ASTM C172 English

Testing fresh concrete in the field By Kim Basham

Proper testing benefits the contractor, ready mix producer, and owner

STM standards define precise procedures for performing field tests to determine the quality of freshly mixed concrete. Usual field tests measure consistency, strength, unit weight, air content, and temperature (see box). Many test details may seem trivial, even arbitrary. However, ASTM standards establish uniformity in the testing methods. By always performing tests the same way, it’s possible to detect changes in fresh concrete that could affect concrete performance. Improper testing or even deviating from standard test methods may cause good concrete to be rejected or bad

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concrete to be accepted. In either case, poor testing penalizes the contractor, ready mix producer, and owner. Understanding the scope, significance, and procedure of each test for fresh concrete is the first step to achieving accurate results. Sampling fresh concrete Follow procedures given in ASTM C 172 to ensure the sample is truly representative of the freshly mixed concrete being tested. This standard gives procedures for obtaining and handling composite samples from stationary, paving, and truck mixers, and from agitat-

ing and nonagitating equipment. Obtain composite samples by sampling the concrete at least twice as the middle portion of the batch is discharged. Mix the samples into one sample for testing. Obtain samples randomly. Do not base sample selection on concrete appearance, convenience, or other biased criteria. Usually concrete is sampled as it’s discharged from the mixer and transported to the forms. Some specifications may require other points of sampling, such as at the discharge of a concrete pump line. Concrete tested at these discharge points is not suitable for making

Figure 1. Slump is the difference between the height of the mold and height of the “true” slump. Shear slump indicates a lack of cohesion. Collapse slump indicates a lean, harsh, or very wet mix (Ref. 1)

• Molding of strength Tips for specimens must begin Proper Sampling within 15 minutes after ■ Obtain at least two samples to making the composite make a composite sample. sample. ■ Do not take samples from the It’s important to carevery first or last portions of the fully observe these time batch. restrictions to achieve the ■ Take samples after all water most accurate test results. and admixtures have been This may be difficult on added to the mix. some jobs. To minimize the elapsed time between ■ Carefully observe time limitaobtaining samples and tions to ensure results of field performing tests, mix the tests are consistent. composite sample, and ■ Protect samples from sun, test and mold specimens wind, rapid evaporation, and as close as possible to the c ontamination. point of sampling. Specifications usually Slump indicate the number of tests to be made. HowevThe slump test (ASTM C 143) e r, ASTM C 94 re q u i re s m e asures concrete consistency, or Figure 2. Remove the slump mold uniformly; don’t air content, slump, and the ability of fresh concrete to flow. use a twisting or jerking motion. Note lines on mold marking heights of equal volume. temperature at the time Slump is commonly mistaken as a of placement and as necmeasure of workability—the ease essary for control checks. in which fresh concrete can be ASTM C 94 also states placed, consolidated, and finished. acceptance tests for consistency, air these tests shall be made when content, or potential strength. The specified and always when manipulation of concrete during strength specimens are made. pumping can significantly change It’s a good, but not required, mix characteristics. When sampractice to run a unit weight pling from a discharge stream, ditest every time strength specivert the discharge into the sample mens are made. container. Do not cause segregaComposite samples used to tion by restricting concrete flow make specimens for strength from the mixer or chute. Regulate tests must be larger than 1 cuthe discharge rate of bic foot. A strength test conrevolving-drum truck mixers by sists of the average compresslowing the rate of drum revolusive strength of two cylinders. tion, rather than by adjusting the Since ASTM C 94 requires gate opening size. that slump, air content, and ASTM C 172 imposes three time t emperature tests be perlimitations on the technician: formed whenever strength • Elapsed time between obtaining specimens are made, obtain a the first and last portions of the sample large enough to make composite sample is 15 minutes. all the tests. ASTM allows smaller samples for routine • Tests for slump or air content air content, slump, and temmust begin within 5 minutes afFigure 3. Always lift filled molds carefully from perature tests. ter obtaining the final portion of the bottom with a large trowel to avoid the composite sample. distorting the mold.

slump, the plus tolerance is 0 inches. When the specifications for slump are not written as a “maximum” or “not to exceed” requirement, ASTM C 94 tolerances are: Specified Slump 2 inches and less More than 2 inches through 4 inches More than 4 inches

Figure 4. Carefully finish the top of cylinders with a tamping rod or, preferably, a wood float.

