ASTM C31 - Making and Curing Concrete Test Specimens in the Field
Significance and Use of ASTM C31
Concrete cylinder samples are made for your lab technician to break in a hydraulic press machine and find the compressive strength of the concrete that is being poured for a specific part of a structure. You will get a representative sample of freshly mixed concrete from the truck or mixer, perform slump, temperature, and air content tests on it, and then make cylinders that will be taken back to the lab. These samples provide quality control for architects and engineers and allow them to determine if they will be able to put that concrete into service for their structure, or if they need to remove what they have poured. While you are out there making cylinders you can keep lookout for errors in the mixing and pouring of the concrete; field techs are expected to report things that go wrong to the site superintendent. The curing process will also help those in charge of the site determine how to best protect the concrete in the structure as it cures and gives them a set of limits for form and shoring removal time.
Equipment Required for Making Concrete Cylinders
Rods - 6x12 cylinders need the large tamping rod and 4x8 cylinders need the small tamping rod. Please make sure your rods are clean and have a smooth hemispherical tip before use, as specified in ASTM C143. Rodding is only used if the slump is greater or equal to one inch; if the mix is very dry and the slump comes out to less than an inch you will have to vibrate the concrete to consolidate it thoroughly.
Cylinder molds – These can be plastic or steel, but we typically use plastic molds. If steel molds are used, mineral oil will also be needed to lightly coat the inside surface of the mold and keep the concrete from sticking to the mold. Whichever mold you are using, they will need to be watertight and resistant to damage from tamping, as specified in ASTM C470. Disposable molds are to be calibrated every time a new lot arrives. I would advise you to carry an extra set of each size of mold to every job, because you never know if you might be asked to make another set.
Large Scoop – Should be clean and big enough so that you get a representative sample with each scoop, but small enough to not pour concrete out on the ground when you’re pouring into the mold. Excessive concrete buildup can be removed from metal scoops and rods by soaking them in vinegar overnight, then washing them off with soap and water and coating them with mineral oil to protect the galvanization.
Strike-off Bar – Must be clean and level and must have a flat and even (plane) surface.
Wheelbarrow – Should be able to hold at least 1 cubic foot of concrete, the minimum size required for concrete strength testing specimens.
Bucket of Water and Rag – Please clean off your equipment after each use to help keep it in spec. You can typically get water from a hose on the side of the concrete truck, or you could possibly bring your own water in a sealed container. It is very important for the integrity of any test to have clean, working equipment.
Vibrator (optional) – Should be internal and operate at a frequency of at least 7000 vibrations per minute. Make sure that the diameter of a round vibrator is no more than ¼ of the diameter of the cylinder mold, and make sure that the length of the vibrating element is more than 3 inches longer than the section of the cylinder that you are vibrating. If you’re using one that is not round, the perimeter of it needs to be equal to the circumference of a round vibrator of similar size.
2-inch sieve (optional) - If the biggest chunk of aggregate you can see in the concrete is larger than 2 inches wide, you will need to wet sieve the portion of the concrete you are using through a 2-in. sieve before you can put it in a 6x12 cylinder mold. The cylinder diameter needs to be at least 3 times as wide as the largest piece of aggregate (the nominal aggregate size), so we only make 4x8s when the nominal aggregate size is less than 1.25 inches wide.
ASTM C31 Procedure
Sample the concrete in accordance with ASTM C172, making sure to mix your concrete sample thoroughly. The ultimate goal of the sampling process is to have a sample that is an average representation of what is going into the pour, to get the most accurate strength value in the lab.
When you sample the concrete, try to get your sample from the middle of the batch and not the first or last clumps of concrete, because the first or last portions may come out of the mixer slightly different from the rest of the batch.
Ask if all water has been added before you get your sample, and also make sure you get the batch ticket from the driver of the truck, because you will need it later on when you are entering the sample into your computer system or otherwise filling out paperwork.
If you are making multiple sets of cylinders with more than one truck, write the number on the batch ticket and on your paperwork so you know which truck goes with which set of cylinders.
If you are also doing slump, temperature, and air content on this sample, note that you must have these cylinders made within 15 minutes of receiving the sample, so plan accordingly and set up your work area beforehand (ASTM C172 Paragraph 4.1.2). It helps to group your equipment by test, so you have everything you need for each test in one place.
When you are picking a place to make your cylinders, make sure it is in a level area that is free of debris, away from vibration, and away from moving vehicles and large construction equipment.
