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A client requires a road that requires little maintenance with reasonable non-skid properties. With the aid diagrams, suggest a type of road and the construction methodology for the proposed road.
Describe the performance and specify the material that can be used to fill the void of disused structures, eg: culvers, redundant sewers, cellars and basements and also for soil structural stabilization, eg:bridge abutments tunnel stabilization and embankments.
Briefly describe the activities involved in external works at the start of the contract?
Part a 4-6
Part b 7-9
Part c 10-12
Part d 13-15
The primary objectives of cladding systems are:
1. Provide enclosure
2. Speed of dry construction (prefab off site)
3. Impose minimal additional dead load
4. Enhance architectural concept
5. Control internal environment
To meet the objectives, cladding system provide for a number of specific performance requirements and functional needs, and the different cladding options available today have evolved to meet these needs. In general they may be considered to include the following:
Strength & stability
Exclusion of moisture/weather protection
Durability & freedom from maintenance
Control of internal temperature
Strength & stability
The achievement of required levels of structural stability is essential if a building is to withstand the loads that are imposed upon it during its life. Vertical, oblique, and lateral loadings must be safely transmitted through the structure to the load bearing stara.
To allow for differential movements between the structural frame and the wall structure there has to be adequate support to carry the weight of the wall in position and at the same time allow differential movements without damage to either the fixings or the wall material.
Exclusion of moisture/weather protection
The ability to exclude wind, rain, snow and excessive heat or glare from the sun is paramount in the list of user requirements, yet this must be achieved while still allowing best use to be made of natural light and ventilation. In these areas, particular care must be taken to exclude moisture, although the nature of the details will naturally vary with differing cladding forms.
It is common practice to solid cladding system as a cavity weight block that acts as a cavity cladding with an outer leaf as a rain screen, a cavity and an inner leaf of lightweight that acts as a thermal barrier and solid inside surface. Non absorbent materials are vulnerable to rain penetration and therefore the joints in the cladding should be watertight and flexible and serve as a protective seal against rain penetration.
Durability and freedom from maintenance
The masonry and stone facing require very little maintenance over the expected life of most buildings. Concrete cladding panels, which weather gradually, may become dirt-stained due to the slow run-off of water from horizontal joints. This irregular and often unsightly staining is a consequence of the panel form of this type of cladding. There must also be access to maintenance. The level of freedom from maintenance is a measure of the frequency and extent of maintenance work required to preserve functional requirements.
Control of internal temperatures
Sealed weather tight restricts natural ventilation. This can be overcome by the use of air conditioning, where hot air rises causing the movement of air; and alternative is a stack system where open plan areas around an atrium are used as an open central core. However, in areas where there is little pollution it is sometimes preferable to allow for opening windows, but this is not suitable in all the cladding form available.
Cladding systems are required to limit external fire spread for a set period of time. The materials that make up the elements of a building are required to have properties that will protect the structure form collapse and must not support spread of flame or fire from one part of the building to another and between adjacent buildings.
Some additional material or materials have to be used to improve the thermal properties of cladding structures built with solid, panel or thin sheet materials. Insulating materials are constructed or formed as an insulating inner leaf or lining behind solid or panel cladding or as a lining to panels. Materials used to enhance the thermal properties of cladding structures should be continuous lining behind or in the cladding structure, and cold bridging must be avoided.
The need to minimize the level of sound transmission through cladding can arise for a variety of reasons, but in general it could be considered necessary when sound levels differ greatly from the inside to the outside of a building. Sound travels in two distinct ways, via a solid material (impact sound transmission) via the air (airborne sound transmission).
Figure 1: Sound insulation system
According to the client requires, a suitable type of a road is rigid pavement. The road design for rigid pavement is quite different with flexible pavement.
Figure 1: Road design for rigid pavement
CONSTRUCTION METHODOLOGY OF THE PURPOSE ROAD
The function of the sub-base is to assist drainage, to protect the subgrade against frost and in the case of fine-grained soils, to prevent pumping (the ejection of water and silt trough joints or cracks caused by the downward movement of the slab due to heavy wheel loads).
The materials used are usually granular, eg crushed rock, crushed slag, crushed concrete, natural sand, gravels or well-burnt non-plastic shale. The materials should be graded. The thickness of the sub-base depends on the type of subgrade and should follow the recommendation of standard design tables. If the subgrade is susceptible to frost, the total thickness of sub-base and concrete slab should be a minimum of 450 mm. After the pavement slab has been designed, the thickness of the sub-base should be increased, if necessary, to gain a total pavement thickness of 450 mm.
