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Eng Materials
Concrete/Timber used in construction
| Question | Answer |
|---|---|
| What are the different equipment used to convey fresh concrete at the construction site? | Mixer truck, Special dump truck (concrete hopper truck, side dumping truck, flat bed dump truck), bucket, buggy |
| What are a few effective measures to reduce segregation of fresh concrete during placement | a) Avoid concrete hitting the form by using a hopper, b) avoid concrete running down a slope, c) use a baffle and drop at the end of a chute. |
| How do we consolidate the fresh concrete that is placed? | a) Hand rodding or tamping, b) internal vibrator, c) external vibrator |
| What the basic steps in finishing a concrete slab? Describe the purpose of each step. | Strikeoff–striking off excess concrete to bring the top surface to proper grade Bullfloating–lvling to eliminate high and low spots Floating–firmly embed the aggregate, to compact the surface and to remove any surface imperfections. Troweling–fnshing t |
| How can we produce the different textures in the concrete surface? | Texturing, like using a broom to provide skid resistance, |
| What are isolation joint, construction joint & control (or contraction) joint? What is the main purpose of each joint and how is it constructed? | a) Isolation joint – permits vertical and horizontal movement b) Construction joint – used for stopping places during construction, prevents bonding c) Control joint – permit thermal shortening |
| Why is it important that a concrete be properly cured? What is the basic principle to follow to ensure proper curing of concrete? | Concrete should be cured properly to prevent loss of moisture, supply additional moisture, and accelerate strength gain. Basic principles to follow: keep concrete moist and control the temp |
| What are the effects of using sea water as mixing water in concrete? | a) strength of concrete may be lower at later ages – higher early strength, lower ultimate strength, b) increase the risk of corrosion of reinforcing steels (do not use rebar) |
| What are the effects of hot weather on concreting? What are the appropriate measures to be taken in hot weather concreting? | a) increased rate of setting, b) slump loss earlier, c) increased possibility of plastic cracking, d) increased loss of moisture during curing, e) difficulties in controlling entrained-air content |
| Measures to prevent hot weather screw ups? | a) Cool the aggregate and the mixing water. Ice can be used as mixing water, b) reduce the time between placement and start of curing, c) cover the concrete during curing to reduce evaporation of water, d) sprinkle additional water to the concrete during |
| What are the effects of cold weather on concrete and the appropriate measures to be taken? | a) lower rate of hydration at low temperature, b) freezing of water will damage the concrete |
| Measures to be taken for cold weather concrete curing? | a) use insulated forms, b) use Type III cement or an accelerating admixture, c) use heaters to keep concrete warm, d) use air-entrainment in concrete, e) allow longer curing time before stripping forms |
| Know how to perform a concrete mix design on paper according to the ACI (Absolute Volume) method. You should know the specific gravity of cement and density of water by heart | Specific Gravity of cement: 140/62.4 = 2.24 Density of water : 62.4 |
| How do we verify or make the necessary adjustments in a concrete mix design? | Design using calculations, then make sure slump and unit weight are the same during testing. Adjust the water content and cement content proportionally (to maintain a water/cement ratio), proportion the fine and coarse aggregates. |
| Water reducing admixture? | Increases the workability of fresh concrete without the addition of more cement and water, achieves higher compressive strength without increasing cement content or reducing workability, reduce cement content without reducing the strength, produce high ea |
| b. Accelerating admixture | accelerates the strength development of concrete at an early age, expedites the start of finishing operation and application of insulation for protection, reduces the time required for proper curing and protection, permits early removal of forms and earli |
| c. Retarding admixture | offsets the accelerating effect of hot weather on the setting of concrete, delays initial set of concrete or grout when more time is needed, offset the accelerating effect of other admixtures |
| d. Gas forming admixture | added to concrete to form air bubbles in the concrete to form lightweight cellular concrete. (aluminum powder) |
| e. Corrosion inhibitor | added to concrete to reduce the corrosion of steel reinforcing bars (reubar) in concrete (calcium nitrite) |
| f. Fly ash | improves the workability of fresh concrete, improve resistance to thermal cracking (due to lower heat of hydration), improve resistance to sulfate, acid water, and seawater, reduce alkali-aggregate expansion, reduce permeability of concrete |
| g. Ground blast-furnace slag | improves the workability of fresh concrete, improve resistance to thermal cracking (due to lower heat of hydration), improve resistance to sulfate, acid water, and seawater, reduce alkali-aggregate expansion, reduce permeability of concrete |
| h. Silica fume | pozzolanic reaction is much faster than fly ash or blast-furnace slag; produces high-strength concrete; improves resistance to sulfate, acid water, and seawater; reduces alkali-aggregate expansion, reduces permeability of concrete |
| What are the differences between a normal water reducing admixture, a superplasticizer and a plasticizing admixture? | Normal: reduces the water demand to produce a consistency in fresh concrete Superplasticizing: reduces the water demand to produce a consistency in fresh concrete by at least 12% Plasticizing: produces a flowing concrete without further addition of wate |
