Unevenness of top surface of concrete specimen results reduction in apparent compressive strength. Any of these three remedial measures can be taken to overcome this problem
Of these three, capping is superior to other options. All means have some limitations of producing scattered results. At first we will go for grinding.
In one word, uneven surface of concrete is grinded to obtain square and plane bearing surface. The grinding is done by abrasive silicon carbide. Grinding offers relatively satisfactory result than that of packing and capping.
Grinding is also relatively expensive than other two measures. It is reported that grinding produces relatively more strength than capping. The reason behind this is elimination of loss of strength involve in capping.
Thus concrete specimen grinded in this way produces identical strength as that of perfect cast surface. So grinding is superior to capping but cost involvement is rather high and properly capped specimen produce rational result.
Packing of specimen is not suggested, as it produces significant reduction in apparent strength of concrete relative to a capped specimen. Even careful rendering of smooth-trawled surface of the specimen will not produce satisfactory result.
Packing produces scattered results (significant variation in concrete strength) of strength as the contact surface have significant amount of defects. The reduction of strength in packing is due to introduction of lateral strains in specimen cylinder by influence of poisson’s ratio of packing material. As poisson’s ratio of packing material is usually greater than concrete, the splitting is initiated.
To understand effect of poisson’s ratio let us consider the cylinder is lubricated at the ends. The lubricating eliminates effect of retraining from friction between the platen and the specimen on spreading of concrete in lateral direction. Lubrication also reduces strength of test specimen but less than that of packing.
Capping also reduces strength, but selecting suitable capping materials will not adversely affect measured strength. Thus scattered result from uncapped specimen is reduced, selecting a material having elastic properties and strength similar to concrete, will eliminated tendency of split. A fairly uniform stress distribution over cross-section of specimen is established. Capping operation of concrete specimen will be discussed in details in upcoming post.