Concrete slab construction
When the sub-base has been prepared, it is common practice to provide an anti-friction membrane over the sub-base before laying the concrete slab. This layer is normally polythene sheeting, which performs the extra function of preventing grout loss from freshly laid concrete. The slab is normally placed by a concreting train which runs on a heavy duty road form to prevent deflection. The concrete train usually includes hopper units which feed the concrete on to the base via a conveyor belt. Alternatively, this operation may be carried out by a screw-type spreader. Concrete is laid to the level of the fabric reinforcement and following the placing of the fabric, a second spreader and compactor unit completes the slab.
Figure 2: Slab preparation prior
to concrete laying
Reinforcement may be either steel fabric or bar reinforcement, the letter being deformed and spaced at centres of not more than 150mm. The diameter of bar to use in lieu of fabric can be established from design tables. Concrete cover to the reinforcement should be 60 mm; useless slabs are less than 150 mm in thickness, in which case 50 mm cover is required. The reinforcement should terminate at least 40mm and not more than 80mm from the edge of the slab and from all joints except longitudinal joints.
Joints are formed in concrete slabs for the purpose of allowing and controlling movement; the movements include expansion, contraction and warping. There are two types of material used in joints; the filler which separates the slabs, and a sealing compound which fills the top 25mm of the joint, thus resisting the entry of water and grit. Materials suitable for joint filling are impregnated fibre board: cork, sheet bitumen and rubber.
Joint sealing compound must have good adhesion to concrete, extensibility without fracture, resistance to flow in hot weather, and durability. There is no perfect solution to all thse requirements but some adequate solution are:
Expansion joints must prevent unrestrained horizontal movements of the slabs; to accommodate the movement a compressible material 25mm thick should be provided between the slab faces. The compressible material must be protected against the ingress of grit by filling the upper part of the joint with sealing compound to a level 5mm below the surface of the slab. An alternative material to sealing compound is a preformed neoprene compression sealing stip.
Contraction joints are similar in construction to expansion joints except that the filler material and dowel bar sleeves are omitted. Dowel bar are use to transfer loads across the joints and one half of each bar is coated with a bond-breaking material to allow contraction to take place. In addition, the interface of the slabs may be coated with bitumen before the second slab is cast.
Warping joints are needed in unreinforced concrete slabs to relieve stresses of restraint due to contraction. Warping is caused by vertical temperature gradients within the slab, and stresses caused by these may be higher than those caused by contraction. These joints, sometimes referred to as tied warping joints, consist essentially of a contraction joint with a special arrangement of reinforcement.
Figure 3: Expansion joints
Figure 4: Contraction joints
Figure 5: Warping joints
On completion, the surface of the slab may be textured by brushing with a wire broom at right angles to the centre line of the carriageway. This gives a better skidding resistance and a uniform appearance. The slab should be cured immediately after brush treatment by spraying with a curing compound.
Aerated concrete is manufactured by adding separately produced foam to a basic mix consisting of cement, water and possibly supplementary material or auxiliary or fillers. The foam must give the cement adequate stability, until the cement matrix itself is stable. Owing to the low density of aerated concrete, the material is used for limiting ground settlement.
The values are strongly dependant on the density and composition of the concrete and also on the age of the material. More accurate values can be quoted by the manufactures. Owing to the fact that the foaming agent only remains stable for a limited period, it must be introduced immediately before processing and hence be added at site. Aerated concrete can only be applied under water if special precautions are taken. The same applies to processing in rainy weather or at temperatures below 5ÌÅ c. The maximum distance over which aerated concrete cement can be pumped is 200-250 m. In order to prevent undue differences in density or cracking through heat of hydration, the maximum layer depth to be casted is limited to 0.30-0.40 m.
Standards can be devised for composition, water assimilation, mechanical properties, density and consistency of aerated concrete. The density of the aerated concrete produced often has to be checked on site.
If the material is used as the foundation for asphalt pavement, it should be born in mind that the aerated concrete offers high insulation values, so that summer conditions can cause stability problem. In the winter, there is the risk of uppermost road surface freezing likewise as a result of the effects of aerated concrete isolation.
Figure 6: General properties of aerated concrete
Properties of aerated concrete
The physical and mechanical properties of aerated concrete may be made to vary over a considerable range, depending on the quantities of the raw materials used, the mix design and curing conditions.
One of the major concerns with aerated concrete is that of cracking, both externally and internally. Cracking may be caused by shrinkage, creep, temperature, moisture effects, or by deformation and settlement of foundations. It is not always possible to separate the causes and effects of cracking.
Factors affecting creep of aerated concrete are stress level, moisture content, ambient temperature and relative humidity.
Density for aerated concrete generally refers to oven-dry material. When delivered to the building site, the practical density is higher because of the moisture content and presence of any reinforcement.
