Construction science and material

Lime is made by heating chalk or limestone (both calcium carbonate, CaCO3) to a temperature of 900� in a kiln. The heating breaks down the limestone to produce quicklime (calcium oxide, CaO) and carbon dioxide gas (CO2). The chemical reaction is shown below:

CaCO3 Heat CaO CO2

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Limestone 900�C quick lime carbon dioxide

When quicklime is added to water the quicklime combines with the water to form hydrated lime (calcium hydroxide Ca (OH) 2.) This process is known as slaking. The chemical reaction, which takes place, is shown below:

CaO H2O Ca (OH) 2

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Quicklime Water Hydrated lime

GYPSUM PLASTER:

Gypsum plaster is made from gypsum, whish is naturally occurring rock (calcium sulphate dihydrate, CaSO4 .2H 20). The gypsum is quarried, crushed and ground to a fine powder. The gypsum is then heated to drive off some or all of the water.

If the gypsum is heated to 150�C, then only some of the water is lost and the gypsum converts to plaster of Paris (calcium sulphate hemihydrate, CaSO4.1/2 H20) as shown below

CaSO4 .2H 20 heat CaSO4.1/2 H20

Gypsum 150�C plaster of Paris

Plaster of Paris sets rapidly (within 10-20 minutes). Because this is too fast for most plastering work, a retarder (e.g. keratin) is often added to slow down the setting.

If the gypsum is heated to 650�C, then all the water is driven off and the gypsum converts to anhydrous gypsum (calcium sulphate, CaSO4).

CaSO4 .2H 20 heat CaSO4

Gypsum 650�C anhydrous gypsum

PORTLAND CEMENT:

Portland cement is made by heating a mixture of limestone (or chalk) and clay (or shale) to 1,500�C in a kiln. This heating causes the material to form a hard granular material called clinker. The clinker is allowed to cool and then ground to a fine powder to produce Portland cement. A small amount of gypsum (calcium sulphate) is added to the resulting clinker to slow down the setting of the cement.

CONCRETE:

Concrete is made by mixing cement, sand and gravel with water. The cement and water form a paste, which surrounds and binds the aggregate particles together. A chemical reaction (called hydration) takes place between the cement and the water. This causes the cement paste to stiffen (or set) in a about 2 hours and then gain strength (or harden) over a period of several years.

The concrete gains strength rapidly at first and then much more slowly as time goes by, about 90% of the concrete’s final strength is gained in the first 28 days. For this reason, a time period of 28 days was selected by specification writers as the age that all concrete should be tested.

TIMBER:

Timber production is the process of managing stands of deciduous trees to maximize woody output. The production process is not linear because other factors must be considered, including marketable and non-marketable goods, financial benefits, management practices, and the environmental implications of those management practices.

Forests include market and non-market products. Marketable products include goods that have a market price. Timber is the main one, with prices that range from a few hundred dollars per thousand board feet (MBF) to several thousand dollars for a veneer log. Others include grazing/fodder, specialty crops such as mushrooms or berries, usage fees for recreation or hunting, and biomass. Forests also provide some non-market values which have no current market price. Examples of non-market goods would be improving water quality, air quality, aesthetics, and carbon sequestration.

The more biodiverse the hardwood-forest ecosystem, the more challenges and opportunities its managers face. Managers aim for sustainable forest management to keep their cash crop renewing itself, using silvicultural practices that include harvesting, promoting regeneration, controlling insects and disease, fertilizing, applying herbicide treatments, and thinning. Fertilization can increase the growth rate and amount of plant material, thus possibly increasing the number of wildlife that can inhabit a site. Invasive species control maintains an area’s structure and native composition.

But management can also harm the ecosystem; for example, machinery used in a timber harvest can compact the soil, stress the root system, reduce tree growth, lengthen the time needed for a stand to mature to harvestability. Machinery can also damage the understory, disturbing wildlife habitat and prevent regeneration.

PAINT:

Paint is an opaque, colour liquid applied to a surface to protect it and make it attractive. It bonds to the surface and dries to a hard, solid finish. Paints can be applied to virtually any material including: timber, metal, brick, plastics, and concrete.

