CONCRETE AND CONCRETING

    INTRODUCTION

Concrete is a widely used construction material. It is produced by mixing properly proportioned quantities of aggregates, cement, and water by weight. Admixtures may be added in the mixture to produce concrete of specified quality. Concrete may be molded into any desired shape or size. Concrete is reinforced with steel to make a composite material that combines the ceramic properties of concrete with the tensile strength of steel. The integrity of reinforced cement concrete depends to a large extent on the reinforcing steel, plain or deformed.

Concrete in its plastic state must remain workable until it is placed in position and compacted. Hardened concrete must be durable against the process of deterioration. there are different types of concrete, such as shotcrete, light-/heavy weight concrete, ready-mixed concrete, high performance concrete, self-compacting concrete, polymer-modified concrete, fiber reinforced concrete. The influence of the elastic incompatibility of steel and concrete is avoidable in pre-stressed concrete.

There are specification and guideline for production transportation, and placing of concrete in the extreme hot/cold weather. Underwater concreting is a special operation that may have to be carried out in remote and difficult areas in unusual environment. Timber or metal forms are used to mould the concrete as per the equipment. Stripping of forms from the set concrete demands the same care that goes into the fabrication and erection of forms. Curing process is a very important factor in deterring the strength of concrete.

Quality of concrete is to be assured right from the stage of procurement of the ingredient of concrete including testing of water available at the site. Quality of concrete is to be checked by carrying out both destructive and non-destructive tests.

2.       DEFINITION OF CONCRETE

Concrete is a versatile construction material. It is defined as a properly proportioned, homogeneous, and dense mixture of fine and coarse aggregates, cement, and water with or without admixtures. The aggregates are considered as economic filler materials generally inert in nature. Cement and water comprised a continuous binder phase that, after hardening, holds the aggregates together into a compact mass having load-bearing capacity. There are many binders, such as asphalt, sulphur, epoxy resin, and so on but the unqualified term ‘concrete’ means that the binder is principally hardened cement paste, which is the product of the chemical reaction of the ordinary Portland cement and water. In common parlance, the term concrete implies ordinary Portland cement concrete despite wide acceptable of the blended cements.

An admixture is a material other than the essential ingredients such as water, aggregates, and Portland cement (ordinary/blended) used as an ingredient of concrete and added to it immediately before or during its production.

Concrete is the most widely used material, second only to water, which is used in larger quantities. Concrete is a material that has been used in some form since the ancient times because:

It possesses excellent resistance to water unlike wood and steel. It can be easily cast or formed into any predetermined shape or size. But the area of modern concrete dates from the middle of the nineteenth century with the advent of the first truly ’Portland’ cement. Although aggregates constitute about 70% of the produced concrete, it is the cement paste that is responsible for most of the good and bad qualities of concrete. For a given aggregate, physical and chemical characteristics of the cement paste determine the workability of plastic concrete such as strength, durability and dimensional stability. However, it has certain weakness. It is brittle and very poor in tension. Ductility and toughness are also poor.

Despite its deceptive uniformity, cement is not a unified chemical entity. It comprises at least four phases, which retain their different chemical identities during the hydration process. Concrete strength and durability are affected significantly by the relative proportion of these four phases of cement with some minor constituents of cement influencing concrete durability. It is well known that reinforced concrete is a composite material, combining the ceramic properties of concrete with the tensile strength of steel. The quality of both these components is essential to increase the maintenance free life of reinforced concrete structures in marine conditions, severe atmosphere pollution, and other extreme conditions.

3.       HISTORY

The word concrete comes from the Latin word" concretus " (meaning compact or condensed), the perfect passive participle of" concrescere ", from " con-" (together) and" crescere " (to grow) Perhaps the earliest known occurrence of cement was twelve million years ago, when a natural deposit formed after an occurrence of oil shale naturally combusted while adjacent to a bed of limestone. These ancient deposits were investigated in the 1960s and 1970s. On a human time-scale, lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of fully waterproof concrete. German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400-1200 BC. Concrete was used for construction in many ancient structures.
The Romans used concrete extensively from300 BC to 476 AD, a span of more than seven hundred years. During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension.

“Concrete as roman knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick”

Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm ²). However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application was also different:

“Modern structural concrete differs from Roman concrete in two important details. First, its mi x consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregates, which in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas roman concrete could depend only upon the strength of the concrete bonding to resist tension.

The widespread use of concrete in many Roman structures has ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges have masonry cladding on a concrete core, as does the dome the Pantheon .After the Roman Empire, the use of burning lime and pozzolana was greatly reduced until the technique was all but forgotten between 500 AD and the 1300s. Between the 1300s until the mid-1700s, the use of cement gradually returned. The Canal du Midi was built using concrete in 1670, and there are concrete structures in Finland that date from the 16th century.

Perhaps the greatest driver behind the modern usage of concrete was the third Eddy stone Lighthouse in Devon, England. To create this structure, between 1756 and 1793, British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. A method for producing Portland cement was patented by Joseph Aspdin on 1824. In 1889 the first concrete reinforced bridge was built, and the first large concrete dams were built in 1936, Hoover Dam and Grand Coulee Dam. Reinforced concrete was invented in 1849 by Joseph Monier.

4.       COMPOSITION OF CONCRETE

Concrete is a composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fills the space between the aggregate particles and glues them together. We can also consider concrete as a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregates. The simplest representation of concrete is:

Concrete = Filler + Binder.

