Building Technology - Lecture 1 PDF

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CONCRETE1. CEMENTINGMATERIALS1 Lime1 Gypsum1 Cement2. STORAGE OFCEMENT3. CONCRETE3 Definition3 Qualities of GoodConcrete3 Materials ofConcrete3 Slump Test3 Proportioning3 Mixing3 Transporting andPlacing3 Shrinkage3 Curing3 Admixtures3 Forms4. PROCESSEDCONCRETE4 Types ofProcessed Concrete4 Aggregates...

Description

CONCRETE CEMENTING MATERIALS

3.09 Curing

1.01 Lime

3.11 Forms

1.

3.10 Admixtures

1.02 Gypsum

PROCESSED CONCRETE

1.03 Cement

4.

STORAGE OF CEMENT

4.01 Types of Processed Concrete

2.

4.02 Aggregates for Light-weight 3.

CONCRETE

3.01 Definition 3.02 Qualities of Good Concrete 3.03 Materials of Concrete 3.04 Slump Test 3.05 Proportioning 3.06 Mixing 3.07 Transporting and Placing 3.08 Shrinkage

Conc.

Cementing Materials 1.

1.01

Quicklime

LIME

One of the oldest manufactured building materials used as a mortar and plaster by all the early civilizations: • Egyptians used lime plaster before 2600 B.C. • Greeks used it extensively for mortars and plasters

Before quicklime can be used, it must first be mixed with water in the process called slaking or hydration. The lime has now become calcium hydroxide

• Romans developed a mixture of lime putty and volcanic ash for the first real cement. Manufactured by the calcination of limestone (carbonates of calcium and magnesium). (Ca(OH)2), known as slaked lime or hydrated lime.

The carbonates decompose into carbon dioxide, which is expelled, and calcium oxide (CaO) called quicklime.

Hydrated lime mixed with water to make lime putty, is used as an ingredient of hardfinish coat for two-and threecoat Portland cement plasters. It is also used for mixing with cement mortar or concrete to:  

increase its workability decrease its permeability to water

 

reduce cracking due to shrinkage

retard the set and improve the setting qualities. Hard

A type of lime which will set under water is hydraulic lime, used only where slow underwater setting is

required.

1.02

GYPSUM

Gypsum, like lime, was used as a plaster by the Egyptians, Greeks and Romans.

wall plaster is harder than lime plaster, sets more quickly and thoroughly. Gypsum plaster is rendered more plastic by the addition of hydrated lime. Fiber or hair is also sometimes added for greater cohesiveness. The fiber may

“Plaster” from the Greek word for both the raw material and calcined product. In architectural terminology the words “Plaster” and gypsum” are often used interchangeably. Gypsum rock is ground fine and heated (calcined) to between 325 F. to 340 F. when it loses about threefourths of its combined water. The remaining product is Plaster of Paris if pure gypsum is used, or hard wall plaster if 39.5 % impurities are present or added to

be hemp, sisal or jute; the hair is generally cleaned goat or cattle hair.

lime with pozzolana (volcanic ash) which hardened pozzolana under water. With the fall of the Roman Empire the art of cementmaking was lost and for several centuries. In 1756, Smeaton, an Englishman, rediscovered hydraulic cement but it was not until 1824 that Aspdin, an English bricklayer and mason, invented and patented Portland cement. Today, the word “cement” generally refers to Portland cement which is the principal type of cement in use.

Portland cement is obtained by finely pulverizing clinker produced by calcining a proportioned mixture of argillaceous (silica, alumina) and calcareous (lime) materials with iron oxide and small amounts of other ingredients. Types of Portland cement: • slow-setting cement 1.03

CEMENT

First developed by the Romans by mixing slaked

• quick-setting high strength cement

early

• sulfate-resisting cement for applications where alkaline water and soils occur • white cement (or stainless cement which is free of iron impurities).

Cement is soft and silky to the touch. If it has lumps that do not readily break, the cement has already absorbed a damaging amount of moisture. Cement should be used as soon as possible after delivery. Piles should be limited twelve sacks in height.

to

Warehouse set - when the cement is stored in high piles

STORAGE CEMENT 2.

OF

Cement should be protected at the building site from injury through contact with dampness. They should be stored in shed with a wood floor raised about 300mm (12”) from the ground.

for long periods, there is a tendency for the lower layers to harden caused by the pressure above.

