دانلود رایگان مقاله اثر مقاومت فشاری بتن در طول انتقال

abstract

This paper examines the effect of concrete compressive strength on the transfer length of prestressing strands. The paper includes the results from several research projects conducted at the University of Arkansas (UA) and from testing reported in the literature. At the UA, 57 prestressed, precast beams have been cast since 2005. The beams were cast with selfconsolidating concrete (SCC), high strength concrete (HSC), lightweight self-consolidating concrete (LWSCC), and ultra-high performance concrete (UHPC). Using data from the UA and from the literature, an equation to estimate transfer length was developed and presented. The results were also compared with the American Concrete Institute (ACI 318) and the American Association of State Highway and Transportation Officials (AASHTO) prediction equations for transfer length, which were designed for conventional concrete. The results also showed that there was little change in transfer length when the compressive strength at release was greater than 34.5 MPa.

6. Summary and conclusions

The research project examined the measured transfer lengths of 57 prestressed concrete beams cast with a variety of different concrete types. The concrete types included normal strength (NS), high strength (HS), self-consolidating concrete (SCC), ultra-high performance (UHP), and light weight (LW) concrete. Fifty one beams were fabricated with 15.2 mm, Grade 270, seven wire low relaxation prestressing strand. The concrete compressive strengths at release for those 51 beams ranged from 23 MPa to 155 MPa. Six beams were fabricated using 12.7 mm diameter strands with concrete compressive strengths at release between 24 MPa and 31 MPa. Measured transfer lengths were determined using concrete surface strains along with the AMS method. The UA data was analyzed using the power regression in order to develop a new transfer length equation. A power regression was chosen to develop this new equation because this repression provided a better fit than the linear regression. This was due to the influence of concrete compressive strength on the transfer length. In addition, measured transfer lengths from the literature were collected and analyzed and compared with ACI 318-14, ACI (50db), AASHTO (60db), NCHRP-603 (40db), equations from the literature, and the proposed equation, Eq. 2. Based on the investigation, the followings conclusions were made: 1. Transfer length in prestressed concrete members decreases as concrete compressive strength increases. Research results also show that the ACI 318-14 and AASHTO equations overestimate transfer lengths in members containing concrete with high compressive strengths. Therefore, concrete compressive strength should be a factor in predicting transfer length. 2. Based on the results of the study, Eq. 2 and the ACI 318-14 equation are recommended when the concrete compressive strength at release is less than 34.5 MPa. Based on the UA experimental data, 40db should be used as minimum transfer length for members containing concrete with compressive strengths at release greater than 34.5 MPa but less than 55 MPa. When the concrete compressive strength at release is greater than 55 MPa, transfer length can be taken as 33db. There is little change in transfer length as concrete compressive strength at release increases beyond 55 MPa. 3. The proposed UA equation, Eq. 2, is based on experimental data with good strand bond (STSB values of 117 MPa or more). For strands with poor surface quality, further investigation is needed in order to determine the applicability of the UA equation.