128 The Masterbuilder - April 2013 • www.masterbuilder.co.in

Pre-Cast Concrete Elements in Construction - Emerging Scenario in India

Pre-cast construction is gaining significance in Indian scenario in general and Urban areas in particular. Pre-cast construction can be broadly classified in to three

categories.

- Project Specific Pre-Cast- General Pre-Cast for Sector Specific- Precast for Non Structural Elements

While the first and last categories are very much prevalent in India for quite some time. The First Category is gaining more popularity with the rapid urban infrastructure growth in India. The Non Structure Category is widely present but still to attract Major organized players. General Pre-Cast which is Sector Specific Such as Buildings, Power Distribution, Water Supply etc., is available scattered in India. Again this Category requires involvement of major players.

Though advantages of Pre-Cast are accepted by every stake Holder, the application is limited because the industry didn’t attain the scale it should be in. The Note presents case study pertaining to Building Precast category, by discussing various elements that can make the total MSB completely pre-cast. Also provides case study or example plants set up in Hyderabad city of India.

General

Cast In Place (CIP) VS. Pre – Cast

The current practice in India is the CAST IN PLACE (CIP) Reinforced Concrete Structures. The other construction method is using structural steel. In general most commercial and residential buildings are of CIP in nature. Steel structures are normally restricted to Industrial Building and sparsely Commercial Buildings. The ratio between Cast In Place (CIP) and steel structures is approximately 70 : 30. The third construction is using Precast Concrete which is primarily limited to Bridges and Railway Sleepers and other non structural elements.

Problems faced in CIP Structures

a) Safety issues: The biggest problem faced during construction

is failure of scaffolding. More structurally stable scaffoldings are cost prohibitive for ordinary commercial structures. (Pic. Of Collapse of Buildings at Khammam, Andhra Pradesh as published in News Papers is placed below.)

b) Concrete Placement using conventional systems in uncontrollable weather conditions.

c) Reinforcement Placement at higher altitudes.d) Quality of finishes highly dependent on working conditions

and form of support structures.e) The current structural system used is a rigid frame with

infill masonry walls. This system is very inefficient in the resistance of lateral loadings like earth quakes etc. Lot of Redundancy in structures.

f) Even most advanced techniques like tunnel forms with 9hrs cycle time take 8 months for 19 storied building.

Why Pre-Cast Construction?

Precast concrete offers solution to all of the problems and setbacks faced by Cast in Place Concrete (CIP).

The following are the main advantages:

a) Rapid Erection: Rapid Speed of Erection & Fast Construction resulting in earlier occupancy and reduced financing costs. Eg. 19 Storied Building can be completed in 3 months time.

b) Quality Assurance: Materials, Workmanship and Finishes. No transit loss in Quality. Greater specialization in trades leading to higher quality in workmanship.

c) Longer spans are possible with Precasting and Prestressing.d) Ease of Construction due to total Pre – Engineering.e) Aesthetics: Both Structural and Architectural finishes are

possible. Architectural finishes may be achieved acid itching, sand blasting or brick veneer or any other finish as specified by the Customer/Architect.

f) No form working is required at the job site. Limited space for Construction and minimum number of workers at site

g) The floor height can be reduced by eliminating the beams and shallow depths of slabs.

h) No prolonged usage of heavy machinery at site.i) No intensive labour at site for construction. The erection

P. Surya PrakashChief Consultant Satya Vani Projects and Consultants, Hyderabad.

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crew is limited 5 to 6 personnel.j) Better coordination with other specialized trades during

the planning and fabrication stage.k) Construction activities are limited to Precasting Erection,

Mechanical & Electrical services and the total coordination is done at the design and fabrication stage.

l) Precast Systems generally use walls for structural stability. The structural wall systems are more efficient and economical than the conventional CIP framed structures.

Pre –Cast Elements:

The following Precast Elements may be commonly used as illustrated in the photographs placed below:

- Columns- Lite wall panels- Structural wall panels- Single Tees, Double Tees & Quad Tees- Inverted Tees- Hollow cores- Spandrels- Rectangular beams- Stairs- Balconies, Architectural elements etc.

Case Study

As Per the request of the Manufactures the Case study is not published but will be presented in the work shop.

Case A: General Precast factory for Columns, Beams, Solid Slabs and Hollow core.

Case B: Specific Plant for Residential shear wall Construction.

Pre-Cast Construction

Modern Construction of the Buildings and various other Structures are done with Reinforced Cement Concrete by freshly mixed material, which can be molded into any shape. The relative quantities of Cement, Aggregates and Water mixed together control the properties in the wet state as well as in the hardened state, which is poured on the Steel Reinforced according to the requirements.

