TESTING OF A NOVEL FLEXIBLE CONCRETE ARCH SYSTEM

Transcription

1 Proceedings of ACIC 7 Advanced Composites in Construction 2 nd 4 th April 7, University of Bath, Bath, UK TESTING OF A NOVEL FLEXIBLE CONCRETE ARCH SYSTEM S E Taylor 1, A Gupta 2, J Kirkpatrick 3 and A E Long 1 1 Queen s University Belfast 2 Macrete Ireland Ltd. 3 Owen William Consulting NI Abstract: This paper describes the further development and testing of a novel flexible concrete arch system which requires no centring in the construction phase nor steel reinforcement in the long-term. The arch is constructed from a flat pack system by use of a polymer reinforcement for carrying the selfweight but behaves as a masonry arch once in place. The paper outlines the construction of a 1m prototype arch and load testing of the backfilled arch system. Some comparison to results from ARCHIE have been made. The arch had a load carrying capacity far in excess of the current design loadings. Keywords: masonry arches, polymer reinforcement, concrete voussoirs 1 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 generally built of reinforced concrete or steel. The introduction of these materials allowed arch bridges to be longer with lower height and can either be manufactured as pre-cast concrete or assembled on site in steel. However, a common problem with such bridges is corrosion of steel and particularly reinforcement. This has led to high repair and 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 as they are unable to meet European loading standards [2 & 3]. In the Roads Service in Northern Ireland, the total number of bridges on motorways, trunk and non-trunk roads is about 6 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. Hence, an arch bridge system with low or zero amounts of corrodible reinforcement which does not require centring 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 ]. 2 Construction detail 2.1 Flexible arch construction There are two options for the construction of the arch unit. The voussoirs can be pre-cast individually, laid contiguously horizontally with a layer of polymer grid material placed on top. The individual voussoirs are then interconnected by an in-situ layer of concrete which is placed on top (Fig. 1). 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 1 Lecturer, s.e.taylor@qub.ac.uk 2 KT Associate, abhey@macere.com 3 Consultant, kirkpatrick@coleraine.fslife.co.uk

2 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. 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 un-reinforced masonry arch. insitu screed cast over voussoirs polymeric reinforcement individually cast voussoirs (a) Construction of arch unit using precast individual voussoir concrete blocks polymeric reinforcement 5.m 2.m (b) Monolithic construction of the arch unit using a special form of pre-cast wedges formwork Figure 2: Arch lifting process and final geometry Figure 1: flexible arch construction 2.2 Prototype arch construction details The arch details were as given in Table 1 Table 1: Arch details Voussoir dimensions Clear span: 5.m Effective span: 5.24m Internal height: 2.m Depth of arch ring: Width of arch ring: 1.m Polymer Reinforcement (as properties outline in previous papers [4 & 5] Arch ring concrete compressive strength* 324mm mm.24m (4mm top screed) / 2 layers over middle 17 blocks and 1 layer for remaining outer 3 blocks ~55N/mm 2 mm Backfill lean mix concrete to.4m above arch extrados In Table 1 * is based upon average 28 days cube results and all concrete in the arch ring in excess of 28 days. 3 Load testing of 1m prototype arch 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 BS811 [ 7]. A simulated static wheel load was applied at the and of the arch ring. It is recognised that, in practice, there will be a dynamic amplitude factor above the static load. The single wheel load and factors of safety used to establish the test loads were based upon the requirements in BD91/4 [8]. Bridges are designed under static load conditions with factors of safety applied to these loads. An impact factor of 1.8 is recommended in BD91/4 and this takes into account dynamic amplitude effects in an arch bridge form. The single wheel load, for this category of bridge, is 5.75t. This equates to an ultimate design load (ULS) of : 5.75 x 1.65 x 1.8 = 17.1t and incorporating a contingency factors of safety of 1.1 for the test gives: 17.1t x 1.1 = 18.8t However, in both tests the ULS design load was exceeded and the maximum applied load was 35t. That is, the full test loads were six times the single wheel load and nearly twice the ULS design load. Digital photos and observations were made throughout the duration of the test.

