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Bearing Capacity of Prestressed Concrete Decks Slabs

Amir, S., van der Veen, C., Walraven, J. C.Department Design and Construction, Structural and Building Engineering, Concrete Structures, Faculty of

Civil Engineering and Geosciences, Delft University of Technology, the Netherlands. 

de Boer, A.Ministry of Infrastructure and the Environment (Rijkswaterstaat), the Netherlands.

Abstract

In the Netherlands, most of the bridges were built more than 50 years ago and it is uncertain if theyare still safe. Experiments carried out to investigate punching shear capacity of transversely prestressed concrete decks under concentrated loads have shown that sufficient compressive

membrane action (CMA) had developed in the deck slab and combined with the prestressing force,the bearing capacity was much higher than predicted by various codes and theoretical methods.

1 Introduction 

There are many bridges in the Netherlands that were constructed in the 60s or 70s of the lastcentury. Since then, the traffic flow has increased making the safety of such bridges questionableaccording to the modern design codes. This paper describes the experimental research beingconducted in the Delft University of Technology, to investigate the capacity of a 1:2 scaled modelof a bridge with thin transversely prestressed concrete deck slab panels cast between precast

concrete girders and subjected to concentrated loads (Fig. 1). The effect of the transverse prestressing level (TPL), the geometry of the deck, the type of loading etc, on the punching shearstrength is investigated. It is expected that in laterally restrained post-tensioned deck slabs, CMAand the deck transverse prestressing will provide an enhanced bearing capacity in flexural and punching shear.

Fig. 1 Top view and front view of the test-setup.

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2  Experimental investigation

In order to simulate an actual bridge as closely as possible, a 1:2 scale was used to design the

 prototype with failure expected to occur in the deck slab (Fig. 1). The deck prototype was 12 mlong and 6.4 m wide consisting of four precast concrete girders placed at 1800 mm c/c distance.The deck slab was cast in-situ with a clear span of 1050 mm, thickness of 100 mm and post-tensioned in the transverse direction. The 28 days concrete compressive cube strength was 75 MPa.Two transverse prestressing levels were applied; 1.25 MPa and 2.5 MPa. Inclined interfaces were

 provided at both sides of a deck slab in two out of three panels of the bridge. Most of the tests weredone with a load applied in-between  two adjacent transverse prestressing ducts. Two transverse

 beams were provided at the end of the girder-slab assembly, post-tensioned as well in thetransverse direction. Static tests were performed at different locations of the deck slab by applyinga concentrated load through a hydraulic actuator. The vertical and horizontal deflections of the slab

at various points and the development of cracks were recorded at various loading intervals.

3 Experimental results

Fig. 2 shows the test results. When a single load was applied at midspan (MSp) of the slab panel orwhen a single or double load was applied close to the girder flange-slab panel interface (INT), a

 brittle punching failure was observed. However, when a double load was applied at the midspan ofthe slab panel, a flexural punching failure was observed. It can be concluded that an increase in

TPL increased the punching capacity of the deck slab. Tests done above the ducts (bounded blackin Fig. 2a) showed a higher capacity as compared to tests done in-between the ducts for the sameTPL. The indented interface was always found to be strong enough regardless of a straight (ST) or

an inclined (SK) interface.

Fig. 2a Single load punching failure tests. Fig. 2b Double load punching failure tests.

4 Results of calculation 

 4.1 Single load tests at midspan with failure in brittle punching shear mode

Fig. 3 shows the punching shear capacity of single load tests calculated with: a) the backgroundreport 25.5-02-37-prENV 1992-1-1(2002) and ACI 318 (2005) that do not consider CMA, b) theUK BD81/02 that considers CMA. Both the background report 25.5-02-37-prENV 1992-1-1(2002)and ACI 318 underestimate the punching shear capacity. This lack of capacity is attributed to theignorance of compressive membrane action and although the UK BD81/02 considers membrane

action but it also predicts a conservative punching capacity probably because it was developed for reinforced concrete only.

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Fig. 3a Code punching capacity without CMA. Fig. 3b Code punching capacity with CMA.

 4.2 Double load tests with failure in the flexural punching shear mode

The flexural punching shear capacity considering CMA for double loads acting at midspan of the

slab panel is calculated by a method outlined by Taylor et al (2002) and gives a fairly good

estimation of the failure loads when compared to those observed in tests.

Table 4

Comparison of test and calculated capacityTest TPL Test load Predicted Flexural Punching load

[MPa] [kN] [kN]

Arching + Bending (Effective width = 1960 mm)

BB11 1.25 377.9 431.4

BB05 2.5 490.4 533

BB16 2.5 553.4 533

5 Conclusions

Ongoing experiments have shown that substantial CMA develops in the deck slab and transverse

 prestressing affects the bearing capacity positively. Failure always occurs in the span of the deckslab panel and the interface between the girder flange and deck slab panel has proven to havesufficient strength. Also, when loaded directly above a prestressing bar/duct, the deck slab

shows a higher punching capacity. Analysis of the current codes shows that the Eurocode 2

(2005) and ACI 318 (2005) have no provision for CMA and therefore underestimate this

effect in the punching shear capacity formula.

References

ACI Committee 318 (2005), Building Code Requirements for Structural Concrete (ACI 318-05)

and Commentary (318R-05), American Concrete Institute, Farmington Hills, Mich.Background report 25.5-02-37 – prENV 1992-1-1:2002, Section 6.4 (2002), J. C. Walraven, Delft

University of Technology, the Netherlands.

Taylor, S. E.; Rankin, G. I. B.; Cleland, D. J. (2002), Guide to Compressive Membrane Action inBridge Deck Slabs, Technical Paper 3, UK Concrete Bridge Development Group/British

Cement Association.

UK Highway Agency, BD 81/02 (2002): Use of Compressive Membrane Action in bridge decks,Design Manual for Roads and Bridges, Vol. 3, Section 4, part 20.