Sustainable bridges - the development of a flexible concrete arch system

Transcription

1 Sustainable bridges - the development of a flexible concrete arch system S E Taylor, A Gupta, J Kirkpatrick, A E Long and I Hogg Abstract This paper describes the further development of a novel flexible masonry concrete arch system which requires no centring in the construction phase or steel reinforcement in the long-term. The arch is constructed from a flat pack system by use of a polymer reinforcement for supporting the dead load but behaves as a masonry arch once in place. The paper outlines the construction of a prototype arch and load testing of the backfilled arch system with some comparisons to the results from ARCHIE. The arch had a load carrying capacity far in excess of the current design loads. Keywords: concrete, masonry arch bridges, polymer reinforcement, Technical session on xxxxx Page 1

2 Dr. Su Taylor is a graduate of University of Bath and completed her PhD in structural analysis at QUB. She is currently a Senior Lecturer in Structural Engineering at QUB. Mr A Gupta, completed his MPhil in Structural Materials at QUB in He is currently a Knowledge Transfer Associate and Project Engineer for Macrete Ireland Limited. Professor A.E. Long, Professor in the School of Civil Engineering, Queens University Belfast Dr. J Kirkpatrick was a Principal Engineer for the DRD Road Services, NI and is currently an independent consulting engineer with Owen Williams Ltd INTRODUCTION Arch bridges have structural efficiency twinned with aesthetic qualities and when constructed from masonry blocks have proved to be highly sustainable. Arch bridges were originally built of stone or brick but are now built of reinforced concrete or steel. The introduction of these new materials allows arch bridges to be longer with lower height and can either be cast on site or manufactured as pre-cast. However, a common problem with such bridges is corrosion of the reinforcement, which further leads to their repair and high maintenance costs. The UK Highways Agency [1] states that consideration shall be given to all means of reducing or eliminating the use of corrodible reinforcement and includes the use of plain concrete structural elements. It also recommends the use of the arch form of construction where ground conditions permit. However, a major constraint in the construction of masonry arch bridges at present is the time and labour costs associated with the temporary formwork, or centring, and the masonry arch formation. Additionally, many of these relatively recent concrete bridges have had to replaced due to lack of load carrying ability to meet European loading standards [2 & 3]. In Northern Ireland the Roads Service, the total number of bridges on motorways, trunk and non-trunk roads is about 6400 of which 64% are masonry structures. The maintenance of these masonry structures is an annual exercise requiring significant funding and staff resources and any saving that can be achieved has significant financial implications. Annual expenditure on the maintenance and strengthening of minor structures, the majority of which are masonry arches, now amounts to about 5 million in Northern Ireland alone. An arch bridge system with low or zero amounts of corrodible reinforcement which does not require centering has been developed under a Knowledge Transfer Partnership (KTP) between Queen s University Belfast and Macrete Ltd. The prototype arch unit has been manufactured in Macrete, lifted successfully and monitored during backfill operations; as outlined in previous publications [ 4 & 5]. This paper presents further development of the arch system. A summary of the results of recent load testing of a prototype arch with concrete backfill and preliminary analysis of the prototype are discussed in this paper. The arch system was analysed using the software package ARCHIE [6]. CONSTRUCTION DETAIL Flexible arch construction There are two options for the construction of the arch unit (Fig. 1). The voussoirs can be precast individually, laid contiguously horizontally with a layer of polymer grid material placed on top. Material tests on the polymer reinforcement have been reported in a previous paper [4] and a summary of the tensile properties is given in Fig.2 and Table 1. The individual voussoirs are then interconnected by an in-situ layer of concrete which is placed on top. Alternatively, the arch unit can be made in a single casting operation by using a shutter with wedge formers spaced to simulate the tapered voussoirs. The arch unit can be cast in convenient widths to suit the design requirement, site restrictions and available lifting capacity. When lifted, the wedge shaped gaps close, concrete hinges form in the top layer of concrete and the unit is supported by tension in the polymer grid (Fig.2). The arch shaped units are then placed on a pre-cast footings or anchor blocks. When in the final arch position, the self-weight is carried by compression in the arch ring and the arch behaves as an unreinforced masonry arch. Technical session on xxxxx Page 2

