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purpose of concrete mix design
Normal water-reducing admixture
reduces the water demand to produce a given consistency in fresh concrete by 5-10%. Same slump.
Plasticizing admixture
added to concrete to produce flowing concrete (with a slump for greater than 7.5 inches while maintaining a cohesive nature) without further addition of water
benefits of water-reducing admixture
Higher compressive strength without increasing the cement content or reducing the workability of the fresh concrete
Improved workability without addition of more cement and water
Produce high early strength by using superplasticizers (combined effect of using a lower w/c and the higher rate of hydration)
Reduce the cement content without reducing the strength or the workability of the concrete
benefits of calcium chloride as accelerating admixture
Aggravates alkali-aggregate reaction
Increases compressive strength and elastic modulus at early age
Improves workability of fresh concrete and reduces bleeding
Improves water tightness and resistance to freezing and thawing at early age
Increases heat of hydration
Increases shrinkage, volume change and creep
Reduces resistance to sulfate attack
Reduces both initial and final setting times (accelerates setting)
silica fume
a pozzolanic material. Due to its high fineness, the pozzolanic reaction of silica fume is much faster than that of fly ash or blast-furnace slag
By-product of the silicon industry. Reduction of quartz to silicon at high temperatures (up to 2000 °C) produces SiO vapors, which oxidize, cool down and condense to tiny spherical particles consisting of noncrystalline silica (SiO2)
Shape: Spherical
Particle size: less than 1 m in diameter
Fineness: 20,000 m2/kg (compared to 10,000 m2/kg for tobacco smoke, 300-400 m2/kg for Type I cement, and 500-600 m2/kg for Type III cement)
Specific gravity: 2.10-2.25, but can be as high as 2.55
types of mineral admixtures
blast-furnace slag, fly ash, silica fume, natural pozzolans
gas forming agents
added to concrete to form air bubbles in the concrete to form lightweight cellular concrete (ex: aluminum powder)
corrosion inhibitors
added to concrete to reduce the corrosion of steel reinforcing bars in concrete (ex: calcium nitrite)
structural lightweight concrete
concrete having a 28-day compressive strength greater than 2500 psi (17 MPa) and an air-dried unit weight not greater than 115 lb/ft3 (1850 kg/m3). Ex: expanded clays, expanded shales, expanded slates, and expanded slags
advantages of structural lightweight concrete
Savings in steel reinforcement
Reduction in dead weight gives better resistance to earthquake loading
Reduced handling, transportation, and construction cost for precast concrete elements
problem with high slump
For fresh concrete with high consistency, the lightweight aggregate tends to segregate and float to the surface
Often necessary to limit the maximum slump and to entrain air to lower the mixing water requirement. ACI 213: maximum slump of 100 mm (4 in). Air entrainment is generally between 4.5-9%
heavyweight concrete for radiation shielding
Produced generally by using natural heavyweight aggregates
Unit weights: 210-240 pcf
Most economical material for radiation shielding, with low initial and maintenance cost
To reduce the chance for segregation, it is desirable that both coarse and fine aggregates be produced from high density rocks
To avoid segregation, a preplaced aggregate method may be used, where the forms are first filled with compacted coarse aggregate. The voids in the aggregate are then filled by pumping in a cement grout
high-strength concrete
High cement content gives high heat of hydration and high drying shrinkage. Partial replacement of cement by a pozzolan can reduce the risk of thermal cracking with no loss in ultimate strength. However, the early strength may be reduced
Compressive strength of more than 6000 psi (40 MPa)
Condensed silica fume as pozzolanic admixture for early strength
Fly ash or ground blast-furnace slag can be used as a partial replacement of F.A. to increase strength
Low w/c is needed to achieve high strength. Means low workability. Workability is achieved by use of high-range water-reducing admixtures
Reducing maximum size of aggregate can improve strength
advantages of roller-compacted concrete
Lower cement content can be used
Lower unit cost, formwork cost and transportation cost due to ease of handling
Higher rate of concrete placement
Higher rate of utilization of equipment and labor
properties of roller-compacted concrete
Strength is dependent on compaction. Better compaction of the concrete gives higher strength
Fly ash is commonly used to improve workability
Lower heat generation
Compressive strength up to 10,000 psi can be achieved
Creep and thermal properties are similar to those of conventional mass concrete
Permeability is = or < that of conventional mass concrete
benefits of mineral admixtures
Improve workability of fresh concrete, when fly ash or blast-furnace slag is used (use of silica fume may increase the water requirement due to its higher surface area)
Improve resistance to thermal cracking, when fly ash or slag is used as partial Portland cement replacement (due to the lower heat of hydration)
Improve resistance to sulfate, acid water and seawater
Reduce alkali-aggregate expansion
Reduce permeability of concrete
Produce high-strength concrete using silica fume
High range water-reducing admixture, or superplasticizer
reduces water demand to produce a given consistency in fresh concrete by at least 12%. More expensive than normal water-reducing admixture
Water demand reduced
Improved workability
Improved freeze-thaw and sulfate resistance
Improved resistance to deicers and salts
Each percent of entrained air reduces compressive strength by 2-6%
Required proportion of sand is reduced
Permeability is not increased
A no-slump, almost dry concrete mixture that is finished by compacting with a vibratory roller
exogenous
grows outward by adding new cells in a layer between the existing wood and the bark
Softwoods: conifers (needles) or cone-bearing, grows year-round (and therefore faster) Ex: pine, spruce, fir, hemlock, cedar, cypress, redwood. Used in construction, less expensive
Hardwoods: broad-leafed, mostly deciduous (sheds its leaves in the wintertime) Ex: oak, maple, ash, walnut, hickory, poplar, gum, birch. Takes ~20 years to grow → more expensive → used to make furniture, cabinets
Leads to higher strength and stiffness of wood
laminated veneer lumber (LVL)
A board product made by gluing pieces of thin lumber or veneer together to make a large member
Grains of all pieces are oriented along the long axis of the panel
Used in structural applications
disadvantage of roller-compacted concrete
Difficult to bond the fresh RCC to another concrete surface due to its dry consistency
Latex-Modified Concrete AKA Polymer-Modified Concrete
A latex usually contains about 50% by weight of spherical and very small (0.01 to 1 m in diameter) polymer particles.