3 mm Φ loading plate Applied or L/2 = L/3 =.4m minimum fill temporary plywood separator for demolition purposes temporary polythene liner for demolition purposes V2 H7 2.m clear span = 5.m effective span, L = 5.235m = transducers = vibrating wire gauges Figure 3: Instrumentation and test-set-up for arch ring and arch with backfill 3.1 Instrumentation The instrumentation set-up for the and load tests is shown in Fig. 3. 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. 3.2 Testing procedure A typical test arrangement is depicted in Fig. 4. A circular concentrated load was applied at the loading position via a mm diameter steel plate bedded on soft board (Fig. 4). The application of load was from an accurately calibrated 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 and position. The load was applied incrementally and deflection and strain measurements were recorded at each incremental loading. A service test load of 1t mm φ was applied twice at each loading plate load position prior to the Figure 4: t loading system application of the full test load. As the loading was increase the load was held at each of the load stages outlined in Table 1. The behaviour of the arch and the backfill was monitored via. 4 Test results 4.1 Observed behaviour For both the and loading, no cracking was observed under the two test loads of 1t. The first cracking appeared at an applied load of 2t in the concrete backfill. In both cases, the crack started on a line adjacent to the perimeter the loaded area and propagated towards the arch ring voussoirs. Under loading, the crack followed the vertical line of the 18mm plywood stop end at the which had been used between the two sides of the backfill to facilitate demolition. Under loading the crack propagated diagonally towards the intrados. 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 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. The openings in the voussoirs also closed.

4 4.2 Deflection measurements Typical load vs. deflection results are given in Fig. 5 and the maximum vertical, horizontal and the overall deflections (that is, the vector sum of the maximum horizontal and vertical deflections) have been summarised in Table 2. The deflections at the maximum applied loads are more clearly illustrated in Figs. 6 and 7 which show the exaggerated deflected shape at the two maximum applied load values. It can be seen that the maximum deflection under the load was 2.3mm inwards at an applied load of 34kN (~34t). The maximum deflection under the loading was 1.3mm outwards at the at an applied load of 352kN. A deflection of 1.3mm is equivalent to the (effective span / 8) 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 mid span 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. Table 2: Summary of maximum deflections Test No. Position Max. Vert. Defln. 1: 2: 3: 4: 5: 6: F = Front face B = Back face Max. Horiz. Defltn B / B B V2/H B Total Defln. B V2/H inwards B inwards B / F / B / F / B V2/H outwards B / inwards + = outwards movement of arch - = inwards movement of arch V2 H7 Full test Third Span Vertical Deflections : Front Full test Third Span Vertical Deflections : Back V Deflection V Deflection TEST 6: Full test : front face vertical defltns TEST 6: Full test : back face vertical defltns Full test Third Span RHS Horizontal Deflections : Back Full test Third Span Horizontal Deflections : Front H Deflection H Deflection TEST 6: Full test : front face horiz. defltns. TEST 6: Full test : back face horiz. defltns Figure 5: Applied mid span load vs. deflection for Test 6: full test load at

5 Deflections x : Full Test Midspan Load = kn transducer position arch Front Deflection 34kN Back Deflection 34kN Figure 6: Test 3 - Full test : Deflection 34kN load Deflection x : Full Test Third Span Load = 352kN transducer position arch Front Deflection 352kN Back Deflection 352kN Figure 7: Test 3 - Full test Deflection 352kN load

