Concrete (betonas)

Content

Introduction.......................1
Composition of concrete..................1
First users of concrete...................1
Portland Cement...................2
Reinforcement....................2
Composition of Portland Cement...........2
Hydration of Portland Cement............3
Water and concrete.................3
Stenght of concrete.................4
Properties of concrete.................7

This project is about concrete and its properties. There is written about concrete composition, who was the first users of concrete. Reading this project you will take much information about how to use concrete and where it is beneficial.

Concrete differs from other construction materials in that it can be made from an infinite combination of suitable materials and that its final prroperties are depended on the treatment after it arrives at the job site. The efficiency of the consolidation and the effectiveness of curing procedures are critical for attaining the full potential of a concrete mixture. While concrete is noted for its durability, it is susceptible to a range of environmental degradation factors, which can limit its service life. There has always been a need for test methods to measure the in-place properties of concrete for quality assurance and for evaluation off existing conditions. Ideally, these methods should be nondestructive so that they do not impair the function of the structure and permit re-testing at the same location to evaluate changes in properties with time.

Concrete is a compound material made fr

rom sand, gravel and cement. The cement is a mixture of various minerals which when mixed with water, hydrate and rapidly become hard binding the sand and gravel into a solid mass. The oldest known surviving concrete is to be found in the former Yugoslavia and was thought to have been laid in 5,600 BC using red lime as the cement.

The first major concrete users were the Egyptians in around 2,500 BC and the Romans from 300 BC The Romans found that by mixing a pink sand-like material which they obtained from Pozzuoli with their normal lime-based concretes they obtained a far stronger material. The pink sand turned out to be fine volcanic ash and they had inadvertently produced the first ‘pozzolanic’ ceement. Pozzolana is any siliceous or siliceous and aluminous material which possesses little or no cementitious value in itself but will, if finely divided and mixed with water, chemically react with calcium hydroxide to form compounds with cementitious properties.

The Romans made many developments in concrete technology including the use of lightweight aggregates as in the roof of the Pantheon, and embedded reinforcement in the form of bronze bars, although the difference in thermal expansion between the two materials produced problems of
f spalling. It is from the Roman words ‘caementum’ meaning a rough stone or chipping and ‘concretus’ meaning grown together or compounded, that we have obtained the names for these two now common materials.
Portland Cement

Lime and Pozzolana concretes continued to be used intermittently for nearly two millennia before the next major development occurred in 1824 when Joseph Aspdin of Leeds took out a patent for the manufacture of Portland cement, so named because of its close resemblance to Portland stone. Aspdin’s cement, made from a mixture of clay and limestone, which had been crushed and fired in a kiln, was an immediate success. Although many developments have since been made, the basic ingredients and processes of manufacture are the same today.

Reinforcement

In 1830, a publication entitled, “The Encyclopaedia of Cottage, Farm and Village Architecture” suggested that a lattice of iron rods could be embedded in concrete to form a roof. Eighteen years later, a French lawyer created a sensation by building a boat from a frame of iron rods covered by a fine concrete which he exhibited at the Paris Exhibition of 1855. Steel reinforced concrete was now born. The man normally credited with its introduction as a building material is William Wilkinson of

f Newcastle who applied for a patent in 1854 for “improvement in the construction of fireproof dwellings, warehouses, other buildings and parts of the same”.
It is not only fire resistance that is improved by the inclusion of steel in the concrete matrix. Concrete, although excellent in compression, performs poorly when in tension or flexure. By introducing a network of connected steel bars, the strength under tension is dramatically increased allowing long, unsupported runs of concrete to be produced.
Steel and concrete complement each other in many ways. For example, they have similar coefficients of thermal expansion so preventing the problems the Romans had with bronze. Concrete also protects the steel, both physically and chemically.
Composition of Portland Cement

Portland cement is a complex mix of many compounds, some of which play a major part in the hydration or chemical characteristics of the cement. It is manufactured commercially by heating together a mixture of limestone and clay up to a temperature of 1300 to 1500°C. Although twenty to thirty percent of the mix becomes molten during the process the majority of the reactions which take place are solid-state in nature and therefore liable to be slow. Once cooled, the resulting clinker is ground to a fine po
owder and a small amount of gypsum (calcium sulphate dihydrate) is added to slow down the rate at which the cement hydrates to a workable level.
The work of early investigators using optical and X-ray techniques, starting in 1882 with Le Chatelier, has shown that most Portland cement clinkers contain four principal compounds. These are tricalcium silicate (3CaO.SiO2), aluminate (3CaO.Al2O3) and a ferrite phase from the (2CaO.Fe2O3 – 6CaO.2Al2O3.Fe2O3) solid solution series that at one time was considered to have the fixed composition
(4CaO.Al2O3.Fe2O3). These phases were named alite, belite, celite and felite respectively by Tornebohm in 1897.
Hydration of Portland Cement

When water is mixed with Portland cement a complicated set of reactions is initiated. The main strength giving compounds are the calcium silicates which react with water to produce a calcium silicate hydrate gel (C-S-H gel) which provides the strength, and calcium hydroxide which contributes to the alkalinity of the cement. Tricalcium silicate reacts quickly to provide high, early strengths while the reaction of dicalcium silicate is far slower, continuing, in some cases, for many years. The other cement compound of particular relevance to steel reinforced concrete is tricalcium aluminate. It reacts rapidly with water to produce calcium aluminate hydrates.

