Preparing for your ACI?

We've compiled helpful information for study whether you're planning on taking your first test or ACI renewal. It's important to review the ACI Certification Technician Workbook, Publication CP-1 (11) in addition to the information presented. But we have taken some of the most important information and compiled it here for easy review.

Materials

Wheelbarrow: capable of holding and transporting 1 cu.ft. of concrete (min. size for sampling)
Temperature Measuring Device: ability to measure 30 - 120 degrees F, within +/- 1 degree F
Slump Cone: 4" (top) x 8" (base) x 12" (height), tolerance of +/- 1/8-inch for any dimension
Tamping Rod (Slump & Pressure Air Meter): 5/8" dia. x 16 - 24" long, with one or both ends having a hemispherical tip
Tamping Rod (Volumetric Air Meter): 5/8" x max 24" (minimum length must be 4 inches longer than depth of bowl) with one or both ends rounded to a hemispherical tip, made of steel or high density polyurethane
Tamping Rod 5/8" x 16"-24": Can be used for ALL ACI ASTM tests requiring 5/8-diameter rod use
Tamping Rod (Test Specimens): 4" greater than depth of mold (not to exceed 24" long); 3/8" dia. for cylinder or width of beam < 6" diameter; 5/8" dia. for cylinder or width of beam 6" or greater diameter
Scoop: a size large enough that each amount of concrete taken from the sampling receptacle is representative, and small enough so it's not spilled during placement into slump cone or bowl
Strike-Off Bar: metal, at least 1/8" x 3/4" x 12" (thickness by length), or (volumetric) polyethylene bar min. 1/4" x 3/4" 12"
Strike-Off Plate: rectangular plate with a length and width at least 2" greater than the diameter of the measure used and: metal, at least 3/4" thick; or acrylic or glass, at least 1/2" thick
Mallet: mass must be 1.25 +/- .5 lb for containers .5 cu.ft. or smaller, and 2.25 +/- .5 lb for containers larger than .5 cu.ft.
Measuring Bowl: min. diameter .75 to 1.25 times the height, capacity of at least 0.20 cu.ft.
Bowl (Volumetric Method): diameter equal to 1 to 1.25 times the height, capacity of at least .075 cu.ft.
Vibrator (Air Meters): vibrating element .75" - 1.5" thick max by min. 24" length; also minimum 7,000 vibrations per minute (only used on concrete slumps 3" or less)
Vibrator (Test Specimens): no more than one-fourth the diameter of the cylinder mold or one-fourth the width of the beam mold; minimum 9,000 vibrations per minute
Trowel: standard brick mason's trowel for finishing
Sieve: 1-1/2" sieve for concrete with aggregate larger than 2"
Alcohol: isopropyl, 70% by volume
Measuring Vessel for Isopropyl Alcohol: min. capacity of one pint with graduates not larger than 4 ounces

Additional materials are used for determining Air Content and described in ASTMs C231 & C173.

Click on link to scroll to details:

ASTM C1064 - Temperature
ASTM C172 - Sampling Fresh Concrete
ASTM C143 - Slump Test
ASTM C138 - Density (Unit Weight), Yield, & Air Content
ASTM C231 - Air Content by Pressure Method
ASTM C173 - Air Content by Volumetric Method
ASTM C31 - Making & Curing Concrete Test Specimens in the Field

ASTM C1064 - Temperature

Temperature Measuring Device must:

Measure to within +/- 1 degree Fahrenheit
Measuring device must read between 30 to 120 degrees
Have 3-inches concrete cover in all directions
Be inserted a minimum of 3-inches
Be left in concrete a minimum of 2 min., maximum of 5 min.
Be calibrated annually, or if there's a question of accuracy

ASTM C172 - Sampling Fresh Concrete

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, 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.

No sampling can be taken from the first 10 percent or last 10 percent of the load. Here are some additional important guidelines.

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
The receptacle or sample container must be dampened
15 min. time limit between grabbing the first and final portions of samples for your composite sample
Composite sample must be at least 1 cubic foot of concrete
Start slump, temperature and air content tests within 5 min. after obtaining your final portion of the composite sample
Start molding strength specimens within 15 minutes after obtaining the composite sample
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

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 answers you are to study and memorize are written in bold.

