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If concrete is mixed perfectly and compacted to a solid impervious, the longevity of all the construction materials is significantly increased in a non aggressive atmosphere. The strength of concrete is influenced by sulphate and sulfuric acid that takes place normally in soils, erosive chemicals existent in industrial waste in fill materials and organic acids and carbon dioxide existent in ground water. A solid, properly compacted concrete can efficiently safeguard the concrete piles, pile cap and ground beams against the attack by sulphates. The low penetrability of dense concrete resists or significantly controls the ingress of the sulphates into the pore spaces of the concrete. That's why high strength precast concrete piles are mostly recommended for application. Although these are not acceptable for all the site conditions and bored cast in situ / driven cast in situ piles, so, at the time of application, these should be designed perfectly to attain necessary degree of impenetrability and defiance to aggressive action. Both high alumina cement and super sulphated cement are not suitable for piling work. As an alternative, reliance is provided on the resistance of solid impervious concrete that is formed with a low water cement ratio. Coating of tar or bitumen on the surface, metal sheeting or glass fibre wrapping filled with bitumen may be chosen. A layer of heavy gauge polythene sheeting provided on a sand carpet or on blinding concrete is arranged to safeguard pile caps and ground beams on the underside. The vertical sides are safeguarded once the formwork is eliminated with the use of hot bitumen spray coats, bituminous paint, trowelled on mastic asphalt or adhesive plastic sheeting. Preventative measures against the aggressive action caused by sea water on concrete should only be taken into consideration with regard to precast concrete piles. Cast in situ concrete is utilized only as a centering to steel tubes or cylindrical precast concrete shell pills. The precast concrete piles for marine condition, a minimum ordinary portland cement content of 360 kg/m3 and a maximum water cement ratio of 0.45 by weight should be chosen.
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Published By
Rajib Dey
www.constructioncost.co
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Cast In Situ belongs to a construction item or structural member similar to a beam or in this case a Pile that should be built up, assembled or poured at site instead of prefabrication in a factory. Normally, in cast in place or cast in situ construction, concrete is delivered from a batch plant to site where it is poured and compressed into the formwork that is fixed in required shape and dimensions at site. Cast in situ or cast-in-place piles are cast in position inside the ground; generally in such type of piles, drilling of necessary diameter and depth into the ground is done with an auger drilling device or a drill bit. There is a helical screw blade generally known as a “fighting blade” inside the device that functions as a screw conveyor to eliminate the drilled out material. Besides, auger drilling an old method known as percussion drilling is also applied for excavating the hole. Under this method, a heavy cutting or hammering bit affixed to a roper or cable is lowered in the open hole or inside a temporary casing. After entering deep into the ground, a temporary steel casing is lowered in the borehole to safeguard loose soil from dropping in the borehole. The verticality of the casing should be examined precisely prior to start. Once the optimal depth is attained, the reinforcement cage with vertical rebars and stiffeners is lowered within the borehole, and the upper part is hanged at the top. The concreting is generally performed with Tremie method of concrete piling. There are normally 6 types of cast in situ piles as below:- 1. Simplex Pile, 2. Franki Pile, 3. Vibro Pile, 4. Vibro Expanded Pile, 5. Raymond Pile, 6. Mac Arthur Pedestal Pipe Benefits of Cast In Situ Concrete Piles: The cast in situ piles are set up with pre-excavation and reduce the vibration because of driving as in case of driven piles. In housing area, sound pollution may occur if the piles are entered by hammering. To get rid of this issue, situ piling is suitable in such areas. For water logged area, cast in situ piling with permanent casing is very effective. The skin friction resistance with the ground is fully used in cast in situ piles throughout the design phases that is not recommended in case of driven piles where only the end bearing is applied. Normally, there is no need of any foreign materials and tools and the original equipments and materials will meet the requirements of a project, as a result the cast-in-situ piles become as a cost-effective and adjustable type of pile foundation. Precast piles should be designed to satisfy the handling and driving stresses thus enhancing the essential reinforcement that does not happen in the case of cast in situ piles and therefore, the amount of necessary reinforcement is minimized. Cast in situ piles are attached over the ground with a pile cap that applies a monolithic approach. The top ground is excavated up to pile cut off level from where the slushy low quality concrete is eliminated with hand hilty to retain developed rebars into the pile cap. Because of this monolithic connection, cast in situ pile provides good resistance against the earthquake and wind forces. As soon as the piles are casted, no maintenance is required. Since the materials and machinery applied are obtained from the local community, local contractors can perform the job and not any skilled labor is required for cast in situ piles. No serious consideration should be provided for joints in cast in situ piles with regards to precast driven piles.
This construction video is based on pile cap construction process for building up a bridge. The video is specifically designed for bridge engineer. The video throws light on the following topics :- a. Pile Head Breaking b. Granular laying c. CC work d. Rebar Fabrication The construction method of bridge is segregated into two parts :- Substructure (pile foundation, pile cap, pier, pier cap) Superstructure (bearing, girder, slab) Definition of pile cap: It belongs to a structural member that is positioned and generally attached on the top of a pile or a group of pier to transfer the loads into the pile or group of piles to relate them into a bent. Functions of pile cap To disperse a single load evenly over the pile group as well as over a larger area of bearing potential. To laterally strengthen separate piles and enhance complete durability of the group. To arrange the required combined resistance to stress organized by the super structure and ground movement. To transfer the loads of the building to the foundations and the ground soil layers despite the loads are vertical or inclined. To facilitate the column or superstructure to stay on a consistent and solid core foundation rather than staying directly on ground. To learn the detail pile cap construction process for a bridge, go through the following video tutorial.
In this construction video tutorial you can get live demonstration of blending and positioning of concrete in the jobsite for slab or road. Mixing: Mixing of concrete is one of the most crucial factors for obtaining good final properties and for this purpose proper equipment should be utilized. So, it is suggested to employ ready-mixed concrete to get the best result. Mixing allocates the aggregate uniformly during the cement paste and it makes sure that all of the cement is completely saturated in water, and eliminates large air voids. Besides, mixing separates agglomerated clusters of cement particles and facilitates air entraining admixtures to produce the perfect air void system. Undermixing can produce large flaws and therefore leads to poor strength, whereas overmixing consumes time and energy and can demolish entrained air voids. If the workability is lower, the more mixing energy and mixing time is necessary. Placing: As soon as the concrete is blended sufficiently, it should be placed into the formwork to determine its final position and shape. If it is required to reinforce the concrete, the rebar should have been placed properly in order that the concrete can flow around it. If the concrete mixing truck is available adjacent to the site, then the concrete should be directly poured into the forms. If it becomes difficult, the concrete is transmitted in buckets through a crane or wheelbarrow. If it is impossible because of the necessary distance or the size of the job, the fresh concrete has to be pumped via a system of pipes or hoses to the site with the special concrete pumps. Concrete that is to be pumped contains more severe needs for workability. If the concrete gets too dry, it will not pump properly, whereas if it gets too wet it will have a tendency to segregate. Segregation may also happen if the concrete is poured into the formwork too rapidly, because larger aggregate particles will be subject to be pushed downward. In slab construction, the placing has to be initiated along the perimeter at one end of the work with each batch arranged against transmitted concrete earlier. It is suggested not to dump the concrete in independent piles and then level and work them jointly; nor should it be placed in large piles and carried horizontally into final position. Watch the following video for more details.
