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8/10/2019 HCS Testing

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Toward a standardized procedure for

hollow-core slab testingMarco di Prisco and Marco G. L. Lamperti

Department of Structural engineering


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Marco di Prisco and Marco G.L. Lamperti

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Aims of Research

To verify reliability of predictions

To optimize experimental lever arm

To investigate the residual shear capacity

EXPERIMENTAL

INVESTIGATION

EC2 MC '90

LITERATURE

Most criticalShear Value

Fibre reinforcement quantification to preventshear failure


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3Experimental Program 1

Four hollow-core slab types from30 to 45 cm thickness

Shear and bending test

Real-scale performed tests: 18


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4Experimental Program

Bending Test set-up

Bending test set-up:

distribuited load controlled vs.

Distributed Load-controlledset-up 4PB Stroke-controlledset-up


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5Experimental Program

Shear Test set-up

with flexural cracking

1.5 < < 3.5

without flexural cracking

3.5 < < 5.0

Stroke-controlled4-Point Bending


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6Material characterization

Tested according UNI specifications gave resultes 1936 < fptk < 2020 N/mm2

Concrete

Steel

Cubic specimens (150 mm side)casted at the same time of slabs

Cores directly extracted from testedslabs


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7Experimental Results

No bond failure was observed

Two different test set-ups wereadopted for only the V35 to induce

shear failure with or without flexuralcracking

After shear crack formation manytest presented a progressive strandslip

Bending tests

Shear tests

The real collapse was not reached insome cases due to the large mid-span deflection obtained


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Bending Test

Although a flexural collapse did never occurred, because of the high deformabilityof hollow-core slabs, closed-loop control allows a better test procedure that caneasily follow structural damage evolution.

0 40 80 120 160

freccia [mm]

0

40

80

120

160

200

P[

kN

]

fr Var30-F1

fr Var30-F2

simul. elastica

simulazione

Pcrcon fctf=1.05fctm


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Shear Test 1Top-Chord collapse

Strand Slip failure


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Shear Test 2

The small load lever arms corresponded to large residual bearing capacities after thepeak load, while larger lever arms caused small residual load capacities.

The higher force value in the chords reduces the shear resistant mechanisms as wellas to the higher elastic strain release in the crack propagation associated to the largerbending deformation.


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11Adopted Design Models

The adopted material models, are suggested from Fib Model Code 90 and EuroCode2.

The moment-curvature relation is derived from a plane-section model.

Concrete - compression Concrete - tension Steel - tension


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12Adopted Design Models

In order to predict the shear loads, both the empirical equations adopted forcracked and uncracked in flexure cases were computed. According to Walraven(1982) and FIB, the following equations are here taken into account:

with flexural cracking

without flexural cracking


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13

Experimental vs.

Theoretical Results

The load displacement curves for bending collapse were very well reproduced byadopting the beam theory.

The cracking moment was calculated by neglecting the tension stiffening and consideringthe tensile strength of the concrete.

The shear tests are not equally well reproduced, especially for small shear span. Thisresult could be mainly related to the not perfectly planar extrados surface of the hollow-core that involves a stiffness reduction.


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Discussion of Results - An analysis performed according to

Kanis valley approach highlights ajump between 3.66 and 4.17 of a/d,strictly related with the passagefrom VRt toVR1 shear resistance.

The 300 mm deep element showeda border condition for a/d = 4.23.


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Discussion of Results -

By taking into account the residual values an evaluation of steel fibre addition could becarried out by computing the maximum between the increase of VR1 needed to reach theultimate bending moment adopting a load lever arm close to 4, and the increase of theresidual shear capacity when VRt takes place. In this case we could expect that steel fibreaddition stabilizes the crack associated to both VRt andVR1 failures, as recently shown byElliott (2002), even if the elastic energy associated to the bending of a real span hollow

core could significantly affect the values investigated.


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Concluding Remarks

The experimental investigation confirmed the predictive relations introduced byprevious authors in the range of 300 450 mm deep hollow core elements.

A closed loop hydraulic jacket, with 1000 kN of maximum load capacity, madedisplacement controlled tests possible.

The residual shear capacity of the structures was exploited in order to preventbrittle failures in these elements.

When the structures were cracked in flexure, the residual values were smaller thanin the case of uncracked structures.

A border value of about a/d = 4 was detected to separate the cracked-in-flexure

cases from the uncracked ones. The use of steel fibre to prevent brittle failure has to be encouraged, even if it

should be validated on real-size beams, in order to take into account the real elasticrelease rate in the crack propagation at failure.

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