The amount of tricalcium aluminate present may well be limited as in the case of sulphate resisting Portland cement, to prevent adverse reactions between the hydrate and sulphates from the environment which can result in swelling and cracking of the cement matrix.

The great advantage of tricalcium aluminate is its ability to combine with chlorides, so removing them from the liquid phase of the cement. Chloride ions, as will be seen, are one of the major causes of corrosion of embedded steel.

Water is the key ingredient, which when mixed with cement, forms a paste that binds the aggregate together. The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure, typically drinkable, in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process. The role of water is important because the water to cement ratio is the most critical factor in the production of “perfect” concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.. walls, domes, etc.). Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.

Strength of Concrete
The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. Some pores will remain no matter how well the concrete has been compacted. The relationship between the water/cement ratio and porosity is illustrated in Figure 3.

Figure 3. Schematic drawings to demonstrate the relationship between the water/cement ratio and porosity.
The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in Figure 4.

Figure 4. A plot of concrete strength as a function of the water to cement ratio.
Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability.
The physical characteristics of aggregates are shape, texture, and size. These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio.
Time is also an important factor in determining concrete strength. Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete’s strength. It is common to use a 28-day test to determine the relative strength of concrete.
Concrete’s strength may also be affected by the addition of admixtures. Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process. Some admixtures add fluidity to concrete while requiring less water to be used. An example of an admixture which affects strength is superplasticizer. This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.

Hoover Dam (4,360,000 cubic yards of concrete)
Photo Courtesy of the US Department of the Interior

Properties of Concrete

Concrete has many properties that make it a popular construction material. The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal.
Good-quality concrete has many advantages that add to its popularity. First, it is economical when ingredients are readily available. Concrete’s long life and relatively low maintenance requirements increase its economic benefits. Concrete is not as likely to rot, corrode, or decay as other building materials. Concrete has the ability to be molded or cast into almost any desired shape. Building of the molds and casting can occur on the work-site which reduces costs.
Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures. It is resistant to wind, water, rodents, and insects. Hence, concrete is often used for storm shelters.
Concrete does have some limitations despite its numerous advantages. Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking. Concrete remains the material of choice for many applications regardless of these limitations.
The compressive strength of concrete is usually at least ten times its tensile strength, and five to six times its flexural strength. The principal factors governing compressive strength are given below:
• Water-cement ratio is by far the most important factor.
• The age of the cured concrete is also important. Concrete gradually builds strength after mixing due to the chemical interaction between the cement and the water. It is normally tested for its 28 day strength, but the strength of the concrete may continue to increase for a year after mixing.
• Character of the cement, curing conditions, moisture, and temperature. The greater the period of moist storage (100% humidity) and the higher the temperature, the greater the strength at any given age.
• Air entrainment, the introduction of very small air voids into the concrete mix, serves to greatly increase the final product’s resistance to cracking from freezing-thawing cycles. Most outdoor structures today employ this technique.

In conclusion I would like to say that concrete is very beneficial building material. All builders and all construction enterprises use it. Concrete is durable building material, so buildings made from it are lasting and these buildings does not need reconstruction for long time.

Keywords:

treatment – apdirbimas
consolidation – sutvirtinimas
durability – patvarumas
compound material – sujungta medžiaga
hydrate – hidratas
binding – jungimas
embed – įstatyti
reinforcement – sustiprinimas
expansion – plėtimasis
spalling – nuolauža
bar – gabalas, luitas
rough – šiurkštus
chipping – skalda
compound – mišinys
intermittently – nenutrūkstamai
a lattice – grotelės
tension – įtempimas
flexure – lenkimas
thermal – šiluminis
expansion – plėtimasis
molten – išlyditas
clinker – šlakas
powder – milteliai
investigator – tyrinėtojas
alkalinity – šarmingumas
matrix – rišamoji medžiaga
consolidate – sutvirtinti
weaker – silpnesnis
porosity – poringumas
fluidity – skysta busena
admixture – įmaišymas
corrode – korozijos veikimas
decay – puvimas
ductility – elastingumas
lasting – patvarus, ilgalaikis

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