3 insitu screed cast over voussoirs polymeric reinforcement individually cast voussoirs (a) Construction of arch unit using precast individual voussoir concrete blocks Figure 1: Two forms of flexible arch construction formwork (b) Monolithic construction of the arch unit using a special form of pre-cast wedges Table 1: Tensile strength of polymeric reinforcement. Sample ID Average tensile strength (kn/m width) 100/ % 150/ % Figure 2: Typical failed specimen of polymer reinforcement Maximum creep at ultimate load 2.0m 5.0m Figure 2: Arch lifting process and final geometry Prototype arch construction details The arch details were: Voussoir dimensions 324mm 300mm 200mm Effective span: 5.24m Internal height: 2.00m Depth of arch ring: Width of arch ring: 1.00m 0.240m (40mm top screed) Polymer Reinforcement 150/100 2 layers over middle 17 blocks and 1 layer for remaining outer 3 blocks Arch ring concrete compressive strength* ~55N/mm 2 Backfill lean mix concrete to 0.4m above arch * information based upon average 28 days cube results and all concrete in the arch ring in excess of 28 days. Technical session on xxxxx Page 3

4 LOAD TESTING ON 1M PROTOTYPE The 1m prototype flexible concrete arch system as shown in Fig. 3 was tested in accordance with the requirements of Macrete Ltd. and following the guidelines in BS8110 [ 7 ]. A simulated static wheel load was applied at the midspan and third span of the arch ring. It is recognised that, in practice, there will be a dynamic amplitude factor above the static load and Table 1 summarises the single wheel load and factors of safety used to establish the test loads. Bridges are designed under static load conditions with factors of safety applied to these loads. The single wheel load, for the intended category of bridge, is 5.75t. However, both of the full test loads were ~35t, six times the single wheel load. Figure 3 : Test set-up for midspan loading Table 1: Summary of test loads Single Axle Load (AL) 11.5 t Single Wheel Load (SWL) 11.5/2 t = 5.75t Dynamic Factor (DF) 1.8 Contingency Factor (CF) 1.1 Load Factor for Ultimate Load (ULF; γ f ) 1.65 Load Factor for Service Load (SLF; γ f ) 1.2 Loading Stages Load applied (t) Stage 1: Single wheel load (SWL) 5.75 Stage2: SWL x CF 6.33 Stage 3: SWLx CF x DF Stage4: Service Load Stage 5: Ultimate Load Instrumentation The instrumentation set-up for the midspan and third span load tests is shown in Fig. 4. Deflection transducers were used to monitor both horizontal and vertical deflections and vibrating wire strain gauges were used to measure crack openings at the joints between voussoirs. Technical session on xxxxx Page 4

