Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Transverse flexural and torsional strength of Prestressed PrecastHollow-Core Slabs

A. PisantyFaculty of Civil and Environmental Engineering, Technion, Haifa, Israel

ABSTRACT: The transverse flexural and pure torsional strength of Prestressed Precast Hollow-Core Slabs(PPHCS) were investigated experimentally. Two series of tests were conducted and their results reported, relatingto: a. the lateral (perpendicular to span) flexural strength, and b. the pure torsional strength. The developmentof a machine for the testing of slab cuts under pure torsion is presented and explained in detail. Evidence ofexisting reliable tensile/shear strength was produced. A size effect was detected indicating a higher strength withthe reduction of the slab thickness, under both bending and torsion.

1 INTRODUCTION

Information about the transverse flexural and torsionalresistance of prestressed, precast hollow-core slabs(PPHCS) is essential for design and the expected per-formance of these slabs, designed as one way spanningelements, however, employed in slabs and horizontaldecks under circumstances of uneven loads and loaddistributions, therefore transverse flexure and torsionare inevitable.

The geometry of PPHCS, in section, as a resultof the industrialized manufacturing system, imposesdifficulties on the investigation of particular, narrowdefined, zones of their sections, therefore most exper-imental investigations were conducted observing theirglobal behavior as one way single spanning element,in service or ultimate states. This was done by allpast code drafts or chapters in codes and to a largeextent in the fib bulletin 6 by Van Acker et al (2000).To the most, transversal behavior of a one way span-ning series of PPHCS was regarded as a chain of nonreinforced segments, interconnected by hinges witha vague knowledge of their transversal strength ortorsional resistance. A recently published experimen-tal and analytical investigation by Broo et al (2007),amongst the very few to employ powerful FE analysisprograms, might allow a better insight into the stressstate of these complex topology sections, however thepresently published evidence still is with regards to theglobal element behavior.

Experimental investigations aimed at a better under-standing of the “micro” behavior of PPHCS wereconducted by Pisanty (1991–1998). The web shearstrength was investigated by Pisanty (1992). The ten-sile strength of the web was reported also by Pisanty &

Regan (1991). Lateral bending stiffness was assessedexperimentally by Pisanty (1997). Last a testing devicewas developed and tests were conducted in order toobtain an insight of the torsional resistance – Pisanty(1998). In spite of the lack of reinforcement (otherthan prestressing) a level of resistance to both lateralbending and torsion was exhibited.

2 EXTRUDED PPHCS

The manufacturing method of extruded PPHCS dif-fers from other production systems, where the concretequality over the thickness of the slabs is fairly uniform.The process of extrusion results in reduced concretequality from the bottom layer (where the prestressingreinforcement is embedded) towards the top layer.Thisfact requires a particular consideration as to the loca-tion of the tensile zone in the investigated model andthe imposed stress state when dealing with extrudedPPHCS. Proof was provided by Pisanty (1991) that thetensile strength of the bottom layer is higher comparedto that of the top layer, testing for flexural bendingal cuts from extruded PPHCS in the direction of thespan.

3 TRANSVERSE FLEXURAL STRENGTH

Transverse flexural strength tests on extruded PPHCSwere carried out following the classical Rilem flexuralbending test of two point loaded segments at 1/3 of thespan (Fig. 1).

For this purpose from 1200 mm wide slabs seg-ments were cut having a length equal to their thickness(whereby prestressing was neutralized): 8 specimens

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Figure 1. Standard Rilem flexural bending test.

Figure 2. 250 mm thick slab specimen on test set.

Figure 3. 300 mm thick slab specimen on test set.

from 300 mm slabs, 4 specimens from 250 mm slabsand 4 specimens from 200 mm slabs. Characteristicconcrete strength for all test specimens was 60 MPameasured in 100 mm cubes For the testing half of thespecimens were positioned so that the bottom layer(originally containing the prestressing strands) was atthe tensile face (marked ↑ in table 1). The second halfof the specimens were positioned so that the originaltop layer would be at the tensile face (marked ↓ intable 1). Illustration of both positions may be seen atFigs. 2 (250 mm) and 3 (300 mm).

