BCT 102 - concrete and reinforcement matls

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Concrete is defined as a mixture of cement, aggregates and water. When first mixed, concrete is semi-liquid. It then hardens (sets or cures) thru a checmical process called ________ into a hard, stone-like mass.: hydration
Portland Cement: is a complex mixture of at least four chemical compounds which interact with water to make calcium silicate hydrate gel. The hydration process produces heat. The faster the hydration the more heat is produced. Note that cement is not concrete and, thus, the 2 terms cannot be used interchangeably.
Hydration: is the catalytic action water has on transorming the chemicals in Portland cement into a hard solid. Prior to the addition of water, Portland cement is a powdery mix of chemicals. Water, which is also a chemical acts as a catalyst to make the other chemicals interact with each other to form the calcium silicate hydrate gel.
Plastic Concrete: is concrete in a semi-liquid, pourable and moldable state.
Green concrete: is concrete after it hardens, but before it gainst structural strength.
Cured concrete: is concrete which has hardened and gained it structural strength
Poured-in-Place Concrete: is concrete which is placed in its final location while in a plastic form. It may be poured (placed in a hole, a mold or a form.
Precast Concrete: is concrete which is cast or molded in one location while plastic, and after it cures is moved to its final location. Such items as blocks, pipes, beams, wall and floor panels can be precast.
Reinforced Concrete: is concrete placed around some type of reinforcing material, usually steel rods or mesh, which gives the structural member greater tensile and shear strength. Almost all concrete is reinforced.
Pre-Stressed Concrete: is concrete which is placed around reinforcement, usually steel rods or cables which are under tension. When the concrete is cured and the tension is released, the unit gains great structural strength. Many structural concrete components are often prestressed.
Post-Stressed Concrete: is concrete which is placed around cables or rods which are kept isolated from the concrete by encasement and/or a lubricant. After the concrete has cured, tension is applied to the rods or cables, to create a permantent tension condition.
The following are 2 properties of plastic concrete:: 1. Moldability: Plastic concrete may be formed into almost any shape. It is often used for decorative effects.
2. Portability: Plastic concrete may be moved in mixign trucks, motorized buggies and wheel-barrows, by belt conveyors & hydraulic pumps.
The following are properties of cured concrete:: 1. High Structural Strength: When concrete is reinforced or pre-stressed, it has hgih structural strenth under compression, tension and lateral pressure.
2. Waterproof: Although water is ued to prepare concrete, the concrete can harden under water. Not always waterproof out of the water.
3. Druability: Properly mixed, well-placed concrete continues to gain strength for several years and becomes almost as durable as the hardest natural stone (Hoover Dam - 1939)
4. Abrasion Resistance: Properly mixed, well-finished concrete will take havy use with little evidence of wearing.
Wet concrete is toxic. Hazards include: wet concrete can be absorbed through the skin thru an open wound and cause blood poisioning. Can cause chemical burns to the eyes. If the concrete enters boots from top, remove boots and rinse them clean as soon as possible.
Cement: is a finely ground powder consisting of lime, silica, alumina, and iron. While dry, it may be moved in bulk, bagged and stored for relatively long periods of time. When mixed with water, the hydration process beings and the mixture soon will solidify.
Although ancient civilizations used forms of clay, lime and gypsum as binding agents for stone structures, after the fall of Rome and during the Dark ages, these formulas were lost. John Smeaton built a lighthouse in English channel of material which har: it produced a yellow-grey concrete that resemled stone quarried on the Isle of Portland.
Today, portland cement is manufactured by heating lime mixed with: clay, shale or slag to about 3000 degrees F. The clinker which is produced is pulverized and gypsum is added to regulate the hydration process.
Types of Portland cement: Type I: Normal - a general purpose cement, used in pavements, sidewalks, bridges, buildings, tanks and pipes.
