Excessive strand end slip in prestressed piles

This paper presents the results of a research project that investigated exc essive strand end slip observed recently in some prestressed piles. From me asurements taken in the field, it is apparent that the problem of excessive initial strand slip is independent of pile shape and size. Strand end slip is evident in piles of different manufacturers in different states in the Southeast. Excessive strand end slip was found in both the top and bottom o f the cross section of the piles, although the top portion of the cross sec tion generally exhibited much higher initial slip. Several preventive measu res ran be adopted to reduce the excessive strand end slip. These preventiv e measures include: a) proper concrete mixture proportioning to reduce top bar effect; b) use of higher-strength concrete with the lowest possible slu mp and setting time; c) assessment of the condition of the strands prior to installation to insure excellent bond characteristics; d) gradual release of prestress, with an optimal release sequence; and e) use of adequate vibr ation to ensure consolidation. The strand end slip measured at five prestressing plants in the Southeast i s considerably higher than the allowable end slip and is expected to affect the pile performance. If the strand slip theory is adopted the strand deve lopment length increases substantially die to the excessive strand end slip . A top bar effect factor similar to the one used in reinforced concrete de sign is recommended. To maintain the excellent quality of precast and prest ressed concrete products, manufacturers should adopt a dynamic quality cont rol process that follows the rapid changes in the industry. More tests are necessary to ensure excellent quality such as the Moustafa or an equivalent test, to assess the bond capabilities of the strands, end slip measurement s. and direct measurement of the transfer length. Installation of piles sho uld proceed in a manner to alleviate the top bar effects by placing piles a lternately in their best and worst directions.