Concrete specimens that are representative of a distinct batch of concrete must be sampled and analyzed for the purpose of quality control.

Tests are performed on concrete cylinder specimens to evaluate the compressive strength of the concrete. The standard cylindrical specimen is 6 in. in diameter by 12 in. long.

Cylinder molds should be made of steel, cast iron, or other nonabsorbent material that does not react with concrete containing portland cement or other hydraulic cements. Molds should hold their dimensions and shapes under conditions of severe use. They should be able to hold, without leakage, the water poured into them. Before using the molds, coat them lightly with mineral oil or a suitable nonreactive form of release material.

FILLING CYLINDER MOLDS. Place the molds on a level, rigid surface, free of vibration or other disturbances, at a place as near as possible to the location where they are to be stored for the first 24 hr.

Fill the molds with concrete specimens (taken as previously described for the slump test). The number of layers is determined by the mold size

Table 15-5.-Numbers of Layers Required for Specimens

(table 15-5). As you fill a mold, rotate each scoopful of the concrete around the top edge of the mold as the concrete slides from it. This ensures a symmetrical concrete distribution within the mold.

Tamp each layer with the tamping rod, distributing the strokes uniformly over the cross section of the mold and penetrating the underlying layer. Tamp the bottom layer throughout its depth. The number of roddings is determined by the diameter of the cylinder. (See table 15-6.) After tamping the top layer, strike off the surface with a trowel or rod so that the concrete fills the mold exactly. Do not add unrepresentative concrete to an underfilled mold. If voids are left by the tamping rod, tap the sides of the mold lightly with your open hand to close the voids. If desired, cap the top surface of freshly made cylinders with a thin layer of stiff portland cement paste, which you should then permit to harden and cure with the specimens. When finished, move the specimens to the storage place and leave them undisturbed for the initial curing period.

CURING AND STORING CYLINDERS. During the initial curing period of test specimens, be sure to take precautions to prevent the evaporation and loss of water in the specimens. Cover the specimens with a sheet of plastic. You may place wet burlap on top of the plastic to help retard evaporation, but be sure that it does not come in contact with the concrete surface. The exterior of cardboard molds must be protected against the absorption of water or molds may expand, allowing the specimens to be damaged. The test specimens now begin the initial curing period of 24 hr 8 hr. Test specimens maybe transported after the initial curing period, providing they remain in the mold. Upon completion of the initial curing period, remove the specimens from the molds and place them immediately in a moist environment with water maintained on their surface at a temperature of 73.4F 3 (23C 1.7). You can also obtain the required condition by immersing the specimens in saturated limewater or by storing them in a moist room or cabinet. Do not expose test specimens to a flow of running or dripping water.