Engineering Materials-concrete.docx

CONCRETE MATERIALS IN CONCRETE Concrete is a conglomerate, stone-like material composed of essentially of three materials: Cement, water, and aggregate. Sometimes a fourth material, an admixture, is added for a variety of specific purposes, such as acceleration or retardation of setting or hardening. The strength and quality of concrete depend not only on the quality and quantity of the materials, but on the procedures used in combining these materials and the skill involved in the placing and curing of concrete. The conglomeration of these materials producing a solid mass is called Plain concrete. Concrete in which reinforcement is embedded in a manner that the two materials act together in resisting forces is called reinforced concrete.

CEMENT The characteristics of concrete vary widely, depending on the composition of the aggregate and chemical and physical properties of the cement paste. The term cement, in its broader meaning, applies to any material that will bind two or more non-adhesive substances together. Hydraulic cement is a bonding agent that reacts with water to form a hard stone-like substance that is resistant to disintegration. The cement is the bonding agent of rock materials called aggregate which act as a filler. Chemistry of Cement The chemical reaction between cement and water is the principal action in the chemistry of concrete. Cement by itself does not provide a cementing binder, the cementing gel is formed by the reaction of cement and water. The cement paste becomes hard within week, but the hardening process may continue to some extent for months or years. Types of Cement 1. Roman Cement Concrete has been used as construction material for centuries. Before 100 B.C., the Romans had developed an excellent concrete, which enabled them to erect vast structures and works of engineering. On the slopes of Mt. Vesuvius and in extinct volcanic areas near Rome they found a light, porous volcanic rock. Its rough surface formed a good bond for cementitious material (a substance capable of acting as a cement), or mortar. The cement was prepared from a mixture of lime and a volcanic ash called pozzolana, named after the village of Pozzouli near Mt. Vesuvius. This is were the Pozzolan Cement originated. 2. Natural Cement. Certain natural rocks, when quarried, crushed, and processed, will produce a natural cement. If enough heat is applied to drive off gases, a hydraulic cement results, but it has a very low strength.

3. Portland Cement In 1824, Joseph Aspdin in England developed and patented a hydraulic cement that was superior to the natural cement of that time. He called this cement Portland cement, because of its resemblance to a grayish limestone mined on the isles of Portland. Portland cement was first manufactured in the United States, in Pennsylvania, in 1872. It was discovered that if a carefully controlled mixture of limestone and clay was burned at a much higher heat than had been used before, the resulting cement had better hydraulic qualities. As this higher heat the clay and limestone fused into hard, marble-sized clinkers composed of two original materials in a new form. These clinkers, when ground, produced Portland cement as we know it now. Portland cement has the following basic composition: Lime 60 – 65 % Silica 10 – 25 % Iron Oxide 2–4% Alumina 5 – 10% Most of the ingredients of Portland cement are found in nature, but they cannot always be used in their natural form. Portland cement is the most widely used in various small and large construction including roads and highways. Portland cement is not a brand but a type of hydraulic cement and is sold in 40 kilograms bags or Bulk into cement trucks. When Portland cement is mixed with a sufficient amount of water and left undisturbed, the paste loses its plasticity and becomes solid. Cement does not harden by drying. It hardens because of chemical reaction called Hydration: the water and the cement combine chemically to form a new compound. This process is called Setting.

Types of Portland Cement Type I

Normal Portland Cement – this is a standard Portland cement for general construction. It is generally grayish in color.

Type II

Modified Portland Cement – this cement has a lower heat of hydration than Type I and generally sets more slowly. It is used in drainage structures, foundations and floor slabs where the soil contains moderate amount of sulfate (sulfate will disintegrate concrete).

Type III

High Early Strength Portland Cement – this type develops approximately 190 percent of the strength of Type I at three days and 90 to 130 percent at 28 days. It is used when it is desired to remove forms at an early stage to speed up construction.

Type IV

Low-Heat Portland Cement – concrete made with this type of cement sets very slowly and generates little heat. It was first developed for the construction of Hoover Dam (dam on the border of the states of Nevada and Arizona, United States, situated in Black Canyon on the Colorado River, near Las Vegas, Nevada). Its slow setting time is an advantage in large construction, where solid one-piece construction is desired.

Type V

Sulfate Resistant Portland Cement – a special cement intended for use in structures exposed to severe surface action of soils or water with high alkali content. It has a slower rate of hardening than Type I.