The Reinforced Concrete combines concrete and steel bars by simply putting them together and letting them act together as they may wish.

Pre-stressed Concrete, on the other hand, combines high strength concrete with high strength steel in an active manner. This is achieved by tensioning the steel and holding it against the Concrete, thus putting the Concrete into compression. This active combination results in a much better behaviour of the two materials. Steel is ductile and now is made to act

in high tension by Prestressing. Concrete is a brittle material with its tensile capacity now improved by being compressed, while its compressive capacity is not really harmed. Thus Pre-stressed Concrete is an ideal combination of two modern high strength materials.

Pre-cast Concrete is a material used to clad the exterior building envelope where each building design can be a custom creation, reflecting desired aesthetic expressions through colours, textures and physical sizing of pre-cast components. One must think of material, initially fluid in nature, with the ability to assume any design form from the mould into which concrete is poured. The subsequent curing, finishing and site installation ultimately provides a wall assembly which could be lean and sleak or strong and massive or perhaps very omate and sculptured emulating detailed stonework found in architecture in previous centuries. Pre-cast can be considered as a plastic material in its uncured stage, with infinite shapes, sizes and panel configurations.

Existing Pre–Cast Technology

Current Precast techniques are limited to specific customized projects and are usually Precast at the job site. Quality of field Precast elements cannot be assured because of batching, placement of concrete, placement of reinforcing, method of compaction, method of forming and finally quality of prestressing. Railway sleepers are also precast.

The application of Precast Concrete in Buildings require special connection details not commonly used in India. This expertise and skill is a common practice in advanced countries in Europe and USA.

This expertise can be acquired through collaborative effort with firms abroad.

Tecnology

It has been envisaged to import the existing Technology and expertise from USA with modifications to suit Indian conditions. Almost all commercial and residential CIP buildings currently built in India can easily be converted into precast, prefabricated concrete structures.

Process Involved in Production of Pre – Cast And Pre – Stressed Concrete Elements:

The following Production Process is recommended to be followed for obtaining the optimum levels. Proper planning of Plant layout and Equipment sizing is very important.

Equipment & Site Preparation

The basic equipment and site preparation required to support Pre-cast & Pre-Stressed Concrete elements production is listed below.

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- Serviced land of a minimum of 20 acres.- Building or production shed with cover is considered.- Levelled compacted storage yard.- Concrete batching plant capable of producing “zero”

slump concrete.- A minimum of 1.5 Cum output capacity is recommended.- Overhead cranes or Gantry cranes of 7 to 10 Tons capacity

of two units are recommended over the production area.- Construction of concrete stressing abutments and casting

pallet platform.- Mechanic shop and concrete testing facility.- Curing system with low pressure, high recovery, hot water

boiler with circulating pump and pipes, steel boiler with feed pipes or electric heating.

- Temperature control capability is recommended.- Yard cranes or Gantry cranes and Fork lift Trucks.- Front end loader for Raw Material handling.- Flat bed trailers or buggies to transport finished product

from production area.

Production Sequence

A typical Hollow core production should follow the recommended setup procedures and production sequences as listed below:

a. Cleaning and release agent application to pallet surface with Ultra-Span Service vehicle.

b. Pulling pre-stressing cables with Service vehicle and stressing cables with hydraulic jack.

c. Place extruder on pallet and prepare unit for extrusion.d. Start-up batching plant, and prepare mix for deliver.e. Deliver the mix to extruder and start production.f. After first line is cast, mark product and cover with curing

blankets.g. When curing is completed, test product for concrete

strength. If concrete strength is at the required level start distressing operators.

h. Distress the cables and start cutting operations on each pallet.

i. Lift off product from pallets and transport to storage area.j. Repeat cycle on a continuous base.

Equipment and Construction Requirements

The model hollow core production plant and other casting yard is based on specifications as follows:

a. Land requirement .b. Factory Building of the size c. Concrete bases for casting palluts and stressing abutments.d. Levelled and compacted storage yard area.e. A Production capacity of f hollowcore slabs per year

based on

f. A hollowcore slab production of the size of 20 Cms x 120 Cms.

g. A batching plant capable of producing “Zero” slump concrete.

h. Concrete curing equipment.i. Boiler and circulating pipes or supply pipes.j. Clean water and utility services.k. Electrical supply of 3 phase 440 Volts.l. Concrete Delivery System or Equipment.m. Cutting, Stripping, Cleaning and Storage Handling

Equipment.n. Overhead cranes or Gantry cranes over production area.o. Yard Material Handling Equipment.p. Clean and graded raw material supply and storage facility.q. Concrete and Material Testing Laboratory Facility.

