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STM standards define pre-cise pro c e d u res for per-forming field tests to deter-mine the quality of freshly mixed c o n c rete. Usual field tests measure c o n s i s t e n c y, strength, unit weight, air content, and temperature (see box). Many test details may seem trivial, even arbitrary. However, ASTM standards establish unifor-mity in the testing methods. By al-ways performing tests the same w a y, it’s possible to detect changes in fresh concrete that could aff e c t c o n c rete performance. Impro p e r testing or even deviating fro m s t a n d a rd test methods may cause good concrete to be rejected or badc o n c rete to be accepted. In either case, poor testing penalizes the c o n t r a c t o r, ready mix pro d u c e r, and owner. Understanding the scope, significance, and pro c e d u re of each test for fresh concrete is the first step to achieving accurate re-s u l t re-s .
Sampling fresh concrete Follow pro c e d u res given in ASTM C 172 to ensure the sample is truly re p resentative of the fre s h-ly mixed concrete being tested. This standard gives pro c e d u res for obtaining and handling composite samples from stationary, paving, and truck mixers, and from
agitat-ing and nonagitatagitat-ing equipment. Obtain composite samples by sam-pling the concrete at least twice as the middle portion of the batch is d i s c h a rged. Mix the samples into one sample for testing. Obtain samples randomly. Do not base sample selection on concrete ap-pearance, convenience, or other bi-ased criteria.
Usually concrete is sampled as it’s discharged from the mixer and transported to the forms. Some specifications may re q u i re other points of sampling, such as at the d i s c h a rge of a concrete pump line. C o n c rete tested at these discharg e 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)
Testing fresh concrete
in the field
By Kim Basham
acceptance tests for consistency, air content, or potential strength. The manipulation of concrete during pumping can significantly change mix characteristics. When sam-pling from a discharge stream, di-vert the discharge into the sample c o n t a i n e r. Do not cause segre g a-tion by restricting concrete flow f rom the mixer or chute. Regulate the discharge rate of
re v o l v i n g-d rum truck mixers by slowing the rate of drum re v o l u-tion, rather than by adjusting the gate opening size.
ASTM C 172 imposes three time limitations on the technician: • Elapsed time between obtaining
the first and last portions of the composite sample is 15 minutes. • Tests for slump or air content
must begin within 5 minutes af-ter obtaining the final portion of the composite sample.
• Molding of stre n g t h specimens must begin within 15 minutes after making the composite s a m p l e .
It’s important to care-fully observe these time restrictions to achieve the most accurate test re s u l t s . This may be difficult on some jobs. To minimize the elapsed time between obtaining samples and performing tests, mix the composite sample, and test and mold specimens as close as possible to the point of sampling.
Specifications usually indicate the number of tests to be made. Howev-e r, ASTM C 94 rHowev-e q u i rHowev-e s air content, slump, and t e m p e r a t u re at the time of placement and as nec-essary for control checks. ASTM C 94 also states these tests shall be made when specified and always when s t rength specimens are made. It’s a good, but not re q u i re d , practice to run a unit weight test every time strength speci-mens are made.
Composite samples used to make specimens for stre n g t h tests must be larger than 1 cu-bic foot. A strength test con-sists of the average compre s-sive strength of two cylinders. Since ASTM C 94 re q u i re s that slump, air content, and t e m p e r a t u re tests be per-formed whenever stre n g t h specimens are made, obtain a sample large enough to make all the tests. ASTM allows smaller samples for ro u t i n e air content, slump, and tem-p e r a t u re tests.
Tips for Proper Sampling
■Obtain at least two samples to make a composite sample. ■Do not take samples from the
very first or last portions of the b a t c h .
■Take samples after all water and admixtures have been added to the mix.
■Carefully observe time limita-tions to ensure results of field tests are consistent.
■Protect samples from sun, wind, rapid evaporation, and c o n t a m i n a t i o n .
Slump
The slump test (ASTM C 143) m e a s u res concrete consistency, or the ability of fresh concrete to flow. Slump is commonly mistaken as a m e a s u re of workability—the ease in which fresh concrete can be placed, consolidated, and finished.
Figure 2. Remove the slump mold uniformly; don’t use a twisting or jerking motion. Note lines on mold marking heights of equal volume.
Figure 3. Always lift filled molds carefully from the bottom with a large trowel to avoid distorting the mold.
Though consistency and workabil-ity are closely related, only consis-tency is considered a fundamental p roperty of the fresh concre t e . Workability must be related to the type of construction and placing and finishing methods.
The slump test is a quality-c o n-t rol n-tesn-t because changes in n-the m e a s u red slump indicate changes in mix proportions, mixing pro c e-d u res, or other factors affecting the n a t u re of fresh concrete. Figure 1 shows three distinct types of slumps. True slump is a general subsidence of the mass without b reaking up. Shear slump usually indicates a lack of cohesion, and commonly occurs with harsh mix-es. Collapse slumps generally indi-cate a lean, harsh, or more likely, very wet mix. Of course, impro p e r testing can also cause concrete to shear or fall away.