5 300mm Φ loading plate Applied midspan or third span L/2 = 2.618m L/3 = 1.745m temporary polythene liner for demolition purposes H V 0.4m minimum fill H7 V3 H V 4 = transducers = vibrating wire gauges temporary plywood separator for demolition purposes H9 2.00m V1 clear span = 5.00m V5 Testing procedure A typical test arrangement is depicted in Fig. 4. A circular concentrated load was applied at the loading position via a 300 mm diameter steel plate bedded on soft board. The application of load was from an accurately calibrated 500 kn hydraulic jack system and the test rig was assembled with the top beam horizontal about both axes thus minimizing eccentricity effects. The simulated static wheel load was applied to midspan and third span position. The load was applied incrementally and deflection and strain measurements were recorded at each increment of load. A service test load of 10t was applied twice at each load position prior to the application of the full test load. During the full test load, the load was held at each of the load stages outlined in Table 1. The behaviour of the arch and backfill was monitored via digital photos and observation made throughout the duration of the test. Observed behaviour effective span, L = 5.235m Figure 4: Instrumentation and test-set-up for arch ring and arch with backfill TEST RESULTS For both the midspan and third span loading, no cracking was observed under the two test loads of 10t. The first cracking appeared at an applied load of 20t in the concrete backfill. In both cases, the crack started on a line adjacent to the perimeter the loaded area and propagated diagonally towards the arch ring voussoirs. Under midspan loading, the crack followed the vertical line of the 18mm plywood stop end at the midspan which had been used between the two sides of the backfill to facilitate demolition. The plywood stop end had been placed across the whole width of the arch (i.e. 1m length) and this probably caused additional cracking in the concrete backfill. There was no visible opening in the joints between the voussoirs and no further cracks developed under the full test load. Under third span loading and at the full test load of 352kN, the joint directly below the plywood stop end had opened by 2mm. This opening was partly due to the differential settlement in the backfill due to presence of the plywood across the full width of the arch unit which provided a shear plane. Cracking also occurred horizontally adjacent to the anchor block. After unloading, there was visible recovery in the deflections and cracking in the arch system. Deflection measurements The overall deflections (i.e. the vector sum of the maximum horizontal and vertical deflections) have been summarised in Table 2 and the deflections at the maximum applied loads are more clearly illustrated in Figs. 5 and 6 below. It can be seen that the maximum deflection under the midspan load was 2.3mm inwards at an applied load of 340kN (~34t). Technical session on xxxxx Page 5

6 The maximum deflection under the third span loading was 10.3mm outwards at the third span at an applied load of 352kN. A deflection of 10.3mm is equivalent to the (effective span / 508) and within acceptable limits for deflection. It should be noted that a plywood stop end had been used between the two sides of the backfill to ease demolition (see Fig.4). This caused a higher degree of cracking at the midspan compared to a continuous backfill and, coupled with the polythene liner, probably gave a conservative prediction of the deflections compared to an arch ring without a liner or plywood at midspan. Table 2: Summary of maximum deflections Test No. Transducer position Maximum Vertical Defltn. (mm) Maximum Horizontal Defltn. (mm) 1: midspan B V4/H B V : midspan B V2/H B V Total Deflection (mm) 3: midspan B V2/H inwards B V inwards 4: third span B V4/H F V5/H : third span B V4/H : third span (unloading after 352kN) F = Front face B = Back face F V5/H B V2/H outwards B V4/H inwards + = outwards movement of arch - = inwards movement of arch 2500 Deflections x 50: Full Test Midspan Load = 340kN H8 V2 H7 V3 H6 V1 V4 H9 V5 500 transducer 0kN arch Front Deflection x 340kN Back Deflection x 340kN Figure 5: Test 3 - Full test midspan: Deflection 340kN load Technical session on xxxxx Page 6

7 H8 V2 H7 V3 H6 V1 V4 H9 V Figure 6: Test 3 - Full test third span Deflection 352kN load From the load versus deflection results, it was noted that there was a shift in the reading at an applied midspan load of 200kN. This was due to cracking in the backfill at the position of the plywood, as discussed earlier. The rate of change in deflection also changed at an applied third span load of 200kN. This was also due to cracking in the concrete backfill. However, the maximum deflections, for both loading conditions, were less than span/500 at an applied load of ~34t (six times the single wheel load). The recovery of the arch after loading was good. For example, for the full test load at midspan, the maximum deflection at V3 was 2.0mm at an applied load of 340kN. After unloading, the permanent deflection was 0.3mm. This equates to an 85% recovery in deflection which is within the acceptable limits given in BS 8110: Pt 2: Section 9 [7]. Strain measurements midspan voussoir The strain measurements were very low with the midspan load indicating low levels of stress in the arch ring. Under the third span loading, the strain measurements were also low. However, the largest opening occurred at the midspan voussoir right hand side joint which did not have a gauge (Figure 7). The joint opening was approximately 1-2mm at the maximum load at third span. Figure 7: Full test load at third span at an applied load of 352kN Technical session on xxxxx Page 7