Paints are made by mixing the pigment with some of the binder. The mixture is ground in mills to ensure all the pigment particles are broken up. The additives and the remaining binder are then added to the paint mixture. Next, the thinner is added finally, the paint is put in to tins.

STEEL:

Steel is an alloy of iron and carbon containing less than 2% carbon. (an alloy is a mixture with metallic properties that contains at least one metal). It is made from iron by the basic oxygen process. Molten iron from the blast furnace and scrap iron is put into furnace. Oxygen is blown into the furnace to reduce the carbon in the molten iron.

The oxygen reacts with the carbon to form carbon dioxide gas:

C + O2 CO2

Carbon oxygen carbon dioxide

And convert impurities (such as silicon and phosphorus) into oxides:

Si + O2 SiO2

Silicon oxygen silicon dioxide

Then, limestone (calcium carbonate) is added to remove the oxide impurities to form slag:

CaCO3 + SiO2 CaSiO3 + CO2

calcium carbonate calcium oxide calcium silicate carbon dioxide

Finally, metals such as titanium, manganese or chromium are added to make a wide range of steels with different properties.

TASK 2:

Deteriororation

CONCRETE:

Sulphate attack:

Sulphates are frequently present in the ground water. The most common are sulphates of calcium, magnesium, and sodium. The sulphate solution reacts with the tri-calcium aluminate in the hardened cement paste and produce calcium sulphoaluminate. This occupies a greater volume than the original tri-calcium aluminate and causes the concrete to crack and lose strength. This chemical reaction is known as sulphate attack.

To reduce the risk of sulphate attack, use sulphate-resisting cement in the concrete.

Sulphate attack can be ‘external’ or ‘internal’.

External: due to penetration of sulphates in solution, in groundwater for example, into the concrete from outside.

Internal: due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate, for example.

External sulphate attack

This is the more common type and typically occurs where water containing dissolved sulphate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include:

* Extensive cracking

* Expansion

* Loss of bond between the cement paste and aggregate

* Alteration of paste composition, with monosulfate phase converting to ettringite and, in later stages, gypsum formation The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste

The effect of these changes is an overall loss of concrete strength.

The above effects are typical of attack by solutions of sodium sulphate or potassium sulphate. Solutions containing magnesium sulphate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.

Other sources of sulphate which can cause sulphate attack include:

* Seawater

* Oxidation of sulfide minerals in clay adjacent to the concrete – this can produce sulphuric acid which reacts with the concrete

* Bacterial action in sewers – anaerobic bacterial produce sulphur dioxide which dissolves in water and then oxidizes to form sulphuric acid

* In masonry, sulphates present in bricks and can be gradually released over a long period of time, causing sulfate attack of mortar, especially where sulphates are concentrated due to moisture movement

Figure 1 Scanning electron microscope image of sulfate attack in concrete. Ettringite (arrowed) has replaced some of the calcium silicate hydrate in the cement paste. As a consequence, the paste will be weakened. Although much of the cement paste here remains apparently unaltered (e.g.: top right), if widespread within the concrete (which in this instance it was) sulfate attack can significantly weaken the concrete.

Internal sulfate attack

Occurs where a source of sulfate is incorporated into the concrete when mixed. Examples include the use of sulfate-rich aggregate, excess of added gypsum in the cement or contamination. Proper screening and testing procedures should generally avoid internal sulfate attack.

Frost attack:

If water soaks into concrete and freezes, it will expand and cause the concrete to crack. This is known as frost attack.

Frost attack can only occur if:

* The temperature is below 0� C

* Water is present, and

* The concrete is permeable.

To avoid or reduce the risk of frost attach, ensure the concrete does not remain saturated for long periods.

TIMBER:

Fungal attack:

The risk of fungal decay depends on the type of timber and the proportion of sapwood and also the possibility of surrounding present spores. Bad dampness along with poor ventilation and a normal internal temperature present ideal conditions for the germination of a fungal attack. This highlights loft and basement space as ideal areas for attack. Wood that has been attacked and damaged by fungus may sound hollow and dull and possibly be discoloured. It will yield easily under pressure and will break against the grain.

Some timbers are more resistant than others.

