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Cladding can be defined as a protective or insulating layer fixed to the outside of the building or any other structure. The objectives of cladding system to be built are:
To provide enclosure.
The cladding was made to provide enclosure to the building which will give the necessary protection against the weather and external changes.
Speed of dry construction.
The usage of off-site prefabrication for the cladding can provide a drier and faster construction.
It imposes minimal additional dead load.
The cladding is usually made from a lightweight material and thus the additional load impose on the building is minimum.
It enhances architectural concept and impress the appearance of the building.
The cladding is made from a different type of materials. So, each of the material used may extrude the appearance of the building according to the concept and function of the building that the architect wants to design.
It can control internal environment.
The cladding system can control the internal environment of the building such as controlling light penetrating into the building, controlling the radiation and conduction of heat from the sun, and many others.
To achieve the objectives, the cladding system should obey some of the functional requirements. The primary functions of cladding system is to separate the indoor environment of a building from the outdoors in such a way that the outdoors environment will not affecting the indoors and the indoor environment can be maintained at levels suitable for the intended use. The other functional requirements of the cladding system are as follows:
The cladding system should have a good strength and stability.
The cladding system must have adequate strength to support its own weight between the points of support or fixings to the structural frame whereas the sufficient stability is needed to against the lateral wind pressure exposed to the building. Both of these features are required in the cladding system to allow the differential movement between itself; in terms of material expansion and contraction, between the structural frame or the differential movement in other adjacent building element.
The cladding system should have exclusion of wind and rain.
To fulfil these criteria, a common practice is to construct a solid cladding system with an outer leaf as a rain screen and inner leaf as insulator. Thus, the materials used to seal joints are required to be resilient enough to accommodate movement and resist weather deterioration. To sustain the pressure and suctions by the wind, the cladding must be adequately strong and stiff. The wind directions and velocities are greater at the upper reaches of building.
The cladding system must have durability and freedom of maintenance.
The minimum frequency and extent of work is necessary to maintain the minimum functional requirements and acceptable appearance of the cladding. As an instance, the glass cladding requires frequent cleaning and renewal of seal to maintain its performance whereas the sheet metal cladding need to be observed and carefully take care of it as it is easily undergo oxidation process and faded.
The cladding system has the control of internal temperatures.
This is due to the solar gain through the glass panels. The internal temperature can be controlled by three ways. First is by using a deep recessed window in conjunction with external vertical fins. Secondly, by using non-transparent external louvers and thirdly is by using a special solar control glass.
The cladding system has a great resistance towards fire.
The system must fulfil the building regulations by the local authority. The materials used as a lining for insulation must be made from a non-combustible material.
The cladding system should include thermal properties by controlling the radiation and conduction of heat.
The lining of the cladding material need to provide additional insulation to control the heat. To radiate the flow of heat from the sun, the interior surfaces of the cladding need to be at the temperature that will not cause radiant discomfort which is neither too hot nor too cold. To control the conduction of heat, the cladding must be made from a low conductivity of heat material to avoid localised condensation on interior surfaces, thermal bridges and stabilised the required degree of the conduction of heat into and out of the building.
The cladding system should be equipped with sound insulation.
The cladding system should isolate the noise outside the building from the interior of the building or the interior of the building from the external noise. The isolation of noise is best achieved by walls that posses the features of airtight, massive and resilient. The used of resilient pad to prevent sound originating within the structure to be transferred vertically through the cladding members. The airborne sound can be prevented by utilising double glazing panel to windows area.
The cladding systems must provide sufficient openings for the admittance of natural daylight and ventilation.
The admittance of the sunlight into the building must be controlled as the sunlight has ultraviolet ray which is harmful towards human skin and must be kept off from inferior material that easily disintegrate or fade. The visible light of sunlight is useful for illumination but it can also be bothersome because it causes glare.
Question 2 (b)
A client requires a road that requires little maintenance with reasonable non-skid properties. With the aid of diagram, suggest a type of road and the construction methodology for the proposed road.
The type of road that requires little maintenance and reasonable non-skid properties is a rigid pavement or also known as concrete road. A rigid pavement consists of a concrete slab resting on a thin granular base. The pavement may be Unreinforced Concrete (URC), Jointed Reinforced Concrete (JRC) or Continuously Reinforced Concrete (CRCP). The concrete slab should be Pavement Quality Concrete (PQC), manufactured, laid and cured according to the specification required.
Figure 1: Cross-section of Pavement Structure
The basic rigid pavement structure consists of subgrade (existing soil), sub-base course, base course and surface course (concrete). The sub-base course and base course are optional depending on the location of the road to be made. Sub-base layer consists of a lean concrete base and a layer of cement-bound granular base or soil cement base. The total thickness of sub-base and concrete slab should be a minimum of 450mm. The function of sub-base is assisting the drainage, protecting the subgrade against frost, and, in the case of fine-grained soils is to prevent pumping where the ejection of water and silt through joints or cracks occurs due to the downwards movement of the slab caused by heavy wheel loads.
The materials used as a granular sub-base are crushed rock, crushed slag, crushed concrete, natural sand, gravels or well-burnt non-plastic shale.
