High Rise Pre-Cast construction

Buildability & Effect on Structural Modelling

This note is prepared as a part of the Lecture in ICICPC-2013 organized by GVPCE, Vizag. The objective is to discuss the Pre-cast technologies available worldwide, type of standard elements for High Rise construction and Buildability. Also, the author will discuss case studies of the effects on Structural Model.

Introduction

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

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 have been in. The Note presents case study pertaining to “Precast Building” category, by discussing various elements that can make the total High Rise Buildings completely pre-cast, also provides case study of effect on structural modelling.

General

Cast In Place (CIP) VS Precast

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 method is using Precast Concrete which is primarily limited to Bridges and Railway Sleepers and other non structural elements.  

Problems faced in CIP Structures

• The biggest problem faced during construction is failure of scaffolding. More structurally stable scaffoldings are cost prohibitive for ordinary commercial structures.
• Concrete Placement using conventional systems in uncontrollable weather conditions.
• Reinforcement Placement at higher altitudes.
• Quality of finishes highly dependent on working conditions and form of support structures.
• 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.
• Even most advanced techniques like tunnel forms with 9hrs cycle time take 8 months for 19 storied building.

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.

Precast Construction

• Modern Construction of the Buildings and various other Structures are done with Reinforced Cement Concrete by freshly mixed material, which can be moulded 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 behavior of the two materials. Steel is ductile and now is made to act in high tension by Pre stressing. 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 sleek or strong and massive or perhaps very ornate 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 Precast 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 pre stressing. Railway sleepers are also precast.
• The application of Precast Concrete in Buildings requires 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.

Technology

It has been envisaged to import the existing Technology and expertise from USA/Europe 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.

Production Process Involved

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.

• 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, and 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:

• Cleaning and release agent application to pallet surface with Ultra-Span Service vehicle.
• Pulling pre-stressing cables with Service vehicle and stressing cables with hydraulic jack.
• Place extruder on pallet and prepare unit for extrusion.
• Start-up batching plant, and prepare mix for deliver.
• Deliver the mix to extruder and start production.
• After first line is cast, mark product and cover with curing blankets.
• When curing is completed, test product for concrete strength. If concrete strength is at the required level start distressing operators.
• Distress the cables and start cutting operations on each pallet.
• Lift off product from pallets and transport to storage area.
• Repeat cycle on a continuous base.

Equipment and Construction Requirements

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

• Transfer of Documents.
• Design Drawings
• Process Sheets
• Tooling Documents
• Technical Standards
• Deputation of Engineering Specialists.
• 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 Precast & Pre-Stressed Concrete Elements within a period of 12 months as detailed below:

• 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 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
• To minimize the stripping operations to save labor and time.
• New methods of Compaction
• New techniques of Surface and needle vibrations. 
• Curing- 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 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 in order not to consume much time of trucks.  
• Erection, Jointing and Finishing 
• Minimum labour 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 withstand and absorb all types of loads and eddy effects
• Smooth finish and good look. 
• Safety certification- Process of inspection  
• Conducting of tests
• Safety measures
• Certification methods  
• Engineering 
• Engineering can be improved through Software.
• 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.
• Planning 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  

Buildability

Buildability is an important factor these days to be considered. The availability of resources such as, Skilled/ semi-skilled workers, Transport trailers, Erection cranes, Availability of working space at construction site, Approach roads to site, Traffic conditions at site location etc., will determine the construction methodology. The size and weight of the elements and reach of the handling cranes with truck parking space and crane location etc will determine the structural system and size of elements. Thus even before conceptualizing the project, unlike conventional construction sites, precast projects require construction engineering to be evaluated. Thus once the elements and Structural system are fixed, the structural modelling will be done.

Conclusion

Precast being emerging construction technology, the need for construction engineers, pre casters, architects and structural engineers to conduct project workshops and consider all aspects of buildability to satisfy the limitations of site conditions, transport equipment, handling equipment. The joint treatment and structural model to be considered shall be decided after elaborate planning of the type of elements, construction sequence, availability or viability of particular elements etc.

Author: P. Surya Prakash, Chief Consultant SatyaVani Projects and Consultants Pvt. Ltd., Hyderabad.