Type I Portland cement must meet the following chemical and physical requirements: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Lime content, percent: 63.0 Silica content, percent: 22.0 Alumina content, percent:6.0 Iron content, percent: 3.5 Magnesia content: percent: 3.0 Sulfur trioxide content: 2.5 Specific gravity(dry), gm per cu.cm.:3.10 Weight of one bag or sack of cement (dry) , lb.: 94 Fineness, sq. cm. per gm, in accordance with ASTM, serial designation C204, 2600 to 3000 , and serial designation C115, 1500 to 1900. Time of setting: Initial set, 1 hr: final set 10 hr Soundness, in accordance with ASTM serial designation C151 ( maximum permissible change in volume after final set) percent: 0.80. Loss on ignition (by weight ), percent: 2

To counteract the normal drying shrinkage of Portland cement, a 10% calcium sulfoaluminate clinker compound mixed with 90% Portland cement by volume is specified on some construction contracts. This type of cement will cause the concrete to expand 0.05% in about 7 days after being mixed with water provided the consistency (slump) is properly regulated and that calcium chloride is not used as an accelerating admixture in cold weather. An additional three to five gallons of water per cu.yd of concrete is also required. Cement is packaged and delivered in sack or by the barrel ( 4 sacks per barrel) with the brand and the name of the manufacturer clearly marked. During shipment and storage, cement must be kept as dry as possible. Other types of Portland cement include airentraining types IA, IIA, and IIIA ( ASTM designation C175), which is used to improve resistance to freeze-thaw action, surface scaling and bleeding; the white cement, which is used primarily for architectural purposes; the blast-furnace slag types I S and I S-A ( ASTM, types I P and I P-A ( ASTM serial designation C205), which are used in lieu of types II and IIA; the puzzolan types I P and I P-A ( ASTM serial designation C340), which are used in lieu of type II , IIA and IV as another means of reducing the heat of hydration; the masonry cements (ASTM serial designation C91) which are used in mixing masonry mortars; the oil well cement ( American Petroleum Institute Standard Specifications 10A), which is used in sealing oil wells; the waterproofed cement, which is manufactured by adding a small amount of stearate to the clinker during final grinding; and the plastic cement, which is manufactured by adding plasticizing agents up to 12% by volume with types I and II and used for mixing plaster, stucco, and tile setting mortars.

AGGREGATES: Aggregates must consist of clean, hard, strong, durable particles, free from loam, alkali organic matter, or other deleterious substances. Normal-weight aggregates such as sand, gravel, crushed limestone, or trap rock and air-cooled blast-furnace slag , which will produce a concrete weighing about 150 lb per cu.ft. must meet the requirements of standard ASTM specifications, serial designation C33. Structural lightweight aggregates such as expanded shale, clay, slate, and slag, which will produce a concrete weighing about 100 lb/cu ft, must meet the requirements of ASTM specifications, serial designation C330. Lightweight insulating aggregates such as pumice, scoria, perlite, vermiculite, and ilmenite are often used in massive concrete structures. In addition, fine and coarse aggregates must meet the following chemical and physical requirements: 1. 2. 3. 4. 5. 6.

7.

8. 9. 10. 11.

12.

Abrasion resistance in accordance with ASTM serial designation C131. Freezing, thawing and weather resistance, in accordance with ASTM, serial designation C290, C291 and C88. Chemical stability, in accordance with ASTM, serial designation C227, C289 and C295. Organic impurities and objectionable fine materials in accordance with ASTM serial designation C40, C117, and C142. Grading ( particle size distribution) by sieve analysis ,in accordance with ASTM , serial designation C136. Fine aggregates must not contain any grains that will not pass #4 mesh sieve or greater, and not contain more than 6% by weight of grains which will pass a 100 mesh sieve or less. Coarse aggregates (normal weight) must not contain any pieces that will not pass through a hole 1 ½ in square or greater, and which will pass through a hole ¼ in. square or greater. Coarse aggregates (lightweight) must not contain any pieces that will not pass through a hole 1 in, square or greater. Coarse aggregates ( heavyweight) must not contain any pieces that will not pass through a hole 2 ½ in. square or greater. Specific gravity ( dry normal weight), in gm/cu.cm: 2.65. Bulk unit weight (dry) , in lb per cu ft.: Lightweight insulating aggregates: 40 Structural lightweight aggregates : 55 Normal-weight aggregates: 90 Heavy-weight aggregates: 110 Maximum allowable moisture absorption, in accordance with ASTM serial designation C70, C127, and C128.

