ASTM Standard Test Method C39: Compressive Strength of Concrete Cylinders
Significance and Use of ASTM C39
Concrete's compressive strength determines whether the concrete placed in a structure can bear the weight of what's on top of it, or if it will splinter into a million pieces and cause the structure to collapse. It is highly important for engineers to know how strong concrete is, and so construction materials testing companies send their field technicians to various construction sites to make cylindrical samples out of the same concrete that is being poured (read ASTM C31 to learn how cylinders are made).
Back at the lab, these samples are cured in a temperature-controlled moisture room with a constant fog spray, and on certain days a couple of samples from that set are loaded to their breaking point with a hydraulic press machine. There is usually a 7 day break and a 28 day break, and if something fails to meet strength a spare sample is set aside for a 56 day break. This way, you have a record of how the concrete gained strength over that period of time, and you may be able to pinpoint problems in the making or curing of the concrete or in the mix itself.
Concrete strength is highly variable and can change with many factors, including the size and shape and condition of the cylinder, the way it was batched and mixed and transported from the concrete plant to the job site, the way it was molded in the field, and the temperature and moisture conditions during the curing process. Lightweight concrete will differ in mix design and strength compared to regular concrete, and smaller samples may be able to handle less load than larger ones.
Engineers can use strength test results to see if the concrete being poured fits with what it is being used for and meets their specification's requirements. These results are their quality control for the whole process of pouring concrete from batching to placement. Strength test information can also help them figure out if admixtures put into the concrete mix at the job site are effective.
Technicians who test these cylinders must be appropriately trained and certified. ASTM C1077 requires that an examiner not related to your company must see you demonstrate this test in order to be qualified to do it. The ACI Lab Technician Certification Course will serve this purpose for lab techs in America.
Equipment for Concrete Strength Testing
To break cylinders, you will need several pieces of equipment.
- Testing Machine - The testing machine is powered by hydraulic fluid, and uses a piston to lift the lower bearing block and push the cylinder into the upper bearing block, loading the cylinder with increasing weight until it ruptures. It is typically operated by a lever or several buttons to retract, hold, or advance the lower bearing block, and its results may be reported by a dial gauge or a digital readout. This is a sensitive piece of equipment and it must regularly be calibrated and maintained. ASTM C39 section 6 goes more into depth about the specifications of the machine's individual parts.
- Calipers or Ruler - Measuring the diameter of each cylinder is vital to the test results, as you will need to calculate the area of the cylinder to find the strength. Keeping a daily record of your cylinder diameters is recommended. No individual diameters on the same cylinder can vary by more than 2%, or the sample is invalid.
- Carpenter's Square - These are useful to check the perpendicularity of the cylinder's axis, making sure that the cylinder does not depart from perpendicularity by more than 0.5 degrees. It helps to get one that comes with a bubble level.
- Straight Edge, 1/8 inch nail and 1/5 inch nail - This is used to check the planeness of the ends of the cylinder. You put the straightedge across the end of the cylinder, and poke the nail at it to see if it goes underneath. The 1/8 inch nail is used if capping with ASTM C617, and the 1/5 inch nail is used for unbonded caps (ASTM C1231).
- Cylinder Wraps - This is safety equipment, and also helps to keep the testing machine and its surrounding area clean. They are rectangular pieces of canvas with velcro on the ends that wrap around the cylinder and keep concrete fragments contained, protecting the machine operator from sudden ruptures shooting concrete everywhere.
- Retaining Rings - If you are using unbonded caps, these contain neoprene pads that help absorb the shock on the cylinder as it breaks, and go over the ends of the cylinder. Make sure they are level when you place them on. If you work in a lab where these are exposed to the elements and you don't want them to rust, regularly clean them with a wire brush and some WD-40. You can learn more about unbonded caps in ASTM C1231.
- Sulfur capping equipment - This equipment consists of sulfur mortar, a sulfur pot apparatus to melt the mortar in, capping plates, spoons, and various other items. Refer to ASTM C617 to learn more about the capping procedure.
- Spacers - Break machines are typically built to break 6x12 cylinders, so if you have smaller samples you will need to put something in there for them to sit on, kind of like a booster seat for a small child. Typically these are made of steel or some other strong material, and are cylindrical in shape, but a little wider than the diameter of the cylinders that sit on them.