Fungi are important organisms and are resent in many situations and play a number of roles. They are responsible for the breaking down of organic matter can cause disease in both animal and man, they are the basis for the fermentation and they help to produce chemicals and medicine (such as anti biotic). They are important factors in the growth of living trees but are responsible for the decay of timber.

Dry rot

This fungus mostly causes decay in areas with restricted ventilation, high humidity and a moisture content of between 30-40%. However, dry rot can remain active in timber with as little as 20% moisture and can also attack dry timber. It can grow through the fabric of a building if conditions are suitable, by penetrating brickwork and masonry and behind plaster, decaying any timber in its path. Decaying timber develops “cuboidal” cracking and is usually overgrown by masses of grey-white mycelium. Plate-like fruiting bodies produce millions of rusty-red spores (seeds) as a reddish dust, and these spores spread the fungal infection to other areas.

Wet rot

Wet rot develops as a result of rain water penetration, such as in the opened mitres of door and window frames, where paint films have broken, where plumbing is faulty or a major leak is occurring. High moisture levels (of 40 – 50%) are necessary for wet rot to develop, and the wood characteristically splits along the grain where decay occurs. There is usually a small amount of surface mycelium, but fruit bodies are not common

Fungi that attack woods can be categorised into two areas and these are wet rot and dry rot. Both dry and wet rots can attack hardwood and softwood and whilst dry rot will not grow in saturated conditions neither will grow in a dry environment.

Fungi usually attack untreated wood because it is easier to penetrate. It develops extensive root systems (mycelium) to remove nutrients and oxygen from the cells. This in turn destroys the chemical structure of the timber. The minute airborne spores will germinate if they land on a damp substrate such as wood. The germinating spores produce this thread like hyphae, which collectively from a mycelium. The hyphae making up the mycelium continue to grow and extend through the timber cells where the timber then begins to disintegrate. The presence of fungi is usually a warning of more serious dampness problem further down the structure and should always be investigated.

Insect attack:

Common furniture beetle

“Woodworm” is the most common cause of insect attack of softwoods in buildings, and is often to be found in structural timbers, roofs, floors and joists. The female adult beetle lays her eggs onto the susceptible timbers, and they hatch into larva which in turn burrows into the timber, gradually weakening it. This process can take at least three years, with the larvae growing to a length of 2.5mm. After the pupal stage, the adult beetles emerge from the timber through a 2mm flight (exit) hole.

Deathwatch beetle

The deathwatch beetle causes deterioration in structural hardwoods such as oak, elm and chestnut, which have already been partly decayed by wet rot. This pest is more of a threat to large timbers in older buildings and occurs mainly in the Southern and Central areas of England and Wales. It has yet to be recorded in Scotland.

Powder post beetle

This beetle attacks the sapwood of larger-pored hardwoods, and is more commonly found in flooring, plywood and furniture.

Wood-boring weevils

After the Furniture Beetle, wood-boring weevils are probably the most common timber pest. They are commonly found attacking partly decayed wood, with both adults’ larvae causing the wood to break down by burrowing, principally along the grain, and leaving thin paper walls of wood separating the borings. of more commonly found in flooring, plywood and furniture. Flight holes are ragged in outline and less than 2.5mm in diameter.

House longhorn beetle

This beetle – a notifiable pest – causes severe damage to the sapwood of softwood roofing timber in parts of Surrey, Hampshire and Berkshire. The larvae burrow into the timber and, when fully grown after anything from 3 to 11 years, can measure over 25mm in length. The adult emerges through a flight hole 6 – 8mm in diameter

BRICKS:

Sulphate attack:

When water soaks into brickwork, the soluble salts will dissolve in the water. This solution reacts with the tri-calcium aluminate in the cement mortar joints. This causes the mortar to expand, crack and crumble leading to bowling of the walls. This chemical reaction is known as sulphate attack.

Example of sulphate attack.

Sulphate attack in mortar is recognised typically by one or a combination of the following: lightening in colour of the mortar joints; horizontal cracking within the mortar bed joints; crumbling of the mortar. When weakened by sulphate attack, mortar also becomes more vulnerable to the effects of frost; the two actions can thus occur simultaneously, aggravating the effects of each. The effects of the reaction are very slow to develop and, if the attack is not too extensive, it may be halted by simple remedial measures such as preventing further saturation of the brickwork. At the other extreme, for well established cases of sulphate attack demolition may be the only answer. However, the need for such drastic action can be avoided by adopting very simple precautions in design and selection of materials and by general good practice.

