PRECAST CONCRETE AND ITS CONNECTIONSAbstractDue to rapid growth in urbanization there is tremendous pressure on industry to deliver faster construction. In such situation, Pre-cast construction technology becomes a popular choice. This technology has already addressed several issues like quality, durability and safety during construction. This paper presents Need of Precast concrete construction, Its Advantages and disadvantages. Different joint connections (Beam-Beam, Beam-slab and column-column connections) and tests conducted to know behaviour and strength of joints in multistory buildings.

CHAPTER 1INTRODUCTIONThere is a large shortage of housing for many millions in India and the increase in the cost of construction is a big problem in overcoming this shortage. It is necessary to develop a large variety of low cost options for different regions of India such that housing becomes affordable for an ordinary citizen. The major cost increase in house built with conventional technology is due to two factors viz.[1] Increase in basic cost of materials like cement, steel, bricks etc. Increase in labour costsTo reduce material consumption and to reduce labour component many cost reducing system and techniques are required. The cost reduction in precast is very essential as it forms a major part of the construction cost.In pre-cast, substantial cost reduction is possible by: Using efficient structural geometry and efficient structural action principles. Good practice from an early stage in the design and planning process.In addition, it is also necessary to reduce the cost of other structural components like walls, foundation, floor, lintels etc. A study on such options will be useful for reducing cost of construction.

1.1 PRE-CAST OR PRE- FABRICATED CONSTRUCTION:In India and other developing countries to accommodate spiraling population, there is a need to construct more and more houses, and other buildings at a faster speed. Traditional in-situ construction is too slow to cope-up with this increasing demand. By pre-fabrication, better quality buildings can be constructed at faster rate.Most of the European countries and USA have set up efficient concrete industries with large scale mechanization of pre-cast operation and site erection. The introduction of large panel and other techniques by these countries has revolutionaries the building industry and an unprecedented rate of construction has been achieved. The various advantages and disadvantages of prefabrication are discussed below:1.1.1 Advantages1. In pre-fabricated construction shuttering and scaffolding are eliminated to greater extent. Apart from saving in the cost of shuttering elimination it gives clear space for other building activities.2. Pre-cast moulds can be used for a large number of times. As the reuse of mould is more the cost of mould per precast unit is less.3. It is easier to produce components with structurally efficient shapes in pre-cast construction. Hence the consumption of scarce materials like cement and steel can be reduced.4. Production can be done independent of the weather condition and hence can be produced even during rainy season or when the climate is extreme. It is possible to produce components under close supervision and test them periodically resulting in better quality and more reliable.5. By adopting prefab construction work site is reduced to minimum and will be clean, the construction is speeded up and occupied earlier which means early return of capital invested.6. In case of prefab construction as the pre-cast components have undergone most of the shrinkage before they are used in the construction, stresses due to shrinkage on the structure are much less compared to in-situ construction.1.1.2 Disadvantages1. To ensure monolithically, free ended condition, more reinforcement is necessary.2. In case of handling and erection stresses are excessive, extra reinforcement may be needed.3. Special care should be taken while handling and erection to avoid damage and breaking.4. Temporary supports may be necessary in some cases. As there is a chance of leakage through joints between precast components, extra care is necessary to make them leak proof.5. Unless adequate quality of reinforcement is provided across the joints between precast components and in-situ concrete, cracks may develop. The concept of precast or prefabricated construction includes those buildings, where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost.The main features of this construction process are as follows:1. The division and specialization of the human workforce.2. The use of tools, machinery, and other equipment, usually automated, in the production of standard, interchangeable parts and products.3. Compared to site-cast concrete, precast concrete erection is faster and less affected by adverse weather conditions.4. Plant casting allows increased efficiency, high quality control and greater control on finishes.This type of construction requires a restructuring of entire conventional construction process to enable interaction between design phase and production planning in order to improve and speed up construction.

Fig. 1.1: Precast plant consisting structure elements

(a) (b) Fig 1.2 : (a) Erection of Gypsum reinforced concrete wall panels construction in IITM-2013, (b) Precast concrete structure at GVP college of Engg, Vishakhapatnam during ICICPC-2013 conference.

