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. 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 to ground level for ease of sampling, as it may modify the air
content of the concrete being sampled. 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.
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.