CGN3501C Test 2 Study Guide

Lecture 9 

• Concrete Mix Design

  • Purpose: to determine the most economical and practical combination of available materials (aggregates, cements, water, admixtures) to produce a concrete that will satisfy the performance requirements

  • Design Considerations

    • Acceptable workability of fresh concrete

    • Adequate durability and strength of hardened concrete

    • Uniform appearance of hardened concrete

    • Economy 

  • Most important factor affecting workability: water content

  • Most important factor affecting strength: water-cement ratio

  • Most important factors affecting durability: water-cement ratio and cement content

  • Most important factor affecting appearance: proportioning of fine and coarse aggregates

• Concrete Mix Design Calculations

Lecture 10

• Admixtures: 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

  • About 71% of the concrete placed in the US contains one or more admixtures

  • Three Categories of Admixtures:

    • Surface-active chemicals 

      • Air-entraining: when the cement hardens, these air bubbles in the cement paste act as a cushion to absorb additional expansion. In cases of freezing water in the concrete, it absorbs the expansion. More durable.

        • Effects: minute air bubbles, 10-100 micrometers in diameter, are entrained in the cement paste. These air bubbles are not interconnected and are well-distributed. 

          • Permeability of the concrete is not increased

          • Water demand for the same workability of fresh concrete decreases as entrained air content increases

          • Required proportion of sand in concrete decreases as the entrained air content increases

          • Improved freeze-thaw & sulfate resistance

          • Improved resistance to deicers and salts (damage due to pressure caused by formation of salt crystals

          • Improved resistance to alkali-silica reactivity

          • Each percent of entrained air reduces the compressive strength of concrete by 2-6%

          • Improved workability of fresh concrete

      • Water-reducing

        • Normal water-reducing admixture reduces the water demand to produce a given consistency in fresh concrete by 5-10%

          • Classified as Type A admixture in ASTM C 494

          • Reduces water content but same slump

        • High range water-reducing admixture, or superplasticizer, reduces water demand to produce a given consistency in fresh concrete by at least 12%

          • Classified as Type F admixture in ASTM C 494

          • More expensive than normal water-reducing admixture

        • Plasticizing admixture is 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

          • Classified as Type I Plasticizing Admixture in ASTM C 1017

        • Benefits: 

          • Improved workability without addition of more cement and water

          • Higher compressive strength without increasing the cement content or reducing the workability of the fresh concrete

          • Reduce the cement content without reducing the strength or the workability of the concrete

          • Produce high early strength by using superplasticizers (combined effect of using a lower w/c and the higher rate of hydration due to the superplasticizer

    • Set-controlling chemicals

      • Accelerating

        • Used to accelerate strength development of concrete at an early age.

        • Applications:

          • To expedite the start of finishing operation and application of insulation for protection

          • To reduce the time required for proper curing and protection

          • To permit early removal of forms and earlier opening of the construction for service

          • To permit more efficient plugging of leaks against hydraulic pressures

        • Classified as Type C in ASTM C 494.

        • Water-Reducing and Accelerating admixtures are classified as Type E in ASTM C 494

        • Most commonly used:  Calcium chloride (CaCl2•2H2O)

          • Reduces both initial and final setting times

          • 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

          • Reduces resistance to sulfate attack

          • Aggravates alkali-aggregate reaction

          • Increases shrinkage, volume change and creep

      • Retarding 

        • Used to retard the rate of setting of concrete.

        • Applications:

          • To offset the accelerating effect of hot weather on the setting of concrete & other admixtures

          • To delay the initial set of concrete or grout when more time is needed before placement and finishing

        • Classified as Type B in ASTM C 494.

        • Water-Reducing and Retarding Admixtures are classified as Type D in ASTM C 494.

        • High-Range Water-Reducing and Retarding admixtures are classified as Type G in ASTM C 494.

        • Plasticizing and Retarding admixtures are classified as Type II in ASTM C 1017

    • Mineral admixtures

      • Blast-furnace slag

      • Fly ash

      • 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 to 2.25 (typically), but can be as high as 2.55

      • Natural pozzolans 

    • Applications of Fly Ash, Blast-Furnace Slag and Silica Fume 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

  • Other Admixtures

    • Corrosion inhibitors: added to concrete to reduce the corrosion of steel reinforcing bars in concrete (ex: calcium nitrite)

    • Gas forming agents: added to concrete to form air bubbles in the concrete to form lightweight cellular concrete (ex: aluminum powder)

Lecture 11

• Challenge:  Compared to steel, normal concrete has a low strength/weight ratio.   Concrete with a higher strength/weight ratio is needed for tall buildings and long span bridges.