| What are the typical ranges of density and 28-day compressive strength of Structural Lightweight Concrete | Density: < 115 lb/ft3 Compressive strength: > 2500 psi |
| What is the difference between an all-lightweight concrete and a sanded-lightweight concrete? | An all-lightweight concrete uses both lightweight coarse and lightweight fine aggregates. A sanded-lightweight concrete uses natural sand and lightweight coarse aggregates. |
| List a few aggregates that are commonly used in making lightweight concretes | Expanded clays, shales, slates and slags |
| How is a structural lightweight concrete different from a normal concrete in terms of the most important factor affecting its strength? | The most important factor in the strength of structural lightweight concrete is the cement content, rather than w/c ratio |
| Are there any difference between the slump requirement for lightweight concrete and that for normal concrete to obtain the same workability during placement? | A lower slump is needed – a slump of 2-3 in has similar workability as a normal concrete with a slump of 4-5 in. |
| What are the possible advantages of using a structural lightweight concrete as compared with a normal concrete? | Reduction in dead weight structure, savings in steel reinforcement, reduction in dead weight gives better resistance to earthquake loading, reduces handing, transportation and construction cost for precast concrete elements. |
| What is the typical usage of Heavyweight Concrete? What is its typical density? How is a heavyweight concrete produced? | Used for Radiation Shielding; Typical density: 210-240 pcf; Heavyweight concrete is made using fine and coarse aggregates from high-density rocks. |
| What is the typical range of 28-day compressive strength of High-Strength Concrete? What are the usages of high strength concrete? | Compressive strength: > 6000 psi; Reduces the volume of concrete, therefore reducing the weight. Used in high-rise buildings to reduce the weight, also bridges and repair work (since it has a high early strength). |
| Describe the typical aggregate, w/c, additives and admixtures used in high-strength concrete. | Smaller aggregate helps because less chance of trapping water; low w/c to achieve high strength; Need to mix with high-range water-reducing admixtures (aka superplasticizers) to increase the workability (low workability caused by low w/c), reducing the ma |
| What is a Roller-Compacted Concrete? What is its typical compressive strength? | A no-slump, almost dry concrete mixture which is finished by compacting with a vibratory roller. Typical compressive strength is up to 10,000 psi |
| What are the possible usages, and possible advantages and disadvantages of roller-compacted concrete as compared with normal concrete? | Advantages: - Lower cement content can be used - Lower formwork cost - Lower transportation cost due to ease of handling Disadvantages: - Difficult to bond the fresh RCC to another concrete surface due to its dry consistency |
| What is a latex-modified concrete? How are its properties different from those of a normal concrete? What are its possible usages? | A latex-modified concrete is a concrete where latex is used as an admixture; A latex usually contains about 50% by weight of spherical and very small polymer particles, tends to incorporate large amounts of entrained air in concrete, 10 to 25% polymer by |
| What is a fiber-reinforced concrete? How are its properties different from those of a normal concrete? What are its possible usages? | - A fiber-reinforced concrete is a concrete containing discontinuous discrete fibers. Property differences: - Higher tensile strain at failure - Higher toughness and resistance to impact - Ultimate tensile strength increased only slightly - Reduc |
| What are the differences between an exogenous and an endogenous tree? Give an example for each class. | Exogenous: Grows outward by adding new cells in a layer between the existing wood and the bark, cross-section has rings (Pine, Oak). Endogenous: Grows inward by adding new cells to the old (bamboo, palm) |
| What are the differences between softwoods and hardwoods with respect to (A) growth of the tree, (B) mechanical and hygroscopic properties, and (C) usages? | Softwood: a) Grows fast, therefore not as strong, b) weaker, c) cheaper, so used for construction Hardwood: a) Grows slow, b) stronger, c) more expensive, used for things like furniture |
| What are the two main components of wood and their contributions to the strengths of wood? | 1) Cellulose and Hemicellulose – provides tensile axial strength and elastic property of wood 2) Lignin – cements the cellulose together to provide compressive strength |
| What is the growth region in an exogenous tree called? | The cambium layer |
| What are heartwood and sapwood? What are their differences in terms of mechanical properties and decay resistance? | Heartwood – inactive inner portion, relatively darker in color, more resistant to decay as compared to sapwood, drier Sapwood – active outer potion, relatively light in color, more sap ::Mechanical properties are about the same |
| What are early wood and late woods in the growth region of a tree? | Early wood – inner light-colored layer, which grows in the spring and grows relatively fast Late wood – outer darker layer, which grows in the summer and grows relatively slower (also known as summerwood) |
| How can the rate of growth of a tree be measured? | By the number of annual growth rings per distance (rings/in) along a line perpendicular to the rings across a right section of the tree. |
