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122 Cards in this Set
- Front
- Back
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Strength of Concrete is mostly a function of what? |
Strength of paste, nature of paste aggregate, interface and the nature of microcracks.
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Strength of concrete paste is dominated by what characteristics? |
Capillary porosity - which in turn is governed by w/c and degree of hydration |
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Given no other information, what is the best parameter to predict compressive strength? |
w/c or w/cm |
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What does Abrams Law state |
That strength of concrete is inversely proportional to w/cm |
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Aggregates in concrete greatly influence the stress strain curve. explain. |
Aggregates have very little influence on strength of concrete. However, stress concentrations occur at the aggregate resulting in progressive internal cracking. This results in a curvilinear stress vs strain curve. |
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In concrete, micro cracks start to occur at what % of ultimate strength? |
25% |
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In concrete, above 75% of ultimate strength what will occur is load sustains? |
Static Fatigue failure |
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The extent of microcracking is mainly a function of what? |
Compatibility between aggregates and pase |
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Explain how curing temperture on strength changes the strength as time curing progresses |
Higher temp = higher initial strength gain. (Final strength is a little less than that of normal temp and cold temp) Normal temp (approx 20) = Typical strength gain Low temp = lower initial strength gain. (Final strength is a little more than that of normal temp and high temp) |
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What is the idea maturity concept is based off of? |
Based on the idea that two concretes of the same maturity should have the same strength |
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Define Creep |
-Ian Mckenna |
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What results in more creep? If loaded and drying at the same time or if dried and then loaded? |
If loaded and drying at the same time will result in higher creep. |
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Where does creep occur in concrete? For creep to be reduced, what needs to be reduced in the concrete? |
Creep occurs in the paste fraction |
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What causes chemical shrinkage? |
Volume reduction due to chemical reaction of cement - volume of the products is less than the volume of reactants. |
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Explain Autogeneous shrinkage |
It is a component of chemical shrinkage, results in external volume change (cracking) |
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Explain plastic shrinkage and cases when it would occur |
-Evaporative loss when concrete is not yet hardened. Occurs generally prior to or shortly after the set of concrete |
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Drying shrinkage - explain what it is and possible mitigation |
-Time-dependent strain due to loss (or gain) of water. |
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What is the most important parameter for shrinkage in concrete |
Amount of water in the concrete |
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Explain water curing |
Moist curing - the net effect is less shrinkage and cracking, allows time for tensile strength to develop |
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Explain what restraint is in concrete |
Cracking resulting from concrete is restrained from moving |
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Explain thermal cracking |
Heat generated during hydration, resulting in temperature differential and thus strain differential between interior and exterior of concrete element |
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What is durability of concrete mean? |
Ability to last for the service life without unacceptable performance. |
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Durability helps the environment by... |
-Conserving natural resources |
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What is the number 1 cause of concrete deterioration |
Corrosion |
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Explain chloride induced corrosion |
-Ingress of corrosive species into the POROUS concrete |
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Explain Carbonation Induced Corrosion |
Naturally, concrete has high pH, atmospheric CO2 diffuses into concrete, reacts with high pH concrete ingredients thus lowering pH and disrupting passive layer on steel |
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Explain freezing and thawing deterioration and how it can be prevented |
- Can occur from cases: 1)Non air entrained w/ high w/c 2) Air entrained w/ low w/c ratio. |
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Explain sulphate attack to concrete |
Sulphur from groundwater, industrial chemicals etc can get into concrete, reacts with aluminate, causes severe softening, expansion and cracking |
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List some general solutions to corrosion/cracking |
-Low w/c through use of a superplasticizer |
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Name some targeted solutions for steel corrosion |
-Coatings, cathodic protection |
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List the factors (design criteria) for proportioning concrete mixtures |
Strength |
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List the types of materials in concrete |
Cement |
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List, in order, Concrete mix design process |
Obtain required material information |
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Why is air content a function of aggregate size? Or is it!????????????!! |
Max aggregate influences the amount of paste needed to get a given workability for a given w/cm (larger aggregate surface area < fine aggregate surface area. Thus needing less paste to wet and get flow). Therefore larger agg mixes have less paste volume & it is the paste fraction that needs to be protected with a specific volume of air bubbles. This results in a mix with less paste needs less total air. |
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List some advantages of using wood |
-Aesthetics |
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List some disadvantages of using wood |
-Bacteria, fungi, insects, etc |
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Define anisotropic |
having a physical property that has a different value when measured in different directions |
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Difference between wood and timber |
Wood is more of a generic term, while timber is wood cut into shapes for structural purposes. |
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Wood is made of aproximately what % of carbon? |
49% |
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List the organic polymers which wood is composed of |