After the placement of sub-base layer, an anti-friction membrane is placed over and normally polythene sheeting that performs extra function of preventing grout loss from freshly laid concrete.
The concrete slab is normally placed by a concreting train that runs on a heavy duty road form to prevent deflection and is bedded in position at least 24 hours before concreting tthe slab. A hopper unit is usually included in the concrete train to feed the concrete on the base through a conveyor belt. This operation may be carry out alternatively by using a screw-type spreader. Then, the concrete is laid onto the fabric reinforcement, followed by the placing of the fabric, a second spreader and compactor unit to complete the slab. Next, a surcharge is placed on the top layer of concrete to gain maximum compaction.
An alternative method to lay the slab is by using a slip-form paver. This plant requires no side forms and mounted on crawler tracks. It is capable of laving pavement at 2m per minute.
The general method of rigid pavement costruction consists of placement, consolidation, finishing, curing and jointing.
This process involves equipments and procedures use to place the pre-cast concrete on a desired thickness of surface. The concrete can be pour and spread by using truck or truck attachments. For a more accurate and even placement, a placementmachine can be use instead. The concrete will become less homogenous or tend to segregate after it has been unloaded from the truck. Thus, the screeding process is carried out immediately after the concrete placement. The excess portion of roughly pre-cast concrete are cut off to the required level of slab elevation. This can be done by dragging a straightedge at the required elevation across the slab.
Figure 2: Placement Over Dowel Bars in an Intersection
Figure 3: Placement in Front of a
This process is use to eliminate undesirable voids in any ways possible. This process removes undesirable air voids and causing it to move around reinforcing steel or other potential obstruction and thus, makes the freshly placed pre-cast concrete more uniform and compact mass. The process of consolidation is completed by using a vibrator that can be characterised as a long and slender vibration rods. The vibrator works by moving back and forth to rotate an eccentric weight which causes the particles in the pre-cast concrete mass to excite to move close together for a better flow around obstruction. However, the vibrator need to be controlled carefully as the exccess or too little vibration can cause the coarse aggregate particles to be non-uniformly distributed.
Finishing process involves all the steps and equipment used to create the final surface finish and texture of fresh pre-cast concrete. This process can be divided into two parts; floating and texturing.
Floating. This process involves different tools and may use multiple passes over the same surface. This is done by running a flat surface across the concrete to remove high and low spots, eliminate the imperfections, and compact the mortar at the surface for texturing.
Texturing. The concrete is quite smooth after the floating. So, texturing process is carried out to create a slip resistance surface for traffic. The pattern is done by dragging a rough-texture item across the surface. There are two types of texturing; microtexture and macrotexture.
Microtexture. The texturing is done by dragging a section of artificial turf or burlap behind the paver. This can enhances frictional force between the tires of vehicles and the surface of pavement and thus increases safety at lower speeds.
Macrotexture. The texturing is done by tining the pavement surface. This type of texturing allows the water to escape from between the tyres of vehicles and the pavement and thus increases the safety at high speeds.
Figure 4: Microtexture Figure 5: Macrotexture
The curing process is the maintenance of required temperature and satisfactory moisture in pre-cast concrete as it hardens to develop desired properties such as strength, durability and density. These properties related to the extent of hydration of concrete which depend on the moisture and temperature of the site. The perfection of the hydration can results in the better concrete's properties.
This process involves the actions to insert purposeful discontinuities in the pavement and seal them appropriately. Joints are formed in slab for allowing and controlling the movements such as expansion,contraction and warping. There are two types of materials used in joints, a sealer that separates the slab and a sealing compound that fills the top of joint. The material used as a joint fillings are cork, rubber or sheet bitumen.
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 to gives a better skidding resistance and uniform appearance. The slab should be cured immediately after brush treatment by spraying with a curing compound.
Question 2 (c):
Describe the performance and specify the material that can be used to fill the void of disused structures, e.g. - culverts, redundant sewers, cellars, and basements and also for soil structural stabilisation, e.g. - bridge abutments, tunnel stabilisation, and embankments.
The material that can be used to fill the voids of disused structure is foamed concrete. Specifically, it is called Highly Air-entrained Mortar (HAM) or also known as aerated concrete. Ready-mix foam concrete is highly workable and contain up to 50 percent air-entrained which lead to a low density material. This type of concrete can self-levelling, self-compacting and can be pumped with the risk of settlement cracking and plastic shrinkage is lower than a normal concrete.
The performances of the foamed concrete can be divided into three categories which are plastic state advantages, working improvements and final concrete performance.
At plastic stage advantages, the foam concrete can be implemented to a variety project needs and operational conditions as it is more practical than any other materials. The features of concrete that satisfy at this stage are as follows:
The concrete can be produced on site or off site. This feature allows an immediate placement on delivery and thus, no space is needed for storage. This will also lead to nice workability retention.
The concrete has wide range of possible mixes of materials according to its usage. The proportion of mixture is adjustable to fulfil required performance. This may results in controlled density and strength of the concrete.
