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
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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
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
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Delaminations
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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.
Other Types and Sources of
Deterioration
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Delaminations
Types and Sources of Deterioration
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Loose connections
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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.
Types and Sources of Deterioration
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Loose connections
Types and Sources of Deterioration
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Surface depressions
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Surface depressions indicate internal collapse, which
could be caused by decay.
Types and Sources of Deterioration
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Fire
Types and Sources of Deterioration
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Impact or Collisions
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Severe damage can occur to truss members, railings, and
columns from floating debris or ice.
Types and Sources of Deterioration
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Abrasion or Wear
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
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Overstress
Types and Sources of Deterioration
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Overstress
Types and Sources of Deterioration
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Weathering or Warping
Other Types and Sources of
Damage
Animal
Wood swell
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NATURAL DEFECTS OF TIMBER
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NATURAL DEFECT DEFINITIONS
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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.
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?
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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.
Life Cycle of Fungus
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Fungus
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DECAY MECHANICS
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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%).
Controlling Decay
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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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Common Areas Where Decay Occur
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
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Where do I look for decay?
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Conditions and locations conducive to decay
can be readily identified:
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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.
Decay Detection Methods
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Method for Detecting Exterior Deterioration
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Method for Detecting Interior Deterioration
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Visual Inspection
Probing
Sounding
Moisture Meter
Drilling and Coring
Method for Detecting Exterior Deterioration
-Visual Inspection
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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.
Method for Detecting Exterior Deterioration
-Visual Inspection
What to Look for:
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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.
Visual Signs of External Decay
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Fungi or fruiting bodies (conks)
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Under the bridge, on girders and sills, anywhere
the wood is subject to wet/dry cycles.
Visual Signs
of External Decay
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Spongy and Rotted areas.
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Look at wood/soil interfaces and anywhere
water collects. Stains may be rot indicators.
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
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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.
Method for Detecting Interior Deterioration
- Pick Test
Decayed wood breaks abruptly
across grain without splintering.
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Sound wood pries out as long
splinters.
External Decay
What are the Rot Indicators?
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External Decay
How Deep is that Rot?
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External Decay
What is Underneath the Surface?
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Method for Detecting Interior Deterioration
- Sounding
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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.
Method for Detecting Interior Deterioration
- Sounding
Interpreting Soundings:
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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
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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.
Method for Detecting Interior Deterioration
- Drilling and Coring
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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.
Method for Detecting Interior Deterioration
- Drilling and Coring
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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.
Other Methods for Detecting Interior
Deterioration
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Some Regions have more sophisticated inspection
tools, such as;
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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.
Resistograph Drill
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Review of Visual Signs of
Exterior Deterioration
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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
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 of
Exterior Deterioration - Fungus/Conks
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Review of Visual Signs of
Exterior Deterioration - Fungus/Conks
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Review of Methods of Detecting
Interior Deterioration
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Soundings
Moisture Meter
Coring and Drilling
END
Timber Deterioration and Decay
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References
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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 MicroDrilling 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