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Concrete

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As a construction material, concrete is eminently suited for many uses. It is durable, sanitary, and fire-resistant. The upkeep cost of concreteis low, and it can easily be made attractive. Because it is plastic when first mixed, concrete lends itself to the construction of diverse objects.

On the other hand, perhaps no other material depends so much upon the user for its success. Good materials, accurate proportioning, and careful control in all operation s are essential to the making of good concrete.

Fundamental Facts About Concrete

Concrete can be considered to be made of two components, aggregates and paste. Aggregates are generally classified into two groups, fine and coarse. Fine aggregates consist of natural or manufactured sand with particle sizes smaller than about 1/4 inch, coarse aggregates are those with particle sizes greater than about 1/4. The paste is composed of cement, water, and sometimes entrained air.

Cement paste ordinarily constitutes 25 to 40 percent of the total volume of concrete and water from 14 to 21 percent.
Air contents in air-entrained concrete range up to about eight percent of the volume of the concrete.

Since aggregate makes up about 60 to 80 percent of the concrete, its selection is important. Aggregate should consist of particles having adequate strength and resistance to exposure conditions, and should not contain materials having injurious effects. A smooth gradation of particles sizes is desired for efficient use of the cement and water paste.

In properly made concrete each particle of aggregate is completely paste-coated. Also, all of the space between aggregate particles is completely filled with past. It is shown that the quality of the concrete is greatly dependent upon the quality of the paste.

Quality of paste is dependent upon the ratio of water to cement used, and the extent of curing. The cementing properties of the paste are due to the chemical reactions between cement and water. These reactions, called hydration, require time and favorable conditions of temperature and moisture. [Heat generated by sunlight, wind, shade, soil moisture all play a roll in hydration.] They take place very rapidly at first and then more and more slowly for a long time under favorable conditions.

More water is used in mixing concrete than is required for complete hydration in order to make the concrete plastic and workable. However, as the paste is thinned with water, its quality is lowered, it has less strength and is less resistant to weather. For successful results, a proper proportion of water to cement is essential.

Resistance To Freezing And Thawing

Concrete is used in structures and pavements that are expected to have long life and low upkeep. One requirement of such concrete is high resistance to anticipated exposure conditions. The most destructive factor of weather is freezing and thawing while the concrete is wet or moist. Deterioration may be caused by expansion of the water in the paste, by expansion of some of the aggregate particles, or by a combination of both. Air entrainment improves resistance to this deterioration. The durability of concrete exposed to freezing and thawing is effected by the quality of the paste. Air-entrained concrete with a low water-cement ratio is highly resistant to repeated freeze-thaw cycles.

Destructive expansion of water in the paste during freezing is accommodated in air-entrained concrete. Air bubbles in the paste provide chambers to relieve the expansive force.

When freezing occurs in concrete exposed to wet conditions for a sufficiently long time to saturate some aggregate particles during the ice formation is unable to escape fast enough through the surrounding paste to prevent such pressure. However, under nearly all exposure conditions, paste of good quality [low water to cement ratio] is able to prevent aggregate particles from becoming saturated. if this paste is air-entrained, it can accommodate the small amounts of excess water that are expelled from the aggregate, thus protecting the concrete from freeze-thaw damage.

Watertight Concrete

Where concrete is exposed to weather or other severe exposure conditions, it is important that it be watertight. Such concrete requires a watertight paste. Tests show that water tightness of paste depends primarily on the amount of cement and mixing water used and the length of the moist-curing period. Test results obtained by subjecting non -air-entrained mortar discs to 20 psi. water pressure that leakage is reduced as the water-cement ratio is decreased and the curing period increased. In these tests, the mortar discs that were moist-cured for seven days had no leakage when made with a water-cement ratio of 0.50. Leakage was greater in mortars made with higher water-cement ratios. Also, for each water-cement ratio, the leakage became less as the length of curing period was increased. In the discs with a water cement ratio of 0.80, the mortar still permitted leakage after being cured a month.

Air-entrainment improves water tightness by increasing density through improved workability and reduced segregation and bleeding. Because the total water requirement of air-entrained concrete is less, the paste will have a lower water-cement ratio and will therefore be more watertight. To be watertight, concrete must also be free from cracks and honeycomb.

Strength Of Concrete

The compressive strength of concrete is important in design of structures. In pavements and other slabs on ground, the flexural strength of the concrete is often used for design purposes. Compressive strength can be used as an index of flexural strength, once the empirical relationship for the materials being used has been established.

The principal factors effecting strength are the water-cement ratio and the extent that hydration has progressed.    Strengths increase as the water-cement ratios decrease, and that strengths increase with age. Flexural and tensile strengths and bond of concrete to steel are similarly influenced by water-cement ratio.

Resistance To Abrasion

Concrete floors, pavements, and dam spillways are subjected to abrasion; hence resistance to abrasion or wear is important in these applications of concrete. Test results indicate that abrasion resistance is dependent principally upon the strength of the concrete. Strong concrete is more resistant to abrasion than weak concrete. The tests were conducted by rolling steel balls under pressure over the surface of concrete specimens. Each specimen was subjected to the same number of revolutions. Since compressive strength is dependent upon water-cement ratio and curing, it is evident that a low water-cement ratio and adequate cuing are necessary for abrasion resistance.

