Masonry Arch Bridges Condition Appraisal and Remedial Treatment

Masonry Arch Bridges: Condition Appraisal and Remedial Treatment CIRIA RP 692 - Examples -

Assessment

Level of Analysis Elements of Analysis Level 1 Basic Analysis Level 2 Detailed Analysis Level 3 Special Analysis Semi-empirical methods should only be considered as part of the appraisal by inspection where the owner considers this procedure as appropriate. Basic 2 D limit analysis methods should be used to assess all structures except significantly skewed bridges, long spans, bridges with unusual geometries and important structures. The bridges excluded from the previous group and those failing its assessment should be analysed using solid mechanics methods. These analyses should be adapted to the available bridge data and combined with site investigations and monitoring as appropriate, if more refined analyses are required to demonstrate the adequacy of the structure to fulfil its purpose. The use of characteristic or worst credible strengths of materials may be used, based on test results from samples taken from the structure. The level of refinement achieved before it is decided that the structure is unfit for purpose will depend on owner needs and constraints. Where the use of refined solid mechanics methods cannot demonstrate structural adequacy, it may be possible to demonstrate its adequacy and inherent safety by comparison of its safety characteristics with other similar structures using stochastic approaches and probability analysis Actual live traffic loadings of the bridge might be determined statistically and used for analysis The safety criteria of the bridge in question might be assessed and specific relaxations considered if this can be justified and the acceptability of risks clearly demonstrated with adequate confidence This level of assessment would require considerable specialist knowledge and research, and the benefit is unlikely to be justifiable only in the most critical of cases.

Method Main Parameters MEXE Applicability Span Rise at crown and quarter point Arch thickness and crown cover Arch material Backfill material Mortar joint depth and condition General condition factor HEYMAN’S LIMIT ANALYSIS METHODS Arch geometry Compressive strength of masonry (in some models) Masonry and backfill densities DISCRETE AND INDISCRETE RIGID BLOCK METHODS CASTIGLIANO’S NON-LINEAR Arch geometry Compressive strength of masonry (in some models) Masonry and backfill densities Dilatancy Angles of friction (radial and tangential) Arch geometry Compressive strength of masonry (in some models) Masonry and backfill densities Only applicable to spans shorter than 18 m Not applicable for flat or appreciably deformed arches Not applicable for multi-span bridges, although the BR version of MEXE accounted for that using an extra factor Advantages Can be applied to multi-ring arches Can be applied to multi-span arches Can produce unsafe results in shallow arches with large spans (bridges where snap-through failure is possible) Some methods might be able to consider skewed arches Cannot be used with skewed If applied by an experienced engineer, the condition factors may allow to account for effects difficult to model Can be difficult to apply to deep arches Can be difficult to apply to shallow arches with large spans Can be difficult to apply to bridges with complex geometries Disadvantages For simple structures, it can produce safe results from very limited input and at a limited cost These methods are very effective when the engineer has a clear idea of the mechanism by which the structure will fail Quick and reliable for a significant range of bridge configurations It is a very versatile tool for an experienced engineer Simple and easy to use The only resisting mechanisms considered are the arch and the weight of the backfill The limiting load criterion is not realistic Unnecessary assumptions on geometry and load locations When applied by inexperienced engineers, some modifying factors can be dangerously subjective Its results are assumed to be conservative, but can be over-conservative as well as unsafe Cannot consider the effect of strengthening measures For Upper bound methods, if some failure mechanisms are ignored, the method would provide an unsafe prediction For Lower Bound methods, if some kinematically admissible equilibrium states are ignored, the method would provide a conservative prediction. Similarly, if an assumed equilibrium is not possible (because some failure criterion has been ignored) the method will produce unsafe results Cannot consider ring separation Cannot consider snap-through failures Cannot consider the contribution of the spandrel walls The separation between rings during cannot be reproduced. Instead, the used has to assume whether ring separation will or will not take place Consideration of masonry compressive failure might increase the computational time The prediction of the in-service behaviour can be quite sensitive to the boundary conditions and the initial stress state, which are very difficult to determine