8 ANALYSIS OF ARCH SYSTEM An analysis of the arch unit was carried out using ARCHIE, a numerical analysis package which allowed for interaction with the arch backfill. It is important to note that this software is also used by the DRD Road Service in Northern Ireland for load assessment analysis of their arch bridges. Therefore, validation of the manufactured arch unit using ARCHIE was a critical task in the development of the arch system. The arch unit was analysed under different wheel loading conditions. A typical case of arch unit analysis is shown in Figure 8. An arch unit of the required geometry can be created and loaded with the standard wheel loads. A line of thrust is indicated in Figure 8. Under design loading, the position of the thrust line in the arch unit gives information about the stability of the unit. Furthermore, it was found that the thrust line is affected by the application of Passive Pressure (PP) and Backing Material (BM) at the springing level. Therefore, for a particular loading condition, using a suitable value of PP and BM, the required thickness of the arch unit can be found. A similar deflected shape was given for the test load condition although the predicted ultimate capacity was conservative based on the actual load carrying capacity of the arch system (which is greater than the applied test loads which were not ultimate loads). Standard Wheel Loading Backing Backfill Arch Unit Thrust Line Passive Pressure Fig 8 Analysis of the arch system using ARCHIE software CONCLUSIONS It can be concluded that the 1m arch ring with concrete backfill was capable of supporting a midspan load of 34t and a third span load of 35t. The arch ring showed good recovery in deflections and cracking after the removal of all load. This was despite the presence of the plywood stop end across the width of the backfill at midspan and the polythene layer between the arch ring and the backfill. These were used to facilitate demolition. The maximum test loads were six times the single wheel load and nearly twice the ultimate load including ULS, dynamic and contingency factors of safety. The maximum deflection, with third span load, was 10mm and is equivalent to (span/508) which is within acceptable limits for deflection. The maximum deflection occurred at the third span and there was a 69% recovery in the maximum deflection after the removal of the all load. This is good recovery for an applied load which is nearly twice the ultimate design load and suggests the maximum loading was in the elastic/plastic range of the load/deflection response of the arch system. The strain values were very low at maximum applied loads indicating low levels of stress in the arch ring. The results form ARCHIE gave a similar deflected shape to the measured results although the predicted load capacities were conservative. Technical session on xxxxx Page 8

9 Acknowledgements The Authors would like to thank the Institution of Civil Engineers, Invest NI and the Knowledge Transfer Partnership scheme for supporting the development of this novel system. References 1 UK Highway Agency, BD 57/95 & BA 57/95, Design for Durability Design Manual for Roads and Bridges, Volume 1, Section 3, Department of Transport, Highway and Traffic, BD37/01, Departmental Standard, Loads for Highway Bridges (used with BS5400: Pt2) Design Manual for Roads and Bridges, Volume 1, Section 3, Part 14, Department of Transport, Highway and Traffic, BD44/95, Departmental Standard, The assessment of concrete highway bridges, Volume 1, Section 3, Department of Transport, Highway and Traffic, Taylor S E, Gupta A, Kirkpatrick J, Long A E, Rankin G I B and Hogg I, Development of a novel flexible concrete arch system, 11th International Conference on Structural Faults and Repairs, Edinburgh, June Gupta, A., Taylor, S.E., Kirkpatrick, J., Long, A.E. and Hogg, I. A Flexible Concrete Arch System for Durable Bridges Proceeding from IABSE Conference, Budapest, ARCHIE-M: Masonry Arch Bridges and Viaduct Assessment Software, Version 2.0.8, OBVIS Ltd. UK. 7 British Standards Institute, BS 8110: Part 2: Structural use of concrete: Code of practice for special circumstances Section 9: Appraisal and testing of structures and components for construction, London, Technical session on xxxxx Page 9