Precast concrete construction

In the mid-1990s, I was running a civil design department for a large EPC contractor in Southeast Asia. We received an order to build a paper plant.

The main building of a paper plant is the paper machine building. The typical paper machine building is about 300 m long. The building normally has two floors, one at ground level and the other at about 7.5 m above sea level. The paper machine is installed on a base that is not connected to the building. The machine is accessible from the machine room at a level of 7.50 m. This building houses other complex and heavy machinery and has very strict requirements for quality, structural design and stability. The ceiling is high and some of the sections of this building are subjected to temperatures between 50 and 60 ° C. A large bridge crane runs the length of the engine room upstairs. 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 the foundations of the equipment, usually 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 recounted a company in the United States that had developed techniques to build a paper machine using precast 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 one year reducing the time by approximately six months. This was despite a delay of about three months due to the learning curve and the time required to install 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 precast elements for bridges, culverts, have been standardized. Precast units are located near major cities that supply these elements 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 sloped construction, module accessories, etc.

I have often wondered why India, with so much construction needed in all construction sectors, has not embraced this technique. Apart from other issues such as the need for repetition, unfavorable taxes, transport requirements or lifting machinery, etc., I think our engineers have not seriously thought about 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 broken down into elements, in a predetermined configuration. It is like making the pieces of a puzzle that when put together will form the complete puzzle. It can be a combination of standard and non-standard parts.

2. God is in the details: Each element so planned has to be detailed to fit all the elements on all sides and the required inlay for public services.

3. Design the Construction and Build the design: Normal structural engineering practice of designing the final product and leaving the “How?” construction personnel, does not work in precast. The structural engineer must be involved in the precast, assembly, and placement process.

As far as I know, the IS codes do not have specific provisions for precast structures unlike the ACI or BS codes. Some of the clauses of ACI may be replaced by provisions of its supplementary publications. Such provisions should be applied prudently after proper evaluation of the life stages of the item. A leading pre-casting expert once said, “Applying the provisions of the RCC code to pre-pitch would be like playing tennis with a baseball bat.”

The structural design of a precast element is done for various stages of its early life. Multiple level controls are required until the element is placed, more controls are required if it is a prestressed element with partial disunity of the tendons.

4. Joints can cause headaches: Solving and setting up a joint between precast elements can be a daunting task. It becomes a heuristic process to balance structural requirement, functionality against basic consideration such as watertightness, and size of elements to which an element is attached under consideration. Joints must be constructed as intended.

5. Cutting your ears off because they stick out not only damages hearing, but also creates difficulties in wearing glasses – this has been known to occur frequently when architectural requirements are of prime importance. Some architects generally dislike some essential arrangements created for better joints. Removing these “hindering” details can lead to a reduction in the 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 was being built in India with prestressed precast concrete arch beams as roof trusses for a fertilizer plant. Of the twelve bowstring girders, six broke while being lifted, while the others were erected without a hitch. The designs were double-checked and double-checked and re-checked. This was before the easy availability of the sophisticated finite element analysis that we have today. Finally, someone realized that the bowstring girders broke because a girder, being lifted in tandem by two cranes, went out of plane due to different lifting speeds. A structural engineer designing precast elements must, therefore, 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 precast elements. But there have been many misalignments, rejects and failures because only the quality of the concrete is observed and less importance is given to the placement of reinforcement embedments and dimensional tolerances.

8. A one rupee increase in production cost can mean one rupee rupee in the end: Due to the repetitive nature of precast cost, a lot of thought must be given to using any “nice to have” component. While looking closely at the most obvious cost elements related to concrete, a small inlay or detail, which is incorporated into the design and casting of an element for probable use, escapes attention. Such inlay that was proposed to be used and has melted into the item has already added to the cost of producing the item. When multiple of these items are thrown in, 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 *