  • Solution: increase the strength/weight ratio, reduce the weight by using lightweight concrete or increase the strength by using high-strength concrete.

• Challenge: Normal concrete has low toughness and low resistance to impact.

  • Solution:  Use fiber-reinforced concrete

• Challenge:  Low permeability and high chemical resistance are needed for durability of concrete.

  • Solution:  Use polymer concrete.

• Challenge: control of temperature rise in mass concrete use for dams and other large structures is needed to reduce the chance of cracking of the concrete.

  • Solution: use techniques for pre-cooling concrete materials for roller-compacted concrete

• Challenge: high density material is needed for use as radiation shielding in nuclear power plants

  • Solution: use heavyweight concrete

• Structural Lightweight Concrete

  • Definition:  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).

  • Composition:  Similar to normal concrete except that it is made with lightweight aggregates or combination of lightweight and normal-weight aggregates. All lightweight concretes use both lightweight coarse and lightweight fine aggregates. Sanded-lightweight concretes used natural sand and lightweight coarse aggregates.

    • Ex: expanded clays, expanded shales, expanded slates, and expanded slags

  • 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 and 9%

  • Workability required: placing, compacting, and finishing a lightweight concrete requires less effort → lower slump is needed

    • Lightweight concrete with slump of 50-75 mm (2-3 in) has similar workability as a normal concrete with a slump of 100-125 mm (4-5 in)

  • Strength: 

    • Compressive strength is usually related to cement content at a given slump rather than to the w/c ratio

    • Common designed strength: 3000 to 5000 psi at 28 days

    • Strength of 6000-7000 psi can be achieved by using good-quality lightweight aggregate of small size (3/8 or 1/2 in maximum) and a high cement content

  • Advantages:

    • Reduction in dead weight of structure

    • Savings in steel reinforcement

    • Reduction in dead weight gives better resistance to earthquake loading.

    • Reduced handling, transportation, and construction cost for precast concrete elements

• 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

  • Definition: concrete with a compressive strength of more than 6000 psi(40 MPa)

  • General considerations in mix design for high strength concrete:

    • Low w/c is needed to achieve high strength

    • With low water content, the fresh concrete will have low workability.  Workability is achieved by use of high-range water-reducing admixtures

    • Reducing the maximum size of the aggregate can improve the strength of the 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

    • To achieve a higher strength at early age, use  condensed silica fume as the pozzolanic admixture

    • To further increase the strength of the concrete, fly ash or ground blast-furnace slag can be used as a partial replacement of the fine aggregate

• Roller-Compacted Concrete

  • Definition: a no-slump, almost dry concrete mixture which is finished by compacting with a vibratory roller

  • Advantages:

    • Lower cement content can be used

    • Lower formwork cost

    • Lower transportation cost due to ease of handling

    • Higher rate of concrete placement, and higher rate of utilization of equipment and labor 

    • Lower unit cost of concrete

  • Mix proportions: Mixture must be dry to prevent sinking of the vibratory roller but wet to permit adequate distribution of mortar throughout the concrete

  • Disadvantage: Difficult to bond the fresh RCC to another concrete surface due to its dry consistency

  • Properties:

    • Compressive strength up to 10,000 psi can be achieved

    • Strength is dependent on compaction.  Better compaction of the concrete gives higher strength

    • Fly ash is commonly used in RCC to improve workability

    • Lower heat generation from RCC

    • Creep and thermal properties of RCC are similar to those of conventional mass concrete

    • Permeability of RCC is equal to or less than that of conventional mass concrete

• Latex-Modified Concrete AKA Polymer-Modified Concrete

  • Materials and production method are the same as those for normal Portland cement concrete except that latex (a suspension of polymer in water) is used as an admixture

  • 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

  • Definition: concrete containing discontinuous discrete fibers. Fibers of various shapes and sizes produced from steel, plastic, glass, and natural materials can be used

  • Properties (compared with 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: 

    • Slabs for parking garage

    • Airport runway, taxiway and parking area

    • Overlay on pavement

    • Shotcrete used for tunnel lining 

    • Repair work

Lecture 12

• Classification of Trees

  • 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

  • ENDOGENOUS: grows inward by adding new cells to the old.