Be sure to write on your paperwork which location you are making or leaving your cylinders at, because someone else may be the one to come pick them up.
For rodding, see 3A. For vibration, see 3B.
If you are rodding the cylinders to consolidate them, the amount of layers will vary by sample size: 4x8 cylinders require 2 equally sized layers, and 6x12 cylinders require 3 equally sized layers.
All cylinders in a set should be started at the same time and have their layers added at the same time (you do not add three layers for one and then move on to the next cylinder, they must all have their first layer added and rodded before you move on to layer 2).
For both sizes, each layer will be rodded 25 times in an even pattern, distributing the holes uniformly over the cross-section of the mold. The bottom layer must be penetrated to the bottom of the mold, and for the layers above it you must go through that layer and about an inch into the layer below it.
It’s especially important to rod the concrete vigorously when you’re dealing with a dryer mix, as the air will be difficult to get out and you do not want large air bubbles in the concrete affecting the breaks.
When you are doing the last layer, if your mold is underfilled you can add extra concrete to keep it properly filled as you rod.
After you rod each layer you need to tap the outside of your mold with an open hand about 10 to 15 times (plastic molds may be damaged if you use a mallet) to let excess air out of the cylinder and close the holes from rodding (ASTM C31 Paragraph 9.4.1).
Number of Layers
Number of Layers
If you are vibrating the cylinders to consolidate them, you will always have 2 equal layers to put into the mold, but for 4x8s you insert the vibrator one time per layer and for 6x12s you insert it twice per layer.
The duration of the vibration will depend on the concrete’s slump: typically no more than 5 seconds is needed for each insertion for concrete with a slump greater than 3, and no more than 10 seconds is needed for concrete with a slump less than 3.
You will know when the vibration is done when the surface of the concrete is relatively smooth and air bubbles have stopped coming up to the top surface.
When you insert the vibrator don’t allow it to touch the bottom or sides of the cylinder mold, and when you pull it out go slowly so that no air pockets are left behind.
When placing the final layer, do not fill it more than ¼ in. over the top of the mold (ASTM C31 Paragraph 9.4.2).
Number of Layers
Number of Vibrator Insertions
After you have consolidated the concrete, you will need to strike off the concrete to provide a smooth surface.
Use a back-and forth left-to-right sawing motion, starting in the middle and working away from you, then come back to the middle and move the strike-off bar towards you, still performing the same sawing motion.
You are trying to produce a flat even surface that is level with the rim of the mold and doesn’t have any holes or indents larger than 1/8 inch (ASTM C31 Paragraph 9.5).
Use your rag to clean around the rim of the mold, wiping away excess concrete from the edge but never touching the material inside the rim.
Label each cylinder mold with the project number, the date and time you made the cylinder, and the sample number.
Cover each cylinder with an appropriate sized cylinder bag and place it in a protected location free of vibration and disturbances.
Transportation and Curing Procedures
Cylinders need to be picked up anywhere from 18 to 48 hours after they are made. Don't leave them out there for a week and forget about them or your lab tech is going to be very confused when they look for cylinders to break and find an empty moisture room. They need time to cure under moist temperature-controlled conditions and they won't get that exposed to the elements.
When transporting cylinders, you will want to make sure they don't bounce around in the truck or fly out, so you should have some cushioned transport racks to hold them in. Keep them away from the sun as well, because it can affect their temperature and change the way they cure. Drive carefully, these are the only samples you've got.
Once you're back at the office, get the cylinders off of your truck and out of the transport racks, removing their bags. Set them on the floor or on the table, whichever you prefer, and grab a stripping tool (looks like a t-shaped metal bar) and a rubber mallet. To strip a cylinder, insert the stripping tool into the top of the mold in the gap between the concrete and the mold, and hammer it all the way to the bottom of the mold. You’ll probably have to do this at least twice to get the cylinder out. You could also use an air compressor to remove the mold quickly, using something to create a small hole at the bottom of the mold and blowing through that. Once the cylinder is out you will need to label it and check it in through whatever system you use (we use Metafield).
After the cylinder has been picked up, checked in, and carefully labelled (make sure you have the right sample number and date!), you will put it in the moisture room to cure it. Ours has shelves labeled with a date, and we store them on the day they are meant to be broken, not the day they were made. Typically for 6x12s we break 2 at the seven day mark and 2 at the 28 day mark and leave a spare cylinder to break on the 56 day mark if the cylinders have still failed to meet the required strength. Once they are in the moisture room you are done.
ASTM C31 Demonstration
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