Title: SHEAR STRENGTH OF PRECAST, PRESTRESSED STEEL FIBER REINFORCED CONCRETE HOLLOW-CORE SLABS

Author(s): Gustavo J. Parra-Montesinos;Luis B. Fargier-Gabaldón;Mohamed Al-Tameemi

Publication: CRC

Volume:

Issue:

Appears on pages(s):

Keywords: precast,prestressed,concrete,hollow-core,slabs,steel,fibers,shear,

DOI:

Date: 11/27/2019

Abstract:
Precast, prestressed concrete hollow-core slabs are commonly used in residential, office and industrial construction because of their light weight, rapid construction, and large span-to-depth ratios. These structural members are typically manufactured using either an extrusion (e.g., Elematic) or a slip-form process. Although most hollow-core slabs have depths not exceeding 12 in. (305 mm), there are cases in which deeper slabs are needed to satisfy strength and serviceability requirements. Results from past research (Hawkins and Ghosh, 2006), however, have indicated that deep hollow-core slabs may fail at shear forces substantially lower than the web-cracking shear strength, Vcw, calculated according to the ACI Building Code. Because of this, ACI 318-14 limits the shear strength of hollow-core slabs with overall thickness greater than 12.5 in. (320 mm) to half of the web-cracking shear strength, unless at least minimum shear reinforcement is provided. In this research, the use of randomly oriented, deformed steel fibers in hollow-core slabs was evaluated as minimum shear reinforcement in extruded and slip-formed prestressed concrete hollow-core slabs. A total of 14 and 16 tests were conducted on extruded and slip-formed hollow-core slabs, respectively. For the extruded slabs, the shear span-to-effective depth ratio was either 3.0 or 3.5. For the slip-formed slabs, on the other hand, the shear span-to-effective depth ratio ranged between 2.0 and 3.0. Two types of hooked steel fibers were evaluated. Type 1 fibers were 1.18 in. (30 mm) long, 0.022 in. (0.55 mm) in diameter, and had a specified tensile strength of 160 ksi (1100 MPa). Type 2 fibers, on the other hand, were 2.36 in. (60 mm) long and 0.035 in. (0.9 mm) in diameter, with double hooks at each end. The nominal tensile strength for the Type 2 fibers was 335 ksi (2300 MPa). In the extruded slabs, Type 1 fibers were evaluated at 50 lbs/yd3 or 290 N/m3 (0.38% volume fraction), while Type 2 fibers were evaluated at 40, 50 and 62 lbs/yd3 (230, 290 and 360 N/m3 ) for fiber volume fractions of 0.30, 0.38 and 0.47%, respectively. In the slabs constructed using a slip-form process, Type 1 fibers were evaluated at 55 and 66 lbs/yd3 (320 and 385 N/m3 ), which corresponded to a fiber volume fraction of 0.42 and 0.50%, respectively, while Type 2 fibers were evaluated at 40 and 50 lbs/yd3 (230 and 290 N/m3 ), which corresponded to a fiber volume fraction of 0.30 and 0.38%, respectively. viii Extruded hollow-core slabs with Type 1 and Type 2 fibers in volume fractions of up to 0.38% (50 lbs/yd3 or 290 N/m3 ) could be mixed without any changes to the original mixture proportions or mixing process. From observations during manufacturing of the slabs and the test results, it appears that some difficulties in terms of mixing and fiber distribution could be encountered when using Type 2 fibers in a 0.50% volume fraction. Manufacturing of slip-formed hollow-core slabs, on the other hand, proved challenging when using Type 2 fibers, particularly in volume fractions of 0.38%. In some instances, manufacturing of these slabs had to be paused to clear fibers trapped in the slip-form machine. On the other hand, no major difficulties were encountered in the manufacturing of slabs with Type 1 fibers at dosages of up to 0.50% by volume. Strand end slip values in the slabs constructed using an extrusion method did not seem to be affected by the presence of fibers. Fifty strand diameters (50db) represented an adequate, and generally conservative estimation of the average transfer length for 0.6 in. diameter strands, and was in all cases conservative for estimating transfer length for 0.5 in. diameter strands. Measured slip values for the slabs manufactured using a slip-form process, on the other hand, showed a much greater variability compared to the measured slips in the extruded slabs. Calculated transfer length for 0.5 in. diameter strands in the slip-formed slabs with Type 1 fibers was close to 50db. Measured slip values for the slabs with Type 2 fibers, on the other hand, were significantly greater than those in the slabs with Type 1 fibers, with calculated transfer lengths exceeding in most cases 80db. This is believed to have been caused by the difficulties encountered in the manufacturing of the slabs with Type 2 fibers, which were twice as long as the Type 1 fibers. Extruded fiber-reinforced concrete slabs with Type 1 and Type 2 fibers at a 0.38% and 0.30% volume fraction, respectively, exhibited peak shear strengths that ranged between 1.08 and 1.20 times the calculated web-cracking shear strength. The two extruded slabs without fibers, on the other hand, failed at shear forces corresponding to 0.93 and 0.87 times the calculated webcracking shear strength, Vcw. The extruded slabs with Type 2 fibers at 0.38% and 0.50% volume fraction showed a greater variability in shear strength, likely due to variations in fiber distribution or concrete compaction. These slabs failed at shear strengths ranging between 0.94 ix and 1.29Vcw. Besides an increase in shear strength, the presence of fibers, particularly of Type 2 fibers, led to a more gradual post-peak strength decay. The test slip-formed slabs that failed in shear exhibited peak shear forces ranging between 0.80 and 1.21Vcw for an average value of 1.04Vcw. The three spans reinforced with Type 1 fibers that failed in shear (all with fibers at 55 lbs/yd3 or 320 N/m3 ) for which transfer lengths were, on average, approximately 50db, exhibited peak shear forces ranging between 1.08 and 1.21Vcw. A greater variability of shear strength was observed in the slabs with Type 2 fibers, for which significantly larger transfer lengths were calculated. Even though no regular concrete slip-formed slab was tested as part of this investigation, a comparison of the shear strength of the spans that exhibited a web-cracking shear failure with that of eight slip-formed slabs tested by Palmer and Schultz (2010) suggests that the use of fiber reinforcement leads to an increase in shear strength, especially when considering the slabs with Type 1 fibers for which similar transfer lengths were calculated. The limited data suggest that slabs with Type 1 and Type 2 fibers in the dosages evaluated should lead to shear strengths of at least Vcw, as long as transfer lengths of approximately 50db or shorter can be achieved.