However, it was noted that complete elimination of friction during cylinder testing had

not been achieved. They also suggested that the behavior of concrete prior to peak stress

was not affected by the amount of friction between the load platens and the cylinder. The

effect of friction on the cylinder behavior was also recognized by Mindess et al. (2003)

who indicated that the confining effect of the platens extends as far as a distance from

each end of approximately (30.5/2) d which meant that for a concrete cylinder with a

diameter of 6 in. the unconfined section of the cylinder was very small, approximately 2

in. Therefore, the damage and failure zone was located towards the mid-height of the

cylinder. Mindess et al. (2003) also indicated that as the confining effect of the load

platens was reduced the failure patterns of the concrete cylinders changed from cone

failure to columnar failure for the cylinders with minor confining effects. This type of

failure was also described in the ASTM C39/C39M standard test method for concrete

cylinders as one of the possible fracture types for concrete cylinders tested in uniaxial

compression (ASTM 2001).

 Landis and Shah (1993) tested small size mortar beams in bending and used

acoustic emissions to map the cracks across the section of the beams. They found that the

cracks were denser and longer on the external layers of the beams and became shorter

closer to the center of the cross-section. The beam cross section had two regions with

regard to the rate of change of the crack length. Region one extended from the outside

face of the beam to a distance approximately one eighth to one quarter the width of the

beam. In region one the crack profile followed an angle of 50 to 64 degrees while in

region two the angle was 7 to 11 degrees (see Fig. 8-3). Similar results were obtained by

Kan and Swartz when they mapped the crack profiles of small beams using a dye