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Precast Concrete Construction
In the mid-nineties, I was heading the civil design department for a large EPC contractor in Southeast Asia. We got 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 meters long. The building usually has two floors, one at ground level, and the other at a level of about 7.5 meters. The paper machine is installed on a base not connected to the building. The machine is accessible from the hall at a level of 7.50 m. This building contains other complex and heavy machinery and has very strict requirements in terms of quality, structural design and stability. The roofs are high and some sections of this building are subjected to temperatures ranging from 50 to 60 degrees Celsius. A large overhead crane swings the machine hall upstairs. The paper machine foundation shall have a differential settlement of less than one mm and an overall settlement of less than 1.25 mm at any point. The building, including all its components and equipment bases, typically takes 18 months to construct.
Our managing director was an innovative person and constantly searched for ideas to speed up construction. One day, he called me into his office and showed me an article about a company in America that had developed paper machine building techniques using pre-cast elements. It is mentioned in the article that the construction of this paper machine was completed in a record time of 6 months. We hired an American company as our consultant and they did the engineering with the help of our engineers at our office. We built our paper machine building in about six months over a year. This was despite a delay of about three months due to the learning curve and the time required to set up the precast plant.
Thus began my twenty-two year long relationship with pre-cast concrete. My old company has built many large industrial plants and other structures since then.
Pre-cast elements have been standardized for bridges, culverts in many first world countries. Pre-casting units are located near major cities that supply these elements to construction sites. This not only reduces the construction time but also reduces the design time as one uses standard elements whose properties are known.
There are variations of precast concrete construction such as tilt up construction, module fitments etc.
I have often asked why India has not adopted this technology when so much construction is required in all areas of construction. Apart from other issues like need for repetition, friendly taxes, transportation or lifting machinery etc., I think our engineers have not seriously thought of developing this technology.
I would like to share some of my learnings.
1. Planning is paramount: A structure to be built from predetermined elements must be broken down into elements in a predetermined configuration. It’s like making pieces of a jigsaw puzzle that when put together will form a complete puzzle. It can be a combination of standard and non-standard pieces.
2. God is in the details: Every element thus planned must be elaborated in all its aspects to fit the necessary embedments for all elements and utilities.
3. Design Build and Design Build: Designing the final product and the “how?” General structural engineering practice to drop. To construction workers, precast does not work. A structural engineer should be involved in the process of pre-casting, erection and placement.
As per my knowledge, IS codes do not have specific provisions for precast structures unlike ACI or BS codes. Some clauses in the ACI may be superseded by provisions in their supplementary publications. Such provisions should be applied judiciously after a proper evaluation of the stages in the service life of the element. A leading expert on pre-casting once said “applying the provisions of the RCC Code to pre-casting would be like playing tennis with a baseball bat”.
Structural design for a precast element is done at various stages of its early life. Several levels of checks are required until the element is placed, if it is a pre-stressed element with partial un-bonding of the tendon, further checks are required.
4. Joints can cause headaches: Fixing and configuring joints between precast elements can be a daunting task. It is a heuristic process to balance between structural requirements, functionality with respect to basic considerations such as water tightness, and the size of the elements to which the element under consideration is attached. Joints should be constructed as they are designed.
5. As the ears stick out, cutting not only disturbs the ear but also creates difficulty in wearing spectacles: this is often the case where architectural requirements are of primary importance. Generally some architects do not like some of the necessary arrangements created for good joints. Removing these “obstructing” details can reduce the functionality of joints or elements. An expensive alternative arrangement is required to restore functionality.
6. Construction method can make or break a project: Many years ago, large bulk warehouses were being constructed with pre-cast pre-stressed concrete bow string girders as roof trusses for a fertilizer plant in India. Six of the twelve bowstring girders were broken during lifting, while the others were erected smoothly. Designs are checked and double checked and checked again. This was before the easy availability of the sophisticated finite element analysis we have today. Eventually someone realized that bow string girders broke because one girder, when lifted in tandem by two cranes, moved out of plane due to different rates of lifting. A structural engineer designing precast elements must, therefore, have knowledge of the lifting process.
7. Quality is the watchword: Consistent product quality is one of the arguments put forward by advocates of precast. But looking only at the quality of the concrete and giving less importance to the placement of reinforcement embeds and dimensional tolerances has led to many mismatches, rejections and failures.
8. A rupee increase in production cost could end up being a crore rupees: Because of the recurring nature of the cost of pre-casting, using any “well done” component requires a lot of thought. While the most obvious cost elements related to concrete are carefully looked at, a small embed or detail, which is included in the design and casting of an element for possible use, remains unnoticed. Such an embed that was proposed to be used and cast into the element is already added to the element production cost. When many such elements are cast, the expense can be substantial. If such redundancies are not removed in time, lakhs of rupees can be wasted.
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