NBM & CW article - Affordable housing in India with precast construction

NBM & CW article - Affordable housing in India with precast construction


Affordable Housing in India With Precast Construction

Introduction

India is facing a housing shortage which is currently estimated at more than twenty five million homes in the country out of which EWS (economically weaker sections) and LIG (lower income groups) account for 95 percent. In 2015 the Indian government launched the Housing for All mission with an aim to provide affordable housing to urban poor. It is proposed to build twenty million houses for the urban poor by the year 2022. To achieve this goal the government and private real estate developers have to launch large scale affordable housing projects all over the country. However the construction industry is facing issues with manpower shortage and rising cost of construction materials. The cost of labor has gone up significantly and availability of construction workers has become problematic. Furthermore the pressure on developers has increased as customers are demanding on time delivery of their homes and good quality construction work. Conventional construction methods will not be able to exploit the advantages of mass housing and in the current scenario it is evident that new construction technologies should be utilized to deliver homes fast with minim labor and zero wastage of materials. Especially in Europe after the Second World War the housing shortage was solved by implementing new technologies like precast concrete construction. Precast concrete construction is a building method where building components are prefabricated off site on an assembly line using advanced tools and equipment and thereafter transported to the building site where they are erected using building cranes. Precast concrete construction has evolved over the past five decades towards a flexible building technology which is used in many infrastructure projects and building projects throughout the world. This article describes the technical details and specifications of three different housing projects in India which are being constructed with precast concrete technology. The first project is an affordable group housing project with medium rise apartment towers located in Ghaziabad, Uttar Pradesh. The second project is also an affordable group housing project with medium rise apartment towers but located in Bangalore, Karnataka. The third project is consisting of low rise villas located in Greater Noida, Uttar Pradesh.


Project 1. Medium Rise Apartment Buildings, Bharat City, Ghaziabad

This first project is located in Ghaziabad at the outskirts of Delhi where the real estate prices are at an affordable level. The project is being developed on 60 acres of land by real estate developer BCC Infrastructure Pvt. Ltd and the first phase of the project which was launched in 2011 comprised of almost 1800 apartments of 1BHK, 2BHK, 3BHK and 4BHK. Phase one was constructed using conventional cast in place reinforced concrete structures with apartment towers of ground plus fourteen floors. Although achieving an affordable costing the real estate developer was not satisfied with the speed and quality of construction. A technical feasibility study was prepared for phase two of the project to investigate the possibilities of constructing the remaining 39 apartment towers with precast technology. The purpose of the study was to present objective information regarding several precast systems and prepare a comparison between the advantages and disadvantages of the systems. Based on the performed investigation and research it was concluded that a large precast panel system would be the most suitable with respect to structural safety, economics, aesthetics and constructability.


Picture 1. Bharat City phase 2, Ghaziabad

Building System

In this project the foundations and basement structure are made entirely as conventional cast in-situ reinforced concrete structure. The basement areas below the towers are utilized for car parking while the driveways are situated in the extended basement areas. The apartment towers which come on top of the basement are made in load bearing precast concrete with the use of large precast elements like slabs and walls. The precast walls are considered to be shear walls and are part of the lateral load resisting system and are adequately connected to the floor diaphragm to achieve an earthquake resistant structure. Due to manufacturing and transportation limitations the precast wall panels are made as one storey high elements and will be jointed at floor level (picture 2). The horizontal and vertical connections between the precast walls are established at the building site only.


Picture 2. Precast concrete wall erection

Adequate buttressing of the external wall panels has been achieved by connecting the internal wall panels to the external wall panels by grouted shear key joints and reinforced wire loop connections. All load bearing elements at the corners of the building have been stiffened by jointing structural elements perpendicular to it (picture 3). The load bearing precast walls are placed on top of each other and are properly connected to each other at floor level and adequately connected to the floor slab with protruding reinforcements. The precast shear wall system is designed to emulate the behavior of cast in-situ concrete construction in terms of stiffness, strength, ductility and energy dissipation. Because of the provisions for car parking the layout of the basement structure is somewhat different from the superstructure and wherever possible the precast load bearing shear walls of the superstructure are supported by cast in-situ RCC walls at basement level and only at a few locations the precast walls are supported on short span transfer beams.