Moist Curing

The increase in strength with age continues as long as drying of the concrete is prevented. When the concrete is permitted to dry, the chemical reactions slow down or stop. It is, therefore desirable to keep concrete continually moist as long as possible.

When moist cuing is interrupt, the strength increases for a short period and stops. However, if moist curing is resumed, the strength will again increase. Although this can be done in laboratory, it is difficult on most jobs to re-saturate concrete. It is best to moist-cure the concrete continuously from the time it is place until it has attained the desired strength.

Concrete Ingredients

The essential ingredients of concrete are cement, aggregate, and water which react chemically in a process called hydration to form another material having useful strength. Hardening of concrete is not the result of the drying of the mix as seen from the fact that fresh concrete placed under water will harden despite its completely submerged state. The mixture of cement and water is called cement paste, but such a mixture, in large quantities, is prohibitively expensive for practical construction purposed.

Cement

Most cement used today is Portland cement, which is usually manufactured from limestone mixed with shale, clay, or marl. The properly proportioned raw materials are pulverized and fed into kilns, where they are heated to a temperature of 2700 degrees and maintained at that temperature for a certain time. As a result of certain chemical changes produced by the heat, the material is transformed into a clinker. The clinker is then ground down so fine that it will pass through a sieve containing 40,000 openings per square inch.

There are a number of types of Portland cement, of which the most common are Types I through V and air-entrained.

Types of Cement

There are five common types of Portland cement in use today. The type of construction, chemical type of the soil, economy, and the requirements for use of the finished concrete are factors which influence the selection of the type of cement to be used. The different types of cement are as follows:

Type I (normal Portland cement) is used for all general types of construction. It is used in pavement and sidewalks construction, reinforced concrete buildings and bridges, railways, tanks, reservoirs, sewers, culverts, water pipes, masonry units, and soil cement mixtures. In general, it is used when concrete is not subject to special sulfate hazards or where the heat generated by the hydration of the cement will not cause an objectionable rise in temperature.

Type II (moderate Portland cement) has a lower heat of hydration than Type I. Lower heat generated by the hydration of the cement improves resistance to sulfate attack. It is intended for use in structures of considerable size where cement of moderate heat of hydration will tend to minimize temperature rise, as in large piers, heavy abutments and retaining walls, or when the concrete is placed in warm weather. In cold weather when the heat generated if helpful, Type I cement may be preferable for these uses. Type II cement is also intended for places where an added precaution against sulfate attack is important, as in drainage structures where the sulfate concentrations are higher than normal, but not usually severe.

Type III (high-early-strength Portland cement) is used where early high strengths are desired. It is used where it is desired to remove the forms as soon as possible, to put the concrete in service as quickly as possible, and in cold weather construction to reduce the period of protection against low temperatures. High strengths at early periods can be obtained more satisfactorily and more economically using high-early-strength cement than using richer mixes of Type I cement. Type III develops strength at a faster rate than other types of cement, such as follows:

Twenty-eight-day strength for Types I and II, which is reached by Type III in about seven days.
Seven-day strength for Types I and II, while Type III takes about three days
.

Type IV (slow-heat Portland cement) is a special cement for use where the amount and rate of heat generated must be kept to a minimum. This type of cement was first developed for use on the Hoover Dam. It develops strength at a slow rate and should be cured and protected from freezing for at least 21 days. For this reason it is unsuitable for structures of ordinary dimensions, and is available only on special order from a manufacturer.

Type V (sulfate-resistant Portland cement) is a cement intended for use only in structures exposed to high alkali content. It has a slower rate of hardening than normal Portland cement. The sulfates react chemically with the hydrated lime and the hydrated calcium aluminate in the cement paste. This reaction results in considerable expansion and disruption of the paste. Cements which have a low calcium aluminate content have a great resistance to sulfate attack. Therefore, Type V Portland cement is used exclusively for situations involving severe sulfate concentrations.

Air-entrained Portland cement is a special cement that can be used with good results for a variety of conditions, It has been developed to produce concrete with a resistance to freeze-thaw action and scaling caused by chemicals applied for severe frost and ice removal. In this cement, very small quantities of air-en training materials are added as the clinker is being ground during manufacturing. Concrete made with this cement contains minute, well-distributed, and completely separated air bubbles. The bubbles are so minute that it is estimated there are many millions of them in a cubic foot of concrete. Air bubbles provide space for freezing water to expand without damage to the concrete. Air-entrained concrete has been used in pavements in the northern states since 1930’s with excellent results. Air-entrained concrete also reduces the amount of water loss and the capillary and water-channel structure. The agent may be added to Types I, II, and III Portland cement. The manufacturer will specify the percentage of air-entrainment which can be expected in the concrete. An advantage of using air-entrained cement is that it can be used and batched like normal cement.  Next >

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