ASTM C143 - Slump Test

All the equipment must be dampened prior to testing, including the base on which the slump cone is set. The cone dimensions must be 4" (top) x 8" (base) x 12" (height), with a tolerance of +/- 1/8"-inch for any of those dimensions. The tamping rod must be 5/8" in diameter by 16 - 24 inches long, with one or both sides having a hemispherical tip. A measuring device at least 12" long with increments of at least 3/4" must be used to measure slump. The slump cone mold must be checked annually for irregularities.

Fill cone in three lifts, rodding each one 25 times, each time penetrating the layer below approximately one inch for the second and third lifts. Do not tap the cone.
Each lift should fill a third of the volume. Height of concrete for first lift should be 2-5/8", and second lift should be 6-1/8".
After the last rod, screed the material off the top (an 1/8" excess concrete above top of mold is optimum) and wipe any concrete from around the base of the cone.
Pull cone straight up with no twisting taking anywhere from 3 to 7 full seconds (specs say 5 seconds +/- 2).
Measure slump from the top of the mold to the displaced original center of the concrete. Record results to the nearest 1/4-inch.
Begin and complete slump test within 2-1/2 minutes, maximum.
Maximum size aggregate for slump test is 1-1/2"; if larger, wet sieve through a 1-1/2" sieve.

Remember that slumps less than 1/2" may not be adequately plastic, and slumps greater than 9" may not be adequately cohesive for the slump test to have significance.

ASTM C138 - Density (Unit Weight), Yield, & Air Content

DENSITY

To figure out the density, you are filling up a container with concrete. This container must have a known volume (4), minimum .20 cubic feet. First weigh the empty container (1) and record that value to the nearest tenth (.1) of a pound. After performing the proper procedures for filling the container (i.e. filling it in three lifts, mallet blows, rodding, striking off with strike-off plate), follow these steps:

Weigh the container with the concrete (2) - record value to nearest tenth of a pound
Subtract the empty container weight from the full container weight

(2) - (1) = weight of concrete (3)
Divide the weight of concrete by the known volume

(3) / (4) = density, or fresh unit weight

A few unique procedures listed in the ASTM C138 to remember:

Slump
- If < 1" = Must use vibrator capable of at least 7,000 vibrations per minute
- If 1" - 3" = Can use vibrator or tamping rod
- If > 3" = Can't use vibrator, only use tamping rod
If using vibrator, fill and vibrate measure in two equal layers, not three
Insert vibrator at three different points for each layer approx. 1 inch into the underlying layer
Mallet mass must be 1.25 +/- .5 lb for containers .5 cu.ft. or smaller
Mallet mass 2.25 +/- .5 lb for containers larger than .5 cu.ft.
Use strike-off plate, not bar

YIELD

Yield is used to determine if the concrete supplier has delivered the amount of concrete ordered. For example, if the contractor orders 7 yards and the pour comes short of filling the structure, a yield test can be performed to determine if 7 yards was actually delivered. To calculate yield, follow these simple steps. Note that you must calculate density prior to calculating yield. You will take the total mass of all materials batched (aggregate, water, cement--this figure will be given to you on the test) to begin. The total mass of the materials is the total weight.

The yield per batch cu.ft.
Total weight / fresh unit weight = yield per batch in feet
The yield per batch in cu.yds.
Total weight / (fresh unit weight x 27) = yield per batch in yards
The yield in cu.m.
Yield per batch in cu.yards x .764 = yield in cu.meters
(1 cu. yd. = .764 cu. meters. This will not be on the test.)
The yield per cubic yard in cu.ft./cu.yd.

Yield per batch [result from (a)] / cu.yds delivered = yield per cu.yd. in cu.ft./cu.yd.

