1 trail bridge inspection timber deterioration & decay
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TRAIL BRIDGE INSPECTION
TIMBER DETERIORATION & DECAY
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TIMBER DETEROIRATION & DECAY
OBJECTIVES: Recognize the Types and Causes of Timber
Deterioration Recognize Natural Defects In Timber Elements Understand What Causes Decay Identify Where Decay Is Likely To Occur Identify Methods Used to Detect Deterioration
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Types and Causes of Timber Deterioration
•Natural Defects In Timber Elements•Fungi (Brown and White Rot)•Insects
•Termites•Carpenter Ants•Powder-post Beetles•Caddis Flies•Marine Borers
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Types and Causes of Timber Deterioration
•Delamination of Glulam Beams•Fire•Impact or Collisions•Abrasion or Wear•Overstress•Weathering or Warping•Animals
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NATURAL DEFECTS OF TIMBER
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Fungi
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Mold and Stain
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Brown and White Rot
Fungi types that weaken the wood include:
Brown rot - degrades the cellulose and hemi-cellulose leaving the lignin as a framework which makes the wood dark brown and crumbly
White rot - feeds upon the cellulose, hemi-cellulose, and the lignin and makes the wood white and stringy
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Brown and White Rot
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Insects
Insects tunnel in and hollow out the insides of timber members for food and shelter. Some common types of insects include:
Termites Carpenter ants Powder-post beetles or lyctus beetles Caddis flies
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Termites
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Carpenter Ants
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Powder-post Beetles or Lyctus Beetles
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Caddis flies
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Marine Borers
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Marine Borers
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Marine Borers
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Types and Sources of Deterioration
Delaminations – Delaminations occur in glued-laminated members
when the layers separate due to failure within the adhesive or at the bond between the adhesive and the laminate.
– They provide openings for decay to begin and may cause a reduction in strength.
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Other Types and Sources of Deterioration
Delaminations
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Types and Sources of Deterioration
Loose connections– Loose connections may be due to shrinkage of
the wood, crushing of the wood around the fastener, or from repetitive impact loading (working) of the connection.
– Loose connections can reduce the bridge’s load-carrying capacity.
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Types and Sources of Deterioration
Loose connections
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Types and Sources of Deterioration
Surface depressions– Surface depressions indicate internal collapse, which
could be caused by decay.
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Types and Sources of Deterioration
Fire
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Types and Sources of Deterioration
Impact or Collisions– Severe damage can occur to truss members, railings, and
columns from floating debris or ice.
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Types and Sources of Deterioration
Abrasion or Wear
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Types and Sources of Deterioration
Excessive Wear
Usually indicated by knots that are above the surrounding wood.
Nails protruding above the surface and bent over.
Rounding on the edges of the planks at the bridge end.
Dished and polished Planks.
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Types and Sources of Deterioration
Overstress
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Types and Sources of Deterioration
Overstress
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Types and Sources of Deterioration
Weathering or Warping
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Other Types and Sources of Damage
Wood swell
Animal
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NATURAL DEFECTS OF TIMBER
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NATURAL DEFECT DEFINITIONS
Check – A separation of the wood normally occurring across or through the rings of annual growth and usually as a result of seasoning.
Split – A separation of the wood through the piece to the opposite surface or to an adjoining surface due to the tearing apart of wood cells.
Shake – A lengthwise separation of the wood which occurs between or through the rings of annual growth.
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Defect - Check
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Defects - Split
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Defect - Shake
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Review of Natural Defects
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WHAT CAUSES DECAY?
Decay detection is a significant part of the timber bridge inspection.
Basic understanding of the decay is essential. The source of the decay is a plant – a fungus that has
tiny microscopic, thread-like roots called “hyphae” that penetrate throughout the wood in search of food.
The hyphae penetrate the wood secreting powerful enzymes that reduce the wood into food for the plant.
This chemical alteration of the wood cell is what we call decay.
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Life Cycle of Fungus
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Fungus
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DECAY MECHANICS
The wood is initially infected when the fungus spores land on the surface or in the checks and splits of wood and geminate.
The following four conditions must exist for germination:– A sufficient supply of oxygen.– A favorable temperature range. (32° - 90° F)– An adequate food supply.– Available water. (Wood must be above the fiber
saturation point of the wood. Moisture content of approximately 25%).
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Controlling Decay
Depriving the spores of any one of these requirements will effectively slow, stop or prevent decay.
For example:(1) If wood is underwater, there is no usable oxygen present and the
wood will not decay; HOWEVER DECAY WILL FLOURISH AT THE WATERLINE
(2) If the wood is treated, there is no palatable food for the fungi; HOWEVER, IF THE TREATMENT ENVELOPE IS BREACHED AS IN A CHECK - -DECAY MAY BE PRESENT.
(3) If the wood is always dry there is no moisture for germination.
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Where do I look for decay?
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Locations to look for decay:•Around Checks•Around Splits•Around Shakes•Around Cracks•Around Fasteners•Areas in contact with soil•Areas where debris and water collect
Common Areas Where Decay Occur
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Where do I look for decay?
Conditions and locations conducive to decay can be readily identified:– Members that are in contact with soil such as posts,
piling, abutments, wing walls, etc. – Sawn timber members that have large checks open
to the weather or stream flow (water entry).– Wood penetrating fasteners and wood to wood
contact.– End grained surfaces.
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Decay Detection Methods
Method for Detecting Exterior Deterioration– Visual Inspection– Probing
Method for Detecting Interior Deterioration– Sounding– Moisture Meter– Drilling and Coring
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Method for Detecting Exterior Deterioration -Visual Inspection
The simplest and most common method for locating deterioration and decay
The inspector observes the structure for signs of actual or potential deterioration and decay
Visual inspection requires a strong light and is useful for detecting intermediate or advanced surface decay
Visual inspection cannot detect decay in the early stages, cannot detect internal decay, and should never be the sole method used.