WATER: Water used in for mixing concrete must be clean and fresh. It is measured and added by the number of gallons specified, depending upon the type of mixture. The effect of

impurities in mixing water on the quality of concrete can be found in the manual of the Portland Cement Association.

ADMIXTURES: 1. Accelerators are used to speed the initial set of concrete. Such material may be added to the mix to increase the rate of early strength development to allow earlier removal of forms and in some cases reduce the whole curing period. 2. Retarders are used to delay the setting time of the cement paste in concrete. In hot weather, hydration is accelerated by heat, thus cutting down the time available to place, consolidate, and finish the concrete. High temperatures, low humidity and wind will cause rapid evaporation of water from the mix during summer. This drying of the concrete will lead to cracking and crazing of the surface. An initial set retarder will hold back the hydration process, leaving more water for workability and allowing the concrete to be finished and protected before drying out. 3. Air-entraining agents contain microscopic bubbles of air formed with the aid of a group of chemicals called surface active agents, materials that have the property of reducing the surface tension of water intended for use when better resistance to frost action is concerned. 4. Dispersal agents: When cement and water are mixed, the cement particles tend to gather in clumps, or to flocculate. As a result, water does not reach some of the particles and some are only partially hydrated. Sometimes, only 50% of the cement is hydrated. Water trapped in these clumps later bleeds to the surface of the concrete, because of the weight of the other materials. The voids left by forcing out of the water later become passages to which water can penetrate the concrete. A cement dispersal agent such as calcium lignosulfanate causes cement particles to separate by imparting like electrostatic charges to them. 5. Concrete Hardeners: Plain concrete surfaces which are subjected to rolling live loads, the impact action of live traffic, and other types of wear begin to dust and crumble at the surface after a period of time. This condition worsens with time, finally resulting in the destruction of the surface. To prevent this, two types of concrete hardeners are used: a. Chemical hardeners – liquids containing silicoflourides or flousilicates and a wetting agent which reduces the surface tension of the liquid and allows it to penetrate the pores of the concrete more easily. The silicoflourides and flousilicates combine chemically with free lime and calcium carbonate which are present in the concrete and bind the fine

particles into flintlike topping, which is highly resistant to wear and dusting. b. Fine metallic aggregates – are especially processed and graded iron particles which are dry –mixed with Portland cement, spread evenly ove the surface of freshly floated concrete, and worked into the surface by floating. The result is a hard, tough, topping which is highly resistant to wear and less brittle than normal concrete. 6. Water reducing admixtures: A material used to reduce the amount of water necessary to produce a concrete of given consistency or to increase the slump for a given water content. A typical one is made from the metallic salts of ligninsulfonic acids. 7. Concrete waterproofers: Materials use to reduce or stop this type of flow are more properly called damproofers. Materials used to reduce permeability and also as damproofers are as follows: a. Air-entraining agents – because it increases the plasticity of concrete and thus help to make placing easier and more uniform. They also reduce bleeding by holding the water in films about the air bbubbles. As damproofer because the small disconnected voids produced by airentrainment break up the capillaries in the concrete and therefore offer a barrier to the passage of water by capillary action.. b. Cement dispersal agent- since it tends to reduce voids formed when water is trapped in groups of cement particles. c. Water repellants – The materials used are compounds containing calcium or ammonium stearate, calcium or ammonium oleate, or butyl stearate. These substances combine with lime or calcium chloride. d. Film applied to surface – The common materials used are those containing asphalt or sodium silicate and one which contains a metallic aggregate. 8. Bonding agents- When fresh concrete is poured against another concrete surface already set and at least partially cured, it is often difficult to obtain a bond between the two surfaces unless especial precautions are taken. Fresh concrete shrinks when setting, and unless there is a very good bond thus shrinkage makes the new concrete pull away from the old surface. Two types : a. Metallic aggregates – iron particles are larger but with the same materials as permeability reducer. Bonding takes place through the oxidation and subsequent expansion of the iron particles. b. Synthetic latex emulsion – consist of highly polymerized synthetic liquid resin dispersed in water. When it is sprayed or painted on a concrete surface, the pores in the concrete absorb the water and allow the resin particles to coalesce and bond.

9. Concrete coloring agents – Application : a. Use concrete paint, applied after the concrete has been neutralized , either by exposure or using a neutralizing agent such as zinc sulfate. b. Integrating color into the surface of the concrete while still fresh. 