RELATIVE YIELD is the ratio of the actual volume of concrete obtained to the volume as designed for the batch. It is understood as a 1:1 ratio. If the relative yield calculates out to 1.00 or greater, then the yards delivered is equal to or greater than what was ordered. If it calculates to less than 1.00, then the yards delivered is less than what was ordered.

yield per batch in yards (b) / cu. yds. delivered = relative yield

AIR CONTENT CALCULATION

Air content can be calculated in the event that your air meter does not work properly. In order to calculate this, you must have the theoretical density. This information can be obtained from the concrete supplier, who has this information on the mix design for the mix you are working with. Again, you will need your results from the fresh unit weight.

theoretical density - actual density (fresh unit weight) / theoretical x 100 = calculated air content

Since the theoretical density from the lab has no air, you are comparing that laboratory weight to the fresh weight which contains air. The air in the fresh weight sample makes it lighter than the theoretical. The formula above allows you to subtract the fresh from the theoretical and come up with a calculated percentage of air content without performing an actual test. For the ACI test, be able to recognize the order of the formula written above.

ASTM C231 - Air Content by Pressure Method

This test method cannot be used for testing lightweight concrete because the aggregate in lightweight is too porous. See ASTM C173 Air Content by Volumetric Method.

Two types of Air:

Entrapped (naturally occurring in the mix)
Entrained (intentionally added air)

Remember, entrained air is necessary for concrete that will be directly exposed to freeze-thaw cycles throughout its lifetime. The air voids allow for collected moisture to expand and contract through freezing without causing the concrete to spall. And as air increases, the ultimate strength of the concrete decreases, just as the increase of water decreases strength.Two types of Pressure Method Air Meters

Type A: Not as commonly used. Uses pressure to displace water into concrete
Type B: Most commonly used. Uses equalization to determine air content in the concrete. Once the valve is depressed and held, equalization occurs when the known air in the air chamber releases into the unknown air content in the concrete sample in the bowl below.

Additional Important Information

Maximum 2" aggregate
Pressure gauge should read to the nearest 0.1 percent.
Mallet mass same as described under ASTM C138
Tamping rod dimensions same as described under ASTM C143
Strike-Off bar made of metal at least 1/8-inch thick and 12-inches long OR
Strick-Off plate that is at least 2-inches wider than diameter of bowl and:
Metal, at least 1/4-inch thick or
Glass or acrylic at least 1/2-inch thick

ASTM C173 - Air Content by Volumetric Method

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.

Important Steps in the Volumetric test

After properly filling bowl, make sure to:

Use strike-off bar
Record results to nearest .25 percent
Insert funnel and add 1 pint of water
Add a "selected amount" of alcohol (no correction factor needed with 2.4 pints of alcohol or less)
Add water until the water can be seen in the neck
Remove funnel and add water until the bottom of meniscus is at the zero mark
Screw lid on tightly
Invert for 5-second intervals, minimum 45 seconds total
Roll for 1 minute at a 45 degree angle tilt completing 1/4 to 1/2 turns, back and forth vigorously
Unscrew cap, allow 2 min. to settle, and take initial reading to the nearest .25 percent
Tighten cap and repeat 1 minute roll, then loosen cap, let settle, and take second reading
If second reading does not differ more than .25 percent from initial reading, then second reading becomes the final reading
If second reading does differ more than .25 percent, a 3rd roll of 1 minute is required
If third reading does not differ more than .25 percent from second reading, then third reading becomes final reading
Note: if the air content is greater than the 9 percent range of the meter, add water using the calibrated cup to bring the liquid level back up into the graduated cylinder. Record the number of calibrated cups used to calculate back in for the final air reading.

Reasons For Invalid Test

If third reading still differs more than .25 percent from second reading, discard test and start new using more alcohol
If at any time during the inversion and rolling process liquid leaks, discard test and start over
If it takes more than 6 minutes for the liquid level to stabilize, discard test
If there is more foam in the neck than the equivalent of 2 percent air content divisions after inversion and rolling, discard test and use more alcohol
If when the test is complete, portions of undisturbed, tightly packed concrete (not sand) is found at the base of the bowl when the contents have been dumped out, discard test and repeat.

ASTM C31 - Making & Curing Concrete Specimens in the Field

When strength is used as a basis for acceptance of concrete, specimens must be molded and cured according to ASTM C 31. Cylinder molds can be metal or plastic, so long as they are non-absorbent, non-reactive to concrete, and maintain their shape and dimensions under all conditions of use. They must not leak when water is poured into them (empty).