It’s important to know the toler-ance on measured slump before making a test. When project speci-fications state a “maximum” or “not to exceed” slump re q u i re-ment, ASTM C 94 sets a 11⁄
2-i n c
h-minus tolerance for specified slumps of 3 inches or less and a 21⁄
2-inch-minus tolerance
other-wise. Whatever the specified
slump, the plus tolerance is 0 inch-e s .
When the specifications for slump are not written as a “maxi-mum” or “not to exceed” re q u i re-ment, ASTM C 94 tolerances are :
Specified Slump T o l e r a n c e s
2 inches and less ±1⁄ 2i n c h
M o re than 2 inches
t h rough 4 inches ±1 inch M o re than 4 inches ± 11⁄
2i n c h
Slump tests are quite sensitive to variations within the test pro c e-d u res given by ASTM C 143. It’s not uncommon for diff e rent tech-nicians to measure slumps that dif-fer by as much as 1 inch for the same concrete (Ref. 1). There f o re , c a refully observe the testing pro c e-d u res to minimize variations.
Tips for Proper Slump Te s t i n g
■Dampen the mold and perform the test on a level, moist, non-absorbent, rigid base. Plywood is not acceptable since its sur-face rapidly becomes rough and uneven when exposed to moisture. Use a 16- t o
2 0-gage metal sheet over a wood base or a premanufac-tured metal base. Do not run the test on the tailgate of a pickup truck.
■Fill mold in three layers, each about 1⁄
3the volume of the
mold, and uniformly rod each l a y e r. Paint stripes around the outside of the mold 25⁄
8and 61⁄8
inches from the bottom to help determine depths for equal volumes (Figure 2).
■Remove the mold by lifting it uniformly in 3 to 7 seconds. Don’t use a twisting or jerking motion (Figure 2).
■Complete the test within 21⁄ 2
minutes to avoid the effects of
slump loss.
Concrete test Cylinders ASTM C 31 covers making, cur-ing, protectcur-ing, and transporting c o n c rete test specimens under field conditions. Pro c e d u res for com-p ression strength testing of these cylinders are described in ASTM C 3 9 .
C o n c rete cylinders are made and tested for two diff e rent pur-poses. Cylinders taken into the lab-oratory the day after casting and s t o red under standardized mois-t u re and mois-temperamois-ture condimois-tions until tested are used to evaluate the quality of concrete as delivere d to the job. These test results are used to determine compliance with strength specifications. Cylin-ders field cured under job condi-tions can indicate strength of in-place concrete at a given time. Tests of these cylinders are some-times used to determine when to remove formwork and shoring.
The standard 6-i n c h-d i a m e t e r, 1 2-i n c h-high test cylinder is used when the maximum size aggre-gate does not exceed 2 inches. When the maximum size
aggre-Figure 4. Carefully finish the top of cylinders with a tamping rod or, preferably, a wood float.
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.
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 re q u i red by project specifi-cations, don’t use cylinders smaller than 6 inches in diameter. Place c o n c rete in the standard cylinder mold in three equal layers, and consolidate by rodding or vibrat-ing. Rod concretes with slumps g reater than 3 inches, rod or vi-brate concretes with slumps of 1 to 3 inches, and vibrate concre t e s with slumps less than 1 inch. ASTM C 31 gives additional de-tails for properly placing and con-solidating the concrete. Almost any deviation from the standard will reduce the apparent concre t e s t re n g t h .
Seal the top of cylinder molds immediately after striking off to p revent 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 fre s h c o n c rete. Leave the molds undis-turbed for 24 ± 8 hours, maintain-ing them at 60° to 80° F. Durmaintain-ing ini-tial storage, every 10° F increase in c o n c rete temperature can re d u c e the compressive strength by as much as 300 psi (Ref. 2). Compre s-sive strength can be reduced by as much as 50% if the test cylinders f reeze. Properly constructed curing boxes are ideal for maintaining curing conditions for the initial storage period.
Don’t demold cylinders if trans-porting them to the lab within 48 hours after molding. Cylinders transported after 48 hours are de-molded after 24 ± 8 hours and placed in saturated lime water at 70° to 76° F until transported. Pro-tect test cylinders from jarring, bouncing, moisture loss, fre e z i n g , and direct sun during transporta-tion. Expect low breaks for cylin-ders tossed into the bed of a pick-up truck and bounced about on the way to the lab.
Tips for Proper Compre s s i o n s t rength Te s t i n g
■Mold cylinders on a level base with the axis of the mold verti-cal 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. ■Avoid overfilling of the last
lay-er because a concentration of large aggregate at the top of the cylinder may occur. ■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).