Though consistency and workability are closely related, only consistency is considered a fundamental property of the fresh concrete. Workability must be related to the type of construction and placing and finishing methods. The slump test is a quality-control test because changes in the measured slump indicate changes in mix proportions, mixing procedures, or other factors affecting the nature of fresh concrete. Figure 1 shows three distinct types of slumps. True slump is a general subsidence of the mass without breaking up. Shear slump usually indicates a lack of cohesion, and commonly occurs with harsh mixes. Collapse slumps generally indicate a lean, harsh, or more likely, very wet mix. Of course, improper testing can also cause concrete to shear or fall away. It’s important to know the tolerance on measured slump before making a test. When project specifications state a “maximum” or “not to exceed” slump requirement, ASTM C 94 sets a 11⁄2-inchminus tolerance for specified slumps of 3 inches or less and a 21⁄2-inch-minus tolerance otherwise. Whatever the specified

Tolerances ±1⁄2 inch ±1 inch ±11⁄2 inch

Slump tests are quite sensitive to variations within the test procedures given by ASTM C 143. It’s not uncommon for different technicians to measure slumps that differ by as much as 1 inch for the same concrete (Ref. 1). Therefore, carefully observe the testing procedures to minimize variations. Tips for Proper Slump Testing ■ Dampen the mold and perform the test on a level, moist, nonabsorbent, rigid base. Plywood is not acceptable since its surface rapidly becomes rough and uneven when exposed to moisture. Use a 16- to 2 0-gage metal sheet over a wood base or a premanufactured metal base. Do not run the test on the tailgate of a pickup truck. ■

Fill mold in three layers, each about 1⁄3 the volume of the mold, and uniformly rod each l a y e r. Paint stripes around the outside of the mold 25⁄8 and 61⁄8 inches from the bottom to help determine depths for equal volumes (Figure 2).

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Remove the mold by lifting it uniformly in 3 to 7 seconds. Don’t use a twisting or jerking motion (Figure 2).

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Complete the test within 2 ⁄2 minutes to avoid the effects of 1

slump loss. Concrete test Cylinders ASTM C 31 covers making, curing, protecting, and transporting concrete test specimens under field conditions. Procedures for compression strength testing of these cylinders are described in ASTM C 39. Concrete cylinders are made and tested for two different purposes. Cylinders taken into the laboratory the day after casting and stored under standardized moisture and temperature conditions until tested are used to evaluate the quality of concrete as delivered to the job. These test results are used to determine compliance with strength specifications. Cylinders field cured under job conditions can indicate strength of inplace concrete at a given time. Tests of these cylinders are sometimes used to determine when to remove formwork and shoring. The standard 6-inch-diameter, 12-inch-high test cylinder is used when the maximum size aggregate does not exceed 2 inches. When the maximum size aggre-

Figure 5. Use a flat plate of glass or metal (at least 1/4 inch thick) to strike off and finish the concrete when running a unit weight test.

Figure 6. Not tapping the sides of the bowl during the pressurization stage of an air test will cause the air content reading to be lower than the true value.

gate exceeds 2 inches, the sample must be wet sieved, or a cylinder with a diameter three times the maximum size aggregate is used. Unless required by project specifications, don’t use cylinders smaller than 6 inches in diameter. Place concrete in the standard cylinder mold in three equal layers, and consolidate by rodding or vibrating. Rod concretes with slumps greater than 3 inches, rod or vibrate concretes with slumps of 1 to 3 inches, and vibrate concretes with slumps less than 1 inch. ASTM C 31 gives additional details for properly placing and consolidating the concrete. Almost any deviation from the standard will reduce the apparent concrete strength. Seal the top of cylinder molds immediately after striking off to prevent moisture evaporation, then label and move to storage. Lift filled molds carefully from the bottom with a large trowel to avoid mold distortion (Figure 3). Keep molds vertical during trans-

port to avoid shifting of the fresh concrete. Leave the molds undisturbed for 24 ± 8 hours, maintaining them at 60° to 80° F. During initial storage, every 10° F increase in concrete temperature can reduce the compressive strength by as much as 300 psi (Ref. 2). Compressive strength can be reduced by as much as 50% if the test cylinders freeze. Properly constructed curing boxes are ideal for maintaining curing conditions for the initial storage period. Don’t demold cylinders if transporting them to the lab within 48 hours after molding. Cylinders transported after 48 hours are demolded after 24 ± 8 hours and placed in saturated lime water at 70° to 76° F until transported. Protect test cylinders from jarring, bouncing, moisture loss, freezing, and direct sun during transportation. Expect low breaks for cylinders tossed into the bed of a pickup truck and bounced about on the way to the lab. Tips for Proper Compression strength Testing ■ Mold cylinders on a level base with the axis of the mold vertical to avoid sloping ends. ■

After rodding each layer, tap the outside of the cylinder with a mallet to close any holes left by the rod. Rod holes or voids cause low cylinder breaks.

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Avoid overfilling of the last layer because a concentration of large aggregate at the top of the cylinder may occur.

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Finish the top of cylinders carefully with a tamping rod or, p r e f e r a b l y, a wood float (Figure 4).

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After making cylinders, store them on a level, vibration-f r e e base and maintain proper temperatures.