Sulphate attack is an uncommon but serious condition, often requiring the re-building of the affected brickwork.

Sulphate attack can only occur if:

* Water is present either from rain or ground water,

* Soluble sulphates are present in the bricks or the ground.

* Tricalcium aluminate in the Portland cement is between 8 and 13%

To avoid or reduce the risk of sulphate attack:

* Ensure the brickwork does not remain saturated for long periods.

* Use a frost resistant, low salt content brick.

* Use sulphate-resisting cement in the mortar.

Test for soluble salts are carried out on a sample of ten bricks. The test involves crushing and analyzing the bricks chemically.

Efflorescence:

Efflorescence is a white powder deposited on the surface of brickwork. It is harmless and usually temporary but it can be unsightly. The salts are generally chlorates or sulphates of calcium, magnesium, potassium or sodium. When the bricks get wet, the salts dissolve in the water. The water carries the dissolved salts to the surface of the brickwork. When the water evaporates, the salts are left behind as a white powder on the brick surface.

Efflorescence will occur in new buildings if the brickwork is not protected during construction gets wet.

Depending on the soluble salt or salts which are causing the efflorescence, you can remove the deposits:

* By brushing with a stiff brush, or

* By washing the salts with water from the wall. However, this process sometimes dissolves the salts and causes them to soak back into the wall.

To prevent efflorescence:

* Cover brickwork to protect it from saturation

* Protect new brickwork from wet weather for at least a week after construction

TASK 3:

Suitable materials for new detached house:

External walls – BRICKS

Facing bricks are best to use for external walls as they are strong, durable with an attractive appearance. They are available in a wide range of colours, textures and strengths.

Properties are:

* Frost resistance

* Compression strength

* Appearance

Ground floor construction – REINFORCED CONCRETE:

This is concrete that has steel embedded into it to increase its tensile strength which makes it ideal for ground floor construction

Properties include:

* It has a good water resistance

* It is cheap and durable

* It can be cast into any shape

First floor construction – RIENFORCED CONCRETE

Again reinforced concrete is the best material for first floor construction too. Main properties are:

* It is cheap and durable

* It is fire resistant and is used to provide fire protection to steel beams and columns

* It has a high density, which makes it good for sound insulation.

Bathroom wall finish – SOLVENT-BASED PAINTS

These paints are both oil-based and alkyd-based paints. they provide a hard and smooth finish which makes it ideal for bathroom wall finish.

Properties include:

* Appearance

* Moisture resistance

* Cost

Gutters and Downpipes – PLASTICS

Plastics are perfect for guttering and downpipes because:

* They are cheap, attractive, and durable

* They are easy to shape and colour

* They are water resistant

* They are light weight

TASK 4:

a)

Causes of deterioration in brickwork

Type of deterioration:

Cause:

Frost Attack:

If bricks are saturated with water, then the water freezing and subsequently thawing can lead to spalling of the brick surface. This is known as frost attack.

Sulphate Attack

When water soaks into brickwork, the soluble salts will dissolve in the water. This solution reacts with the tri-calcium aluminate in the cement mortar joints. This causes the mortar to expand, crack and crumble leading to bowling of the walls. This chemical reaction is known as sulphate attack

Efflorescence

Efflorescence is a white powder deposited on the surface of brickwork. It is harmless and usually temporary but it can be unsightly. Efflorescence is due to the presence of soluble salts in clay bricks

TASK 4:

b)

Likely mode of failure in following situations:

* Bubbles on external painted woodwork – this is caused by water trapped beneath the paint film evaporating.

* Paint lifting away from a painted surface in flakes – this is caused by breakdown of the adhesion, mainly when painting damp or powdery surfaces.