Monitoring and Inspection

General Inspection Techniques Specialist Inspection Techniques Testing Techniques Monitoring Techniques Bridge records Visual observation Hammer tapping Surveying Photography Sonics Conductivity Radar Ultrasonics Infra-red thermography Impact Echo Tomography GPR Techniques Photogrammetry Laser scanning Strain measurement Acoustic emission Electrical conductivity Endoscope Scour detection Coring Petrological examination Geotechnical investigation Load testing Flat-jack testing Materials testing Crack monitoring Strain measurements Displacement measurement Scour detection Fibre-optic sensors

METHOD VISUAL COMMENTS GOOD MODERATE POOR Traditionally, visual inspection has been the first level of inspection. Visual signs of deterioration have usually led to further inspection and/or repair. Visual inspection has the major disadvantage of only recording that which can be seen which may of course be a consequence of that which cannot be seen. All the basic dimensions of the structure should be recorded. The type of material from which the structure is constructed, its general condition and any defects should be mapped such as cracks, settlements, distortions etc (including the location of any slipped voussoirs or bricks). Any repairs or previous works should also be recorded. The presence of water should be recorded. Material: Hard stone Engineering class bricks Sealed surface Well pointed Shape: Arch barrel defined shape Walls, abutments and piers plumb (or as built). Fabric condition: Units and mortar in good condition. Cracks: Longitudinal: None present Transverse: None present Diagonal: None present Abutment, Pier and Wall Cracks: None present (Less than 1 mm measured over a 1 m gauge length in any direction) Settlement: Longitudinal and transverse relative settlement less than 1 in 100. Vehicular surface: No significant defects Wall Alignment: No evidence of wall sliding, bulging or tilting Vegetation: None present Water: None present and no evidence that water has been present and caused deterioration and/or damage Material: Medium stone Building brick Up to 20% not well pointed Shape: Arch barrel some movement over up to 25% of the arch surface but no areas flatter than a 0. 1 m offset on a 2 m straight edge set longitudinally or 0. 05 m offset on a 2 m straight edge set transversely. Fabric condition: Up to 5% of the mortar joints displaying signs of deterioration ie missing or crumbling Up to 5% of the units displaying signs of deterioration ie spalling, crumbling, fissures. Cracks: Longitudinal: Outside the middle third of the arch, less than 1/10 of the span in length Transverse: None present Diagonal: None present Abutment, Pier and Wall Cracks: Some present over 20% or more of the surface such that they are up to 6 mm measured over a 1 m gauge length in any direction. Settlement: and surfacing. Material: Soft stone Weak brick(fk less than 20 N/mm 2) Up to 20% not well pointed. Shape: Arch barrel has general movement over more than 25% of the arch surface. Areas identified than are flatter than a 0. 1 m offset on a 2 m straight edge set longitudinally or 0. 05 m offset on a 2 m straight edge set transversely. Seriously misshaped arch barrels and distorted walls can be dangerous and require immediate investigation. Fabric condition: More than 5% of the mortar joints displaying signs of deterioration ie missing or crumbling. More than 5%of the

Specialist Inspection Techniques Comments Sonics This technique is dependent on measuring changes in the velocity of sonic pulses traveling in a solid material, on the basis that velocity is dependent on the density and elastic properties of the material. Internal discontinuities (e. g. cracks, voids, boundaries between material types) can potentially be detected using this technique. Conductivity Electrodes are inserted into the structure or ground in order to determine its electromagnetic conductivity and hence estimate the moisture content of the masonry or the presence of voids etc. Radar is an echo sounding technique that involves the transmission of short duration pulses of radio energy into a structure and measurement of the reflected signals. The speed, strength and frequency content of the reflected signals can be used to determine moisture content. Ultrasonics The velocity of ultrasonic pulses travelling in a solid material depends on the density and elastic properties of the material. Pulses are not transmitted across voids, so by measuring apparent speeds of pulses it is possible to determine the competence of the material and the location of delaminations etc. Infra-red Thermography involves the measurement of small variations in surface temperature (0. 1 degree Celsius) which are used to predict internal conditions. Impact Echo This technique can be used to determine the depth of delaminations (e. g. ring separation) and the thickness of structural elements. It is based on the use of impact-generated stress waves that propagate through the structure and are reflected by internal flaws and/or external surfaces. Tomography Involves the measurement of a net of stress waves (sonic) through the structure. This gives information in 3 -dimensions that enables an assessment of the location of possible defects to be made. GPR Techniques Ground Penetrating Radar (GPR) is an echo sounding technique where electromagnetic impulses are transmitted into the bridge and a receiver detects reflections from material boundaries. It can be used to determine construction details and conditions including delamination and voiding. Photogrammet Several digital images are recorded of the structure from different locations. Using the collected information a 3 -dimensional image of the structure can be created using specialist computer