Aerated concrete does not provide the same corrosion protection of reinforcement as that provided by dense concrete. Thus it is recommended that reinforcement should be protected against corrosion by adequate surface treatment. In areas of relative humidity less than 50% at equilibrium moisture content, corrosion of reinforcement is reported likely to be negligible.
There also may be a risk of corrosion of nails and steel anchors because of the initial moisture content of the material, or moisture from condensation or rain penetration. Acids in the form of liquid and gases penetrate aerated concrete more easily than dense concrete and their deteriorating effect is more rapid than for dense concrete. Aerated concrete should therefore be protected from acidic liquids or fumes. The porous structure of aerated concrete allows air to penetrate the material readily and depending on the availability of carbon dioxide and atmospheric humidity, a decrease in alkalinity of the material occurs.
The extensive use of aerated concrete in countries with cold climates indicates that the material has resistance to frost. Nevertheless, a risk of frost damage exists if the moisture content in any part of construction unit exceeds the critical moisture content of the aerated concrete. The critical moisture content can be considered as the moisture content of the material at which, upon freezing and thawing, a loss in strength properties or damage occurs.
Aerated concrete is non-combustible. Because of its low thermal conductivity, heat migration takes place at a much lower rate than in dense concrete. Consequently, the fire ratings for aerated concrete units issued in some countries in Europe indicate that this material provides for good fire resistance compared with other building materials.
Moisture expansion and shrinkage
Aerated concrete is similar to dense concrete in that it expands on wetting and shrinks on drying. Drying varies with density and method of manufacture. Values of drying shrinkage range from 0.1 to 0.5% when measured from saturated condition to a condition of equilibrium. The low drying shrinkage of an aerated product compared with that for air-cured cellular has been attributed to chemical reactions that take place during autoclaving. Conventionally cured aerated would have high shrinkage but this can be reduced by high-pressure steam curing. Hence many commercial precast blocks are autoclaved and these would also normally contain a pozzolanic material, such as pulverized fuel ash, which contributes to strength.
The permeability of aerated concrete to air varies with the moisture content of the material. It decreases with an increase in moisture content in the pores.
The water vapor permeability by diffusion also decreases with an increase in the moisture content of the material. The diffusion factor of aerated concrete (ratio between the diffusion of water vapor through a layer of air to that through a layer of material of the same thickness) has mum water saturation (40 to 42% by volume) was 40 to 50% less than that of the air-dried material.
ACTIVITIES IN EXTERNAL WORKS AT THE START OF CONNTRACT
The drainage systems of building are two kinds: underground water and surface water. Underground water points include underground pipelines, manhole, access point, and inspection chambers. Surface water is rainwater drained from roofs and paving. The underground and surface water drains are connected to the public sewage system or in rural areas to cesspool or septic tank.
Where the local authority agrees that there is adequate capacity, surface water is drained into either a combined sewer or a separate surface water sewer. Surface water from garage forecourts and car parks is run in open gullies to an interceptor chamber. The interceptor chamber is an underground storage tank of concrete and bricks, which allow separation of the clean water from the oily scum remaining on its surface. The discharge drain to the sewer is turned downwards to near the bottom of the interceptor and three separate chambers are used in series.
Underground drainage systems are designed to operate without the input of energy, wherever possible, to be reliable and to require little, if any, maintenance. Their layout has to be such that drains are not subject to undue stress from foundations or traffic and are fully accessible for occasional clearance.
The first access point close to the building is a gulley, a removable WC or a shallow access chamber just after the base of the internal drainage stack. It is not necessary to fit access points at every change in drain direction, but pipe junctions are made with access chambers. The maximum spacing between access points is 12 m from the start of the drain to the first access, 22 m from rodding eye to a shallow access chamber, 45 m from rodding eye to an access chamber or manhole and 90 m between manholes.
Figure 7: typical site layout showing access
An inspection chamber incorporating a vertical drop for the purpose of connecting to a sewer or drain at high level to one at a lower level is known as a back-drop inspection chamber, or tumbling bay. The vertical drop is usually constructed outside the chamber, but may be constructed inside provided there is sufficient space in the chamber. If the vertical drop is constructed outside the chamber an access branch should be provided through the inspection chamber wall. If the vertical drop is constructed inside the chamber then a cast-iron access bend should be provided at the top of the vertical pipe.
Figure 8: (a)Section through back-drop inspection chamber (b) back inlet gulley (c) grease trap (d) drainage channel
Temporary works are an essential part of the construction process. No construction is possible without some form of temporary works being involved. Temporary works can be defined as ââ‚¬Ëœany temporary construction necessary to assist the execution of the permanent works and which will be removed from the site on completionââ‚¬â„¢, the use and type of temporary works covers a wide field.