In Fig. 4 a 300 mm specimen after rupture is given.Tests were controlled using a load cell having an accu-racy of 10 N, and the average loading rate was 50 N/sec(duration was 4–5 minutes).

The tests results are presented in table 1. Two ten-dencies are clearly demonstrated: a. tensile strengthgrows with diminishing overall thickness. b. there isa very substantional difference between the tensilestrength of the top and bottom layer of the slabs (aspreviously mentioned the manufacturing process issuch that the layer containing the prestresing strand

Figure 4. 300 mm thick slab specimen after rupture.

Table 1. Tests results for transverse flexural bending tensilestress at rupture (MPa).

300 mm 250 mm 200 mmSpec.no. ↓ ↑ ↓ ↑ ↓ ↑

↓ 19/1/B 2.70↓ 19/2/B 2.71↑ 19/3/B 2.65↑ 19/4/B 3.25↓ 10/5/B 2.27↓ 10/6/B 2.56↑ 10/7/B 3.60↑ 10/8/B 3.08↓ 10/9/B 2.06↓ 10/10/B 2.73↑ 10/11/B 4.61↑ 10/12/B 4.00↓ 10/1/B 3.81↓ 10/2/B 3.62↑ 10/3/B 4.39↑ 10/4/B 4.39Average 2.56 3.15 2.40 4.31 3.72 4.39

is the bottom layer). The 250 mm thick slabs resultsdo not exactly fall in line, however it is believed thatan increased number of test specimens of both 200and 250 mm thickness should improve the averageand demonstrate well a systematic development ofthe trends cited here. In terms of characteristic ten-sile strength fctm the results indicate 75% to 100% forthe bottom layer and 60% to 80% for the bottom layer,depending on the slabs thickness.

4 TESTING MACHINE FOR TESTINGTORSION STRENGTH

A simple machine for testing torsional strength wasdeveloped. It allows testing of 1200 mm long speci-mens (cuts from 1.2 m wide slabs) of a square sectionup to 300 mm thick. The main parts are three concen-tric frames (Fig. 5) installed at 440 mm distances that

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Figure 5. Conceptual design of the testing machine.

Figure 6. Sections A-A (left) and B-B (right).

serve as vertical supports. Two end frames are fixed bywelding on a horizontal frame – section A-A (Fig. 6)and the central frame is supported vertically on a seg-ment of a circular arch, however free to rotate – sectionB-B (Fig. 6). A specimen is inserted and fixed in posi-tion at the two end frames that serve as supports fortorsion. On the central frame the specimen is supportedvertically as well.At an eccentricity of 750 mm a verti-cal load is applied. The spans of 440 mm are extremelysmall, so that pure bending and corresponding shearare negligible. The actual stress state is one of almostpure torsion. The load cell accuracy is 10 N. To allowfor carefully monitored loading, load rate may be keptto a low 50 N/second. It should be clear that since thetested specimens are short the effect of prestressing islost, therefore a non reinforced specimen is actuallytested. An actual view of the machine may be seen inFigs. 7 & 8.

5 TESTING UNDER PURE TORSION

18 specimens were cut from 1200 mm wide slabs:10 specimens from 300 mm, 5 from 250 mm and 3from 200 mm thick slabs. Specimen’s length was equalto the slab thickness.Tests were conducted with the aidof the machine described previously, monitored by a

Figure 7. Overview of the testing machine (top).

load cell having an accuracy of 10 N, at an averagerate of 50 N/second. In the case of testing for torsionconsideration about top or bottom layer is irrelevant.Fig. 9 and Fig. 10 show fractured specimens of 200 mmand 300 mm thick slabs respectively.An inclined crackis clearly visible in both cases though the complexityof the sections does not allow a simple link betweeninclination angle and any of the section properties.

