessel before each test procedure.
Slump Test - ASTM C143: The slump test is a test of the consistency of the freshly mixed concrete.
The tools required to do a slump test include the slump cone, a metal conical shaped mold that is 12" high, with a top opening diameter of 4" and a base diameter of 8", a steel rod, and a scoop. This rod is 24" long and 5/8" in diameter with hemispherical tips on each end. The test also requires a smooth, flat surface. A slump kit comes with a base for such a purpose. Some sort of measuring device is also necessary. Some technicians may have a rod that is marked in ¼" increments, but it is wise to have a tape measure handy.
After the concrete sample has been properly consolidated in the dampened mold, the mold is pulled straight up in one steady motion, being careful not to twist the cone. The slump is a measure of the vertical distance that the concrete settles (or slumps) from the top of the slump cone to the displaced center of the specimen. This measurement is recorded to the nearest ¼".
Temperature Measurement - ASTM C1064: The temperature of the fresh concrete sample is measured and recorded to the nearest degree. This reading is of particular importance, since temperature influences so many of the properties of plastic as well as hardened concrete. The sensing portion of the thermometer should be surrounded by a minimum of three inches of concrete in order to get an accurate reading. Once the test has started, it should be complete within 5 minutes. The ambient temperature should also be recorded at the time of the test.
The only tool involved in this test is a concrete thermometer. This instrument should be accurate to within plus or minus one degree. The way the accuracy of a thermometer is calibrated is through the use of a reference thermometer.
Bulk Density (formerly Unit Weight) and Yield - ASTM C138: The results of these tests are used to determine a reasonable approximation of the quantity of concrete produced in the batch sampled. Bulk Density is expressed in pounds per cubic foot. Yield is expressed in cubic feet. One cubic yard equals 27 cubic feet. To calculate yield, divide the total weight of the materials in the batch by the bulk density or unit weight of the fresh concrete.
This test method may also be used to give an indication of the air content in the batch, provided that the specific gravities of the constituents are known. This method for determining air content is known as the gravimetric method.
The quality tools required to determine the bulk density of concrete are, a scale that can register to at least 0.1 lb., unit weight bucket, steel rod for rodding, a rubber mallet for consolidation, a scoop, and a strike off plate.
Air Content - The air content should be determined anytime concrete test cylinders are to be made. This rule applies to non-air entrained as well as air entrained concrete. Air test should be started within 5 minutes of the collection of the concrete sample. There are two other techniques in addition to ASTM C138 for determining the air content of plastic concrete. These are the pressure method and the volumetric method.
Pressure Method - ASTM C231: The tool used to conduct this test is known as a pressure-type meter, sometimes referred to as a pressure or air pot. This method is based on Boyle's law, which relates pressure to volume. After the base is filled with consolidated concrete, the lid is latched on. Water is injected through one of the petcocks on the lid until it flows clear from the adjacent petcock. Pressure is then applied by the pump located at the top of the neck to a predetermined reading. This is the calibrated hands free pressure. What is happening inside the pot is that the applied pressure compresses all the air within the concrete sample, including the air in the pores of the aggregates. Once the initial pressure has been reached, the petcocks are closed. To get the reading, the lever on top of the neck is depressed and held. While depressing the lever, the base needs to be rapped by a rubber mallet and the back of the gauge is tapped. Once the needle settles, the lever may be released and the reading is recorded.
The pressure method of determining the air content of concrete may not be used where lightweight concrete is concerned.
In this particular example, the pressure pot is known as a Type-B Pressure Meter. Until tools needed are a bulb syringe, strike off bar, scoop, rubber mallet, steel rod, a sponge or rag for wiping the lip, and a bucket of water.
Volumetric Method - ASTM C173: This method is primarily used to determine the air content of concrete containing lightweight aggregates, but may also be used to determine the air content of concrete containing any type of aggregate. It works through removing all of the air from a known volume of concrete by agitating the sample in water. This apparatus is known as a roll-a-meter. Rocking the vessel and shaking it upside down initiates the agitation. Then, after a one-minute rest period, the vessel is rolled back and forth on the ground, shaking it by the neck. This is done until the readings stabilize. The test is very physically demanding and, if done correctly, very accurate.
The tools used to conduct a volumetric air test include; a Roll-a-Meter, long funnel, scoop, rod, strike off bar, measured ounce brass cup, isopropyl alcohol, and a bucket of water.
Making and Curing Concrete Test Specimens in the Field - ASTM C31 or Making and Curing Test Specimens in the Laboratory - ASTM C192: This is the molding of the specimens, the curing and transport of these specimens, and the breaking or crushing of the samples at preappointed days of curing.
The test molds, known as cylinders, must meet the requirements of ASTM C470. These days, the cylinder molds are usually made of plastic and may be either 6" x 12" in size or 4" x 8". Other tools needed to make a set of cylinders include, a scoop, steel rod with hemispherical tips, a strike off bar, and a cover or cap for each cylinder. A record sheet is also filled out recording all the information from the other tests mentioned, as well as pertinent information identifying the cylinders in the set. Cylinders should be made within 15 minutes of obtaining the sample.
Once these samples are brought back to the lab and stripped they are immersed in a curing tank and/or moist curing room.
The samples are taken out of the tank and/or moist curing room at the appointed day and time of testing. Test intervals will vary, according to the purpose of the testing. A set of field test is usually comprised of two 7-day compressive tests, two 28-day compressive tests, and one held in reserve for a 56-day compressive test. The specimens are loaded one at a time into the compression machine, where they are tested. The compressive machine records the resistance to loading at which the specimen failed. This load is the resistance in pounds divided by the cross sectional area of the specimen which is the compressive strength recorded in PSI (pounds per square inch).
Although many other concrete tests are provided for in ASTM standards, these six tests are the concrete quality control testing basics. To learn more about the specifics of each test mentioned, reference the ASTM procedure number and read the standard. Each standard spells out the steps to apply a certain test in meticulous detail.