6 From the load versus deflection results, it was noted that there was a shift in the reading at an applied load of kn. 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 load of kn. This was also due to cracking in the concrete backfill. However, the maximum deflections, for both loading conditions, were less than span/ 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, the maximum deflection at was 2.mm at an applied load of 34kN. After unloading, the permanent deflection was.3mm. This equates to an 85% recovery in deflection which is within the acceptable limits given in BS 811: Pt 2: Section 9 [7] Table 3 summarises the recovery rate for each of the tests based upon the maximum deflections. From the load versus deflection, it was noted that there is a shift in the reading at an applied load of kn. This was due to cracking in the backfill as discussed in Section 3. The rate of change in deflection also changed at an applied load of kn. This was also due to cracking in the concrete backfill. However, the maximum deflections, for both loading conditions, were less than span/ at an applied load of ~34t (six times the single wheel load). Table 3: Summary of recovery in deflections Test No. 3: 6: thirdspan Position Maximum Vertical Deflection B V2/H % B % B V2/H7 5. 7% B / % Recovery Strain measurements The strain measurements were very low with the load indicating low levels of stress in the arch ring. Under the loading, the strain measurements were also low. However, the largest opening occurred at the voussoir right hand side joint which did not have a gauge (Figure 7). The joint opening was approximately 2mm under the maximum applied load at. Figure 7: Full test load at at an applied load of 352kN 4 Analysis of the arch unit An analysis of the arch unit was carried out using ARCHIE [6], a numerical analysis package which takes into account 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, assessment of the load carrying capacity of the flexible arch system 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 gave information about the stability of the unit. Furthermore, ARCHIE was able to demonstrate the change in the thrust line by changing the height of the Backing Material (BM) at the springing level and the effect of changing the Passive Pressure (PP). The passive pressure factor is equivalent to coefficient is equivalent to the Therefore, for a particular loading condition, arch ring depth and using a the appropriate for BM the passive pressure required to resist the arching thrust is given. Alternatively, the passive pressure factor can be fixed and the minimum arch ring depth established for the given loading conditions. A similar deflection response was given in the ARCHIE analysis in comparison to the test load results. However, the predicted ultimate capacity was conservative based on the actual load carrying capacity of the arch system (which was greater than the applied test loads in excess of the design ultimate loads).

7 4 References Figure 8: Typical ARCHIE analysis results Conclusions It can be concluded that the 1m arch ring with concrete backfill was capable of supporting a load of 34t and a load of 35t. The arch ring showed good recovery in deflections and recovery in the joint opening after the removal of all load. This was despite the presence of the plywood stop end across the width of the backfill at 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 load, was 1mm and is equivalent to (span/8) which is within acceptable limits for deflection. The maximum deflection occurred at the 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 was twice the ULS design load (which included a 1.8 dynamic impact load factor) and suggests the maximum loading was not the ultimate capacity 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 from ARCHIE gave a similar deflected shape to the measured results although the predicted load capacities were conservative. [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, [2] BD37/1, Departmental Standard, Loads for Highway Bridges (used with BS5: Pt2) Design Manual for Roads and Bridges, Volume 1, Section 3, Part 14, Department of Transport, Highway and Traffic, 1 [3] BD44/95, Departmental Standard, The assessment of concrete highway bridges, Design Manual for Roads and Bridges, Volume 1, Section 3, Department of Transport, Highway and Traffic, 1995 [4 ]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 6. [5 ]Gupta, A., Taylor, S.E., Kirkpatrick, J., Long, A.E. and Hogg, I. A Flexible Concrete Arch System for Durable Bridges proceeding form IABSE Conference, Budapest, 6. [6] ARCHIE-M: Masonry Arch Bridges and Viaduct Assessment Software, Version 2..8, OBVIS Ltd. UK [7] British Standards Institute, BS 811: Part 2: Structural use of concrete: Code of practice for special circumstances Section 9: Appraisal and testing of structures and components for construction, London, [8] BD 91/4 Departmental Standard, Unreinforced masonry arch bridges, Design Manual for Roads and Bridges, Volume 2, Section 2 Special Structures, Part 14 Acknowledgments The Authors would like to would like to express their sincere appreciation to the DoT, Invest NI and ICE for supporting this Knowledge Transfer Project.