2 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
3 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
4 Types and Causes of Timber Deterioration Delamination of Glulam Beams Fire Impact or Collisions Abrasion or Wear Overstress Weathering or Warping Animals
8 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
10 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
18 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.
19 Other Types and Sources of Deterioration Delaminations
20 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.
21 Types and Sources of Deterioration Loose connections
22 Types and Sources of Deterioration Surface depressions – Surface depressions indicate internal collapse, which could be caused by decay.
24 Types and Sources of Deterioration Impact or Collisions – Severe damage can occur to truss members, railings, and columns from floating debris or ice.
25 Types and Sources of Deterioration Abrasion or Wear
26 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.
27 Types and Sources of Deterioration Overstress
28 Types and Sources of Deterioration Overstress
29 Types and Sources of Deterioration Weathering or Warping
30 Other Types and Sources of Damage Wood swell Animal
32 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.
37 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.
41 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%).
42 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.
44 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
45 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.
46 Decay Detection Methods Method for Detecting Exterior Deterioration – Visual Inspection – Probing Method for Detecting Interior Deterioration – Sounding – Moisture Meter – Drilling and Coring
47 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.
48 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.
49 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.
50 Visual Signs of External Decay Spongy and Rotted areas. Look at wood/soil interfaces and anywhere water collects. Stains may be rot indicators.
51 Visual Signs of External Decay - Discoloration
53 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.
54 Method for Detecting Interior Deterioration - Pick Test Decayed wood breaks abruptly across grain without splintering. Sound wood pries out as long splinters.
57 External Decay What is Underneath the Surface?
58 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.
59 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.
60 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.
61 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.
62 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.
63 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.
65 Review of Visual Signs of Exterior 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
66 Review of Visual Signs of Exterior Deterioration – Brown Rot
67 Review of Visual Signs of Exterior Deterioration - Probing
68 Review of Visual Signs of Exterior Deterioration – Brown Rot
69 Review of Visual Signs of Exterior Deterioration - Fungus/Conks
70 Review of Visual Signs of Exterior Deterioration - Fungus/Conks
71 Review of Methods of Detecting Interior Deterioration Soundings Moisture Meter Coring and Drilling
73 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