Natural metallic oxides of cobalt, chromium, iron, etc. have distinctive colors. The ochres and umbers are fine dry powders. They are usually mixed into a topping mix, since this is the best way of distributing the color evenly throughout the concrete. The coloring agents made with synthetic oxides are usually a mixture of the oxide with one or more additional drying ingredients. The color is sometimes mixed with pure silica sand and applied by shaking the mixture over the freshly poured and floated surface.

10. Set-inhibiting agents - Specifications sometimes require that concrete surfaces be produced in which the aggregates are exposed for architectural effects. Certain inhibiting agents will prevent the cement paste from bonding to the surface aggregates but will not interfere with the set throughout the remainder of the pour. Two materials are used for this purpose: a. a liquid is applied to forms for vertical surfaces immediately before pouring concrete b. powder which is applied directly to the freshly poured horizontal surfaces. The depth of penetration of the inhibitor depends on the amount used per square foot. Usual rates of application will vary from 1 ½ to 3 lbs per sq ft of surface. After three or four days of curing, the retarded surface concrete should be hosed or brushed off exposing clean aggregate and leaving a rough cast effect. 11. Non-skid surfaces – To avoid slippery concrete surfaces, use wood or cork floats which will leave a rough surface instead of steel trowelling operation during the floor-finishing process. Another method is to use an abrasive material in the topping, applied as a dry shake in much the same way as metallic-aggregate topping is applied. The abrasive material is floated into the top and steel trowel operation is omitted. Materials used for this purpose are fine particles of flint, aluminum oxide, silicon carbide and emery. 12. Surface sealing agents – Used for two purposes: a. To form a watertight coating which will prevent water from evaporating from a concrete surface and allow it to be retained for hydration.

b. To seal the pores of the concrete surface after it has hardened in order to prevent the passage of water and the absorption of spilled materials such as oil, grease or paint. Sealing agents used to prevent water evaporation are usually liquid waxes which can be sprayed over the surface but which are easily removed after curing is complete. 13. Gas-forming agents – under normal conditions, concrete undergoes settlement and drying shrinkage, which in some situations, can result in undesirable characteristics in hardened concrete. For example, voids in the underneath side of forms, blockouts and reinforcing steel or other embedded parts such as machinery bases may interfere with the bond and allow passage of water and reduce uniformity and strength. One method of reducing such voids is to add an expanding agent to the concrete. Aluminum powder when added to mortar or concrete , react with the hydroxides in hydrating cement to produce very small bubbles of hydrogen gas. This action, when properly controlled, causes a slight expansion in plastic concrete or mortar and thus reduces or eliminates voids caused by the settlement. 14. Puzzolanic admixtures – Materials sometimes used in structures where it is desirable to avoid high temperature or in structures exposed to seawater or water containing sulfates. These puzzolanic materials are generally substituted for 10 to 35% of the cement. Puzzolans may be added to concrete mixes-rather than substituting for part of the cement- to improve workability, impermeability and resistance to chemical attack. A number of natural materials such as diatomaceous earth, opaline cherts and shales, tuff and pumicites, and some artificial materials such as fly ash are used as puzzolans.( Fly ash is a fine residue which results from the combustion of powdered coal and may contain various amounts of carbon, silica, sulfur, alkalis and other ingredients).

Design of Mix The proper proportioning and selection of materials of a given concrete mixture will determine its strength. The amount, type, and size of the various aggregate will determine how the concrete will flow or react when it is placed on forms. Well-graded aggregate will produce dense, strong concrete. A concrete that has too large a percentage of coarse aggregate may contains excessive voids. Excess fine aggregates, there may be too much surface area for the paste to coat each particle, may be smooth and strong but it will not be economical.

Proportioning of Materials In the proportioning of materials, a great attention should be given to the water-cement ratio which has been found to govern the strength of finished concrete. Concrete Proportions Class of Mixture

Cement 40 kgs/bag

Cu. ft.

Sand Cu. m.

Cu. ft.

