PRECAST CONCRETE CONSTRUCTION
In the mid-nineties, I was heading a civil design department for a large EPC contractor in South East Asia. We had received an order to build a paper plant.
The primary building in a paper plant is the paper machine building. A typical paper machine building is about 300 m long. The building typically has two floors, one at ground level, and one at about 7.5 m level. The paper machine is installed on a foundation that is not connected to the building. The machine is accessible from the machine hall at 7.50 m level. This building houses other complex and heavy machinery and has very stringent requirements with respect to quality, structural design and stability. The roof is high up and some of the sections of this building are subject to temperatures between 50 to 60 0 C. A large overhead crane straddles the upstairs machine hall. The differential settlement in the paper machine foundation has to be less than one mm and overall settlement at any point less than 1.25 mm. This building, with all its components and the equipment foundations, normally takes 18 months to build.
Our managing director was an innovative man and constantly sought ideas to speed up construction. One day, he called me to his office and showed me an article narrating about a company in the US that had developed techniques to build a paper machine building using pre cast elements. This paper machine building was completed in a record time of 6 months, said the article. We appointed the US company as our consultants and they did the engineering with the help of our engineers in our office. We built our paper machine building in a year cutting down the time by about six months. This was despite a delay of about three months due to the learning curve and the time required for setting up a precast plant.
Thus began my twenty two years long association with pre-cast concrete. My old company has built several large industrial plants and other structures since then.
In many first world countries pre cast elements for bridges, culverts have been standardized. Pre-casting units are located near major cities that supply these elements to the construction sites. This not only reduces the construction time but also the design time as one uses standard elements whose properties are known.
There are variations of the precast concrete construction such as tilt up construction, module fitments etc.
I have often wondered why India, with so much construction needed in the all the sectors of construction, has not embraced this technique. Apart from other issues like need for repetition, unfriendly taxation, requirement of transport or lifting machinery etc., I think our engineers have not given a serious thought to developing this technique.
I would like to share some of my learnings.
- Planning is Paramount: The structure to be built from precast elements has to be broken down in elements, in a pre-determined configuration. It is like making the pieces of a jigsaw puzzle that when put together will form the completed puzzle. It can be a combination of standard and non-standard pieces.
- God is in details: Each element thus planned has to be detailed out to fit all the elements on all its sides and the embedment required for utilities.
- Design the Construction and Construct the design: Normal structural engineering practice of designing the final product and leaving the “How?” to the construction personnel, does not work in precast. The structural engineer has to stay involved in the process of pre casting, erection and placement.
To the best of my knowledge, IS codes do not have specific provisions for pre cast structures unlike ACI or BS codes. Some of the clauses in ACI can be substituted by provisions in their supplementary publications. Such provisions have to be applied judiciously after a proper assessment of the stages in the service life of the element. A foremost expert on pre-casting once said “Applying provisions of R.C.C code to pre-casting would be like playing tennis with a baseball bat”
The structural design for a precast element is done for various stages of in its early life. Multiple level checks are required till the element is placed, more checks are required if it is a pre-stressed element with partial un-bonding of tendons.
- Joints can cause headaches: Resolving and configuring a joint between precast elements can be an arduous task. It becomes a heuristic process to balance between the structural requirement, functionality with respect to basic consideration as water tightness, and the size of the elements to which an element in consideration is attached. Joints have to be constructed the way they have been envisaged.
- Cutting off ears because they stick out, not only impairs hearing but also creates difficulty in wearing spectacles: This is known to occur frequently where architectural requirements are of primary importance. Typically some architects do not like some essential arrangements created for better joints. Doing away with these “hindering” details may lead to reduced functionality of the joints or the elements. Expensive alternate arrangements are required to restore functionality.
- Construction Methodology can make or break a project : Many years ago, a large bulk warehouse with pre-cast pre stressed concrete bow string girders as roof trusses was being constructed in India for a fertilizer plant. Out of twelve bowstring girders, six broke while being lifted while the others were erected smoothly. Designs were checked and double checked and checked again. This was before the easy availability of the sophisticated finite element analysis that we have today. It finally dawned on someone that the bow string girders broke because a girder while being lifted in tandem by two cranes, twisted out of plane due to different rates of lifting. A structural engineer designing precast elements should, therefore, have the knowledge of the lifting process.
- Quality is the watchword: Consistent Quality of production is one of the arguments put forward by the advocates of precast. But many a mismatches, rejections and failures have occurred due to watching only the quality of concrete and giving less importance to placement of reinforcement embeds and the dimensional tolerances.
- A one rupee increase in the production cost can mean a crore of rupees at the end: Due the repetitive nature of the cost of pre-casting a lot of thought has to be given to use any “nice to have” component. While the most obvious cost elements related to concrete are watched vigilantly, a small embed or a detail, that is incorporated in the design and casting of an element for a probable use, escapes attention. Such an embed that was proposed to be used and has been cast in the element has already added to the cost of producing the element. When a number of such elements are cast, the expenditure can be substantial. If such redundancy if not eliminated in time, it can waste lakhs of rupees.
Pre-casting has its advantages and disadvantages but till a superior technique is found, for fast repetitive construction, it remains the best solution.