Picture 3. Precast concrete load bearing structure on top of basement

The vertical dead loads and live loads are carried by the floors and transferred towards the load bearing structural elements. The floor slab system of the superstructure is made of precast concrete planks on which a reinforced cast in-situ concrete topping will be poured. This system is commonly known as the precast half slab system (picture 4). The slab bottom reinforcement is placed within the precast planks and the slab top reinforcement is placed within the RCC topping. Proper connection between the precast planks and the RCC cast in-situ topping is achieved by providing lattice girders made of reinforcement bars which are partially placed inside the precast plank and partially sticking out at the top. The composite slab system provides continuity and connection to all the shear walls and the slab behaves as a rigid floor diaphragm. The precast planks are made as room sized slabs and are supported on the interior and exterior load bearing walls or beams. Due to continuation of the top reinforcement over the interior supports the slab system has structural behavior of continuous floor span system. The bottom reinforcement of the slabs will be anchored at interior supports by placing extra bottom reinforcement directly on top of the precast slabs. At exterior supports the extra bottom reinforcement shall be provided in the form protruding bars from the walls which will be bend down directly on top of the precast slab.



Picture 4. Precast half slabs with lattice girders

All the balconies are cantilevered precast solid slabs and are connected by protruding reinforcement to the cast in situ RCC topping of the floor (picture 5). Bathroom slabs are also made a precast solid slabs to achieve a waterproof slab and are connected to the half slab system by protruding reinforcement at the slab edges.



Picture 5. Cantilever precast balconies

The lateral load path of the precast superstructure is formed by a large number of precast concrete shear walls which are placed in x-direction and y-direction. The precast shear walls are solid reinforced concrete panels and due to their large in plane stiffness most of the lateral forces are resisted by these structural elements. Load transfer at horizontal joints is achieved by fully filling the 20mm gap between the precast panels with non-shrink high strength grout. Furthermore reinforcement starter bars are passing through the horizontal joints which will ensure a proper connection between one precast wall panel to the next precast wall panel. The starter bars are placed in the center of the precast walls and are connecting inside oversized steel corrugated dowel tubes which are later fully filled with non-shrink grout (picture 6). The starter bars are anchored inside both the precast walls and are lapping indirectly with the reinforcement meshes of the precast wall panels. At the horizontal joints the shear forces between the precast concrete wall panels are transferred by the friction of the joint interfaces and by the dowel action of the starter bars.



Picture 6. Erection of load bearing precast wall panel

The precast buildings have been designed with proper structural integrity to avoid situations where damage to small areas of the structure or failure of single elements may lead to collapse of major parts of the structure. This has been achieved by providing effectively continuous peripheral tie reinforcement near the edge of the building perimeter. Furthermore internal tie reinforcement has been provided at each floor and roof level in two directions at right angles. The peripheral and internal tie reinforcement has been placed as extra reinforcement inside the RCC cast in-situ topping. Also all the external load-bearing members such as precast walls have been anchored or tied horizontally into the structure at each floor and roof level by providing uniform reinforcement protruding from top of precast walls and bend down inside the RCC cast in-situ topping (picture 7). Each precast wall carrying vertical load has been tied continuously from the foundation to the roof level. At all horizontal precast joint locations the starter bars haven been placed staggered by using a combination of normal length grout tubes and longer grout tubes in combination with continuous dowel reinforcement running from bottom to top inside the precast wall panels.



Picture 7. Half slab top reinforcement lapped with protruding wall and balcony reinforcement

The staircases are made of precast solid slab landings at floor level and mid landing level. The flights are made as precast stair elements which are resting on the precast landings and connected by dowel connections (picture 8).



Picture 8. Precast staircase with landings and stair flight

Overview of Precast Components For The Superstructure

Item

Specifications

Load bearing precast walls

160mm thick solid precast walls

200mm thick solid precast walls

Partition walls

100mm thick solid precast walls

Shaft walls

160mm thick L-shaped precast walls

Slabs

50mm thick precast half slab with lattice girders

Bathroom slabs

Solid precast slabs with protruding reinforcement

Balconies

Precast solid balcony slabs with protruding reinforcement

Staircase

Precast stair flights

Precast solid slab landings

Beams

Precast beams 200x600mm