Gravel Cu. m

AA

1

1½

0.043

3

0.085

A

1

2

0.057

4

0.113

B

1

2 1/2

0.071

5

0.142

C

1

3

0.085

6

0.170

Example of Class “A” mix: One part cement is to two parts sand plus four parts gravel. The designing of concrete mixtures is based primarily on the water-cement ratio theory, which states that the strength of concrete is inversely proportional to the amount of water used per unit of cement. This means that if, for example, 65 lb. of water per lb. of cement will produce concrete capable of developing 2,500 psi in 28 days, then less water per bag will produce stronger concrete and more water will produce concrete of lesser strength. Maximum Permissible Water-Cement Ratios for Concrete (28 days) Compressive Strength (fs)

Non-air entrained concrete

Air-entrained concrete

Psi

Kg/cm2

Absolute ratio by weight

Liters per bag of cement

Absolute ratio by weight

Liters per bag of cement

2500

175

0.65

27.6

0.54

23.1

3000

210

0.58

25

0.46

19.7

3500

245

0.51

22

0.40

17

4000

280

0.44

19

0.35

15.1

5000

315

0.30

16.3

0.30

12.9

Control of Concrete Mixes In the actual construction, concrete should undergo test especially for those made of various proportions. The building official has the right to order the testing of any material used in concrete construction to determine if the concrete conforms with, the quality specified. The complete record of the tests conducted shall be maintained and made accessible for inspection during the progress of the work and for a period of 2 years after all the construction work are completed and shall be preserved by the architect or engineer for reference purposes.

The various tests conducted are: 1. Slump Test When the freshly mixed concrete is checked to ensure that the specified slump is being attained consistently. A standard slump cone is fabricated with the following dimensions: 12 in. high, 8 in. diameter at the bottom and 4 in. diameter at the top which open on both ends.

The cone is filled in three equal layers, each being filled in three equal layers, each being tamped or rodded 25 times with a standard 5/8 “ diameter bullet nosed rod. When the cone has been filled and leveled off, it is lifted carefully and the amount of the slump is measured.

Allowable Deflection x Beams and Columns Slabs and Tunnel Inverts Tops and Walls, Piers, Parapet & Curbs Side Walls and Arch in Tunnel lining Canal Lining Heavy Mass Construction

7.5 cm. 5.0 cm. 5.0 cm. 10 cm. 7.5 cm. 5.0 cm.

(0.075 m.) (0.05 m.) (0.05 m.) (0.10 m.) (0.075 m.) (0.05 m.)

3” 2” 2” 4 3” 2”

2. Compression Test Common quality control test for concrete, based on a 7 and 28 days curing periods. Specimens are usually cylindrical with a length equal to twice the diameter. Standard size is 12 in. high and 6 in. diameter. A cylindrical mold is filled just the way the slump test and the specimen is taken out of the mold within 24 hours. The specimen is then taken to the Testing Laboratory for compression test using a compression testing machine.

MIXING OF CONCRETE Another factor in the workability and strength of concrete is the method used to mix the ingredients. It is essential that all ingredients be thoroughly mixed to ensure uniformity. Prolong mixing, however can decrease workability. Mixing Time The mixing time required depends on the size and efficiency of the mixer. The time of mixing should not be less than 1 minute for concrete of medium consistency mixed in a 1 cubic-yard (0.765 m3) or smaller mixer. Larger mixers require 15 seconds additional mixing for each additional yard of concrete. The mixing time is calculated from the time all solid particles are in the mixer. All water should be added before one-fourth of the mixing time has elapsed.

Model: Prime Mover: Capacity:

CM5 and CM7 Air-cooled Gasoline Engine 5 – 7 Horsepower. One-Bagger (19.0 cu.ft. bowl capacity)

Model: Prime Mover: Capacity:

CM-T4W (Tilting Type) CM-NT4W (Nose-Tilting Type) 16 HP Diesel Engine 300 liters. 2 - bagger

Prolonged mixing will not affect the strength of the concrete as long as the mixture remains plastic and additional water is not added to increase the slump. The speed of the mixer is not as critical as the mixing time. The peripheral speed of the mixing drum and blades should be between 100 and 200 fpm (30.5 and 61.0 meters per minute). Manufacturers of mixers specify the number of revolutions per minute the drum should turn to obtain these speeds. It is important that mixers not be loaded beyond their capacity. Job-mixed Concrete Concrete materials may be mixed in a rotating drum batch mixer at the job site. However, this is not recommended unless the job is of sufficient size to warrant proper devices for the measuring of materials or, on a small jobs materials are sometimes measured by shove-full. The measuring of materials by volume may be quite inaccurate. The moisture content of sand and coarse aggregate will vary and effect the water/cement ratio.

