Concrete capping is an essential procedure in the strength testing process of concrete. When preparing a cylindrical concrete test core for a compressive strength test, you must ensure the cylinder or core has a smooth, parallel and uniform bearing surface perpendicular to the applied axial load for the best results. If the cylinder ends are not perfectly uniform and parallel to one another, the measured strength of the concrete becomes weaker due to built-up stress within the cylinder. Concrete capping is the preparation of these cylindrical ends to ensure they can pass compressive strength testing and bear the weight of the appropriate load.
To properly put this vital procedure into practice, you'll need the best equipment for the job. Keep reading to find out the methods of capping concrete and the benefits they offer.
Concrete Capping Methods & Equipment Needed
When looking to perform a compressive strength test on your concrete capping procedure, it's essential to understand the two most popular options for preparing the cylinder ends. Proper preparation of your cylinder ends for concrete capping ensures they have smooth, parallel bearing surfaces to provide a uniform distribution of forces during testing. The two methods of capping concrete used most frequently are sulfur capping and neoprene pad capping.
Sulfur Capping Concrete Cylinders
Concrete capping of cylinders and drilled concrete cores with a sulfur capping compound is an excellent method of preparing your concrete for compressive strength testing. This method of capping concrete is widely recognized as being highly accurate, making it one of the most popular options. You'll be required to invest in all the equipment needed at first, but you'll only need to buy the sulfur capping compound for jobs after your initial purchases.
The ASTM C617 (the standard practice for capping cylindrical concrete specimens) notes that sulfur mortar is best used for testing specimens with more significant compressive strengths than its own – which you can check manually. When you use this method of concrete capping, it's important that you periodically review the concrete for voids by tapping the hardened caps with the edge of a coin and listening to the sound it produces. Capping concrete with this method means it must cure for at least two hours before testing, or 16 hours if the design strength is 5,000psi or greater. The equipment you'll need for this concrete capping procedure includes:
- Sulfur Capping Compound: Sulfur capping compound is sold as ultra-thin, low-odor flakes, carefully formulated from sulfur and mineral filler.
- Melting Pot: A melting pot is used to prepare the concrete capping compound.
- Capping Ladle: A capping ladle is used for transferring the molten capping compound.
- Cylinder Capper: Cylinder cappers are used to position cylinders properly for insertion into the compound and during hardening.
- Capping Thermometer: It's essential to use a capping thermometer to ensure the temperature of the compound is at proper levels.
- Gloves & Eye Protection: Used to promote safe handling during the capping procedure.
Neoprene Pads for Capping Concrete Cylinders
The other method of concrete capping is the use of neoprene pads. For capping concrete cylinders, neoprene pads are best for tests between 1,500 and 7,000psi. You can also use them for concrete strengths of up to 12,000psi with proper verification. Neoprene pads for capping concrete spread the compressive force evenly across the concrete's surface, similar to the sulfur capping method. Neoprene pads are the more cost-effective method because you can use a single set of pads up to 100 times before requiring replacement. They also provide a faster turnaround and can be tested immediately rather than waiting for a specified curing time. You must match the durometer hardness of neoprene pads to the strength range of the concrete for proper compressive testing. The equipment you'll need for this concrete capping method include:
- Neoprene Pads: You can buy neoprene pads in sets based on durometer hardness.
- Retainer Rings: Retainer rings are placed on both ends of concrete cylinders and the confine pads.
- Polysaccharide Powder: Polysaccharide powder is used to promote the quick release of neoprene pads from the retainer rings of unbonded capping sets for easy pad change out.
- Gloves: To promote safe handling during testing.
Whether you use the sulfur capping method or neoprene pads for capping concrete cylinders, you're guaranteed to have a smooth, parallel bearing surface for uniform distribution of forces during your testing.
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