engineering approaches to the condition assessment of stone arch masonry … · condition...

Engineering Approaches to the Condition Assessment of Stone Arch Masonry Bridges

November 2017Engineered to deliver

A case study review of the inspection and condition assessment of six (6) stone masonry bridges throughout regional Victoria.

Engineered to deliver

“Sterling is a specialist provider of structural engineering and asset management consulting to the

road and rail transport industries. We are a client-focused business.”


• Overview of Stone Arch Bridges in Victoria

• Australian guidance and approaches to the condition assessments

• Masonry - Stone vs. Brick

• Case study review of inspection findings for six stone masonry arch bridges

• Summary of Structural Maintenance

• Key Outcomes

Outline

Overview of Stone Arch Bridges


• Some structures in Australia over 150 years old.

• Relatively young age of Australian Infrastructure has led to a small portion of stone arch bridges constructed.

• Generally constructed from stone sourced from nearby quarries–Bluestone / Granite.

• Understanding of material behaviour must consider separation of clay brick and stone masonry.

Brick & Stone Masonry – What’s the difference?


Stone

• Large unit size

• Thin mortar bonds

• Arch barrels consist of a single arch ring course

• Liable to fault lines in stone

• Uneven protruding surfaces where not dressed

Brick

• Small unit size – 215mm x 112.5mm x 65mm

• Mortar thickness = 10mm (Typically)

• Arch barrel consists of multiple arch rings

• Liable to expansion overtime

• Weathering of clay brick units

Overview of Stone Arch Bridges in Australia


• Stone arch bridges demonstrated life-span in excess of 2000 years overseas.

• Increase in loading over time.

• Comparatively limited knowledge within Australian Engineering sector.

• Broad guidance outlined in AS5100.8 for masonry bridges, covers approaches to:• Crack diagnostics and management prior to treatment

• Mortar repair guidance – ‘soft’ lime vs. ‘hard’ mortars.

• Overseas practices for the management of stone arch bridges.• VicRoads Inspection Manual Based on Ontario Ministry of Transportation, Ontario Structure Inspection

Manual

Case Study – Six Arch Rail Bridges


Inspection of Six Stone Arch Bridges


Six (6) stone arch bridges located West of Melbourne constructed circa 1860s.

Five (5) bridges single span supporting single track only.

One (1) three span bridge supporting two tracks North-East of Melbourne (pictured right).

Span lengths range from 3m up to 15m.

Methods of inspection include height access via scaffolding or EWP access.

Summary of Defects Identified


1. Accumulation of Salt Deposits and Stalactite Formations

2. Stone Masonry Wall Distortions

3. Longitudinal Cracking in Arch Soffit

4. Cracking and Spalling of Stone Masonry Units

5. Dropped Masonry Arch Units.

Accumulation of Salt Deposits and Stalactite Formations


Accumulations of calcified minerals to the exposed arch barrel face – causing stalactite formations

Caused by the leaching of water containing salts through the mortar joints from retained soils.

Forms a hard crust layer on the arch barrel surface.

Stalactite layers and mineral deposits can hide possible stone cracking and mortar separation defects.

Presence of mineral deposits prevents accurate inspection of mortar for cracking and fretting.

Should be addressed at early stages.

Stone Masonry Wall Distortions


Wall separations and bulging.

Due to increased retaining wall pressures resulting from:

• Increased live load surcharge through fill material

• Poor water drainage leading to increased hydrostatic pressure

• Poor quality fill/embankment material leading to retaining wall settlement

• Loss of mortar leading to loss of bond strength –not generally applicable to stone masonry walls which are primarily gravity walls

Longitudinal Cracking in Arch Soffit


Cracks propagate generally in the direction of the vehicle movements.

Longitudinal cracks caused by shear and torsional flexure through arch barrel occurring near to or around the bridge centre line.

Cracking identified in inspection located between the loading paths of the tracks above.

Caused by lack of stress transfer transversely across the arch barrel under alternating live loading.

Cracking separations primarily through mortar joints – difficult to view from ground level.

Cracking and Spalling of Stone Masonry


Cracking and spalling through arch barrel units – in this case due to stress increases to stone units resulting from stone settlements.

However, causes may vary – important to identify cause prior to development of remedial actions

Likely causes may be:

• Differential settlement

throughout structure foundations

• Stress re-distributions due to loss

of mortar.

Dropped Masonry Units


Caused by deterioration of mortar leading to loss of bonding between units -

• Cracking thorough arch.

• Fretting / loss of mortar.

Wedge shape of arch barrel units settle into position.

Units jam into position.

Dropped units susceptible to cracking.

Weight of these units and their size presents challenges with re-instatement (approximately 200 – 600kg)

Due to crack sensitivity of stones, cannot drill fix into the units.

Initiate monitoring regime to understand if movement is active over short-term period (i.e. 12 months)

Summary of Structural Maintenance


Provision of defect mapping sketches assisted in defect pattern identification.

Direct inspection of the arch allowed for identification of minor crack patterns.

Initiate monitoring regime to detect movements over short term period (i.e. 12 months) and identify movement patterns for:

• Dropped stones units to identify severity of defect.

• Wall distortions throughout to understand rate of settlement if still occurring.

• Longitudinal cracking though arch to determine if relative movements between arch sections are ongoing.

Full depth injection of longitudinal cracks to reinstate stone bond reinstating stress distributions post monitoring regime.

Epoxy injection to cracked stone units

Repoint arch units to reinstate mortar – full depth repointing using similar mortar is essential (AS5100.8)

Key Outcomes


➢ Review of historical records of defects essential in determining rate of deterioration.

➢ Need for deeper experience of arch structures can be overlooked.

➢ International practices for the management of arch masonry structures should be sought for proven and best practices.

➢ Inclusion of new age technology such as drone inspection and 3-D Scanning/Point Cloud data should be considered for adding value in the management of these arch structures.

➢ Considerations of stone damage from remedial measures.

➢ Young engineers should be aware of the engineering principles and practices pertaining to old stone arch bridges. Bridge terminology can be lost.

engineering approaches to the condition assessment of stone arch masonry … · condition...

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