- Cellulose (40-44% vol) -Extractives (1-5%) |
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What molecular building block is cellulose made of? |
Glucose |
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What does cellulose provide to wood? |
Strength and framework of cells |
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Hydroxyl units can be found in what? What are they responsible for |
-Cellulose -Attracting other cellulose molecules making microfibrils -Attract water which largely is responsible for swelling and shrinking |
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What surrounds cellulose like a "matrix" |
Hemi-cellulose |
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What is responsible for the fibre-to-fibre bonding of wood |
Hemi-cellulose |
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What permeates the cellulose microfibrils in the cell walls? |
Lignins |
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What organic polymer fills up the space between wood cells? |
Lignins |
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Acting together, what organic polymers surround the cellulose units bonding them together? |
Lignins and hemicellulose |
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What organic polymer imparts rigidity and compressive strength to cell walls? |
Lignins |
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What organic polymer adds to woods toxicity, making wood resistant to decay and insect attack |
Lignins |
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Describe microfibrils |
Threadlike bundles of cellulose molecules that are arranged approximately parallel. Bound together by hemi-cellulose and lignin |
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What wall of the cell wall consists of three layers? |
Secondary wall |
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Cell walls consist of what layers? |
Middle lamella Primary wall Secondary wall |
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What part of wood is responsible for moisture conduction and food storage |
Sapwood |
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What part of the wood is the inner non-living core |
Heartwood |
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Describe difference between heartwood and sapwood |
Heartwood is more resistant to decay than sapwood and is drier and harder |
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What part of wood transports sap from the leaves to the growth parts |
Inner bark |
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Explain the difference between softwood and hardwood |
Softwood - Needle like or scale like leaves - conifers. (Spruce, pine, fir, douglas fir, etc) Hardwood - Broadleaf trees, used more for furniture, exterior finishing, not commonly used for structural purposes |
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What cell is responsible for the mechanical support of tree and vertical conduction of water |
Tracheids |
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What cell makes up 90% volume of wood in softwoods |
Tracheids |
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What cells are located in the rays of wood and store food and transport it horizontally |
Parenchyma |
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What cells exist in hardwood only and not softwood |
Fibres (Mechanical support in hardwoods)
Vessels or pores (Transport in hardwoods) |
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What parameter controls tree density |
Porosity (space) |
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Relatively small changes in what parameter in wood can result is drastic changes in mechanical properties of wood |
Porosity |
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What two states does moisture exist in wood |
Free water (liquid of vapour) with the cell cavities |
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Why does green wood (new wood) usually have a high moisture content |
Because it contains sap |
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As green wood dries, what evaporates first |
Free water |
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What is FSP. Explain what it is |
FSP = Fibre saturation point. The condition at which all the free water is evaporated, but cell walls are still fully saturated |
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What is the typical FSP of wood |
25% to 30% |
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As drying occurs below the FSP, what happens? |
Molecular structure compacted and as a result, the wood becomes stronger and starts to shrink |
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Above and below the FSP, what happens to the mechanical properties of wood |
Above - Have little to no effect on mechanical properties Below - Mechanical properties generally improved |
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What is EMC of wood? |
EMC = Equilibrium Moisure Content Wood gives off or takes up water until it is in equilibrium with the environment its located |
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Below the FSP, the volumetric shrinkage is approximately proportional to what? |
To the volume of water lost |
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Explain why wood shrinks more transversely than longitudinally |
As water is lost, microfibrils are attracted closer together (side to side attraction) Since most microfibrils are orientated vertically, there is more shrinkage in the lateral direction |
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List some mechanical properties of wood |
Modulus of elasticity Tensile strength Compressive strength flexural strength shear strength effects of temperature effects of rate and duration of loading |
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Is modulus of elasticity of wood linear or non-linear? |
Non-linear in all stress cases |
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Is wood tensile strength stronger parallel to grain or perpendicular to grain? explain answer |
- Parallel (70-150MPa & small failure strain) & perpendicular (2-9MPa & large failure strain). This is because failure parallel to grain requires breaking primary bons. Perpendicular requires breaking secondary bonds which are much weaker |
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Is wood compressive strength stronger parallel to grain or perpendicular to grain? explain answer |
-Parallel (25-60MPa) Wood develops small kinks withing microfibrils at failure and localized buckling of cell walls -Perpendicular (3-10MPa). Cells collapse or flatten at low stress, then load picks up again and can go very high |
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What failure occurs first for flexural strength of wood |
Failure will occur at the compression side. Neutral axis will lower and eventually tensile will occur at bottom of beam |
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List the three main groups of shear failure in wood |
-Shear perpendicular to grain Shear parallel to grain Rolling shear. |
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Describe how wood reacts to high temperatures (for long periods of time and short periods of time) |
Wood degrades in both cases, however, short term the response is reversible, while long term exposure the degrading is irreversible.