 According to the type of binder used, there are many different kinds of concrete. For instance, Portland cement concrete, asphalt concrete, and epoxy concrete. In concrete construction, the Portland cement concrete is utilized the most.

The composition can be presented as follows  

Cement (+ Admixture) +Water→ Cement paste + fine aggregate →mortar+ coarse aggregate →concrete

 

Here we should indicate that admixtures are almost always used in modern practice and thus become an essential component of modern concrete. Admixtures are defined as materials other than aggregate (fine and coarse), water, fibre and cement, which are added into concrete batch immediately before or during mixing. The widespread use of admixture is mainly due to the many benefits made possible by their application. For instance, chemical admixtures can modify the setting and hardening characteristic of cement paste by influencing the rate of cement hydration. Water-reducing admixture can plasticize fresh concrete mixtures by reducing surface tension of water, air-entraining admixtures can improve the durability of concrete, and mineral admixtures such as pozzolans (materials containing reactive silica) can reduce thermal cracking. A detailed description of admixtures will be given in latter sections

There are many types of concrete available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cementitious and aggregate phases, the finished products can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.

“Aggregate " consists of large chunks of material in a concrete mix, generally coarse gravel or crushed rocks such as limestone, or granite, along with finer materials such as sand.

“Cement ", commonly Portland cement, and other cementitious materials such as fly ash and slag cement, serves as a binder for the aggregate. Water is then mixed with this dry composite, which produces a semi-liquid that workers can shape (typically by pouring it into a form). The concrete solidifies and hardens to rock-hard strength through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust stone-like material.

“Chemical admixtures" are added to achieve varied properties. These ingredients may speed or slow down the rate at which the concrete hardens, and impart many other useful properties.

"Reinforcements” are often added to concrete. Concrete can be formulated with high compressive strength, but always has lower tensile strength. For this reason, it is usually reinforced with materials that are strong in tension (often steel).

“Mineral admixtures" are becoming more popular in recent decades. The use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation, and the discovery that such materials often have complimentary and valuable properties. The most conspicuous of these are fly ash, a by-product of coal-fired power plants, and silica fume, a byproduct of industrial electric furnaces. The use of these materials in concrete reduces the amount of resources required as the ash and fume acts as a cementer placement. This displaces some cement production, an energetically expensive and environmentally problematic process, while reducing the amount of industrial waste that must be disposed of.

5.       THE MIXING DESIGN AND PROCESS

The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure. Thorough mixing is essential for the production of uniform, high quality concrete. For this reason, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work. Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete. The paste is generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.

5.1. MATERIALS USED ON SITE

At the site there was concrete plant where all processes of mixing were done; here are some materials used at the site and some materials described below were not used for our site but because of their need in some cases of concrete mixing some was planned and available on site.

Cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminum. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker) with a source of sulfate (most commonly gypsum). In modern cement kilns many advanced features are used to lower the fuel consumption per ton of clinker produced.

Water

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. A lower water to cement ratio yields a stronger, more durable concrete, while more water gives a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure. Hydration involves many different reactions, often occurring at the same time.

As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.

Reaction:
Cement chemist notation: C 3 S + H   →   C-S-H + CH

Standard notation: Ca₃SiO ₅ + H₂O    →  (CaO) · (SiO ₂) · (H ₂O) (gel) + Ca (OH) ₂

Balanced: 2Ca ₃SiO₅ + 7H ₂O   →   3(CaO) ·2(SiO ₂) ·4(H ₂O) (gel) +3Ca (OH) ₂

Aggregates

Crushed stone aggregate

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted. The presence of aggregate greatly increases the durability of concrete above that of cement, which is a brittle material in its pure state. Thus, concrete is a true composite material. Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients. Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers. In addition to being decorative, exposed aggregate adds robustness to a concrete driveway.

Reinforcement
Installing rebar in a floor slab during a concrete pour. Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds steel reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads.

Chemical admixtures

In some cases of concrete at the site the use of admixture was needed, for example when the concrete was needed at the site of morocco the retarders was added in the concrete. Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than5% by mass of cement and are added to the concrete at the time of batching/mixing.  The common types of admixtures are as follows.

1. Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl ₂, Ca (NO 3) 2 and NaNO3. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.

2. Retarders slow the hydration of concrete and are used in large or difficult pours where
partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar,
sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

3. Air entrainments add and entrain tiny air bubbles in the concrete, which reduces damage during freeze-thaw cycles, increasing durability. However, entrained air entails a trade off with strength, as each 1% of air may decrease compressive strength 5%. Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated
naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers Pigments can be used to change the color of concrete, for aesthetics. Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete. Bonding agents are used to create a bond between old and new concrete (typically a type of polymer). Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.

6. EQUIPMENT AND MACHINERY USED

Ø  Mixer machine

Ø  Cement pump

Ø  Water pump

Ø  balances

Ø  Admixes pump

Ø  Computer

Ø  Generators

Ø  Bulldozers

Ø  Concrete mixer trucks

Ø  Concrete pump

 

7.       Skills and man power used

·         Engineers

·         Concrete specialists

·         Foremen

·         Students

·         Skilled and unskilled labourers

8.       SAFETY PRECAUTIONS

Because cement is dangerous for human body; during concrete work all workers at site were obliged to wear over coats, boots, and helmet and grooves to prevent them against any kind of harm.