Type II - modified portland cement: A cement producing less heat during hydration and having some resistance to sulfate attack. Used in large structures such as piers and heavy retaining structures. It is also used in warm weather and where sulfates are found in the soil or water. Firebird lake - Phx
Type III - Early Strength portland cement: This cement gains strength in from one to 3 days. This enables cold weather protection and forms can be removed sooner than with Type I cement.
Type IV - Low Heat portland cement: This cement is used where the heat produced by hydration must be kept to a minimum. This cement also gains strength more slowly than normal. It is used in very large structures such as dams. - 61 years - Hoover Dam
Type V - sulfate resisting portland cement: This cement is used where severe sulfate conditions exist. It gains strength slowly.
Other types of portland cement: Air-entrained: materials are added which produce many small individual air bubbles in the concrete. This improves the concrete's ability to resist freezing and thawing; reduces the amt of water needed in the mix, thereby producing stronger concrete and makes the plastic concrete more workable. Air-entraining is available in cement types I, II, and III. It is designated by an "A" after the type nubmer: IA, IIA and IIIA.
Other types of portland cement: White: This cement is regulated during manufacture to produce a white cured concrete. It is used for decorative effects and is recommended when admixing colors.
Other types of portland cement: Pozzolan: Elements such as volcanic ash are added in the manuf. to produce a superior underwater concrete. It is designated as "IP" or "IPA" if also air-entrained.
Concrete Mix Measurements: The strength of concrete is measured at ___ and ___ days after placing by a compression test.: 7 and 28 days
The strength meausrement of concrete is determined by the: amount of cement in the concrete and is expressed by the pounds per square inch that the cured concrete can withstand. The ACI (American Concrete Institute of American) published a manual that gives design mix numbers for concrete so you can select the mix adequate for the strength of concrete specified.
Aggregates such as _____, _______, and ____________ are added to concrete to provide a volume of low-cost filler material.: sand, gravel and crushed stone Volume and qualtiy of aggregate is determined by use for the concrete.
Aggregates must be _______, of proper _______ and ________, of proper ______ and ________.: Aggregates must be strong, proper size and strength, of proper weight and clean.
Fine aggregates are ______ or less in diameter. Coarse aggregates are more than ___ in diameter.: 1/4" or less in diameter. Natural or manufactured sand is normally used.
Coarse aggregates are more than 1/4" in diameter and are usually gravel or crushed stone. Aggregates that arer randomly mixed often provide inferior concrete in that they pack so tightly that the binding mixture of cement and water cannot flow freely and coat all surfaces of the aggregate.
Shape of aggregate - both gravel and crushed stone produce concrete of approximately the same strength; however, crushed stone requires more water in mixing and is often harder to place and finish. Aggregate particles that are _________ and ___ are struc: flat and long
Weight of aggregate will be based upon the use for which teh concrete is intended and is as follows:: 1. Very light concrete weights from 20 to 70 pounds per cubic food and has almost no structural strength. Used ofr sound and temp. insulation and contains cincders pumice, perlite and other lightweight substances for aggregates.
2. Lightweight concrete, weighing from 85-115 pds per cubic foot has structural strength sufficient for roofs. It contains shale, clay or slate as aggregates.
3. Normal weight concrete weighting between 140 to 160 pds per cu. foot is produced by using sand, gravel or crushed stone for aggregates.
4. Heavyweight concrete uses such materials as iron and steel particles or other such heavy materials for aggregates.
Aggregates coated with foreign materials such as rust, loam silt, clay, oil, etc.: prevent the adherence of the cement binders and thus should not be used.
As soon as the cement contacts the water, the hydration process begins. Sufficient water to begin hydration and produce a workable mix must be used. This amt is from about ___% to ___% of the concrete.: 15% to 20% of the concrete. Too much water in the mix produces weak concrete.
If questions exist to the quality of water to be used, mortar cubes should be made and tested aginst control cubes made with drinkable water. These tests hould at ___ days show ___ of the design strength.: 28 days show 100% of the design strength. Certain limits have been established on the use of water containing concentrations of chlorides, sulfates, alkalines and solids.