| What is the typical specific gravity of cellulose in wood | 1.5 |
| Describe the factors affecting the density of a wood | Moisture content; more wet = increase density, usually trees that grow slower; also has more cellulose |
| Explain how wood contains moisture in its structure. What is the fiber saturation point? | Woods contains moisture in its cell walls and cavities. The fiber saturation point (FSP) is the where the moisture content at which all of the free water is removed – all cell cavities are empty – but the cell walls are still completely saturated. No fre |
| What are the relative magnitudes of shrinkage and expansion due to moisture changes in wood in the (A) longitudinal, (B) radial and (C) tangential directions? | Wood expands when the moisture content increases and shrinks when the moisture content decreases. Shrinkage also increases with density of the wood. Shrinkage is caused by lateral contraction of the cell walls as they dry out. a) very little contraction, |
| Describe the thermal expansion, heat conductivity and electric conductivity of wood. | Thermal expansion – small compared with that due to moisture effects; heat conductivity (thermal) – low as compared with that of concrete or steel; Electric conductivity – increases with moisture content; moisture content of wood can be estimated from mea |
| Describe the effects of density and growth rate on the strength of wood | Stiffness and strength increase with density; the slower the growth rate, the stronger the wood |
| Describe the effects of moisture on the strength of wood, including the significance of the fiber saturation point. | Below the fiber saturation point, decreases in moisture content increases the strength and stiffness of wood. |
| Describe the relative magnitudes of compressive, tensile and shear strengths of wood in the directions (A) parallel to the grain | a) Parallel to the grain – Strength is greatest in this direction, tensile strength is 2 to 3 times the compressive strength, shear strength is about the same as the compressive strength perpendicular to grain -> therefore ¼ to 1/3 of its own compressive |
| Describe the relative magnitudes of compressive, tensile and shear strengths of wood in the directions perpendicular to the grain. | b) Perpendicular to the grain – Tensile strength is less than 1/10 of tensile strength parallel to grain; compressive strength is about ¼ to 1/3 of compressive strength parallel to grain |
| Give typical strength values for a high quality Southern Pine. | c) Southern Pine – Tension: 1050 psi, Compression parallel: 1450 psi, Compression perpendicular: 565 psi, Shear: 105 psi, Bending: 1850 psi |
| What are the three commonly used methods for chemical treatment of wood for protection against decay? | waterbourne, creosote, Pentachlorophenol |
| What are three methods for improving the water resistance of wood | Paint, varnish, zinc chloride |
| What is a FRT wood? | Fire-retardant treated – pressure treated with fire retardant chemicals to reduce the spread of flame in a fire |
| Knots | Formed at the base of the branches extending into the wood of the tree. Causes stress concentrations. |
| Shakes | Cracks along the grain, originating in the growth of the tree |
| Checks | Longitudinal splits across the growth rings resulting from uneven drying |
| Waynes | Areas where the lumber has been cut too close to the edge of the log and there is bark on the boards |
| Pitch pockets | Accumulations of resins in openings between the annual rings |
| Compression wood | Formed on the lower side of branches, darker than normal wood, high lignin content, higher specific gravity, greater longitudinal shrinkage, not as tough as normal wood |
| Warping | Caused by unequal shrinkage |
| Softwoods | Structural light framing – No.1, 2, or 3; Structural joint and planks – No.1, 2, 3; Light framing – 1) Construction, 2) Standard, 3) Utility, 4)Economy; Studs – grade: stud |
| Hardwoods | Grading is based mainly on the amount of clear area without defects – 1) FAS (first grade), 2) FIF (second grade), 3) Selects, 4) No. 1 common, 5) No. 2 common, 6) Sound wormy, 7) No. 3A common, 8) No. 3B common |
| Why is plywood usually made up of an odd number of veneers? | To arrange the grain direction of each veneer at right angles to the layer next to it. Odd numbers equalize the strength; the grains in the top layer running parallel to the grains on the bottom reduces the warping because they expand in the same directi |
| What is the specific gravity of a typical plywood? | 0.7 to 0.75 |
| Describe how a plywood grade is designated by the American Plywood Association. What does the following designation mean? A-B G2 INT - APA | A – Grade of veneer on front panel B – Grade of veneer on back panel G2 – species group number 2 INT – interior used APA – American Plywood Association |
| Laminated Veneer Lumber (LVL) | A board product made by gluing pieces of thin lumber or veneer together to make a large member. The grains of all pieces are oriented along the long axis of the panel. Used in structural applications |
| Laminated Strand Lumber (LSL) | Lumber made by wood strands mixed with a water-proof adhesive. These products are available in sizes larger than sawn lumber and tend to be significantly stronger than lumber of equal size (due to minimization of defects. Used in construction where high |
| Particle Board | A flat board made from wood flakes missed with an adhesive and formed under pressure. It is not used for structural purposes. Usually used in making furniture and associated products. |
| Oriented Strand Board (OSB | A board made from large wood flakes mixed with a waterproof adhesive and oriented to be parallel to the long axis of the panel. Used in roof sheathing and floor sheathing. |
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