    • Ex: bamboo, palm

• Wood Structure

  • Cellulose & hemicellulose (55-80%)–Wood cells

    • Cellulose provides tensile axial strength & elastic property of wood

  • Lignin (15-30%)

    • Lignin 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

• Two growth regions in an annual ring:

  • Early wood (springwood): Inner light colored layer, which grows in the spring and grows relatively faster

  • Late wood (summerwood): outer darkerlayer, which grows in the summer and grows relatively slower

  • Rate of Growth: 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

• Properties of Wood

  • DENSITY

    • Solid wood substance (cellulose) specific gravity = 1.5

    • Wood (with air-filled cavities) specific gravity = 0.3-0.9

      • A function of moisture content

  • MOISTURE CONTENT

    • Wood contains moisture in 2 forms:

      • Absorbed water in the capillaries of cell walls

      • Free water in cell cavities

    • Fiber-saturation point: moisture content at which the cell walls are saturated & no free water exists; 25-30%

      • Ranges of moisture content (expressed as % of oven-dry weight): 

        • Green 30-250%

        • Air Dry 12-15%

        • Kiln Dry 6-7%         (optimal moisture content)

        • Oven Dry 0%            (strongest wood, but too brittle)

  • SHRINKAGE OF WOOD

    • Wood shrinks when its moisture content decreases and expands when moisture content increases. Shrinkage of wood is caused by the lateral contraction of the cell walls as they dry out. Very little contraction occurs in the longitudinal direction (direction of the cellulose or vertical direction of a tree) 

    • Ranges of shrinkage between fiber saturation point and oven-dry condition: 

      • Volumetric 7-21%

      • Longitudinal 0.1-0.3%

      • Radial 2-8%

      • Tangential 4-14%

    • Shrinkage increases with density of wood

  • STRENGTH

    • Density: Strength & stiffness ↑ density ↑ 

    • Growth: The slower the growth, the higher the strength & stiffness 

      • Latewood has higher strength & stiffness than earlywood

    • Moisture: Below the fiber saturation point, decreases in moisture content increase the strength & stiffness 

    • Orientation 

      • 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

      • Axial strength perpendicular to the grain

        • Tensile strength perpendicular to the grain is less than 1/10 of tensile strength parallel to grain - 800 psi

        • Compressive strength perpendicular to the grain is about 1/4-1/3 of compressive strength parallel to grain - 1200 psi

      • Shear Strength Parallel to the grain

        • About the same as the compressive strength perpendicular to grain - 1200 psi

  • THERMAL EXPANSION/CONTRACTION

    • Small compared with that due to moisture effects

      • Coefficient of thermal expansion of pine wood (parallel to the grain) = 3 X 10^-6 / ºF

  • THERMAL CONDUCTIVITY: how well it conducts heat

    • Low compared with concrete or steel

      • Steel = 328 Btu in / hr ft² ºF

      • Concrete = 6 Btu in / hr ft² ºF

      • Pine Wood = 1 Btu in / hr ft² ºF

      • Good Insulator = 0.2-0.3 Btu in / hr ft² ºF

  • ELECTRIC CONDUCTIVITY 

    • Increases with moisture content

      • Moisture content of wood can be estimated from measuring the electric resistance of the wood

• 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 (based on strengths and amount of defects)

• Grading of Hardwoods (based on the amount of clear area without defects)

• Preservation of Wood Against Decay

  • CHEMICAL TREATMENTS: usually by pressure

    • Waterborne Solutions

      • used inside the house and outdoors

      • Chromated Copper Arsenate (or CCA) had typically been used as a preservative (EPA has banned the use of CCA treated wood in residential environment, including playground, where it may have human contact)

      • New wood preservatives (such as ACC & ACQ) are being introduced

      • Creosote or mixture of Creosote and Coal Tar

      • Pentachlorophenol

        • This and creosote can only be used in an outdoor environment with no contact with human or drinking water

  • WATERPROOFING

    • Paint

    • Varnish

    • Zinc chloride

• Treatment of Wood to Retard Fire 

  • Wood can be pressure treated with fire retardant chemicals to reduce the spread of flame in a fire.  Fire-retardant treated lumber is marked with a label “FRT”

• 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

    • 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)

• 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

• 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