Ready-mixed Concrete In most areas concrete can be purchased from a central plant. These plants are equipped to furnish concrete, conforming to a given mix or guaranteed to meet a specified strength, readymixed to the job site. Delivery of Concrete Ready-mixed concrete is delivered by special trucks (Transit Mixers) designed for the purpose. The initial mixing may done at the central plant, with the remainder accomplished in the truck on the way to the job site, or the entire mixing process is done in transit. The materials are combined in the truck, and the mixing will rotate not less than 50 times nor more than 100 times. The truck manufacturer designates this mixing speed.

PLACING CONCRETE Quality concrete depends on proper placement, finishing, and curing. For uniform results these operations should be directed by an experienced supervisor. Several steps are necessary achieve a strong, lasting, and finished surface for concrete structure. The site and forms must be properly prepared. The concrete must be placed so that it is uniform throughout, and it must be finished so that the surface is compact and has the desired characteristics. It must then be allowed to cure so that a minimum number of cracks develop and the surface has a lasting finish, free of defects. Site Preparation The forms within which the concrete is to be placed and the soil on which it will be deposited should be properly moistened or protected with form oils or plastic liners so that they do not so soak up necessary water from the concrete mix. All vegetable matter and loose material that could become mixed with the wet concrete should be removed. Muddy and soft spots should be compacted to provide a good base for the concrete as it is poured. For best results concrete should be placed on the base of sand or gravel.

Setting of concrete forms with a transit

Forms must be true to shape and tight enough to retain the water in the concrete. They must also be strong and well braced in order to withstand the pressure of the concrete and the vibration that may be necessary to consolidate it. Forms may be made of wood, either prefabricated or job built, hardboard, or metal. There are some recently developed special materials that serve as forms and are then left in place as a finish material. Plastic-coated wood forms are used to produce smoothly finished surfaces on concrete. Reinforcing must be clean, free of rust, and securely anchored in place. Bolts, anchors, sleeves, and inserts which are to be cast in the concrete must be in place.

Method of Placement If the concrete cannot be placed directly where it is needed by the chute of the mixer, it must be conveyed as close as possible to the final location by pumping, belt conveyors, concrete buggies, or buckets moved by cranes. Concrete should be placed in horizontal layers of 6 to 18 inches (152 to 457 mm). If it is piled in one spot and worked or allowed to flow to distant parts of the form, the coarse aggregate usually segregates. The lighter materials flow faster than the deficient in cement paste. If concrete is allowed to fall freely for a distance of more than 3 or 4 ft (914 to 1219 mm), the aggregate also tends to segregate. The heavier particles are concentrated at the bottom of the pour, leaving the upper layer with an excess of fine aggregate. When concrete is to be cast into deep forms, drop chutes may be used. These chutes are lowered into the forms to reduce the free fall of the concrete. Windows may be built into the forms. The concrete is then placed through the side of the form to reduce the amount of free fall. Pumping Pumping ready-mixed concrete through pipes is not a new development. However, until recently this method was limited to large-volume jobs using 6- or 8-in. (152- or 203-mm) fixed pipes. New type of pumps capable of pumping concrete through small-diameter flexible lines has greatly extended this technique. It is now possible to pump concrete 500 ft (152 m) horizontally or 100 ft (30.5 m) vertically. The pumps are either self-powered trailer units or units mounted on the body of a truck and operated by a truck engine. The concrete is received in a hopper from the readymixed truck and is pumped through rigid pipe or flexible hose. The flexible hose allows the concrete to be placed exactly where it is needed with minimum labor

Pneumatic Placement Air pressure has been used for many years to place concrete. A dry mixture of cement and sand is shown is blown through hoses, and water injected at the nozzle. This called gunite, pneumatically placed mortar, or sprayed concrete, and is referred as shotcrete by the American Concrete Institute (ACI). Recent construction of domes, concrete-shell structures, and swimming pools has shown the adaptability of pneumatically placed concrete. Instead of being placed in the forms, the concrete may be shot into two sides of metal lath to form the finished structure. Equipment manufacturers have developed compact mobile units that consist of a mixer, pump, and air tank necessary to place concrete.

Consolidation of Concrete Several types of vibrators run by compressed air, electricity or gasoline engines may be immersed directly to the concrete. Immersing vibrators consists of revolving eccentric elements, turning at 7000 rpm or more, enclosed in watertight cylinders 1 to 4 in. (25 to 102 mm) in diameter and approximately 18 in. (457 mm) long. On deep-section of the concrete the vibrator is inserted vertically into the concrete at points 18 to 30 in. (457 to 762 mm) apart. The concrete is vibrated from 5 to 15 seconds at each spot. In thin slabs the vibrator is inserted into the fresh concrete horizontally or at a very slight angle. Vibration and consolidation of concrete may also be accomplished by vibrating devices attached to forms or applied to the surface of the concrete.

