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longitudinal strain behavior of both tested light-weight concretes is significantly linear until failure, thus reducingits ductility factor compared to NC’s, as explained below. Besides, the longitudinal deformation modulus of both lightweight con-crete mixes are less than or equal to NC’s.Table 2 shows the longitudinal and transverse ductility factorsDF long. and DF trans., observed at 28 days via the compressivetesting of cylinders prepared using the different concrete mixesThe ductility factor is defined as the quotient between the finalstrain and the elastic deformation of the material, according toMeli [24]. The longitudinal ductility factor of LC10-1 is 70% of thatof NC, whereas that of LC10-2 is 75% of NC. Both the strength andthe ultimate strain of concrete under compression determine theflexural capacity of the beams, which were designed to fail undercompression on the upper part of the typical cross section withthe lower prestressing strands being fully developed, being thecompressed concrete unconfined. Thus, it is the concrete’s com-pressive failure that determines both the ductility and the flexuralbearing capacity of the beams.Fig. 4 shows the compressive stress vs. transverse strain dia-gram of the three concrete mixes tested, which corresponds tothe modulus of deformation test shown in Fig. 2. The transversemodulus of deformation is similar for both lightweight concretemixes. The NC concrete has a transverse ductility factor of 6.9which was calculated relative to the ascending iterative branchof the modulus of deformation test. The transverse ductility factorof LC10-1 is 1.7, and that of LC10-2 is 1.8, which represent 24% and28% of the transverse ductility factor of NC, respectively [6].During the prestress release, due to the Hoyer effect, high trans-verse tensile strains are imposed on the concrete that surroundsthe strand along the transfer length. These tensile strains are crit-ical at the end of the beam, where the strand diameter recoverydue to the Hoyer effect is at its maximum. For this reason, if theconcrete that surrounds the strand does not withstand thesestrains, splitting cracks will initiate, and if the aggregate interlockis not able to stop them, these cracks will propagate to the beamsurface. This reduced transverse deformability of lightweight con-crete is due to the low tensile strength of lightweight aggregates,which results in both reduced tensile strength and reduced frac-ture energy in lightweight concretes. Therefore, Fig. 3 depicts thefragility of the stress vs. the transverse strain behavior of light-weight concrete compared to that of NC. In addition, the sustainedtensile strength of lightweight concrete is also lower than that ofnormal weight concrete. These properties of the studied light-weight concretes make splitting cracking in pre-tensioned con-crete elements more likely to occur [25].3.3. Design of the beamsThe design criteria of the beams were as follows:1.
To avoid the effects of flange-web interactions at the endsof the beams, rectangular cross section beams weredesigned (Fig. 5). Concrete longitudinal stresses were lim-ited after prestress release according to Eq. (1). No tensilestresses were allowed in the cross section due to the afore-mentioned reduced quotient between the sustained tensilestrength and the instantaneous tensile strength of light-weight concrete [25,26].0:40f0c rc 0 ð1Þwhere f0c = average concrete compressive strength at the time ofprestress release (positive tension) and rc = stress in any fiber of agiven cross section located at x m from the end.1. The width of the adopted cross section was 200 mm toaccommodate three lower prestressing strands that werealigned horizontally with the customary spacing and coverused in the factory, which was 50 mm as measured fromthe center of each strand (Fig. 2). Three aligned strandswere chosen to evaluate the tendency of lightweight pre-stressed concrete prism beams to develop splitting crackson the beam lower side [7,16].2. To ensure ductile flexural behavior of the beams, a crosssection of 200 400 mm (Fig. 5) was designed with a sin-gle upper prestressing strand that was centered horizon-tally and designed to introduce the total prestressingforce in the central core of the section. Two 16-mm diam-eter rebars were also placed in the upper section toincrease both the load-bearing capacity and the ductilityof the beams under positive flexure [16].3. The minimum cover, the overlap and development lengths,and the bending radii of the passive reinforcement corre-sponded to the requirements of Eurocode 2 parts 1–4 forlightweight concrete and were applied to all of the testedbeams [26,27].4. In the first series of LC10-1 concrete beams, differentamounts of confining reinforcement were placed over a dis-tance of 1.5 m from both ends of the beam (upper limit of轻质和正常重量的预应力混凝土梁受弯性能的比较研究