Tamping Rod must be 3/8" dia for molds less than 6" in diameter or width (beam molds)
Vibrators must produce at least 9,000 vibrations per minute, and vibrating element must be no bigger than one-fourth the diameter of the cylinder mold or one-fourth the width of the beam mold. Vibrators or rodding are entirely interchangeable, but vibrators must be used for slumps 1" or less.

Cylinders

When rodding cylinders, fill in: 2 layers (25 rods) for 4" dia cylinders; 3 layers (25 rods) for 6" dia cylinders; 4 layers (50 rods) for 9" dia cylinders
Tap outside of molds 10 - 15 times with a mallet or an open hand for each lift
When vibrating cylinders, fill in: 2 layers for all cylinder diameter sizes, but insert vibrator once for 4" dia cylinders, twice for 6" dia cylinders, and four times for 9" dia cylinders for both layers
Insert rod or vibrator 1 inch into the underlying lift
Cylinder must be at least 3 times the size of the course aggregate (aggregate larger than 2" must be wet-sieved out of the mix)
Cylinder molds must be twice the length of the diameter

Beams

When rodding beams, fill in: 2 layers (rod once for every 2 sq. in. of top surface area of beam) for beam widths 6 - 8 inches; 3 or more equal layers, layers not to exceed 6" depth (rod once for every 2 sq. in. of top surface area of beam)
When vibrating beams, fill in: 1 layer for beam widths 6 - 8 inches, 2 or more layers for widths greater than 8 inches (inserting vibrator uniformly until the surface of the concrete has become relatively smooth and large air bubbles cease to break through the top surface).
Tap outside of molds 10 - 15 times with a mallet for each lift
Beams must be at least 3 times the size of the course aggregate in the smallest cross section (ex. a beam 6 x 9 x 20, smallest cross section is 6 inches, which means aggregate cannot be larger than 2 inches for this size mold).
Beam mold dimension--the length must be at least 2 inches greater than three times the depth (ex. if depth is 6", length must be 20" min.). Width must not exceed 1.5 times the depth (ex. depth is 6", width can be 6" to 9" max.). Unless specified by the project, depths and widths must be at least 6 inches.

When planning to mold and cure concrete specimens, it is required that tests for slump, temperature, and air content be performed (unless entrained air is not of concern to the project).

Standard Curing

During initial curing, the cylinders must be stored in a temperature range from 60 - 80 degrees F in an environment that prevents moisture loss for up to 48 hours. If the concrete design strength is 6,000 psi or greater, the initial curing temperature must range from 68 - 78 degrees F. Specimens should be shielded from direct sunlight or radiating heating devices, if used. A minimum-maximum thermometer shall record the temperature of the initial curing period, and then recorded upon retrieval of cylinders.

For final curing, cylinders or beams must be placed in their curing storage no later than 30 minutes after removing from the molds. Cylinders shall have free water maintained on their surfaces at all times and remain in a constant temperature of 73.5 degrees F, +/- 3.5 degrees F. Cylinders and beams can be placed in moist rooms or water storage tanks, but beams must be moved into water saturated with calcium hydroxide at the same temperature at least 20 hours prior to testing. Prevent the drying of the surfaces of the beams between the removal of the water storage tank and testing.

Field Curing

Store cylinders or beams in or on the structure representing the area from which the sample was taken. Protect the surfaces of both in the same manner the structure is protected, as best as possible. For beams, at the end of the initial 48 hours (+/- 4 hours) after molding, remove the molds and place them at the structure location they represent. If representing pavements of a slab on grade, bank the sides and ends with earth or sand that must be kept damp, leaving the top surface exposed. As with standard curing, remove the beams from the field and store in water saturated with calcium hydroxide for 24 hours (+/- 4 hours) prior to testing.

Transporting

Protect cylinders with suitable cushioning material to prevent damage from jarring. Also prevent moisture loss by wrapping specimens in wet burlap or plastic, if not contained in plastic already. Transportation time of cylinders shall not exceed 4 hours.

Reporting

Report the following for each sample brought into the laboratory:

ID number (such as project number written on specimens)
Location of concrete represented by the samples
Date and time
Name of individual molding the specimens
Slump, air content, and concrete temperature
Curing method