Generally, mechanical properties of wood will decrease with increase in temperature. |
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Apparent strength of wood does what as rate of load increases |
increases |
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Define defect of wood: Ribre and ring orientation |
Grain direction does not coincide with axis of board |
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Define defect of wood: Knots |
Portion of limb surrounded by subsequent wood growth. The shape and type of knot is important and changes how knot influences the mechanical properties |
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Define defect of wood: Checks aka "Seasoning checks" |
lengthwise separation of wood which usually extendsacross the growth rings |
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Define defect in wood: Wanes |
lack of wood on the face of a piece for anyreason at all |
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Define defect in wood: shakes |
separation along the grain between the annualgrowth rings (often caused by wind damage) |
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What defect in wood is typically caused by wind |
Shakes |
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Define wood defect: Pitch pocket |
Opening between growth rings containing resins or bark |
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Define wood defect: warp |
-Bow (deviation flatwise from a straight line drawn end to end of piece) -Crook (deviation edgewise from a straight line drawn end to end of a piece) |
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Who apprves "graders" who work at mills |
Lumber grading agencies |
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Dimension lumber is divided into four categories of use. Name them |
-Structural light framing -Structural joist and plank -Light framing -Studs |
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Dimension lumber is divided into four categories: Structural light framing, structural joist and plank, light framing, studs. Each of these categories are further divided into what? |
Strength grades |
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List some parameters for visual grading of lumber |
-Density(related to rate of growth) -Decay -Heartwood or sapwood -Slope of grain -Knots -Shakes, checks, splits -Pitch pockets |
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Explain what "machine stress" grading |
-machine measures the stiffness of a piece oftimber -empirical relationship between stiffness(modulus of elasticity) and strength is used togive a strength rating |
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Explain KH (Factor) |
-Load sharing factor -To account for redistribution of load from a highly stressed member to surrounding members ( such as one joist fails load will redistribute to the adjacent joist) -Can increase allowable stress |
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Explain KD (Factor) |
-Load duration factor
- Reduce or increase allowable stress if loading is longer or shorter (respectively) |
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Explain KT (Factor) |
-Treatment factor -Chemicals for preservation or fire retarder may lessen the strength |
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Explain KS (Factor) |
-Service condition factor
-used to modify strength for conditions other than dry (average moisture content = 15% and never greater than 19%) |
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Explain KZ (Factor) |
-Used to further account for the effect of size on strength |
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Manufactured wood products can also be referred to as |
"Engineered" wood products |
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Give a rough definition of manufactured wood products aka "Engineered" wood products |
fibres, stands, or pieces heldtogether with a glue in various configurations toprovide better use of natural recourses and/orimproved structural properties |
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What are fingerjoined lumber commonly used for |
Vertical or "stud" type uses |
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Explain Fingerjoined lumber |
Short pieces finger cut and glued together to make long pieces |
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Explain Glued Laminated Timber |
- AKA glulam -Old traditional and still most used type of SCL -wood pieces glued together with adhesive tomake long, curved beautiful structures |
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What does SCL stand for in wood |
Structural composite Lumber |
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Explain what LVL is |
-Laminated Veneer lumber -Layered composite of wood veneers and adhesive -The grain of each later of veneer runs in the same (long) direction with the result that it is strong when edge loaded as a beam or face loaded as a plank |
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What is LVL commonly used for |
Headers and beams, flanges for prefab wood I-joists |
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Explain what LSL is |
-Laminated Strand Lumber (LSL) -Oriented strands (similar to OSB) typically made from aspen or poplar |
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What is LSL commonly used for |
Headers, beams, studs and columns |
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Explain what PSL is |
-Parallel strand lumber -structural composite lumber product made by gluing long strands ofwood together under pressure. (proprietary name ParaIlam®.) |
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What type of "Engineered" wood products is less prone to shrinkage and warp than lumber |
PSL - Parallel strand lumber |
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List some advantages of wood I-joists |
- They are made by made by gluing solid sawnlumber or laminated veneerlumber (LVL) flanges to aplywood or OSB panel web ADVANTAGES - dimensionally stable, known engineering properties, 'I' shape gives high strength to weight |
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Explain what sheathing is |
-Flat plates or sheets that "sheath" or "sheet" roofs, walls, and floors |
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List the types of sheathing |
-Plywood -OSB = Oriented Strand Board (OSB) - Wafer Board - Sawn lumber boards |
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Describe what plywood is
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-Built up from sheets of softwood veneer, glued together with adhesive -Direction of grain on each sheet is rotated 90 degrees |
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Describe what OSB and Waferboards are |
- OSB = Orientated Strand Board -Laminate together thin chips of poplar wood - waferboard = chips are smaller and randomly oriented - OSB = chips are oriented in the long direction |
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List some factors that could destroy the durability of wood |
-Decay -Fire -Termites |
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Wood will ignite if exposed to an open flame at what temperature? |
250 Degrees C |
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Wood will self ignite at what temperature |
500 Degrees C |
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What is the best protection for wood from termites? |
Block the access for the soil to the wood through appropriate construction techniques |
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Wood rot or decay is caused by what? |
Fungi |