3.

CONCRETE

3.01

DEFINITION

Concrete is:

QUALITIES OF GOOD CONCRETE 3.02

Concrete should be: • Strong • Durable

• a proportioned mixture of cement, aggregate and water.

• of uniform quality, and

• a plastic mass which can be cast, molded or formed into predetermined size or shape

These are obtained through:

• upon hydration, becomes stone-like in strength, hardness and durability. The hardening of concrete is called setting. • when mixed with water and a fine aggregate of less than 6mm (¼“) is known as mortar, stucco or cement plaster. • when mixed with water, fine aggregate and a large aggregate of more than 6mm (¼”) in size produces concrete. • when strengthened by embedded steel, is called reinforced concrete. • when without reinforcement, is called plain or mass concrete.

• thoroughly sound.

• careful materials

selection

of

• correct proportioning • thorough mixing • careful transporting and placing • proper curing or protection of the concrete after it is placed

MATERIALS CONCRETE 3.03

OF

a. Cement In reinforced-concrete construction should be highgrade Type 1 Portland cement type C-150 conforming to the “Standard Specifications and Test for Portland Cement” of the American Society for Testing Materials (ASTM).

• The kind of tests usually made are: ✓ soundness, or constancy of volume ✓ time of setting ✓ fineness ✓ tensile strength Each bag of cement is equivalent to approximately 1 cu. ft. and weighs 94 lbs. b. Aggregates are: Inert mineral fillers used with cement and water in making concrete, should be particles that are durable strong, clean, hard and uncoated, and which are free from injurious amount of dusts, lumps, soft and flaky particles, shale, alkali, organic matter loam or other deleterious substances. • Fine aggregates (aggregates smaller than 6mm (¼”) in size) consist of sand, stone screenings or other inert materials of similar characteristics. Specs: 80 to 95% shall pass a No. 4 wire cloth sieve and not more than 30% nor less than

10% shall pass a No. 50 sieve.

• Coarse aggregate (aggregate larger than ¼” in size) consists of crushed stones, gravel or other inert materials of similar characteristics.

They may range up to 50mm (2”) for less highly reinforced parts of the structures such as footings, thick walls, and massive work.

• Special aggregates, such as cinders, blast furnace slag, expanded shale or clay, perlite, vermiculite, and sawdust, may produce: lightweight, concrete

nailable

- thermal insulating concrete.

Coarse aggregates should be well graded in size to a size which will readily pass between all reinforcing bars and between reinforcement and forms but not exceed 25mm (1”) in size for reinforced beams, floor slabs, & thin walls.

- should be free from oil, acid, alkali, vegetable matter, or other deleterious substances - should be reasonably clear and clean. - The use of sea or brackish water is not allowed. - Water combines with the cement to form a paste which coats and surrounds the inert particles of aggregates. - Upon hardening, it binds the entire mass together. - The strength of the mixture therefore depends directly upon the strength of the paste. If there be an excess of water the paste becomes thin and weak and its holding power is reduced. - The water-cement ratio is the amount of water used per bag of cement. - This usually varies from 5 to 7 gallons, with 6.5 gallons as average for ordinary job conditions. The less water used in mixing, the better the quality of concrete. - The ideal mix is one that is plastic and workable. It should not be too dry that it becomes too difficult to place in the forms, nor too wet that separation of the ingredients’ result.

WATER – CEMENT RATIO Assumed 28- Maximum Pounds of day waterwater per 100 Compressive cement lbs. of cement strength (lbs. ratio per sq. inch) U.S. gallons of water per sack Cement of 94 lbs. 2,000

7.00

62.0

2,500

6.50

57.5

3,000

5.75

51.0

3,750

5.00

44.5

3.04

SLUMP TEST

- used for measuring the consistency of a concrete mix. - Consistency may be defined as the “state of fluidity of the mix”, and it includes the entire range of fluidity from the wettest to the driest possible mixtures. In this test the tendency of a mix to “slump”, or reduce its height due to gravity action,

is measured. The apparatus consists of metal cone, the bottom opening being 200mm (8”) in diameter, the top opening being 100mm (4”), and the height exactly 300mm (12”).

