ASTM C172: Standard Practice for Sampling Freshly Mixed Concrete
Significance and Use of ASTM C172
ASTM C172 gives the requirements and procedures for sampling concrete out of different mixers and receptacles used in the making and moving of concrete. It is important to sample concrete in a way that gives you a portion of concrete that is representative of the entire batch, so that the cylinders you make are just like what's being used in the building that you're working on.
There are many different ways to mix and pour concrete, and this guide will teach you how to sample from all types of containers, from small stationary mixers that batch on site to large concrete trucks that pump concrete down a chute. In all cases, the time between receiving the first and last parts of the sample should not go over 15 minutes. Also, if you are making cylinders, you need to start molding them within 15 minutes after you have mixed the composite sample, and your sample should be at least 1 cubic foot in volume. Whatever container you use to collect the concrete must have a nonabsorbent surface.
ASTM C172 Procedure
Sampling From Stationary Mixers
Take two or more portions at regularly spaced intervals during the discharge of the middle portion of the batch, and mix them up into one composite sample with a shovel. No samples should be taken before 10% or after 90% of the batch has been discharged. To sample the concrete, pass the container through the discharge stream, or completely move the discharge stream into the sample container. If the concrete is coming out too quickly to send the stream entirely into the container, discharge the stream into a large container that could hold the entire batch, and then sample from that container. Restricting the flow of concrete out of the mixer could cause the concrete to separate into its components, so do not stop the concrete from flowing out.
Sampling From Paving Mixers
Sample the concrete after the paving mixer has discharged, not during the discharge like with other methods. Take your samples from at least five different portions of the pile, and then mix them up into one composite sample. Try to avoid contaminating the concrete with the subgrade material, especially prolonged contact with subgrade material that can absorb a lot of moisture, like clay. To avoid this, you can place containers on top of the subgrade and catch the concrete in them. In some cases you will have to support the containers so that they don't spill. They must be big enough to hold a representative sample of the concrete.
Sampling From Revolving Drum Truck Mixers
Sample the concrete by collecting two or more portions of concrete at regular intervals during the discharge of the middle part of the batch. Take these samples within a 15 minute period, and mix them up into one composite sample. Do not take your samples until all water has been added to the mixer, and also do not get your samples from the first 10% and last 10% of the batch. You should sample it by either diverting the entire stream into your container, or by passing your container through the stream at regular intervals. The rate of discharge of the batch should match the rate of revolution of the drum.
Sampling From Open Top Containers
For open top containers, you are allowed to take samples of the concrete by any of the previous three procedures. Choose the method that fits the situation the best.
Procedure for Concrete With Large Maximum Size Aggregates
When the concrete you are sampling has aggregate in it that's larger than the appropriate size for the molds or equipment you're using, you will need to run most of your sample through a sieve to take out the extra large aggregate, and perform a unit weight test on a small portion of the concrete with the large aggregate included.
Additional Equipment Required
- A sieve of the suitable size to remove the unwanted large aggregate
- A container of suitable size with a nonabsorbent surface
- A shovel to mix the composite sample
- Rubber gloves
- A concrete scoop that can hold a representative scoop of the concrete
Wet Sieving Procedure
1. After you sample the concrete, pass a scoop of concrete over your sieve. Place only enough concrete on it so that after sieving, the thickness of the layer of aggregate left on top is no more than 1 piece thick.
2. Shake or vibrate the sieve until no undersize material remains on the sieve. The material that falls through should fall into a container of suitable size that has been dampened before use. The surface must be clean, moist, and nonabsorbent.
3. Remove and throw out the aggregate that stays on top of the sieve. You don't have to wipe off the smaller material sticking to the oversize aggregate.
4. Remix the batch of concrete with the shovel as little as possible to where it is uniform, and proceed to testing. For the unit weight procedure, see ASTM C138.
A Video of the ASTM C172 Procedure
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