Repair

Repair methods for consideration in the repair of arch barrels (for single-span bridges) Is it possible to excavate back to the arch barrel extrados? YES NO It is assumed that the spandrel wall is in good condition. Any bulging, tilting, sliding etc has been appropriately repaired using tie-bars and patress plates or other means. START NOTE: The repairs identified in this flowchart will change the nature of the structural behaviour of the arch and consequently appropriate assessment techniques should be used to determine the new carrying capacity of the bridge. YES Is the structural integrity of the arch barrel very poor? NO Does the arch barrel need waterproofin g? NO Is the cracking of the arch barrel extensive? YES Saddle, waterproof and repoint arch barrel Install relieving slab, waterproof and repoint arch barrel Stitch cracks or reconstruct NO Grout arch barrel and/or retrofit reinforcement YES Use spray/cast insitu concrete repair to arch barrel intrados Reconstruct YES Is the fault due to active subsidenc e? NO Is the repair required to be permanent? YES Is the arch heavily distorted? NO NO YES Use sprayed concrete repair to arch barrel intrados Install steel plate lining to arch barrel intrados NO Underpin structure and/or support arch with steel ribs Is the span of the arch >5 m? YES Install corrugated steel lining to arch barrel intrados

Technique Concrete Saddle Category Structural Defect & Location Strengthening Inadequate overall load carrying capacity of arch barrel in conjunction with spandrel wall and waterproofing failures Engineering Aspects Parapet Upgrading Strengthening Inadequate impact resistance of parapet(s) Repointing Remedial Deterioration of mortar joints Advantages / Disadvantages The adequacy of the existing structure should be checked to ensure that it is capable of sustaining the enhanced loading with the saddle in place. Decide whether the saddle is to act compositely with the existing structure or not. Check the structural interaction, bearing in mind that the barrel is particulate and heterogeneous whilst the reinforced concrete saddle is not. Particular attention should be given to the load paths through the abutments, piers and their foundations. Advantages • No change to appearance as hidden • Facilitates other repairs/parapet upgrades/waterproofin g • Enhanced live load capacity Disadvantages • Traffic disruption during construction • Relative cost • Increase in crown depth possible. The consequence of impact should be given detailed consideration particularly in respect of vehicular containment and falling material. Where the parapet is a new build, its interaction with the existing structure should be checked. Advantages – - Enhanced vehicle containment and/or reduction in likelihood of falling material Disadvantages – - Traffic disruption during construction - Provision of access - Relative cost Careful consideration should be given to the Advantages- Simple established

Concrete saddle Cost Band **** Special Considerations Temporary Works design for all stages Lightweight Concrete supply Waterproofing Heritage Proposed remedial treatment to be assessed for impact on bridge’s heritage value Durability Structural concrete saddle to be designed to give a minimum design life of 120 years. However, the overall life of the structure will be governed by the current level of dilapidation. Inspection Not applicable as the saddle will be buried. Routine visual and tactile inspection of the structure in accordance with the asset steward’s requirements. Performance Effective implementation and inspection / maintenance will enhance structural performance in line with the strengthening or repair design life. Further Guidance BA 16/97, BS 5400 Part 4, BS 5628