Plan for Transfer of Technology

The transfer of Technology for production of Pre-Cast & Pre-Stressed Concrete Elements, visualised being:-

a) Transfer of Documents

- Design Drawings - Process Sheets- Tooling Documents - Technical Standards

b) Deputation of Engineering Specialists

c) Training of Operational Staff in the New Technology Process

Plan for Construction of Project Infrastructure

It is proposed to complete the infrastructure facilities for setting up of the Plant for the manufacture of Pre-Cast & Pre-Stressed Concrete Elements within a period of 12 months as detailed below: -

Plant Machinery & Equipment

It is Proposed to complete the process of selection of the Plant Machinery & Equipment within 3 months from the date of go ahead of establishing the Project. Thereafter the process of Ordering, Procurement, Erection and Installation of the Plant Machinery & Equipment will be completed within a period of 9 months.

Civil Works

The Construction of the Factory Buildings and the Auxillary Buildings including Concrete Bases and Storage Yard shall be planned to be completed within a period of 12 months from the date of go ahead of the Project.

Plan for Manufacture

It is envisaged that the Pilot Project would commence within 12 months from the date of go ahead of the Project and the

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Trial runs will commence thereafter. It is expected that the Process of Production will stabilize within 24 months from the date of go ahead of the Project by which time all the facilities required would be installed and the training of the Personnel for the Production would be completed.

Market Potential

Ready Mixed Concrete (RMC) is being used by about 80% of the Potential Users. This conversion process has taken over 15 years. The Potential Users have stated that survey products increase the convenience at site. The total sales+production of RMC in Hyderabad are expected to be 1,500,000 cubic mts./ annum with a growth of 15% year to year. This constitutes about 8% of cement usage out of total sales of 1.5 million tons/annum in Hyderabad.

The price and product flexibility for different buildings of the survey products is the crux of the problem. The larger contractors, consultants and builders like L&T, MECON, TCE, TPL and NCC may start suggesting or forcing their sub-contractors to use and using these products quickly. Using it for lower value mass housing projects where standard products are required will also improve market potential.

There is regular demand for pre-stressed concrete poles by railways and electricity board and concrete sleepers from railways. Since it is a regular market and there is severe competition for this range of products. These are not part of the survey products.

- There may be different kinds of machinery, which could not be supplied by one single agency. In such a case, all the agents shall be engaged.

- Design, Supply, erection, supply of shop drawings, their approvals shall be in the scope of Agencies.

- However, the Architect shall approve the shop drawings.

- The ancillary mechanical items required for erection of the machinery shall be procured by the concerned Agency only.

- Necessary Base Plates, Bed Foundations etc. required for erection of machinery and machinery parts shall be executed by respective agencies only in collaboration with the Civil or Other Contractors / Agencies.

- Synchronization of two or more machines or machine parts of different companies shall be made in presence of respective agencies, architect, client and engineer-in-charge and they shall be finished to the satisfaction of all the concerned.

Technology

Technology can be improved in operations of all the Processing Plants and Tools

- To avoid under utilization of efficiency of machinery - To save energy, fuel and lubricants wherever possible - To have optimum utilization of men, material and machinery - To have risk free operations - To minimize rejections - To have improved quality of out comes.

Batching plant

- Optimum utilization of materials to increase the production, crushing strengths etc.

- More production in minimum time with technically accepted out come.

- Minimum operations to have the increased out put - Conveyance of ingredients and final mix with easy and

minimum operations to required spot in minimum time either through conveyer belt, buckets or hopper system.

Forming

Forming requires Transfer Of Technology for improving methodology, equipment and optimization.

- Minimum shuttering to use for required casting- Reusing the same shuttering for more number of times- Same shuttering patterns to use for different patterns of

castings- Minimum operations of shuttering for different patterns of

castings

Fabrication

- Optimum tonnage of steel to obtain required strength to castings

- Minimizing number of rows of steel bars to get required stressing

- Cutting of bars without wastage

Pre-Stressing

Pre-Stressing requires Transfer Of Technology for improving methodology, equipment and optimization. This is proposed to be had from R.A.I.

- Conservation of energy in obtaining required stressing

Casting

Casting requires Transfer Of Technology for improving methodology, equipment and optimization. This is proposed to be had from R.A.I.

- Conveying the mix to casting spots.- Minimizing the wastage of mix during casting- To minimize the rejections to maximum extent

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- To minimize the stripping operations to save labor and time.- New methods of Compaction- New techniques of Surface and needle vibrations

Curing

Curing requires Transfer of Technology for improving methodology, equipment and optimization.

