Real Estate

Precast concrete construction

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 one at about 7.5 m high. The paper machine is installed on a base that is not connected to the building. The machine can be accessed from the machine 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 bridge crane straddles the machine room above. The differential settlement at the base of the paper machine must be less than one mm and the total settlement at any point less than 1.25 mm. This building, with all its components and equipment foundations, typically 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 American 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 the time by about six months. This was despite a delay of about three months due to the learning curve and the time required to set up a precast plant.

Thus began my twenty-two year association with precast concrete. My old company has built several large industrial plants and other structures since then.

In many first world countries, prefabricated 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 whose properties are known are used.

There are variations of precast concrete construction, such as sloping-up construction, module fits, etc.

I have often wondered why India, with so much need for construction in all construction sectors, has not adopted this technique. Apart from other problems such as the need for repetition, hostile taxes, transportation requirements or lifting machinery, etc., I believe 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 prefabricated elements must be divided into elements, in a predetermined configuration. It’s like making the pieces of a puzzle that, when put together, make up the whole puzzle. It can be a combination of standard and non-standard parts.

2. God is in the details: Each element thus designed has to be detailed to fit all the elements on all sides and the embedment required for utilities.

3. Design the Construction and Build the 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 my knowledge the IS codes do not have specific provisions for prefabricated structures unlike the ACI or BS codes. Some of the ACI clauses may be superseded by provisions in its companion publications. Such provisions must be applied judiciously after a proper assessment of the life stages of the item. A leading prefabrication expert once said, “Applying the provisions of the RCC code to prefabrication would be like playing tennis with a baseball bat.”

The structural design of a prefabricated element is carried out at various stages of its initial life. Multi-level checks are required until the element is placed, more checks are required if it is a prestressed element with partial tendon detachment.

4. Joints can cause headaches: Troubleshooting and configuring a joint between precast elements can be a daunting task. It becomes a heuristic process to balance between structural requirement, functionality against basic consideration like watertightness, and size of elements to which an element under consideration is attached. The joints must be constructed in the manner in which they have been considered.

5. Cropping ears because they protrude not only impairs hearing but also makes it difficult to wear glasses – this is known to occur frequently when architectural requirements are paramount. Usually, some architects don’t like some essential fixes created for better joints. Removing these “obtrusive” details can lead to reduced functionality of the joints or elements. Costly workarounds are required to restore functionality.

6. Construction methodology can make or break a project: Many years ago, a large bulk warehouse using precast prestressed concrete beams as roof beams for a fertilizer plant was being built in India. Out of twelve bowstring beams, six broke while being erected while the others were erected without issue. The designs were revised and revised twice and revised again. This was before the easy availability of sophisticated finite element analysis that we have today. Finally, someone realized that the bowstring beams broke because one 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: the constant quality of production is one of the arguments put forward by the defenders of prefabricated products. But many mismatches, rejects and failures have occurred because only the quality of the concrete is looked at and less importance is placed on the placement of reinforcing embedments 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 pre-fab cost, a lot of thought needs to be put into using any ‘nice to have’ component. While the more obvious cost items related to concrete are closely watched, a small inlay or detail that goes into the design and casting of an item for a likely use escapes attention. Such an inlay that was intended to be used and cast into the element has already added to the cost of producing the element. When multiple such items are released, the expense can be substantial. If such redundancy is not removed in time, you can waste thousands of rupees.

Leave a Reply

Your email address will not be published. Required fields are marked *