ASTM C617: Capping Concrete Cylinders With Sulfur Mortar
Significance and Use of ASTM C617
Capping concrete cylinders and grout prisms with sulfur mortar helps to give them a plane, level surface, so that force is applied evenly to the entire end surface when the specimen is broken during the ASTM C39 procedure to test compressive strength. Capping will help you to get more accurate results for the strength of your specimen than if you were to break it uncapped.
Sulfur mortar is a good material to use for capping because it melts quickly and cools even more quickly, and it is bonded to the specimen itself so your caps will not move or slide around during the breaking process.
To cap a specimen with sulfur, you will melt sulfur in a pot and then pour the sulfur onto a plate with a recessed area that is the shape of your specimen but a bit larger in diameter. Then you dip the specimen into the molten sulfur and hold it there, making sure it’s vertical. The molten sulfur will cool around the end of your sample and form a cap with an even surface, covering any jagged edges or points on the end of your specimen.
Equipment Needed For Capping With Sulfur
- Sulfur Melting Pot - must be capable of heating the sulfur to a temperature of 265-290 degrees Fahrenheit. The melting pot must also be equipped with automatic temperature controls, and must be made of a metal that won't react with molten sulfur. It is recommended that you use a melting pot with peripheral heating; otherwise, accidents may happen if you are reheating a cooled sulfur mixture that has crusted over on the surface. This can also be avoided by using a metal rod or ladle to contact the bottom of the pot and stick out over the surface of the molten sulfur. This conducts heat to the top of the rod, causing a ring around the rod to melt first and relieve pressure that is building up at the bottom of the mixture.
- Fume hood - a fume hood is necessary for safety, as heating sulfur over an open flame is very dangerous in a couple of ways. The flash point of sulfur is around 405 degrees Fahrenheit, and it can ignite suddenly if it goes over the flash point temperature. If your sulfur mixture starts to burn, covering it will snuff out the flame, and then you should recharge the pot with fresh material after the fire goes out. Also, fume hoods suck out the toxic gases that molten sulfur can produce, such as hydrogen sulfide. Make sure your capping area is well ventilated, as hydrogen sulfide gas can be lethal in high concentrations, and in lower concentrations it will make you nauseous, dizzy, cause stomach issues, give you a headache, and cause eye irritation. It is a colorless gas and smells like rotten eggs, but your sensitivity to the odor may disappear with exposure, so take frequent breaks in fresh air while you cap.
- Thermometer - must be able to measure the sulfur's temperature accurately to the nearest degree, going up to at least 350 degrees Fahrenheit, and be made of a metal that is not reactive with molten sulfur.
- Capping plate - must be made of metal or stone that won’t react with sulfur and must be plane to 0.002 inches over at least a 6 inch area. The surface of your capping plate must be smooth and have minimal gouges, grooves, and indentations that could create a bumpy surface on your caps. Gouges, grooves, and indentations should be less than 0.01 inches deep and less than 0.05 square inches in surface area, or the plate will need to be resurfaced or replaced. In all cases, capping plates should be at least 1 inch wider than whatever you are capping. If your plate has a recessed area to pour sulfur into, that area should not be deeper than 1/2 an inch.
- Alignment device - cylinders can be aligned either with a guide bar attached to the capping plate, or aligned with a bubble level placed on top of the cylinder. If you are using a guide bar, it has to be located so that no cap will be off centered on a test specimen by more than 1/16th of an inch. Also, no cap should depart from perpendicularity to the axis of the specimen by more than 0.5 degrees.
- Ladle - must be made of a metal that won't react with the sulfur and large enough to get a big scoop of sulfur that is not easily spilled on the floor.
- Safety gear - Working with sulfur can be dangerous. It is important to wear the proper safety gear to protect you from burns: gloves, an apron, eye protection, and arm protectors, too.
ASTM C617 Procedure
1. Prepare your sulfur mortar mixture for use by heating it to a temperature between 265 to 290 degrees Fahrenheit. For our melting pot, this takes about 90 minutes, but this could vary depending on what kind of pot you have. Every hour or so, you should check the temperature of the sulfur to make sure it is getting into range and staying around that temperature while you cap.
Note: you should make sure you empty the pot and recharge it with fresh material every 5 times you cap. If you are capping cylinders with a strength of 5000 psi or greater, you cannot reuse old material, so put in some new sulfur if you are capping something with a high specified strength.
2. When the sulfur is properly heated, you should check the ends of the specimen and make sure they are dry, because water will make sulfur foam. Similarly, do not throw damp sulfur mortar into the melting pot, because it will foam and steam, reducing the strength of your caps and causing a safety issue.
3. Oil the surface of your capping plate with mineral oil in order to keep the sulfur from sticking to your capping plate. Before you do any actual capping, you will want to warm the capping plate up, so stir the molten sulfur with your ladle, and then pour some onto your capping plate. Once it has cooled, remove it, and now you are ready to cap.
4. Oil your capping plate again, and then stir the molten sulfur. You will need to stir before every cap poured. Next, pour the sulfur into the recessed area of your capping plate. When pouring, keep the ladle away from your body and hold it steady. You will want your pot within reaching distance of your capping plate, so that there is minimal risk of spilling the sulfur. Pour slowly to avoid splashing; sulfur burns are extremely painful. Read your Material Safety Data Sheet to learn how to treat injuries before they happen.
5. Get your alignment device (we typically use a bubble level), and put it on the center of the cylinder's end. Gently lower the cylinder into the sulfur, so that the bubble is in the middle of the bubble level, or if using an alignment bar, keep contact with the alignment bar as you lower the cylinder. You will want to do this relatively quickly, as molten sulfur hardens and cools fast.
6. Hold the cylinder still until it does not shift, and let the cap cool.
7. Remove the capped cylinder from the capping plate with a hammer, and brush away any pieces of sulfur. Repeat the previous steps to cap the other end.
8. Check your caps for planeness with a 0.002 inch feeler gauge and straightedge, taking at least 3 measurements on different diameters. Also, check for hollow areas under your cap by bouncing a quarter off of the surface. If you hear a light "ding!" sound, your cap is solidly bonded to the cylinder, but if you hear a low "thunk!" sound, there is a hollow area under the cap and you will need to remove the cap and try again.
9. Once both ends of the specimen are capped, put it in the moisture room for at least 2 hours or wrap it with a double layer of wet burlap. It is not recommended to immerse the capped sample in water. Remove the cylinder when you are ready to test.
A Video of How Sulfur Capping Works
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