Effective and accelerated curing with minimum time period of not more than 9 (nine) hours

- Minimum quantity of steam, degree and intensity of super-saturation of steam (if steam is used)

- Optimum time requirement for quick and accepted setting with acceptable results of strengths

- To minimize the types of chemicals used, if chemicals are used

Stripping

- New methods to strip the castings without causing damage- To minimize the labor and time - To strip more number of castings at a time

Handling

Handling requires Transfer of Technology for improving methodology, equipment and optimization.

- Easy handling and conveying the material and castings to required spot with minimum labor, time and energy.

- Risk free handling during loading and unloading

- Usage of hardware like Wire ropes, slings, hooks, D-shackles, clamps, different types of knots to tie the blocks etc.

Stacking

- To stack more quantity of material and castings in minimum place without forming a heap of abnormal size.

- Stacking to have easy loading and unloading- Stacking to have easy movement of vehicles- Stacking materials in order of their movement and priority

of use.

Loading & Shipping

- The equipment can be of indigenous.- Loading into trucks with maximum castings to the full

capacity of the truck- To cause trouble free and effective operations - To minimize the loading and shipping time- To have shipping formalities in minimum time inorder not

to consume much time of trucks.

Erection, Jointing and Finishing

Erection, Jointing and Finishing require Transfer Of Technology for improving methodology, and optimization

The equipment can be indigenous.

- Minimum labor and time to erect as per the drawings and requirements

- Risk and trouble free operations- Causing no damage to the castings- Firm and Effective jointing to with stand and absorb all

types of loads and eddy effects - Smooth finish and good look

Safety certification

Safety Certification requires Transfer Of Technology for improving methodology. Process of inspection

- Conduction of tests- Safety measures - Certification methods

Engineering

- Engineering can be improved through Softwares.- Different kinds of packages of various makes can be studied.- New systems and packages can be improved though the

existing resources.- The information can be had through Foreign and Inland

Companies.- Tie-ups can be had with Foreign Companies for supply of

new technology and engineering.- Techniques can be improved in Engineering.- Preparation and study of Drawings like Construction

Drawings, Structural Drawings, Architectural Drawings, Shop Drawings and As-built Drawings.

- Drawings of all kinds like Civil, Electrical, Mechanical, Plumbing etc.

- Detailing the drawings- Arriving quantities from drawings with ease and

systematically.- Optimum number of engineering personals to be engaged

to commensurate the workload.- Planing the process of works, procurement of materials

and monitoring the achievements by conducting review meetings and emphasizing the need of achieving targets fixed.

- Estimating, Rate Analysis, Tendering, Documentation shall also be improved through Transfer of Technology.

Management

- Managerial skills shall be updated and improved for

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effective running of the Plant.- Demand, Supply and penetration of new product into

the market shall be studied with respect to other similar category of products of other companies.

- Necessary promotional activities and strategies shall be implemented to promote the product in the market.

- Customers who have accustomed to use conventional products shall be tuned to new product.

- Administration shall learn the techniques of extracting more out turn with minimum core staff.

- Shifting, transferring, removing and recruiting new staff frequently may effect the production.

- Finance wing shall update their knowledge in Finance Management.

- Methods to minimize or avoid transfer of funds from one head to other shall be improved.

- Technical staff shall also be trained in principles of finance management to avoid wrong bookings / under wrong heads.

- Sales tax, income tax, excise duty etc.are to be maintained duly updating the knowledge for not to be answerable to respective Govt. Departments.

Training

- Training is an effective mode of transfer of technology.- It can be of practical and theoretical.- Training can be had either at the Factories of concerned

companies or at the plant site.- Training shall be given to the staff and workers in various

fields in rotation system. - It shall be given for every year to update the techniques.- Training in erecting, commissioning and running the

machinery, their maintenance and attending the repairs that occurs generally.

- Smooth and effective running of machinery and plant shall be the main motto of the training.

- Training to all categories of staff and workers shall be given in their respective fields.

- Staff and workers deal with handling, erection, jointing, loading etc. laborious jobs shall be trained not to face unpleasant situations.

- Engineering Personal, Production Managers, Production workers, Marketing, Finance and Administration staff and Managers shall also be sent for trainings in their respective fields.

- Technical staff and workers shall be trained in reading the gauges, measures and meters for measuring various parameters and variables

- They shall be trained in using various instruments used for different purposes and at different places.

Publishers Note: This paper was presented at the Proceedings of the One Day National Workshop on Precast Concrete Buildings in India Practices, Possibilities & Prospects Held in ICSR Auditorium, IIT Madras, Chennai. The Masterbuilder was the official Media Partner for the above event.

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