Concrete Vibrator Vibratory Finishing Screed Laitance In wet concrete mixes a soupy mixture of extremely fine sand, cement, and water will sometimes float, or bleed, to the surface of a pour. This is called laitance. Laitance will show up as a whitish scum on the surface of the concrete or as light streaks in finished concrete. These light streaks or poor-quality concrete are very susceptible to failure when exposed to freezing and thawing and must be removed before the pour is made. Laitance can be controlled by using stiffer mixes or an air entraining admixture.

CONSTRUCTION JOINTS When fresh concrete is poured against hardened concrete, it is usually necessary to produce a good bond and a watertight joint between the new and old concrete. Only a limited quantity of concrete can be placed in one working day, so the concrete must be cast in sections. The design and location of these joints between hardened and fresh concrete, called construction joints, must be considered carefully. Before fresh concrete can be placed, the surface of the hardened concrete must be roughened and cleaned. This may be done before or after the concrete has reached initial set. The concrete may be washed with a jet of water from 4 to 12 hours after it is placed in order to expose a clean surface of sound concrete. The surface of the freshly placed concrete may be brushed with a stiff broom or a steel brush before initial set, to form a roughened surface ready to receive the new concrete. After hardening the concrete may be wet sandblasted and washed to provide the roughened clean surface necessary for good bond when new concrete is poured. On deep section concrete, when the new concrete is placed against hardened concrete it is necessary to provide a cushion of mortar. This mortar consists of a ½- to 1-in. (12.7- to 25-mm) layer of cement, sand, and water, with the same water/cement ratio as the concrete. This mortar

must be applied immediately before the fresh concrete is poured and worked into the irregularities of the hardened concrete to ensure a good bond. Relief Joints may be constructed as built-in strips of elastic material, openings to be filled later with an elastic material, or false joints cut or molded in the concrete surface. In order to keep the two portions of the slab or wall in alignment, keyways may be cast in each section of the wall or slab Concrete expands and contracts with temperature changes. Although this expansion and contraction is only 55-millionth of an inch per degree (2.5  m/0C) of temperature, this amounts to over 1.2 in. (12.7 mm) in each 100 ft. (30.5 m) of structure for a 1000F (55.50C) temperature change. Concrete shrinks when it dries and expands when it absorbs moisture. This contraction and expansion may be as great as that cause by temperature change. A combination of the two factors could double the expansion or contraction of a concrete structure. For this reason, properly designed relief joints must be included to prevent unsightly random cracking. Relief joints to maintain proper alignment as the concrete moves or works. Steel dowels may be provided to bridge the joint for the same purpose. One side of the dowel is anchored firmly in the concrete; the other end is coated with mastic or encased in plastic tube so that it will not bond to the concrete, but will allow for movement while maintaining alignment.

Built-in Joints Preformed rubber or plastic shapes of many designs may be used to bridge expansion and contraction joints. These long flexible strips are cast into the concrete. The dumbshell-shaped or serrated edges of the strips are gripped by the freshly poured concrete. This type of joint can move but still remain watertight. Filled Joints Joints left open for sealing at a later time can be filled with elastic material that will allow movement. These joint sealants, classed as neoprene foams, or polyurethane foams. The materials are delivered to the job in liquid form and, when mixed properly and forced into the joint, provide an elastic, waterproof joint. False Joints Relief joints may be formed in the concrete by the use of metal or wood strip fastened to the inside of forms. These strips are removed after the concrete has hardened. Weakened planes are thus formed where the strips have been removed, confining the cracks to that area. The weakened plane joins can be design and detailed in such a manner as to become an architectural feature in the overall design of the structure. The strips can be located so that each pour of concrete will be stopped at these strips. By the use of this method, irregular lines of construction joints on plain surfaces can be avoided. Relief joints on plain surfaces can be avoided. Relief joints may also be formed by sawing grooves in the concrete. This is usually done as soon as the concrete has set sufficiently to support the weight of the concrete saw. Relief joints must penetrate the slab for a distance of one-fifth the slab thickness to be effective.