CHAPTER 2CONNECTIONSThe structural integrity of precast concrete buildings depends mainly on the connections between the precast structural components. A sequence of particular construction causes joints in a structure. Connection can be between old and fresh concrete and can be between two parts of a structure. Construction joints must be so positioned that the strength of a completed member is not affected. The most suitable place for a construction joint in a simple structure is where a bending moment is zero or a shear force is maximum. A construction joint may be at the junction of a rib and slab of a T-beam or a smaller beam at a short distance from the junction of intersecting beams. Joints can also be possible where columns at different floors are to be integrated with slab or beam construction. In the completed building the structural connections will form an essential part of the structural system. The structural response will depend on the behaviour and the characteristics of the connections. The structural layout, the arrangement of stabilizing units, the design of the structural system and the design and detailing of the connections must be made consistently and with awareness of the intended structural behaviour. To achieve a satisfactory design the designer should understand how the connections influence the flow of forces through a structure under vertical and horizontal loads. The main purpose of the structural connections is therefore to transfer forces between the precast elements in order to enable the intended structural interaction when the system is loaded. Precast concrete building structures are composed of some basic types of structural systems. These systems can be combined in different ways to obtain an appropriate and effective structural concept that fulfils the needs of specific buildings. The most common systems are:1. Beam to beam connection2. Beam to column connection3. Column to column connection4. Column beam floor connection5. Floor and roof system

Fig 2 : Different type of connections column to footing, column to column and column to beam.

2.1 Beam to beam connectionsThe figure shows the composite connections between two identical precast beam elements. Both the isometric view and the sectional elevation show the two beams with opposite notches or nibs connected by mechanical fastenings or bolts. Shear stresses at the nib are reduced by the introduction of the bent bars. In addition, a bar cast in one end is projected outside to be inserted into the hole left in the other element. The two elements will achieve a monolithic structure of higher efficiency. A similar method is adopted by connecting two precast concrete beam elements by means of high tensile barsas tie rods leaving grooves at the top of the concrete and filling them with a specified filler.

Fig. 2.1: Beam to beam connection2.2 Beam to column connections.At the bottom a steel bar, as shown on figure 2.2, is welded to a steel plate which is then welded to a bearing plate on top of the precast column bonded by a steel bar.

Fig. 2.2: Beam to column connection

Fig. 2.2.1: Showing beams resting on column

Fig. 2.2.2 Configuration of beam and column

2.3 Column to column connectionsSometimes factory-made column elements are to be connected to make larger columns of specified lengths. Dowel bars, as shown on fig 2.5, are used to erect such columns. Injection holes are left to fill in hollow areas with grout.

Fig. 2.3: Column to column connections

Fig. 2.3.1: Details of column connections 2.4 Floor and roof systemThe main purpose of floor and roof systems is to carry vertical load to the vertical load-resisting structural elements. Besides, precast floors and roofs are often used as essential parts of the stabilizing system to transfer horizontal loads by diaphragm action to the stabilizing units. The most common floor systems are hollow-core floors and double-tee floors, Double-tee units are also used in roof systems. The elements in a floor system will not resist loads separately from each other, but a degree of interaction between adjacent elements is desired. To obtain a transverse distribution of load effects in case of concentrated loads and prevent uneven vertical displacements at the longitudinal joints, the floor connections must be designed to develop shear key action that ensures the interaction between adjacent elements. In composite floor plate floors precast concrete floor plates are used as formwork for the cast in-situ part and remain integrated in the composite floor section. Composite action depends on the shear transfer in the horizontal joint between the precast plate and the cast in-situ concrete part. There is no requirement for design of longitudinal joint between the plates as the topping is continuous over the precast joint lines.

