Precast/prestressed concrete girders can be made continuous for live load by connecting the girders at the support. The purpose of NCHRP Project 12-53 was to investigate the strength, serviceability, and continuity of connections between precast/prestressed concrete girders made continuous. For live-load continuity, a negative moment connection is usually made through a composite, cast-in-place, reinforced concrete deck. A diaphragm is usually cast between girder ends. However, once the girders are connected, they may camber upward due to the effects of creep, shrinkage, and temperature. These effects cause the formation of a positive moment at the diaphragm. If no positive moment connection is supplied, the joint cracks and continuity may be lost. Positive moment connections are usually made either by extending the prestressing strand from the girder into the diaphragm or by embedding reinforcing bar from the end http://gulliver.trb.org/publications/nchrp/nchrp_rpt_519.pdf This report may be accessed by Internet users at of the girder into the diaphragm. In the first phase of the research, surveys were conducted to establish the frequency of use of positive and negative moment connections and how they were constructed. The survey examined the type of negative moment connection; the use and type of positive moment connection (extended bar or extended strand); connection details; construction sequence; the age at which continuity is established; constructability issues; and design methodologies. A spreadsheet program called RESTRAINT was developed to conduct parametric studies of the continuous system. Using the results of the surveys and the parametric studies, six positive moment connection details were developed: extended strand, extended bar, extended strand with the girder ends embedded into the diaphragm 6 in. into the diaphragm, extended bar with the girder ends embedded into the diaphragm 6 in. into the diaphragm, extended bar with the girder ends embedded into the diaphragm 6 in. into the diaphragm and additional stirrups in the diaphragm, and extended bar with the girder ends embedded into the diaphragm 6 in. into the diaphragm and horizontal bars placed through the web of the girder. All details had a strength of 1.2 Mcr, where Mcr is the positive cracking moment of the composite cross section. The details were tested using short (16-ft.) Type II AASHTO girders, with a composite slab, attached to a diaphragm. The test results showed that both the extended strand and the extended bar connections developed sufficient strength. Embedding the girders into the diaphragm seemed to improve the connection performance, but the improvement was difficult to quantify. Adding additional stirrups in the diaphragm area did not improve strength, but did improve ductility and may be beneficial in seismic applications. Placing horizontal bars through the webs of the girders improved strength, stiffness, and ductility, but the failure mode was cracking of the girders. In a second experimental phase, 50-ft-long Type III AASHTO I girders were assembled into two-span, 100-ft-long, continuous-for-live-load specimens. The first specimen used a reinforced concrete deck for the negative moment connection and an extended bar for the positive moment connection. The positive moment connection had a strength of 1.2 Mcr. Part of the diaphragm was cast 28 days before the slab was cast. It was thought that weight of the slab would cause the girder ends to rotate into the partial diaphragm, precompress the diaphragm, and prevent cracking if positive moments formed due to creep and shrinkage. However, very little precompression was found. When the slab was cast, the concrete temperature increased due to heat of hydration. Later, the slab cooled and contracted. This contraction caused negative moment to form at the diaphragm. The specimen was monitored for 120 days. It was expected that additional negative moment would form due to differential shrinkage of the deck, but this did not occur. Instead, positive moment from creep and shrinkage developed. Thermal effects were found to be significant. Temperature effects caused daily variations of the moments at the diaphragm of ±250 k-ft. This was approximately 50% of the capacity of the connection. During the monitoring period, positive moment cracking occurred at the connection. After monitoring, the specimen was loaded to test for continuity. Initial loading showed the system remained continuous even though the connection had cracked. A post-tensioning system was used to create additional positive moment at the connection and to open the cracks. Subsequent loading showed the system maintained continuity even though positive moment cracks as large as 0.01 in. had formed. A second 100-ft.-long, continuous-for-live-load specimen was created. This specimen used an extended strand connection. As with the first specimen, a post-tensioning system was used to create positive moment at the connection, and additional positive moment was created by jacking up the ends of the specimen. The combination of the post-tensioning and jacking up the ends of the specimen caused positive moment cracks as large 0.07 in. to form. There were also signs that the connection was about to fail. However, subsequent loading showed that the connection maintained 70% continuity. The second specimen was tested for negative moment connection capacity. The negative moment connection was shown to have sufficient strength. The negative cracking moment was lower than predicted, but this was due to pre-existing cracks in the slab that occurred during positive moment testing. The results of the survey, the parametric study and the experimental work have been incorporated in proposed changes to the AASHTO Load Resistance Factor Design Specifications. The results have also been used to create four design examples. (Author/publisher)
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