The tests results are given in Table 2. In view of thecomplexity of the ruptured inclined section, which isdifficult to define, a simple model was adapted, given

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Figure 8. Overview of the testing machine (side).

Figure 9. 200 mm thick specimen.

Figure 10. 300 mm thick specimen.

in Fig. 11. Obviously the model does not representthe real rupture mode, however it does provide a sim-ple assessment tool. A uniform stress distribution isassumed over both layers, in opposite directions. Theshear stresses so derived are given in Table 2. Hada parabolic distribution been assumed for the shearstresses a maximum shear stress 50% higher would be

Table 2. Fracture stress results (MPa) testedin pure torsion.

Slabs ThicknessTestSample 300 mm 250 mm 200 mm

19/1 2.0619/2 1.8519/3 1.7619/4 1.9010/1 1.8410/2 1.3010/3 1.542/1 1.352/2 1.452/3 1.452/4 2.902/5 3.212/6 2.642/7 1.9530/1 2.5530/2 2.5230/3 2.5130/4 1.58Average 1.67 2.19 2.92Standard 0.273 0.471 0.285deviation

Figure 11. Simplified model for shear stress distribution.

obtained. It was decided not to search for an optimumor better suited stress distribution considering the factthat any distribution along a section as given in Fig. 11is not representing the true rupture section, so thatTable 2 provides values that can be useful for designand also provide an insight for a size effect. Stressesare in the range of 40% to 75% of the average con-crete tensile strength fctm. Again it is clear that torsionresistance grows with reducing thickness of the slabs.Again it is seen that 250 mm slabs exhibit (probablydue to geometry of the section reasons) some smallweakness. The standard deviation for the shear stressresults due to torsion is of the order of magnitude ofthat for concrete tensile strength, however, the standarddeviation for 250 mm slabs is considerably higher.

6 CONCLUSIONS

Experimental analysis of the transverse flexural bend-ing and of pure torsion of Prestressed Precast Hollow

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Core Slabs was carried out and the results presentedhere. In flexural bending a reliable average rupturestrength of 75% to 100% of fctm for the bottom layerand 60% to 80% of fctm for the top layer were demon-strated (300 mm reducing to 200 mm thick slabs). It ispossible that improved manufacturing methods result-ing in better uniformity may reduce these differences.Torsion rigidity was proved to exist also. Tests resultsshow maximum shear stresses of 40% to 75% of fctm(again growing with reduced thickness) in a very sim-plified model. The testing machine developed for thetorsion tests proved to be efficient, for testing fullscale specimens, reliable and simple. Both tests seriesprovide useful tools for design and research.

ACKNOWLEDGEMENT

The technical and financial sponsoring of SpancreteIsrael and the invaluable assistance of Eng. YoramGotlieb M.Sc. P.E. are gratefully acknowledged.

REFERENCES

Broo, H., Lundgren, K. & Engstrom, B. 2007. Shear andTorsion in Prestressed Hollow Core-Units: Finite ElementAnalyses of Full-ScaleTests. Structural Concrete, fib 8(2):97–100.

Pisanty, A.& Regan, P.E. 1991. Direct Assessment of the Ten-sile Strength of the Web in Prestressed, Precast, Hollow-Core Slabs. Materials & Structures, RILEM, 24(144):451–455.

Pisanty, A. 1992. The Shear Strength of Hollow Core Slabs.Materials & Structures, RILEM, 25(148): 224–230.

Pisanty, A. 1997. Assessment of the Lateral Bending Stiff-ness of Prestressed, Precast, Hollow-Core Wide Slabs.(Res. 010–153) National Building Research Station, FinalReport, Nov. 1997, (in Hebrew).

Pisanty, A. 1998. Development of Testing Machine for Inves-tigation of theTorsional Resistance of Prestressed, Precast,Hollow-Core slabs. Report to Spancrete Israel, 1998. (InHebrew).

Van Acker, A. et al. 2000. Special design considerationsfor precast prestressed hollow core floors. Fib bulletin6. International Federation for Structural Concrete, Lau-sanne.

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