Concrete Cylinder Capping

• Irregular surfaces will adversely affect the strength of the specimen, leading to an inaccurate representation of the in-place concrete's strength.
• ASTM Practice C1077 necessitates certification for laboratory technicians involved in concrete cylinder capping.
• Certification as an ACI Concrete Strength Testing Technician meets this requirement.

Preparation

Personal Protection Equipment (PPE)

• Safety glasses or face shield
• Heat-resistant gloves and arm protection
• Apron or lab coat
• Steel-toed boots or shoes

Safety

• Fresh sulfur added to the pot must be dry to prevent foaming.
• Keep water away when heating or when sulfur is molten to avoid accidents.
• Never heat a melting pot over an open flame to avoid overheating and ignition of the sulfur.
• Hydrogen sulfide gas can be produced during capping if sulfur is contaminated with organic materials, being colorless and potentially lethal at high concentrations.
• Lower concentrations of hydrogen sulfide gas can cause nausea, stomach distress, dizziness, headaches, and eye irritation.
• The smell of hydrogen sulfide, similar to rotten eggs, is not a reliable indicator of its presence due to rapid desensitization.
• The use of a sulfur pot under a hood with an exhaust fan is important.

Equipment

• Technicians must confirm their supervisor/employer provides equipment compliant with the governing Standard for the procedure.
• Understand equipment requirements to identify when equipment may yield inaccurate results or invalidate tests.
• Capping plates for neat cement paste or high-strength gypsum paste caps can be made from glass, metal, granite, or diabase.
• Glass plates need to be at least 1/4 in. [6 mm] thick, metal plates need to be 0.45 in. [11 mm] thick, and granite or diabase plates need to be at least 3 in. [75 mm] thick.
• The plate must also be at least 1 in. [25 mm] larger than the cylinder and plane.
• Capping plates for sulfur mortar need to be metal, granite, or diabase, not glass.
• Metal plates need to be 0.45 in. [11 mm] thick, and granite or diabase plates need to be at least 3 in. [75 mm] thick.
• The plate needs to be at least 1 in. [25 mm] larger than the cylinder and plane to 0.002 in. [0.05 mm] in over 6 in. [150 mm].
• Capping plates need a smooth surface without gouges, grooves, or indentations. Plates for sulfur capping need a recessed area to hold sulfur no more than 1/2 in. [12 mm] deep.
• Alignment devices are required to keep caps perpendicular to the cylinder's axis within 0.5 degrees.
• Bulls-eye levels or other types of levels can be used when capping with gypsum to meet alignment device requirements.
• A melting pot with automatic temperature controls to melt and heat sulfur mortar is needed
• The pot must be metal or lined with a non-reactive material with sulfur.
• Pots should use peripheral heating to avoid pressure build-up under the sulfur's surface during reheating of cooled sulfur, and should never be heated over an open flame.
• The sulfur pot needs to be under a hood with a functional exhaust fan for ventilation, as capping outdoors without a hood is unacceptable.
• Metal (brass or stainless steel) cube molds produce 2 in. [50 mm] cubes used to test the capping material's strength.
• Metal cover plates are also needed to make sulfur cubes, along with a tamper and trowel for neat cement or gypsum cubes.
• Accessories like a straight edge and a 0.002 in. [0.05 mm] feeler gauges ensure cap planeness, and coins or metal washers can be used to check caps for hollow areas.
• 1/8 in. [3 mm] diameter metal rods or nails with a carpenter square can be used to check resulting cap perpendicularity on a 12 in. [300 mm] tall cylinder.

Capping Materials

• Three capping materials: neat cement paste, high-strength gypsum, and sulfur mortar.
• Capping materials for cylinders with an expected strength of 500 to 7,000 psi [3.5 to 50 MPa] needs a minimum strength of 5,000 psi [35 MPa] or the cylinder strength, whichever is greater.
• Sulfur mortar and gypsum used to cap cylinders above 7,000 psi [50 MPa] needs to be as strong as the cylinder or be qualified for high-strength concrete.
• Qualified sulfur mortar and gypsum have documentation that is acceptable for use by the manufacturer or user
• Make 30 cylinders, cap 15 with the to-be-qualified material, either cap the other 15 with neat cement paste and test or grind their ends plane to 0.002 in.
• Compare the average results from the sets of 15.
• For the material to be considered qualified, the average strength of the cylinders capped with the material being tested needs to be at least 98% of the average strength of the companion cylinders.
• The standard deviation of the strengths of the capped cylinders cannot be greater than 1.57 times the standard deviation of the companion cylinders.
• Records need to indicate the curing time used for caps and that cap thickness requirements were met during qualification testing.
• Qualified capping material can be used on cylinders with strengths up to 20% greater than the concrete tested in qualification tests.
• All capping materials are tested for compressive strength using 2 in. [50 mm] cubes every three months and on a new lot.
• Tests should be thoroughly documented.
• Neat cement paste consists of hydraulic cement and water, being the only capping material for freshly molded cylinders.
• The caps may shrink and crack if they are used on dry cylinders.
• The strength of neat cement paste depends on the water-cement ratio, the age of the cap, and the type of cement used, typically Types I, II, or III.
• The compressive strength of 2 in. [50 mm] cubes made of neat cement paste needs to be determined using ASTM Test Method C109/C109M.
• High-strength gypsum cement paste isn't as common for capping as sulfur, but some labs prefer it for safety.
• Low-strength molding plaster and Plaster of paris are not adequate substitutes for high-strength gypsum.
• High-strength gypsum cannot be cut or mixed with any material
• Water-gypsum cement ratio must be between 0.26 and 0.30.
• A lower ratio will yield higher strength but less workability, while a higher ratio will yield the opposite.
• Water-cement ratio and curing time should meet laboratory needs to achieve the standard of testing 2 in. [50 mm] cubes per ASTM Test Method C109/C109M.
• Sulfur mortar can be reused for cylinders tested under 5,000 psi [35 MPa], after being broken up and cleared of concrete and foreign material.
• It is required to refill or empty the pot if the oldest material isn't being used at least five times.
• Unlike gypsum and neat cement paste, 2 in. [50 mm] sulfur cubes aren't molded per ASTM Test Method C109/C109M, but they require a cover plate.
• Ensure the sulfur mortar in the pot is between 265 to 290°F [130 to 145°C] and stirred prior cube forming.
• Before forming the cubes, coat interior surfaces of the cube mold and cover plate with a release agent.
• Sulfur shrinks a lot, so the cubes must be topped off after approximately 15 minutes to prevent voids.
• Test the cubes per ASTM Test Method C109/C109M for compressive strength at the desired age.

