Concrete

Front 1

concrete

Back 1

fine aggregate, coarse aggregate, cement, and water

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ingredients

Back 2

inert ingredients: aggregates; active ingredients: cement and water

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hydration

Back 3

chemical reaction when water is added to the dry mixture of aggregate and cement

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1850

Back 4

development of reinforced concrete

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1918

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water-cement ratio law

(Duff A Abrams)

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1867

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iron mesh reinforcing

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1928

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long-span bridge design

(Eugene Freynninet used high-strength steel wire to counter the effects of creep in concrete)

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Portland cement

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manufactured from lime, silica, iron oxide, and alumina. proportioned, ground, and burned to form clinkers - then pulverized to produce cement

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aggregates

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affect the quality of the concrete, reduce shrinkage of concrete, and serve as filler

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aggregates sizes (3)

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1. fine aggregate: 1/4" dia or less

2. coarse aggregate: 1/4" dia - 1-1/2" dia.

3. economy: up to 6" dia.

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mix

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the proportioning of the ingredients that comprise concrete

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1:3:5 mix

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1 part cement; 3 parts fine aggregate; 5 parts coarse aggregate

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water-cement ratio

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perhaps most. expressed as the number of gallons of water for each sack of cement. major factor controlling concrete strength and durability

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laitance

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a chalky surface deposit of low strength that comes from excess water (must be removed before any new concrete is poured)

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strength

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compressive strength always refers to its strength 28 days after being placed

usual range 3,000psi - 6,000psi

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ready-mix concrete

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mixed completely at a central mixing plant and transported to the site in an agitator truck. must be placed within 1-1/2 hours after water is added to the mix

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transit-mix concrete

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mixed completely in truck mixer. dry materials are picked up and placed in mixer truck that carries a water tank. once arrived at site the water is added and then thoroughly mixed

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reinforcing

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form of round bars or welded wire fabric

designated by numbers representing the diameter in eights of an inch: #3 (3/8" dia.) - #11 (1-3/8" dia.) and #18 (2-1/2" dia.)

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ASTM A615

ASTM A616

ASTM A617

ASTM A706

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A615 - billet steel

A616 - rail steel

A617 - axle steel

A706 - low-alloy steel

(most common ASTM A615 grade 60)

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lightweight concrete

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- made from shale or clay aggregates.

its weight 90-115 pounds per cubic foot.

- max. size of aggregate 3/" dia.

- modulus of elasticity is lower - thus deflection greater

- drying shrinkage is slightly greater

- thermal insulation better

- cost always greater

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honeycomb effect

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when air bubbles are trapped in the body of the mixture and allowed to remain. they cause a substantial reduction in concrete strength and watertightness

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slump test

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measures consistency and workability of the concrete mix, performed in the field

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cylinder test

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measures the compressive strength of concrete and utilizes standard test cylinders 6" in diameter and 12 inches long. laboratory cured for 7 - 28 days and tested in a crushing machine

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curing

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1. supplying additional moisture to the surface

2. using a wet covering, moist sand, burlap

3. covering with a membrane or curing compound that prevents evaporation

4. leaving wood forms in place and keeping them moist

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length of curing period

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3 - 14 days (should be as long as possible)

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curing temperature

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50 and 70 degrees F

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joints

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1. expansion joints - allow free movement, complete separation

2. control joints - tooled, sawed, or premolded. create weakened sections that induce cracking

3. isolation joints - provide a separation between a slab on grade and columns or walls so that each can move independently

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prestressed concrete

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placed in compression by applying a tensile force to prestressing steel before the external loads are applied. purpose to cause stresses in the concrete that are opposite in direction from those caused by external loads

stiffer (because the entire section is effective) and have greater shear strength

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apply prestress

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1. pretensioning

2. posttensioning

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precast concrete

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greatest economic advantage when there are identical member to cast, other advantages...

1. better quality control of concrete

2. better control over curing

3. members castable in all weather

4. members erectable in all weather

5. faster actual construction time