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Method for Detecting Exterior Deterioration -Visual Inspection
What to Look for:– Fruiting bodies almost always indicate a severe decay problem. – Staining and discoloration indicate areas of potential decay. – Sunken faces and localized surface depressions can indicate
decay.– Horizontal checks that can entrap water.– Interfaces between different wood members, wood and concrete,
etc.– Where dirt and debris has been placed against the wood.– At fastener locations– Insect activity – Most insects prefer the softer decayed wood to
the hard dry wood.
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Visual Signs of External Decay
Fungi or fruiting bodies (conks)
Under the bridge, on girders and sills, anywhere the wood is subject to wet/dry cycles.
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Visual Signs of External Decay
Spongy and Rotted areas.
Look at wood/soil interfaces and anywhere water collects. Stains may be rot indicators.
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Visual Signs of External Decay - Discoloration
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Visual Signs of External Decay - Brown Rot
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Method for Detecting Interior Deterioration -Probing
Probing with a pointed tool such as an awl will locate decay near the wood surface.
Decay will be evidenced by excessive softness or lack of resistance to the probe penetration and the breakage pattern of the splinters.
A brash break (straight line break directly above and parallel with the axis of the awl) indicates decayed wood, whereas a crisp splintered break with the splinter hinging from one end indicates sound wood.
However, care must be taken to differentiate between decay and water-softened wood.
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Method for Detecting Interior Deterioration - Pick Test
Decayed wood breaks abruptly across grain without splintering.
Sound wood pries out as long splinters.
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External DecayWhat are the Rot Indicators?
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External DecayHow Deep is that Rot?
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External DecayWhat is Underneath the Surface?
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Method for Detecting Interior Deterioration - Sounding
Sounding the surface by striking it with a hammer is one of the oldest and most commonly used inspection methods.
Although sounding is widely used, interpretation is VERY subjective.
Soundings are based on the tonal quality of the ensuing sounds and the rebound of the hammer.
Practical experience has shown that sounding is only useful for members less than 4 inches thick.
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Method for Detecting Interior Deterioration - Sounding
Interpreting Soundings: Sound timber gives a crisp sound. Defective timber gives a dull sound. Loose hardware will vibrate.
Note: A 2 inch thick shell of competent wood is sufficient to mask any interior rot.
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Method for Detecting Interior Deterioration -Moisture Meters
As wood decays, electrolytes are released and the electrical properties are altered. Based on this phenomenon, a resistance-type moisture meter can be used to detect these changes.
Although the moisture meter does not detect decay, it does identify wood that has a moisture content high enough for decay growth. Moisture contents of greater than 20% to 30% would indicate conditions suitable for decay.
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Method for Detecting Interior Deterioration - Drilling and Coring
Drilling and coring are probably the most common methods used to detect internal decay in wood members.
Drilling is usually done with a battery powered reversible drill (12 volt minimum, 18 volt to 24 volt recommended) using a 14 inch long, 3/8 inch wood auger.
The inspector drills into the wood member under examination, noting the depths at which the resistance to drilling may become easier and observes the drill shavings for decay.
The drill shavings for each hole usually caught and stored in a plastic bag for future examination with a magnifying glass and possible testing with decay identifying chemical dyes.
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Method for Detecting Interior Deterioration - Drilling and Coring
To facilitate the capture of the wood shavings, the plastic bag is usually thumb tacked to the wood just below the core hole and the bag held open with one hand while the other hand operates the drill.
The color, luster, fracture characteristics, hardness, smell, and moisture are all indicators of the wood fiber condition.
The presence of common wood defects, such as knots, resin pockets, and abnormal grain, should be anticipated while drilling and should not be confused with decay. Inspection holes may also be probed with a bent piece of wire to measure shell thickness.
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Other Methods for Detecting Interior Deterioration
Some Regions have more sophisticated inspection tools, such as;
– Resistograph Drill – Stress Wave Tester
These tools are beyond the scope of major and minor trail bridge inspection.
They may be demonstrated during the classroom session, if your Region has any of these tools.
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Resistograph Drill
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Review of Visual Signs ofExterior Deterioration
Fungus/Conks – under bridge, on girder bottoms and sills, on wet wood
Rot – look for at wood/soil interfaces and anywhere water collects. Indicated by:
Discoloration
Cube or Alligator Patterns Insect activity
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Review of Visual Signs of Exterior Deterioration – Brown Rot
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Review of Visual Signs of Exterior Deterioration - Probing
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Review of Visual Signs of Exterior Deterioration – Brown Rot
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Review of Visual Signs ofExterior Deterioration - Fungus/Conks
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Review of Visual Signs ofExterior Deterioration - Fungus/Conks
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Review of Methods of Detecting Interior Deterioration
Soundings Moisture Meter Coring and Drilling
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END
Timber Deterioration and Decay
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References
AGRI Handbook No. 557 Wood Bridges – Decay Inspection and Control FHWA Bridge Inspection Reference Manual FHWA Field Manual for Timber Bridge Inspection, Draft FPL Condition Assessment of Timber Bridges 1. Evaluation of a Micro-
Drilling Resistance Tool FPL Condition Assessment of Timber Bridges 2. Evaluation of Several
Stress-Wave Tools FPL Controlling Decay in Water Front Structures FPL Timber Bridges Design, Construction, Inspection and Maintenance FPL Wood Handbook FPS Wood and Timber Condition Assessment Manual NAVFAC Inspection, Maintenance and Procurement Procedures for
Wood Poles R6 Trail Bridge Inspection R10 Training PowerPoints