Concrete Cutter

FINISHING AND CURING CONCRETE Leveling When concrete has been placed and consolidated, it is brought to the proper level by means of screeds. Screeds are guides placed on both sides of the slab, and sometimes within the perimeter of large slabs, with their tops at the desired level of the finished concrete. A long plank is then rested on the screeds and pushed back and forth in a sawing motion to strike off excess concrete and show up any areas that are low. After the concrete has been brought to the proper level, any screeds within the slab are removed, and the depressions they have left are filled with fresh concrete. Floating After the concrete has been brought to its final level, while it is still plastic, the surface is floated with a long-handled flat surfaced wood tool called a bull float. There are machines with rotating wood blades that can be used for floating and initial troweling. The proper use of the bull float, either manual or low spots, and eliminate the high spots. Final Finish When the edges and joints have been rounded and the slab has lost its sheen, it is time to begin the final finishing. This may be 4 to 6 hours after placement, depending on the job and the weather conditions. Timing of the final finishing process is critical in producing a sound, defectfree surface. This operation should be delayed until fine particles and water are no longer brought to the surface by trowel testing. Many workers tend to start the final finishing too soon. The final desired will govern the number of times the slab is to be troweled. The often the surface is troweled, the smoother and denser it becomes. If a non-slip finish is desired, the surface may broomed after the floating operation. The coarseness and stiffness of the bristles in the broom and the length of time after floating will govern the final appearance of the slab. If a finer texture is desired, brooming may follow the first steel troweling. Special Toppings and Finishes Materials have been developed that can be troweled into the freshly floated concrete to densify the surface or produce a non-slip or decorative finish. These materials are usually sprinkled on the slab after the floating operation. Extremely hard non-metallic, non-rusting abrasive granules are used as hardeners and for non-slip surfaces exposed to the weather or subject to heavy traffic. Products are available which both densify and decorate. They are sometimes used on walks, steps, and floors, where durable decorative finishes are desired. Salt Finish A texture finish can be produced on concrete slabs by sprinkling rock salt over a freshly troweled surface. The salt is pressed into the concrete with a trowel and the slab is allowed to set. After the concrete has completely hardened, the salt is washed away by thorough flooding with water. The dissolved salt will leave pits or holes in the surface. CURING Proper curing of concrete is an important factor in achieving satisfactory, waterproof, strong concrete free of surface defects. The watertightness and strength of concrete improve rapidly when it is first placed and continue to increase at a slower rate as long as conditions are

favorable. Enough water must be retained in the concrete to allow complete chemical reaction. The temperature of the concrete must be maintained between certain limits to assure a proper chemical reaction. When temperature are below 70 0F (21.10C), chemical action slows. It takes concrete twice as long to set up and gain strength at 50 0F (100C), and practically no chemical action takes place at 300F (-1.10C) or below. Water Curing Covering a flat slab with a thin layer of water is one way to prevent the evaporation ofmoisture from concrete. The layer of water will also help keep the concrete cool in hot weather. Earth dikes may be built around the slab during the curing period. Burlap Curing Wet burlap is often is used to cover the concrete during curing. The burlap must cover all the concrete, including the sides of members, and must be kept moist at all times. Paper Curing Waterproof paper can also be used as cover to prevent evaporation. The paper must cover the concrete completely, with the edges and joints taped to make a continuous cover. A layer of wet sand placed on top of the waterproof paper will hold it in place and help to control temperature. Plastic Curing A lightweight plastic sheathing has been developed for curing concrete. This material is almost impermeable to moisture and is sufficiently flexible to seal in intricate shapes. It can be obtained in rolls up to 32 ft. (9.8 m) wide, in several thicknesses. The plastic sheet used for curing concrete is usually white or milky rather than clear. This helps reflect the suns rays, thereby lowering the temperature somewhat during the hot weather. The edges adjoining sheets can be sealed by heat or with special solvents to form a completely airtight and watertight cover. Curing Compounds Curing compounds, sprayed on freshly placed concrete with a hand or machine sprayer, will form a continuous membrane which assures proper curing. Some curing compounds dry very rapidly and develop a thin, tough membrane in short time. These compounds may be clear or pigmented. The clear compounds are usually used where the appearance of the finished concrete is important.

Aging of Concrete Concrete generally increases strength with age. The increase is rapid at first then becoming more gradual later. Shown below is a table showing the variation of strength of concrete with age

7 days

1 month

2 months

6 months

1 year

5 years

16.5 MPa

25 Mpa

27.5 MPa

30 Mpa

31 MPa

33 Mpa