ASTM 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 bad

c 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 s .

Sampling fresh concreteFollow 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 nonagitating 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 ofcohesion. Collapse slump indicates a lean, harsh, or very wet mix (Ref. 1)

Testing fresh concrete in the fieldBy Kim Basham

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

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 re q u i re 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 tomake a composite sample.

■ Do not take samples from thevery first or last portions of theb a t c h .

■ Take samples after all waterand admixtures have beenadded to the mix.

■ Carefully observe time limita-tions to ensure results of fieldtests are consistent.

■ Protect samples from sun,wind, rapid evaporation, andc o n t a m i n a t i o n .

SlumpThe 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’tuse a twisting or jerking motion. Note lines on moldmarking heights of equal volume.

Figure 3. Always lift filled molds carefully fromthe bottom with a large trowel to avoiddistorting 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 test because changes in 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 s2 inches and less ±1⁄2 i n c h

M o re than 2 inches

t h rough 4 inches ±1 inch

M o re than 4 inches ± 11⁄2 i 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 performthe test on a level, moist, non-absorbent, rigid base. Plywoodis not acceptable since its sur-face rapidly becomes roughand uneven when exposed tomoisture. Use a 16- t o2 0-gage metal sheet over awood base or a premanufac-tured metal base. Do not runthe test on the tailgate of apickup truck.

■ Fill mold in three layers, eachabout 1⁄3 the volume of themold, and uniformly rod eachl a y e r. Paint stripes around theoutside of the mold 25⁄8 and 61⁄8inches from the bottom to helpdetermine depths for equalvolumes (Figure 2).

■ Remove the mold by lifting ituniformly in 3 to 7 seconds.Don’t use a twisting or jerkingmotion (Figure 2).

■ Complete the test within 21⁄2minutes to avoid the effects of

slump loss.

Concrete test CylindersASTM C 31 covers making, cur-

ing, protecting, 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 temperature conditions

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 ofcylinders with a tamping rod or,preferably, a wood float.

Figure 5. Use a flat plate of glass ormetal (at least 1/4 inch thick) to strikeoff and finish the concrete whenrunning 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. During 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 ns t rength Te s t i n g

■ Mold cylinders on a level basewith the axis of the mold verti-cal to avoid sloping ends.

■ After rodding each layer, tapthe outside of the cylinder witha mallet to close any holes leftby the rod. Rod holes or voidscause low cylinder breaks.

■ Avoid overfilling of the last lay-er because a concentration oflarge aggregate at the top ofthe cylinder may occur.

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

■ After making cylinders, storethem on a level, vibration-f r e ebase and maintain proper tem-p e r a t u r e s .

■ Use a curing box that securesand cushions test cylindersduring 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 consolidatethe concrete, tap the sides ofthe container sharply with amallet 10 to 15 times after rod-ding each layer to close voidsleft by the tamping rod. Vo i d scause the measured unitweight to be lower than thetrue value.

■ In the final filling of the con-t a i n e r, if it is necessary to addor remove material, use con-crete, not mortar, so the por-tions remain the same.

■ Use a flat plate of glass ormetal (at least 1/4 inch thick)to strike off and finish the con-crete (Figure 5). A tampingrod, trowel, float, or straight-edge will leave high spots, re-sulting in a high estimate ofunit weight.

Figure 6. Not tapping the sides of thebowl during the pressurization stage ofan air test will cause the air contentreading to be lower than the true value.

■ After strike off, clean excessconcrete from the exterior ofthe measuring container.

Air content by the pressuremethod

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 thesides of the air bowl sharply10 to 15 times with a mallet af-ter rodding to close voids leftby the tamping rod. Vo i d scause a higher air contentreading than the true value.

■ If vibration is used (for slumpsless than 3 inches), be carefulnot to overvibrate causing aircontent readings to be lowerthan the true value.

■ The internal surface of thebowl and cover need to beclean and wet before the test,

so air bubbles will not stick tothe cover and be difficult tod i s l o d g e .

■ When pressurizing the bowl,tap its sides with a mallet 10 to15 times to help particles ofconcrete move to find equilibri-um with the new pressure(Figure 6). Not tapping thebowl at this point in the test willcause the air content readingto 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 specifications place

minimum and maximum limits on

f resh concrete temperature. Te m-

p e r a t u re 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 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 temperaturemeasuring device must haveat least 3 inches of concretecover in all directions.

■ Leave the measuring device inconcrete for at least 2 minutesor until temperature readings t a b i l i z e s .

■ Take temperature within 5 min-utes of obtaining sample andrecord to nearest degree.

Use an accredited testinglab

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 testing.

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.

References1. S. Mindess and J. F. Young, Con-crete, Prentice-Hall Inc., EnglewoodCliffs, NJ, 1981.2. ACI Committee Report 305, “HotWeather Concreting,” ACI, Detroit.

Kim Basham Ph. D., P.E. is dire c t o rof 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 gTechnician—Grade I cer t i f i c a t i o nprogram.

P U B L I C ATION #C930550

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