In making the test, the slump tester is placed on a flat, smooth surface and is filled with newly mixed concrete from mixer. In filling the mold with concrete, the latter is tamped in with a 12mm (½”) rod pointed at one end and the top of the concrete is smoothed off exactly level. The mold is then slowly raised vertically and the height deducted from the original height of 300mm (12”) represents the slump.

A harsh mix is efficient for slabs, pavements, or mass concrete where the lowest possible water-cement ratio is desirable. The following table gives the permissible slump for various types of concrete in relation to their uses:

PROPORTIONING OF CONCRETE 3.05

Briefly stated, the principles of proper proportioning are as follows: a. Use good quality materials: Portland cement, water, and aggregate. b. Determine the strength of the concrete using the water-cement ratio. (The strength increases as the water-cement ratio decreases). c. Determine consistency of the

the mix

using the slump test using as dry a mix as practicable. d. Add correct proportions of aggregates to the cement and water as will give a mix of the desired consistency. e. Make a mix that’s workable, not harsh. The strength of a workable concrete mix depends upon the water-cement ratio. The economy of the mix depends upon the proper proportioning of the fine and coarse aggregates.

There are several methods of proportioning concrete:

Proportioning arbitrary proportions a.

by

Proportioning concrete by the arbitrary selection of the proportions is the oldest, the most commonly used, the most convenient and the least scientific method. In this aggregates are loose volume, volume as it is measuring box.

method, the measured by that is, its thrown into a

One sack of cement is taken as 1 cu. ft. Enough water is used to give the desired consistency.

CONCRETE PROPORTIONS Class 1 : For concrete “AA” 1.5 : 3 under water, retaining walls suspended Class 1 2 : 4 For slabs, beams, “A” : columns, arches, stairs, walls of 100mm (4”) thickness Class 1 : For walls thicker “B” 2.5 : 5 than 100mm (4”), footings, steps, reinforced concrete slabs on fill. Class 1 3 : 6 For concrete “C” : plant boxes, and any non-critical concrete structures. Class 1 : For mass “D” 3.5 : 7 concrete works.

The proportion is to be read:

Common mixes expressed in proportions by volumes of cement to fine aggregate to coarse aggregate are as follows:

Class A: 1 part cement is to 2 parts sand is to 4 parts gravel. Each ‘part’ is equivalent to one cubic foot which is the measure of the box constructed to be 1 foot (12

inches) on each of the three sides. Each bag of cement is equivalent to approximately one cubic foot.

Proportioning by the water-ratio and slump test b.

There are two steps to be observed: - Select the amount of water to be added to the cement to give the desired strength (see Table) - Add just enough mixed aggregate to the water and cement to give a concrete mix the desired consistency. It is customary to specify

This method is the same as the second except that the proportions of the fine and coarse aggregate are determined by the fineness modulus method. For economy, proportion the fine coarse aggregates so that the largest quantity of mixed aggregate may be used with a given amount of cement and water to produce a mix of the desired consistency of slump. Comparatively, the coarse aggregate has a lesser total surface to be covered with cement paste and, therefore, is more economical.

Proportions of cement to fine aggregate to coarse aggregate may be given if desired.

However, there must be enough fine aggregate present to fill the voids in the coarse aggregate, or extra cement paste will be needed for this purpose. A wellgraded aggregate contains all sizes of fine and coarse particles in such proportions that the voids in the combined aggregate will be a minimum.

c. Proportioning by water-

3.06

- the cement in sacks - the water in gallons per sack of cement and - the mixed aggregate in cu. ft. per sack of cement.

ratio, slump and fineness modulus

MIXING CONCRETE

OF

• Reinforced-concrete work should be mixed by machine • Machine-mixed concrete is usually of more uniform quality than that mixed by hand and is generally less expensive when in large volume. • The strength of concrete is very largely dependent upon the thoroughness of mixing.

Batch mixers - into which sufficient materials are placed at one time to make a convenient size batch of concrete, the whole amount being discharged in one mass after it is mixed.

Continuous mixers - into which the materials are fed constantly and from which the concrete is discharged in a steady stream.

a. MACHINE MIXING In machine-mixing, the mixing of each batch should continue not less than one minute after all the materials are in the mixer and whenever practicable, the length of the mixing time should be increased to 1.5 or 2 minutes. The entire contents of the drum should be discharged before recharging the mixer. The mixer should be cleaned at frequent intervals while in use.