Fig. 2.4: Hallow slab connection2.5 Aspect of design connections.There are several ways to obtain tensile capacity in connections. Considering the safety aspect only, the order of preference is as follows:1) Bolting2) Grouting reinforcing bars at the site3) Embedding reinforcing bars in epoxy or polyester at the site4) WeldingThe use of bolts is a simple way to establish a safe connection, but it generally puts strict requirements on the tolerances. The bolts can be threaded rods or regular bolts.Grouting at the site will to some extent depend upon the weather to be successful. Generally the requirements on tolerances are not severe. When the result from a casting operation is successful, a very sound connection is established. It also provides fire and corrosion protection for the steel details.Mix proportions, aggregate size and casting procedures will vary with the size, location and orientation of the operation. Patches permanently exposed will often not be acceptable. Anchoring the grout to relatively large steel surfaces is a problem that is often overlooked. Large elements such as steel haunches can be wrapped with mesh or wire. For recessed plates and similar elements headed studs or wiggled refractory anchors can be welded to plate to provide anchorage for the grout. Gluing with epoxy or polyester is very dependent upon weather conditions to be successful, especially temperature. Also the workmanship is of great importance to secure a satisfactory result, like the mixing of the ingredients and the preparatory cleaning and drying of the contact surfaces. Some types of glued connections also put some requirements on tolerances. Most types of glue loose most of their strength when heated to about 80 0C.Welded connections will in most cases fit without any problems, but the quality of the weld is totally dependent upon the skill of the welder. When welding outdoors the welder must have extensive knowledge about what kind of electrodes to use under different weather conditions on different material qualities, and of treatment of the electrodes and materials prior to welding. Also the actual work may be difficult and laborious to perform; like when fixing a small plate with only ladder access, with heavy cables and no place to put the clamp. The heat generated when welding may also damage the quality of the concrete close to the weld. Welded connections will in many cases have to be fire protected.

CHAPTER 3Tests carried out:-Beam-column connection:-Types of Connections1. Monolithic Specimen (ML):- The monolithic reinforced concrete test specimen (ML) was designed according to IS: 456-2000 and detailed according to IS:13920-1993.

2. Precast connection using Tie Rod (PC-TR):- The main connecting elements were tie rod and steel plates. The beams are connected to the column using tie rods and are supported on concrete corbels as shown[2]

FIG 3

RESULTS

Various observations were made from the experimental investigations and are presented in the following sections. The parameters considered for the present study are Ultimate Load and Moment Carrying Capacity.

Table 1 Comparison of Experimental Yield and Ultimate Loads of the SpecimensSl.nospecimenExperimental Yield load(kn)Experimental ultimate load(kn)

Upward directionDownward directionUpward directionDownward direction

1ML8.89.4011.2911.75

2PC-TR6.58.627.6210.77

Table 2 Comparison of Experimental Ultimate Moment Carrying Capacity of the SpecimensSl.nospecimenExperimental Ultimate moment(kn-m)

Upward directionDownward direction

1ML6.216.47

2PC-TR4.195.92

Column slab connection All the test units had 275mm x 300mm columns and 275mm deep x 200mm wide beams, with different slab parameters as given in Table 3. The values of concrete compressive strength at 28 day are listed in Table.[4]Table 3Unit descriptionSlab 1Slab 2Slab 3

Slab thickness(mm)505063

Reinforcement6@250mm c/c

6@165mm c/c

6@200mm c/c

28-day strength(mpa)fc'

39.0237.0638.38

fcu

44.8243.9343.33

FIG 4 :-Crack patterns in the beam-column intersection zone and slab

CHAPTER 4Discussions and ObservationsThe construction of precast elements is easy simple and economical. The main advantage of precast construction is time saving. This type of construction is lighter in weight and provides better sound and thermal insulation to the buildings. It also ensures rapid construction and eliminates the use of shuttering which are essentially required to support slabs and beams in the conventional type of construction.There is a general concern regarding the seismic performance of precast construction. It is noticed that large panel construction performs better than frame system. However, in areas of high seismic risk, structures must be designed to respond safely to the dynamic forces imparted into the structure. Innovations in joint design are improving the connection systems in precast concrete structures and making them increasingly suitable for use in such areas. Connections should be carefully designed where the impact or dynamic forces are much higher.It is concluded that special Code provisions are required for seismic performance of connections in precast concrete construction. More experimental studies are required for a full scale precast concrete structure to understand the strength, deformation, ductility, dissipation and damage of the structure. [3]

REFERENCES1. Mathur, G.C. (1993), Low Cost Housing in Developing Countries, Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi.2. Behaviour of precast beam-column tie-rod Connection ,R. Vidjeapriya1 and K.P. Jaya23. Seismic Safety of Joints in Precast Buildings A State-of-theart Literature Review Ravikanth Chittiprolu & Ramancharla Pradeep Kumar Earthquake Engineering Research Centre, IIIT Hyderabad.4. Saddam m, Ahmedi, Umarani Gunasekaran, A Parametric Study of R.C Slab in Beam-Column Connection Under Cyclic Loading.5. Code of practice for precast concrete construction (2003).

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