In the mid 1990s, I was running a civil design department for a large EPC contractor in Southeast Asia. We had received an order to build a paper mill.
The main building in a paper plant is the paper machine building. A typical paper machine building is about 300m long. The building usually has two floors, one at ground level and another at about 7.5 m above ground level. The paper machine is installed on a base that is not connected to the building. The machine can be accessed from the engine room at a level of 7.50 m. This building houses other heavy and complex machinery and has very strict requirements regarding quality, structural design and stability. The ceiling is high and some of the sections of this building are subject to temperatures between 50 and 60 0 C. A large overhead crane spans both sides of the engine room above. Differential settlement at the base of the paper machine should be less than 1mm and total settlement at any point less than 1.25mm. This building, with all its components and equipment foundations, normally takes 18 months to build.
Our managing director was an innovative man and was constantly looking for ideas to speed up construction. One day, he called me into his office and showed me an article that told about a company in the US that had developed techniques to build a paper machine building using prefabricated elements. The construction of this paper machine was completed in a record time of 6 months, the article says. 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 one year, cutting time by about six months. This was despite a delay of about three months due to the learning curve and time required to set up a precast plant.
Thus began my twenty two year association with precast concrete. My former company has built several large industrial plants and other structures since then.
In many first world countries, precast elements for bridges and culverts have been standardized. Precast units are located near major cities that supply these items to construction sites. This not only reduces construction time, but also design time, since standard elements are used whose properties are known.
There are variations of precast concrete construction such as sloped up construction, module adjustments, etc.
I have often wondered why India, with such a need for construction across all construction sectors, has not embraced this technique. Apart from other issues like the need for repetition, hostile taxes, transportation requirements or lifting machinery, etc., I think that our engineers have not given serious thought to developing this technique.
I would like to share some of my learnings.
1. Planning is paramount: The structure to be built from precast elements must be divided into elements, in a predetermined configuration. It’s like making puzzle pieces that, when put together, make the whole puzzle. It can be a combination of standard and non-standard parts.
2. God is in the details: Each element thus projected has to be detailed to fit all the elements on all sides and the embedment required for utilities.
3. Design Construction and Build Design: Normal structural engineering practice of designing the final product and leaving the “How?” to personal construction, does not work in prefabricated. The structural engineer must participate in the process of prefabrication, assembly and placement.
To the best of my knowledge, the IS codes do not have specific provisions for precast structures unlike the ACI or BS codes. Some of ACI’s clauses may be superseded by provisions in its supplementary publications. Such provisions should be applied judiciously after a proper assessment of the life stages of the item. A leading precast expert once said, “Applying the provisions of the RCC code to precast would be like playing tennis with a baseball bat.”
The structural design of a precast element is done at various stages of its initial life. Checks of several levels are required until the element is placed, more checks are required if it is a prestressed element with partial detachment of the tendons.
4. Seams Can Cause Headaches: Solving and setting a seam between precast elements can be a chore. It becomes a heuristic process to balance between the structural requirement, the functionality with respect to the basic consideration such as watertightness, and the size of the elements to which an element under consideration is attached. The joints must be constructed in the way they have been considered.
5. Cropping ears because they stick out not only impairs hearing but also makes it more difficult to wear glasses: This is known to occur frequently where architectural requirements are paramount. Usually, some architects do not like some essential arrangements created for better joints. Removal of these “disturbing” details can result in reduced functionality of joints or elements. Expensive workarounds are required to restore functionality.
6. Construction methodology can make or break a project: Many years ago, a large bulk warehouse was being built in India using precast pre-stressed concrete beams as roof beams for a fertilizer plant. Out of twelve bowstring girders, six broke while being erected while the others erected without issue. The designs were reviewed and revised twice and revised again. This was before the easy availability of the sophisticated finite element analysis that we have today. Finally, someone realized that the bowstring beams broke because a beam while being lifted in tandem by two cranes, twisted out of plane due to different lifting speeds. Therefore, a structural engineer designing precast elements must have knowledge of the lifting process.
7. Quality is the watchword: constant production quality is one of the arguments put forward by precast defenders. But many mismatches, rejects and failures have occurred due to looking only at the quality of the concrete and giving less importance to reinforcing embedment placement and dimensional tolerances.
8. A one rupee increase in production cost can mean crores of rupees in the end – Due to the repetitive nature of the prefab cost, a lot of thought must go into using any ‘nice to have’ components. While the more obvious cost items associated with concrete are closely watched, a small embed or detail that is incorporated into the design and casting of an item for likely use escapes notice. Such an inlay that was intended to be used and has been cast into the item has already added to the cost of producing the item. When several such items are released, the expense can be substantial. If such redundancy is not removed in time, you can waste thousands of rupees.