SCOPE
This procedure provides instruction for obtaining samples of fresh concrete for new construction or repair. Sources covered include grade, ready mix truck, mobile mixer, pump or conveyor placement systems, and concrete slab as placed.
SIGNIFICANCE
Testing fresh concrete in the field begins with obtaining and preparing the sample to be tested. Standardized procedures for obtaining a representative sample from various types of mixing and/or agitating equipment have been established. Specific time limits regarding when tests for temperature, slump, and air content must be started and for when the molding of test specimens must begin are also established.
Technicians must refrain from obtaining the sample too quickly. Doing so would be a violation of the specifications under which the concrete is being supplied and it may result in a nonrepresentative sample of concrete. Every precaution must be taken to obtain a sample that is truly representative of the entire batch and then to protect that sample from the effects of evaporation, contamination, and physical damage.
PROCEDURE
A. Apparatus
1. Wheelbarrow or other nonabsorbent container
2. Cover for wheelbarrow or container (plastic, canvas, or burlap)
3. Shovel
4. 5-gallon bucket for water
B. Testing Procedure
For acceptance testing, obtain representative samples from the last practical point before incorporation, but before consolidation. Sample at the truck, prior to placement, when the sampling location it is not practical or safe to obtain a sample. The alternate method of acceptance for bridge deck pumping may be utilized when at least two inspectors are present. The DME may adjust sample location and target air content, to ensure safe sampling location and adequate in place air content is achieved for freeze-thaw durability.
1. Sampling from Grade
Sample after the concrete in the transport vehicle has been discharged onto the grade. To ensure a representative sample, obtain concrete from at least five different locations in the pile and combine into one test sample. Avoid contamination with subgrade material or prolonged contact with absorptive subgrade.
2. Sampling from Ready Mix Truck
Sample the concrete after a minimum of 1/2 yd.3 of concrete has been discharged. Do not obtain samples until after all of the water has been added to the mixer. Do not obtain samples from the very first or last portions of the batch discharge. Sample by repeatedly passing a receptacle through the entire discharge stream or by completely diverting the discharge into a sample container. Regulate the rate of discharge of the batch by the rate of revolution of the drum and not by the size of the gate opening.
3. Sampling from Mobile Mixer
Discharge the concrete into a container or power buggy sufficiently large enough to accommodate the entire batch. Secure a representative sample after the batch has been deposited by obtaining one shovel full, more or less, from each of at least three different positions in the container or power buggy.
4. Sampling from Pump or Conveyor Placement Systems
Sample after a minimum of 1/2 yd.3 of concrete has been discharged. Do not obtain samples until after all of the pump slurry has been eliminated. Sample by repeatedly passing a receptacle through the entire discharge system or by completely diverting the discharge into a sample container. Do not lower or raise the pump arm from the placement position for ease of sampling, as it may modify the air content of the concrete being sampled. Sample at the truck, prior to introduction into pump, for drilled shafts placements, soldier piles, pier caps, or other placement where it is not practical to sample within the unit. Do not obtain samples from the very first or last portions of the batch discharge. To reduce variability in air tests, ensure that the pump configuration is such that sufficient back pressure is achieved and a constant flow is being discharged before sample is obtained.
5. Sampling from Concrete Slab as Placed
Mark the approximate location of concrete placed on grade and sampled for air content. After the paver has passed the marked location, remove the sample from the slab, approximately in line with a vibrator and within an 18 in. x 18 in. square area to a depth approximately two-thirds of the pavement thickness. The sample should be obtained a minimum of 12 in. from the edge of slab to prevent extra handwork in maintaining the pavement edge.
6. Alternate Acceptance of Air Content for Concrete Pumping on Bridge Decks
The following procedures may be utilized for acceptance of concrete air content on bridge decks when pumping is the method of placement. This procedure promotes a safer option than performing all testing performed on the deck. This procedure requires a minimum of two inspectors on the project, one for inspection on the deck and one for testing at the pump. If only one inspector is available, then testing will need to be performed on the deck.
Testing.
a. Acceptance testing air and slump will be performed at the truck, prior to introduction into the pump. If cylinders are to be tested for specification compliance, cast from concrete placed on the deck. Cylinders used for information only may be cast at the truck. There is no requirement to check for loss of slump through the pump.
b. Testing air content on the deck will be considered quality control testing and used to determine the target air content at the truck.
c. Initially, target an air content of 7.5±2%, near the midpoint or higher. Determine air loss through the pump on the deck for the first two trucks and average the results. Use the table below to establish the target air content at the truck, prior to introduction into the concrete pump. Complete adjustments to the target air content within the next two trucks delivered after adjustment.
|
Average Air Loss |
Target Air Content at Truck |
Tolerance |
|
>0.0% to 1.5% |
7.5% |
±2% |
|
>1.5% to 2.5% |
8.5% |
±2% |
|
>2.5% to 3.0% |
9.5% |
-2%/+1.5% |
d. Check loss through the pump at the beginning, middle, and prior to the end of the last third of the bridge deck placement, typically when the pump boom angle become less than 45 degrees. If loss increases more than 1% from previous testing, check again and average results, before adjusting target air content. Use this procedure for all pumps used on the project.
e. Increases in air content are not considered entrained air. Increase in air content is likely due to leaking seals in the pump piping. The increased air is entrapped air and should be eliminated during vibration. Do not adjust target air content at the truck based on an increase on the deck. If concerns with air too high on deck, sample concrete that has been placed on deck after vibration. Air test between 5 and 10% is considered satisfactory. Cast cylinder for hardened air analysis, if there is a discrepancy between test results.
f. Limit air loss through the pump to 3 percent and air content gain to 2 percent or less. If concrete pump continually produces concrete outside this range, adjustments to the pump are required prior to use on another placement or another project.
Transport samples to the place where fresh concrete tests are to be performed and specimens are to be molded. Protect the sample from direct sunlight, wind, rain, and sources of contamination.
Complete test for temperature within five minutes of obtaining the sample. Start tests for slump and air content within five minutes of obtaining the sample. Complete tests as quickly as possible. Start molding specimens for strength tests within 15 minutes of obtaining the sample.