Concrete mixers may be divided into two general classes:

Concrete mixers may also be classified as: - drum mixers - trough mixers - gravity mixers, and - pneumatic mixers. The drum mixers are the most common type.

b. HAND MIXING - hand-mixing must be done on a water-tight platform. - cement and fine aggregate shall first be mixed dry until the whole is a uniform color. - water and coarse aggregate shall then be added and the entire mass turned at least three times, or until a homogeneous mixture of the required consistency is

- Regaging or retempering of concrete that has been allowed to stand more than ½ hour is not to be permitted.

TRANSPORTING AND PLACING OF CONCRETE 3.07

• Fresh concrete should be transported from the mixer as rapidly as practicable by methods that will permit the placing of the concrete in the forms before initial set occurs and without loss or separation of materials. • The delivery of the concrete from the mixer to the forms should be fairly continuous and uninterrupted. • The time of transportation should not exceed 30

obtained.

- since initial set of concrete takes place 1 to 3 hours after mixing, a batch may be used anytime before initial set takes place, provided that the mix is plastic.

minutes.

• The concrete may be transported by means of barrows, buggies, buckets,

cableways, hoists, belts and pipes.

chutes,

• When chutes are used, the slope should not be more than 1 vertical to 2 horizontal or less than 1 vertical to 3 horizontals. The delivery end of the chutes shall be as close as possible to the point of deposit.

• Before placing concrete, the forms shall be cleaned and inspected, surfaces wetted or oiled, and reinforcement properly secured. • Concrete should be deposited in approximately horizontal layers in wall, column and footing forms. They should not be piled up in the forms which may result in the separation of the cement mortar from the coarse aggregate. • Concrete should never be allowed to drop freely over 5 ft. for unexposed work and over 3 ft. for exposed work.

SHRINKAGE CONCRETE TEMPERATURE CHANGES 3.08

OF &

• Shrinkage of concrete due to hardening and contraction from temperature changes, causes cracks the size of which depends on the extent of the mass. They cannot be counteracted successfully but they can be minimized by placing reinforcement so that large cracks can be broken up to some extent to smaller ones. • In long continuous length of concrete, it is better to place shrinkage or contraction joints. Shrinkage cracks are likely to occur at joints where fresh concrete is joined to concrete which has already set, and hence in placing the concrete, construction joints should be made on horizontal and vertical lines.

CURING CONCRETE 3.09

OF

• Concrete must be allowed to “cure” or harden after it is placed. • Hardening is a rather slow process in which the cement and water unite to form compounds that give strength

and durability to the concrete. It continues as long as the temperatures are favorable and moisture is present. • Three main factors that affect hardening are: - age or time - temperature, and - moisture. • In order that the hardening may proceed favorably, the fresh concrete, for about 7 days after placing, should be protected from excessive vibration, loads, extreme heat or cold, too rapid drying, and contact with impurities which may interfere with the chemical action. • The strength of the concrete increases with age when the curing conditions remain favorable.

• The increase in strength is rapid during the early ages and continues more slowly as time goes on. The compressive strength reaches about 60% of its own maximum value at an age of 28 days and about 80% at an age of 3 months.

Curing consists primarily in keeping the concrete from drying out too rapidly. This may be done by: a. Covering the concrete. Floors shall be covered with paper sacking wetted down at the edges or with burlap, sand or earth that is kept moist, after the concrete is hard enough to walk on. b. Removal prescribed shall not be after the time

of forms at time. Forms removed until specified.

c. Sprinkling with water. Beams, columns and walls are sprinkled or sprayed with

water as soon as the forms are removed. d. Using curing compounds (see ADMIXTURES).

Substances added to cements, mortars, and concrete for the purpose of improving or imparting particular properties, such as:

• those for surface application or finish. Admixtures come in powder, paste, and liquid form, and are usually patented and sold under trademark names.

• To improve workability of concrete, e.g. hydrated lime • To improve durability by entrainment of air • To accelerate setting or hardening (accelerators) e.g. calcium chloride • To retard setting (retarders). • To improve wear resistance • To impart water-repellant or water-proofing qualities e.g. hydrated lime, KAOLINE, CELITE. • To impart water-repellant or waterproofing qualit...