Adminixtures are elements that are added to the concrete at its mixing to produce special desired qualities in the plastic, green or cured concrete. Before admixtures are used, answers to the following questions should be obtained: 1. Will they cause corrosion in the reinforcement or imbedded metals?
2. If color is a consideration, will they change the color of the cured concrete?
3. Will they lower the strength of the cured concrete?
4. What special conditions must be met in their mixing, handling and finishing?
Types of Admixtures: 1. Air entraining agents 2. Accelerating admixtures
3. Retarding admixtures
4. Water reducing admixtures
5. coloring admixtures
6. Workability agent admixtures
7. Pozzolan
Air entraining agents: may be added at mixing as well as at cement manufacturing. As a rule of thumb, air entraining is required for exterior concrete that is exposed to freezing and thawing.
Accelerating admixtures: speed the hardening of the concrete. Calcium Chloride used to be added as an accelerator. It raised the hydration temperature and caused early strenth. However, calcium chloride should not be used in concrete because of corrosion to steel reinforcement materials.
Retarding admixtures: slow the rate of hydration. Recommended for concrete placed in hot weather or when difficulties in transportation, placement or finishing occur. Methods: 1. cool mixing water; 2. add retardants to mix; 3. brush or spray retardants on the surface of placed concrete. May reduce strength of concrete while curing
Water reducing admixtures: increase slump, allowing less water to be used in concrete mix. Produces greater strength. Some act as retardants and slow setting time. Advisable to use these admixtures rather than adding more water to the mix when greater workability is desired.
Coloring admixtures: can be used for decorative, warning or identification purposes. If for decoration, samples must be made and approved. Sufficient color admixtures should be ordered from the same manufacturer's batch to complete the entire job.
Workability agent admixturees: such as alginatees and cellulose derivatives are used to increase working time and effort.
Pozzolan: which is volcanic ash, and other substances are used as admixturees to reduce interal heat caused by hydration. They are of benefit in pouring such large structures as dams.
Reinforcement Concrete has good comrpessive strength but is relatively weak in _____. Reinforcemnt in concrete increases its ___ strength and gives it _____ and tension strength.: Concrete has good compressive strength, but is relatively weak in tension. Reinforcement in concrete increases its compression strength and gives it shear and tension strength.
(concrete - kinds of reinforcement) Reinforcing bars, also called __________ and ______ are steel rods of 20', 40' or 60' in lenth with diameters from 1/4" to 2-1/4". All except the 1/4" size have patterns stamped on their surface to: re-bars and re-rods
(concrete - kinds of reinforcement) Re-bars are given number designation of from __ to __. The number identifying the bar multipled by 1/8" equals the approximate diameter of the bar.: 2 to 18
(concrete - kinds of reinforcement) Re-bars are connected by being: overlapped a minimum of 12" on the smaller diameter sizes and at least 24 times the bar's diameter on larger size. All tying is done with steel tie wire. Bars can be connected by butt-welding them but this is not recommended.
(concrete - kinds of reinforcement) Welded wire mesh: is actually a steel wire cloth. It is usually identified by numbers that indicate the spacing of the wires and the gauge of the wire. Wire mesh comes in rolls or in mats. Mesh joints are usually overlapped one square and tied with tie wire.
(concrete - kinds of reinforcement) Steel cables: are often used for reinforcement in both pre-stressed and post-stressed concrete. It has strength, flexibility and elasticity suited to these uses.
(concrete - kinds of reinforcement) Glass mat is another type of reinforcement.: It is a fiber mesh that comes in a package about the size of a coffee can. You throw it in the truck while the mixer is turning, and it expands in the concrete. It eliminates the need for steel reinforcement. It consists of fibers about 1-1/2" long. It should be noted that once glass mat reinforced concrete curees, it is extremely hard to demolish.
The placement of the reinforcement is determined by a structural engineer. Reinforcement is generally placed __ inches above the ground and ___ inches from the form faces. The major portion of the reinforcement is placed in thetension side of the structu: 3 inches above the ground and 2 inches from the form faces.

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