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Use a curing box that secures and cushions test cylinders during transport to the lab.

Unit weight Aggregate amount and relative density, air content, and water and cement contents determine the unit weight of fresh concrete. Unit weight for conventional concrete can vary from 140 to 150 pounds per cubic foot when measured according to ASTM C 138. Slight changes in batch weights or air content can be detected by this simple test. An increase in water content, decrease in cement content, or an increase in air content will cause a measurable reduction in the unit weight of fresh concrete. A unit weight variation of more than 1 pound per cubic foot probably indicates changes in the contents of the mix. Tips for Proper Unit Weight Testing ■ Recalibrate (determine the volume) of the measuring container at least once a year. ■

When rodding to consolidate the concrete, tap the sides of the container sharply with a mallet 10 to 15 times after rodding each layer to close voids left by the tamping rod. Vo i d s cause the measured unit weight to be lower than the true value.

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In the final filling of the cont a i n e r, if it is necessary to add or remove material, use concrete, not mortar, so the portions remain the same.

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Use a flat plate of glass or metal (at least 1/4 inch thick) to strike off and finish the concrete (Figure 5). A tamping rod, trowel, float, or straightedge will leave high spots, resulting in a high estimate of unit weight.

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After strike off, clean excess concrete from the exterior of the measuring container.

Air content by the pressure method Air tests cannot distinguish between larger air pockets and very fine bubbles that improve frost resistance. Unless set by the project specifications, ASTM C 94 gives a ± 1.5% tolerance from the specified value when concrete is sampled at the point of discharge from the transportation unit. The ASTM C 231 procedure to measure air content of freshly mixed concrete observes the change in volume of concrete with a change in pressure. This change in volume is assumed to be caused entirely by compression of the air in the concrete. Voids in the aggregate affect the result of an air test based on pressure, resulting in a higher air content than the true value. Consequently, the pressure method is most applicable for concretes made with relatively dense aggregates. Use the volumetric method, ASTM C 173, for concrete made with lightweight aggregates or aggregates with high porosity. Tips for Proper Air Testing ■ When rodding is used to consolidate the concrete, tap the sides of the air bowl sharply 10 to 15 times with a mallet after rodding to close voids left by the tamping rod. Vo i d s cause a higher air content reading than the true value. ■

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If vibration is used (for slumps less than 3 inches), be careful not to overvibrate causing air content readings to be lower than the true value. The internal surface of the bowl and cover need to be clean and wet before the test,

so air bubbles will not stick to the cover and be difficult to dislodge. ■

When pressurizing the bowl, tap its sides with a mallet 10 to 15 times to help particles of concrete move to find equilibrium with the new pressure (Figure 6). Not tapping the bowl at this point in the test will cause the air content reading to be lower than the true value.

Temperature Because of the important influence temperature has on the properties of fresh and hardened concrete, many specifications place minimum and maximum limits on fresh concrete temperature. Temperature primarily affects rate of hydration, which influences properties such as slump loss, setting and finishing times, and rate of strength gain. Fresh concrete temperature also affects the performance of admixtures (especially the effectiveness of air-entraining agents) and resistance to early-age freezing during cold weather. Tips for Proper Temperature Testing ■ The sensor of the temperature measuring device must have at least 3 inches of concrete cover in all directions. ■

Leave the measuring device in concrete for at least 2 minutes or until temperature reading stabilizes.

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Take temperature within 5 minutes of obtaining sample and record to nearest degree.

Use an accredited testing lab To receive the benefits of proper testing, owners, engineers, and contractors should use a

NVLAP-accredited (or equivalent) testing laboratory that employs American Concrete Institute (ACI)-certified technicians. The National Institute of Standards and Technology operates NVLAP (National Voluntary Laboratory, Accreditation Program). NVLAP accreditation means the testing lab has successfully passed an on-site assessment of facilities, including competence of the staff, and is participating in an ongoing proficiency testing program. ACI-certified technicians have passed written and field performance examinations to obtain the Concrete Field Testing Technician—Grade I Certification. Using an accredited lab and certified technicians helps assure proper testing. NVLAP publishes an annual “Directory of Accredited Laboratories,” that lists all accredited labs and specific tests for which they are accredited. It’s available from National Technical Information Service, (NTIS), 5285 Port Royal Rd.,, Springfield, VA 22161. References 1. S. Mindess and J. F. Young, Concrete, Prentice-Hall Inc., Englewood Cliffs, NJ, 1981. 2. ACI Committee Report 305, “Hot Weather Concreting,” ACI, Detroit.

Kim Basham Ph. D., P.E. is director of engineering for CTC-G E O T E K , D e n v e r, Colo., a testing and engineering firm specializing in fore nsic-type work. He also is an instructor for the ACI Concrete Field Testing Technician—Grade I cer t i f i c a t i o n program.

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