* A timber window sill is soft and has large cracks along the grain and smaller cracks across the grain – When water soaks into brickwork, the soluble salts will dissolve in the water. This solution reacts with the tri-calcium aluminate in the cement mortar joints. This causes the mortar to expand, crack and crumble leading to bowling of the walls. This chemical reaction is known as sulphate attack

* White fluffy powder appears on the surface of recently completed external brick wall – When the bricks get wet, the salts dissolve in the water. The water carries the dissolved salts to the surface of the brickwork. When the water evaporates, the salts are left behind as a white powder on the brick surface, this is known as efflorescence

* Some clay bricks are crumbling in an external wall – If bricks are saturated with water, then the water freezing and subsequently thawing can lead to spalling of the brick surface. This is known as frost attack.

* The cement mortar in external brickwork is soft, crumbling and cracks horizontally along the joints – cement is liable to attack by acids and soluble sulphates and it can easily crack due to shrinkage on setting.

TASK 5:

Methods of applying preservatives to timber to protect against wood-rotting fungi and wood-boring insects

All measures that are taken to ensure a long life of wood fall under the definition wood preservation (timber treatment). Apart from structural wood preservation measures, there are a number of different (chemical) preservatives and processes (also known as timber treatment or lumber treatment) that can extend the life of wood, timber, wood structures or engineered wood. These generally increase the durability and resistance from being destroyed by insects or fungus.

BRUSHING AND SPRAYING:

Brushing preservatives is a long-practiced method and often used in today’s carpentry workshops. Through technology developments it is also possible to spray preservative over the surface of the timber. Some of the liquid is drawn into the wood as the result of capillary action, but this penetration is insignificant and not suitable for long-term weathering. By using the spray method, coal-tar creosote, oil-borne solutions and water-borne salts (to some extent) can also be applied. A thorough brush or spray treatment with coal-tar creosote can add 1 to 3 years to the lifespan of poles or posts. Two or more coats provide better protection than one, but the successive coats should not be applied until the prior coat has dried or soaked into the wood. The wood should be seasoned before treatment.

DIPPING

Dipping consists of simply immersing the wood in a bath of creosote or other preservative for a few seconds or minutes. Similar penetrations to that of brushing and spraying processes are achieved. It has the advantage of minimizing hand labor. It requires more equipment and larger quantities of preservative and is not adequate for treating small lots of timber. Usually the dipping process is useful in the treatment of window sashes and doors. Treatment with Copper salt preservatives is no longer allowed with this method.

HOT AND COLD TREATMENT

This process achieves treatment by immersing seasoned wood in successive baths of hot and cold preservatives. During the hot baths, the air expands in the timbers. When the timbers are changed to the cold bath (the preservative can also be changed) a partial vacuum is created within the lumen of the cells, causing the preservative to be drawn into the wood. Some penetration occurs during the hot baths, but most of it takes place during the cold baths. This cycle is repeated with a significant time reduction compared to other steeping processes. Each bath may last 4 to 8 hours or in some cases longer. The temperature of the preservative in the hot bath should be between 60 to 110 �C (140 to 225 �F) and 30 to 40 �C (85 to 105 �F) in the cold bath (depending on preservative and treespecies). The average penetration depths achieved with this process ranges from 30 mm to 50 mm (1 to 12/3 in.). Both preservative oils and water-soluble salts can be used with this treatment. Due to the longer treatment periods, this method finds little use in the commercial wood preservation industry today.

PRESSURE IMPREGNATION

Pressure processes are those in which the treatment is carried out in closed cylinders with applied pressure and/or vacuum. These processes have a number of advantages over the non-pressure methods. In most cases, a deeper and more uniform penetration and a higher absorption of preservative is achieved. Another advantage is that the treating conditions can be controlled so that retention and penetration can be varied. These pressure processes can be adapted to large-scale production. The high initial costs for equipment and the energy costs are the biggest disadvantages. These treatment methods are used to protect ties, poles and structural timbers and find use throughout the world today.

The various pressure processes that are used today differ in details, but the general method is in all cases the same. The treatment is carried out in cylinders. The timbers are loaded onto special tram cars, so called “buggies,” and into the cylinder. These cylinders are then set under pressure often with the addition of higher temperature. As final treatment a vacuum is frequently produced to extract excess preservatives. These cycles can be repeated to achieve better penetration.

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