Procedure

Specimens and Preparation

• Specimens that can be capped are either hardened concrete specimens or freshly molded cylinders.
• Hardened concrete specimens can consist of 6x12 in. [150 x 300 mm] and 4 by 8 in. [100 by 200 mm] concrete cylinders, concrete cores, grout prisms, and others.
• Prior to capping, the ends of the cylinders should be prepared and checked to make sure that the ends are dry enough to avoid formation of steam or foam pockets under or in the cap larger than 1/4 in. [6 mm] when capping with sulfur.
• Use a rub brick or brush to remove any waxy or oily materials.
• Check the ends of the cylinder to ensure difference in height over the cylinder's surface by using a carpenter square across the top and down the side of the cylinder.
• Differences in height over the cylinder's surface should be less than 1/8 in [3 mm] to avoid inconsistencies within the cap
• A 1/8 in. [3 mm] diameter metal rod or nail can act as a go/no-go gauge.
• Cylinders with careless ends such as concave or convex ends should be saw cut first.
• Prior to capping, inspect the surface of the capping plate for any gouges, grooves, and indentations.

Capping Freshly Molded Cylinders

• Freshly molded concrete cylinders must only be capped with neat cement paste, as gypsum will deteriorate due to contact with water.
• Leave the concrete 2-4 hours to settle in the mold before using neat cement paste on the end.
• Cement paste needs to be mixed 2-4 hours before use to settle and not bleed after application.
• To reach the target water-cement ratio, small amounts of water can be added to the paste for retempering.
• The paste must be remixed to create optimum consistency
• Table 1.2 shows guidelines for achieving optimum capping consistency for different types of cement.

Capping Hardened Cylinders

• Hardened cylinders can be capped with sulfur mortar, high-strength gypsum, or neat cement paste.
• Neat cement caps must only be used on moist cured hardened cylinders to prevent caps from shrinking and cracking if dried.
• Cylinders capped with high-strength gypsum cannot be stored in water storage tanks or a moist room for more than 4 hours.

Capping Hardened Cylinders with Sulfur

• Ensure that the capping plate is warm before capping and heat the sulfur to between 265 and 290 °F [130 to 145 °C] .
• Verify the temperature with a metal thermometer due to the inaccuracy of temperature dial and continue to check every hour.
• The melting pot temperature should be verified with a metal thermometer if the compound is not fluid enough or if fuming occurs at the recommended setting.
• To get the correct thickness from the resulting caps, capping plates need to be warm so that the sulfur hardens more slowly.
• The plate can be warmed in either a laboratory oven or by pouring a ladel full of sulfur in the oiled capping plate to cool.

Cap Checks

• Checking caps is a very important step
• Irregular patterns cause stress in the cylinder that results in a break
• Check the planeness of a minimum of three caps per day using a straight edge and 0.002 in [0.05 mm] feeler gauge
• Use a metal washer to easily locate hollow sounds.
• Caps with hollow areas need to be completely removed and recapped.
• The thickness can only be measured after the cylinder is broken.
• Use a caliper or micrometer to measure 6 pieces from the top of the cylinder
• Record the thickness and cap thicknesses to ensure that they follow the conditions in table 1.1.

Protecting and Storing Specimens After Capping

• Sulfur-capped specimens need to be wrapped, placed back into a moist storage room, or into water storage tanks.
• Gypsum-capped cylinders need to be protected to allow to cure and cannot be contained in a water storage tank for more than 4 hours.

Reporting

• A report is not required as a result of capping cylinders.
• The daily cap checks need to be documented.
• Results need to record the checks for hollow areas and average thickness for each cap.
• Documentation needs to be stored in a daily cap check record.
• The ACI performance examination has a summary and checklist for key steps in this type of practice.
• Inspect the capping plate for gauges, grooves, or indentations.

Form Cap on Cylinder

• Do not apply the end or paste until the concrete has ceased to settle in molds with freshly molded cylinders.
• Clean specimens using free water and laitance.
• Gently push a freshly oiled capping plate onto the ridge until the plate matches the mold's rim
• Use layer burlap and a polyethylene sheet wrapped to stop drying of the layer
• To remove high strength material make sure to remove all material.
• If using sulfur mortar, prepare at 265 and 290 °F [130 to 145 °C].
• Remove any material.
• If a point exceeds 1/8 in correct it.

Extra

• Ensure the ends of all specimens contain excess moisture.
• The device and all plates need to be warm to touch.
• Apply oil to all plates Stir the mortar before each pour.
• Form cap on a specimen.
• Using a straight edge and feeler gauge make sure there are no hollow areas.
• To prevent drying protect and cover all specimens.