■After making cylinders, store them on a level, vibration-f r e e base and maintain proper tem-p e r a t u r e s .
■Use a curing box that secures and cushions test cylinders during transport to the lab. Unit weight
A g g regate amount and re l a t i v e d e n s i t y, air content, and water and cement contents determine the unit weight of fresh concrete. Unit weight for conventional concre t e can vary from 140 to 150 pounds per cubic foot when measured ac-c o rding 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 con-tent, or an increase in air content will cause a measurable re d u c t i o n in the unit weight of fresh concre t e . A unit weight variation of more than 1 pound per cubic foot pro b a-bly indicates changes in the con-tents of the mix.
Tips for Proper Unit Weight Te s t i n g
■Recalibrate (determine the vol-ume) of the measuring con-tainer 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 rod-ding 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.
■In the final filling of the con-t a i n e r, if icon-t is necessary con-to add or remove material, use con-crete, not mortar, so the por-tions remain the same. ■Use a flat plate of glass or
metal (at least 1/4 inch thick) to strike off and finish the con-crete (Figure 5). A tamping rod, trowel, float, or straight-edge will leave high spots, re-sulting in a high estimate of unit weight.
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.
■After strike off, clean excess concrete from the exterior of the measuring container. Air content by the pressure method
Air tests cannot distinguish be-tween larger air pockets and very fine bubbles that improve frost re-sistance. Unless set by the pro j e c t 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 pro c e d u re to m e a s u re air content of fre s h l y mixed concrete observes the change in volume of concrete with a change in pre s s u re. This change in volume is assumed to be caused e n t i rely by compression of the air in the concrete. Voids in the aggre-gate affect the result of an air test based on pre s s u re, resulting in a higher air content than the tru e value. Consequently, the pre s s u re method is most applicable for con-c retes made with relatively dense a g g regates. Use the volumetric method, ASTM C 173, for concre t e made with lightweight aggre g a t e s or aggregates with high poro s i t y.
Tips for Proper Air Te s t i n g
■When rodding is used to con-solidate the concrete, tap the sides of the air bowl sharply 10 to 15 times with a mallet af-ter rodding to close voids left by the tamping rod. Vo i d s cause a higher air content reading than the true value. ■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 d i s l o d g e .
■When pressurizing the bowl, tap its sides with a mallet 10 to 15 times to help particles of concrete move to find equilibri-um 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 v a l u e .
Temperature
Because of the important influ-ence temperature has on the pro p-erties of fresh and hardened con-c rete, many specon-cificon-cations placon-ce minimum and maximum limits on f resh concrete temperature. Te m-p e r a t u re m-primarily affects rate of hydration, which influences pro p-erties such as slump loss, setting and finishing times, and rate of s t rength gain. Fresh concrete tem-p e r a t u re also affects the tem- perfor-mance of admixtures (especially the effectiveness of air- e n t r a i n i n g agents) and resistance to early-a g e f reezing during cold weather.
Tips for Proper Te m p e r a t u re Te s t i n g
■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 s t a b i l i z e s .
■Take temperature within 5 min-utes of obtaining sample and record to nearest degree. Use an accredited testing lab
To receive the benefits of pro p e r testing, owners, engineers, and contractors should use a
N V L A P-a c c redited (or equivalent) testing laboratory that employs American Concrete Institute ( A C I )-certified technicians. The National Institute of Standard s and Technology operates NVLAP (National Voluntary Laboratory, A c c reditation Program). NVLAP a c c reditation means the testing lab has successfully passed an on-s i t e assessment of facilities, including competence of the staff, and is par-ticipating in an ongoing pro f i c i e n-cy testing program. ACI-c e r t i f i e d technicians have passed written and field performance examina-tions to obtain the Concrete Field Testing Technician—Grade I Certi-fication. Using an accredited lab and certified technicians helps as-s u re proper teas-sting.
NVLAP publishes an annual “ D i rectory of Accredited Laborato-ries,” that lists all accredited labs and specific tests for which they a re accredited. It’s available fro m National Technical Information Service, (NTIS), 5285 Port Royal Rd.,, Springfield, VA 22161.
References
1. S. Mindess and J. F. Young, Con-crete, Prentice-Hall Inc., Englewood Cliffs, NJ, 1981.
2. ACI Committee Report 305, “Hot Weather Concreting,” ACI, Detroit.
Kim Basham Ph. D., P.E. is dire c t o r of engineering for CTC-G E O T E K , D e n v e r, Colo., a testing and engi-neering firm specializing in fore n-sic-type work. He also is an instruc-tor for the ACI Concrete Field Te s t i n g Technician—Grade I cer t i f i c a t i o n program.
P U B L I C ATION #C930550
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