10 to 25% polymer by weight of cement is used
Commonly used polymers: styrene-butadiene (SB), polyacrylate copolymers
Latex tends to incorporate large amounts of entrained air in concrete → air detraining agents are usually added
Compressive strength is similar to that of normal concrete.
Tensile and flexural strengths are significantly higher than those of normal concrete
Good bonding with old concrete
Used for rehabilitation of deteriorated floors, pavements, and bridge decks
fiber-reinforced concrete
concrete containing discontinuous discrete fibers. Fibers of various shapes and sizes produced from steel, plastic, glass, and natural materials can be used
properties of fiber-reinforced concrete (compared to normal concrete)
Higher tensile strain at failure
Higher toughness and resistance to impact
Ultimate tensile strength increased only slightly
Reduced workability of fresh concrete
Increase fatigue life
Similar elastic modulus
Similar drying shrinkage
Similar compressive creep, but lower tensile creep and flexural creep
applications of fiber-reinforced concrete
Slabs for parking garage
Airport runway, taxiway and parking area
Overlay on pavement
Repair work
Shotcrete used for tunnel lining
endogenous
grows inward by adding new cells to the old.
Ex: bamboo, palm
Wood Structure
Cellulose & hemicellulose (55-80%) provides tensile axial strength & elastic property of wood
Lignin (15-30%) cements the cellulose together to provide compressive strength
Cross-Section of a Tree
Cambium Layer: layer of new cells beneath the bark (growth region of the tree)
Pith: innermost ring
Heartwood: inactive inner portion, relatively darker in color (more resistant to insects & decay as compared with sapwood)
Sapwood: active outer portion, relatively lighter in color
Medullary Ray: group of cells in the radial direction, adding strength to the radial direction
rate of growth of trees
measured by the number of annual growth rings per distance (rings/inch) along a line perpendicular to the rings across a right section of the tree.
Wood is relatively stronger when the rate of growth is slower
density of wood
Solid wood substance (cellulose) specific gravity = 1.5
Wood (with air-filled cavities) specific gravity = 0.3-0.9
effect of orientation on strength: axial strength parallel to the grain
Strength is greatest in this direction (wood fibers run mostly in this direction) - 5000 psi
Tensile strength in this direction is about 2-3 times the compressive strength - 10,000 psi
effect of orientation on strength: axial strength perpendicular to the grain
Tensile strength is less than 1/10 of tensile strength parallel to grain - 800 psi
Compressive strength is about 1/4-1/3 of compressive strength parallel to grain - 1200 psi
effect of orientation on strength: Shear Strength Parallel to the grain
About the same as the compressive strength perpendicular to grain - 1200 psi
defects in wood
Knots: formed at the base of the branches extending into the wood of the tree. Cause stress concentrations (if it will break, will break at the knot)
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
Decay: caused by insect attack
grading of softwoods is based on
strengths and amount of defects
Grading of hardwoods is based on
amount of clear area without defects
waterproofing wood
paint, varnish, zinc chloride
plywood
Laminated wood usually made of an odd number of thin veneers (thin sheets of wood) bonded with synthetic resin. The grain of one ply is at right angles to the next
properties of plywood
The alternating arrangement of the veneers equalizes the strengths in both directions.
Less warping occurs because the top and the bottom veneers have grains running in the same direction. The shrinkage or expansion in the top and bottom layers are similar and thus warping is minimized.
Density = 0.5 Mg / m3
Strength characteristics depend on the species of the wood used (group 1 to 5) and the grade of the veneer (grade A to D)
particle board
a flat board made from wood flakes mixed with an adhesive and formed under pressure
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 an waterproof adhesive and arranged in layers at right angles to one another
Cheaper alternative to plywood
Used in roof sheathing and floor sheathing
laminated-strand lumber (LSL)
Lumber made by wood strands mixed with a water-proof adhesive
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 strength and large size are required
two ways to preserve wood against decay
chemical treatments (usually by pressure) or waterproofing
early wood (springwood)
Inner light colored layer, which grows in the spring and grows relatively faster
late wood (summerwood)
outer darker layer, grows in the summer and relatively slower
most important factor affecting durability
water cement ratio and cement content
most important factor affecting appearance
proportioning of fine and coarse aggregate
admixture
a material other than water, aggregate, hydraulic cements and fiber reinforcement, used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing
air-entraining admixtures
when the cement hardens, minute air bubbles, 10-100 micrometers in diameter, are entrained in the cement paste. they act as a cushion to absorb additional expansion. In cases of freezing water in the concrete, it absorbs the expansion
Note 
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