Precast concrete construction
In the mid-nineties, I headed the civil engineering department of a large EPC contractor in Southeast Asia. We had an order to build a paper mill.
The main building in the paper mill is the paper machine building. A typical paper machine building is about 300 m long. The building usually has two floors, one at ground level and one at a level of about 7.5 m. The paper machine is mounted on a foundation that is not connected to the building. The machine is accessible from the machine hall at an elevation of 7.50 m. This building houses other complex and heavy machinery and has very strict requirements in terms of quality, structural design and stability. The roof is high and some of the sections of this building are subjected to temperatures between 50 to 60 0 C. A large overhead crane moves in the engine room on the upper floor. The differential settlement at the base of the paper machine should be less than one mm and the total settlement at any point less than 1.25 mm. The construction of this building, with all its components and the basics of the equipment, usually takes 18 months.
Our managing director was an innovative person and was constantly looking for ideas to speed up construction. One day he called me into his office and showed me an article about a company in the US that had developed techniques for building a paper machine using precast elements. This paper making machine was completed in a record time of 6 months, the article said. 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 building in a year, cutting the time by about six months. This happened despite a delay of about three months due to the learning curve and time required to set up a prefab plant.
Thus began my twenty-two year relationship with precast concrete. My old company has since built several large industrial plants and other structures.
In many first world countries precast elements for bridges, culverts are standardized. Precast units are located near major cities that supply these elements to construction sites. This not only reduces build time, but also design time, as standard elements whose properties are known are used.
There are variations of precast concrete construction, such as tilt-up construction, modular assembly, and more.
I have often wondered why India, with so much need for construction in all sectors of construction, has not embraced this technique. Apart from other problems such as need for repetition, unfavorable taxation, requirement for transportation or lifting machinery, etc., I think our engineers have not given serious thought to the development of this technique.
I would like to share some of my knowledge.
1. Planning is paramount: A structure to be built from prefabricated elements must be divided into elements in a predetermined configuration. It’s like making puzzle pieces 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: Every element so planned must be detailed to fit all the elements on all its sides and the built-in required for utilities.
3. Design the structure and construct the design: Normal civil engineering practice to design the final product and leave the “How?” of construction personnel, does not work in prefab. The structural engineer should continue to be involved in the pre-casting, assembly and placement process.
To my knowledge, the IS codes do not have specific provisions for precast structures, unlike the ACI or BS codes. Some of the clauses in the ACI may be superseded by provisions in their supplementary publications. Such provisions should be applied judiciously after a proper assessment of the life stages of the element. A leading pre-casting expert once said, “Applying the provisions of the RCC Code to pre-casting would be like playing tennis with a baseball bat.”
The structural design of a precast element is done for different stages of its early life. Repeated level checks are required until the member is in place, more checks are required if it is a prestressed member with partial tendon debonding.
4. Joints can cause headaches: Resolving and configuring joints between prefabs can be a difficult task. It becomes a heuristic process to balance between the structural requirement, the functionality in relation to the main consideration such as watertightness and the size of the elements to which the element in question is attached. Joints must be constructed as intended.
5. Cutting off the ears because they stick out not only impairs hearing but also creates difficulty in wearing glasses: This is known to happen frequently when architectural requirements are paramount. Usually, some architects don’t like some basic arrangements made for better joints. Removing these “obstructive” details can lead to reduced functionality of the joints or elements. Expensive alternative measures are required to restore functionality.
6. Construction methodology can make or break a project: Many years ago a large bulk warehouse with pre-cast pre-stressed concrete bow beams as roof trusses was being built in India for a fertilizer plant. Of twelve tendon beams, six broke when lifted, while the others straightened smoothly. The designs were checked and double checked and checked again. This was before the easy availability of the sophisticated finite element analysis we have today. It finally dawned on someone that the chord beams had broken because a beam, while being lifted in tandem by two cranes, had become out of plane due to different lifting speeds. Therefore, a structural engineer who designs precast elements must have knowledge of the lifting process.
7. Quality is the password: Consistent manufacturing quality is one of the arguments advanced by proponents of prefab construction. But many non-conformances, rejections and failures have occurred due to looking only at the quality of the concrete and giving less importance to reinforcement placement and dimensional tolerances.
8. A one rupee increase in production costs can mean a whole crore of rupees in the end: Due to the recurring nature of pre-casting costs, a lot of thought needs to be given to using any ‘nice to have’ component . While the most obvious consumables associated with concrete are closely watched, a small embedded part or detail that is incorporated into the design and casting of an item for likely use escapes notice. Such an inlay that is proposed to be used and cast into the element has already added to the cost of manufacturing the element. When several such elements are cast, the cost can be significant. If such cutback is not removed in time, it can waste thousands of rupees.
#Precast #concrete #construction