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CAPPING CYLINDERS. The
ends of compression- test
specimens must be planed within
0.002 in. and within 0.5 degrees of being
perpendicular to the axis of the cylinder.
Specimens formed in strong metal molds
having accurately flat baseplates can be capped
with neat cement at 2 to 4 hr after molding. A
stiff paste of portland cement and water is made
at the time the cylinder is molded so that the
capping mixture will have shrunk before application.
Any free water or laitance (layer of fine
particles on the surface) is removed from the end
of the specimen. The paste is applied to the top
of the concrete and worked with a flat plate until
it is smooth and level with the top of the mold.
Hardened concrete specimens may be ground
to place ends or capped with a material having
greater compressive strength than the concrete.
Prepared mixtures of sulfur and granular materials,
special high-strength gypsum plasters, and
neat high-early strength cement are satisfactory
capping materials (ordinary low-strength plaster
of paris, compressible rubber, or fibrous materials
are not suitable for caps). You should apply these
materials in a plastic state and finish them to the
desired plane surface by applying glass or metal
Table 15-6.-Number of Roddings to be Used in Molding Cylinder Specimens plates and squeezing out excess material to provide a cap that is as thin as possible. Sulfur caps may be applied in time to harden for at least 2 hr before testing. Plaster caps cannot be stored over 4 hr in a moist room. Neat cement caps must be aged 6 days or more in a moist room (2 days when Type II cement is used). During capping, protect moist, cured specimens against drying by covering them with wet burlap.Beam Specimens Tests are performed on concrete beam specimens to evaluate the flexural strength of the concrete. The standard beam specimen is 6 in. by 6 in. by 21 in. (152 mm by 152 mm by 532 mm) for concrete in which the maximum size of the coarse aggregate is 2 in. (50 mm). When the maximum size of the coarse aggregate exceeds 2 in. (50 mm), the smaller cross-sectional dimension is to be increased to at least three times the nominal maximum size of the coarse aggregate. All beam specimens prepared in the field are to be at least 6 in. wide and 6 in. deep unless required other-wise by project specifications.STANDARDS FOR BEAM MOLDS. The beam molds are to be smooth on all interior sur-faces and free from warpage. The molds are to produce specimens that do not exceed the required cross-sectional dimensions by 1/8 in. The length of the specimens is not to be more than 1/16 in. shorter than the specified length, but it may exceed that length.RODDING. Place the concrete in the mold in the required number of layers. (See table 15-5.) Rod the bottom layer throughout, distributing the strokes uniformly over the cross section of the mold. When rodding the upper layers, allow the rod to penetrate the previous layer 1/2 in., providing the previous layer is 4 in. or less and 1 in. if the previous layer is greater than 4 in. The number of strokes per layer is one for each 2 in.2 (13 cm2) of the top surface area of the specimen. After each layer is rodded, spade the concrete with a trowel along the sides of the mold to help in the removal of surface voids. Strike off the top surface with a straightedge, and finish it with a wooden float.CURING. YOU should cure the beam specimens in the same manner as the cylinder specimens with the following exceptions: (1) ex-tend the initial curing period to 48 hr 4 hr and (2) do not allow the surface of the beam specimen to become dry between the time of removal from curing and the completion of testing. When transporting specimens from the field to the laboratory, be sure they are sufficiently cushioned to protect them from damage by jarring. Additional measures are required to prevent damage by freezing temperatures and moisture loss. You can prevent moisture loss by covering the specimens with plastic or surrounding them by wet sand or wet sawdust. |
 
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