The concrete has a stable chemical structure. So, no soil analysis and moisture checks between layers are required. This is due to its compatibility with almost all building products and materials.
The concrete has a cellular fill. This may cause the external contamination face difficulties to penetrate into the concrete.
At the working improvements category, the free-flowing properties of these concrete contribute to the following features and benefits:
The concrete is easy to place and finish. It uses only a single process of installation where the concrete is pour and then levelled. This lead to a reduction in cost for labour and supervision. Besides, the specialist machinery such as compactors or vibrators is not needed.
The foam concrete is pumpable. They can be pumped at a significant distance by using a simple conventional concrete pump.
The concrete is self-compacting and self-levelling. This feature makes it ideal for an inaccessible trench where the compaction is difficult to carry out. It allows the concrete to discharge into narrow space and fully fills the void.
At final concrete performance category, the plastic state qualities have finally translated into significant final performances. The performances are as follows:
The concrete has high entrained air content. This feature makes the concrete becomes more resistance towards freezing or thaw damage. The entrained air content also acts as a good thermal and acoustic insulation.
The foam concrete has a good cohesion. The cohesion makes the concrete as a stable foam structure that reduces settlements. The cohesion also helps in the reduction of bleeding and segregation of concrete
The density and strength of the foam concrete can be controlled. The range of densities and strengths is available for each type of foam concrete. The lower strength concrete allows removal of subsequent access to services
The foam concrete has a stable structure which makes it can be surfaced after twenty-four hours.
Question 2 (d):
Briefly describe the activities involved in external works at the start of the contract.
External works can be generally defined as the construction works done externally from the main building. The external works can be divided into two; at contract commencement and at the end of contract. The activities involved in external works at the start of the contract are drainage, temporary access and buildings, and public utilities whereas the activities involved at the end of contract consists of road and pavement, fencing, landscaping and minor external works.
Drainage can be divided into two types; underground/subsurface drainage and surface drainage.
Subsurface drainage was made to collect the water from where it is not wanted to some other place such as removing and disposing of surplus groundwater from gardens and other plots of open land. The system usually deals with foul water from kitchen, toilets and any industrial process or combining foul water with surface water. The subsurface drainage system consists of underground pipe line, manhole/inspection chamber and culvert.
The underground pipe line pipes and sewer pipes can be made of vitrified clay, uPVC, concrete, high density polyethylene (HDPE) and many others. The jointing of plain ended pipes is made by means of a coupling where the socketed pipe that has distinguishable 'male' and 'female' ends must be laid with 'female' end pointing upstream.
To construct underground pipe line, the pipes should be laid in straight line from point to point with a fall to a steady gradient. The type of bedding is depending on the pipes material to protect the pipes from ruptures or breakage. The fittings and access points must be installed at head of run, bend or change of direction, change in pipe diameter and at junction, unless all runs connected to junction can be rodded from another access point.
The second part of subsurface drainage system is manhole, access point and inspection chamber. The access chambers are intended to provide simple access for cursory inspection and access for drain rods or other maintenance equipment. The inspection chambers (IC) are larger than access chambers. It provides access for maintenance equipment, but tends to have more branches feeding into them. The manholes (MH) are the largest chambers providing access to sewer or drain for maintenance equipment. The manholes can be made from brick (brick manhole) or pre-cast concrete manhole. The brick manhole typically have 215mm thick brickwork with a mass concrete as benching whereas the pre-cast concrete manhole only 50-60mm thick, although those built beneath vehicular trafficked areas should have been haunches with mass concrete at least 150mm thick.
Figure 6: The Cross-section of Pre-cast Concrete Manhole
The third part of subsurface drainage system is culvert. Culvert is a structure which provides a waterway or other opening under a road. The type of pipe culvert should be class Z spun reinforced concrete with spun concrete collars or spigot and socket type.
Figure 7: The Culvert
The second type of drainage is surface drainage system. The system collects water from the roof and the paving, often discharging this relatively clean water into a local watercourse to reduce demand on the effluent treatment plant. The surface water systems are gulley and access point, continuous grating over pre-cast concrete channels and combined kerb and drainage systems.
The gulley and access point can be classed into individual trapped gulley and road gullies. The individual trapped gully commonly used with rainwater downspouts for draining large area of paving. The road gullies are basically much larger and used within carriageways.
The continuous grating over pre-cast channels can be made from wide range materials such as HDPE, polymer concrete and stainless steel.
The combined kerb and drainage systems act as road isolator and discharge point of collected surface water.
Figure 8: The Kerb
Second external works done at start of the contract is the construction of temporary access, storage area, car parking and site facilities. These are the major components to be built before any construction of building can take places in provision of difficult access to all parts of the site, difficulties in storage of materials and for the site facilities and car parking.
For the construction of temporary access, the contractor will lay the base course of permanent road to minimise the cost. The site temporary road can be kept dry by laying the drainage system as soon as possible.
The third external works at the commencement of contract is the public utilities services. The services that involves are water system, electricity and cabling for telephone. The public utilities need to be planned ahead before any major structures are built. Any constructions of service mains and ducts should start concurrently with foundation construction stage so that it will not hinder the construction activities.