- Brush and dustpan - Keeping the bearing surface of the testing machine clean and clear of debris is very important, because it needs to be plane and level for each cylinder to break properly. It is recommended that you sweep it clean after each break.
- Wheelbarrow - A wheelbarrow can be used to hold broken samples to throw them away after you are done testing. Don't let it get too full or you might spill it and leave concrete fragments all over the lab that will take forever to clean up.
- Safety goggles - Wear eye protection, as this can get messy!
ASTM C39 Procedure
1. Bring the cylinders out of the moisture room, keeping them covered with wet burlap to keep them moist. Check the cylinders over for defects (holes, cracks, crumbliness) as you set them on the table, use your straight edge and nail to check for planeness, and set the ones with ends that are not plane aside to be saw cut. You will want to look at the perpendicularity of the cylinder as well, to make sure it does not depart from a vertical axis by more than half a degree. If you want to break cylinders uncapped, they must be plane within 0.002 inches. Most cylinders don't meet this requirement, so you'll want to either cap them with sulfur or gypsum paste (ASTM C17), or unbonded neoprene caps (ASTM C1231).
2. Measure the diameter of each cylinder twice, in the center of each cylinder at 90 degree angles. Make sure that your two diameters are not off from each other by more than two percent, or a test on that cylinder would be considered invalid. With the average diameter, calculate the surface area of each cylinder:
Diameter / 2 = Radius
Area of the cylinder face = Pi * Radius * Radius
3. Make sure that the bearing surfaces of the machine are clean and free of debris, and if you are using unbonded caps, check the cleanliness of your neoprene caps. You should have a record at your break station of the number of cylinders that have been broken on those particular caps. Discard the caps and put new one in the retaining rings if there are large cracks or gouges in them, or if you have broken over 100 cylinders on those caps. It is also recommended that you flip the caps at 50 cylinders.
4. Put the neoprene caps on the ends of your cylinder, and check to make sure they fit right and are plane and level. Place the specimen on the lower bearing block (or on a centered spacer, if breaking a 4x8 cylinder) and align it with the upper bearing block, using the rings on the bottom block to center it.
5. Zero out the machine, and then apply a load at full advance until you get to about 10% of the estimated load. A good spot is around 11000 lbs for a 6x12 cylinder breaking at 4000 psi. Remember that psi is load divided by the area, so you could calculate this for any size cylinder and any specified strength. Put the machine on hold and check the cylinder's alignment with your carpenter's square, making sure it doesn't depart from vertical by more than 0.5 degrees. If everything is good, proceed to the next step, but if the cylinder is off center, remove the load and readjust the position of the cylinder.
6. You can now apply load to the cylinder. It is permissible to go faster than the recommended rate of about 28-42 psi/second for the first half of loading. Switch to a metered advance around 50% of the estimated strength of the cylinder. This will look like an increase of 1000 lbs/second for a 6x12 cylinder, and 500 lbs/second for a 4x8 cylinder.
7. Don't mess with the loading rate after the halfway point, as the cylinder approaches its peak load. The cylinder will hit a peak, then drop. If it drops slightly, the load may begin to increase again, so let it go until the load is decreasing steadily and you can see clear evidence of a forming fracture pattern, and then turn the lever back to the off position.
8. Pull the cylinder out of the machine, and then remove the caps. Carry it over to your wheelbarrow and remove the wrap, letting the pieces fall into the wheelbarrow. Determine the type of fracture and then write down the load and the type of fracture. Calculate the strength of the cylinder, reporting it to the nearest 10 psi:
Strength in psi = load in pounds / area in square inches
Cylinder Fracture Types
A Video of the ASTM C39 Procedure
ASTM C39 Quizview quiz statistics
Questions & Answers
What is the highest strength that you have seen a concrete cylinder break at?
We had a cylinder that unexpectedly broke at 7830 psi, when our neoprene pads were supposed to cap out at 7000 psi and the specified strength for that set was only 4000 psi. The force of the break melted the pad caps a little bit! After that, we bought some stronger pad caps, although I haven't had a cylinder break nearly as high since. If breaks are unusually high, you will need to tell the project engineer, because overly high strength concrete tends to fail in a brittle manner, breaking suddenly and quickly.Helpful 5
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