Sustainable Precast Concrete Practices In The Hydrocarbon Sector

 

Madhavi Latha, Senior Manager, Structural Department, Engineers India Limited, New Delhi

Papia Mandal, Deputy General Manager, Structural Department, Engineers India Limited, New Delhi

Anurag Sinha, Chief General Manager, Structural Department, Engineers India Limited, New Delhi

If the current trends are anything to go by, fast-track execution and implementation is the need of the hour in every field. The Hydrocarbon sector is no different, as the demands of this sector are becoming increasingly dynamic by the day. The conventional practices of cast-in-situ construction that have been witnessed in the past have now become a bottleneck, posing hindrances while observing a stringent project schedule. Sustainability applies to not only the existing competitive business scenario, but also while ensuring continuity in terms of viable and environment friendly construction methods. Precast concrete technology may be one of the best solutions for the same. Even though there are some criticalities involved in adopting a new methodology, such as precast concrete in terms of its execution, joint detailing and expertise of the design engineer is the best solution for its varied advantages like faster construction, assured quality, cleaner and safer environment. This paper is a case study of adopting precast concrete practices in piperack construction, executed in one of the Oil Refinery projects in North India in the recent past.

 

Introduction

As witnessed in the last few decades, two main methods were adopted in the construction industry, viz. Method 1 - RCC cast-in-situ Construction and Method 2 – Structural Steel construction. While the former has an advantage of being a cost-effective solution despite its messy and time-consuming construction, the latter demands substantial funds, though it is construction friendly and encourages fast execution. By overcoming the shortfalls of both the aforementioned methods together, precast concrete technology is one of the best solutions in the construction world for achieving economical and faster execution. Additionally, precast concrete construction aids in providing a cleaner and safer environment at site and also the same has been widely acknowledged by the owners/clients. The basic design concepts of precast elements are similar to those of well-established cast-in-situ concrete elements, which serve as a better alternate for designers.

 

In a large-scale project, such as refinery constructions and expansions, meeting the schedule is of utmost importance. Completion of civil works in the stipulated time is a major contributing factor in ensuring that the schedule is being complied. In structures that require huge quantum of concrete, trimming down the construction time of the same would definitely be advantageous for downstream activities. This need for cutting down on concrete construction time, triggered the idea of adopting precast construction in the project.

 

                                              

1: Precast RCC Trenches                                       Fig. 2: Precast RCC Pipe Sleepers
(Proform Construction Fig Products                        (Century group-Pipeline/sleepers)
: Calgary, Alberta Branch)

 

With this purpose in mind, precast concrete construction was adopted in one of the Oil Refinery projects in North India. Structures like piperack in offsites as well as units, pipe sleepers, pipe way bridges, box culverts, trenches, manholes, etc. were constructed using precast concrete technology. Fig. 1 to Fig. 4 show the adoption of precast construction for RCC trenches, pipe sleepers, box culverts and piperacks respectively in refineries.

This paper is a case study of the precast concrete piperack, starting from conceptualisation to execution, along with examination of all the advantages and limitations encountered at different stages of the project.

 

                    

Fig. 3: Precast RCC Box Culverts                Fig. 4: Precast Piperack
(stanton-bonna.co.uk/drainage-
systems/box-culverts)

 

Conceptualisation

Selection Of Material

In refineries, constructability is one of the major aspects in deciding the material of construction. Construction of piperacks was preferred to be done in RCC, instead of steel due to following reasons:

  • high cost of steel in comparison to concrete
  • extra time required for procurement of steel
  • corrosion of steel
  • steel being more susceptible to fire hazards demanding fireproofing measure and additional time/cost thereof.

 

In addition, precast technology was adopted for the construction of RCC piperack instead of cast-in-situ concrete because the latter requires:

  • extensive labour
  • elaborate shuttering arrangements, which are time consuming
  • pouring concrete at elevated heights, which is highly risky
  • honeycombing at the joints due to improper compaction
  • requirement of concrete setting time does not allow for simultaneous construction activities

 

Following are the additional advantages in adopting precast concrete technology:

  • faster construction
  • sustainable technology
  • lower material wastage
  • cleaner and safe environment at job site
  • reduced noise and air pollution at site
  • assured quality

 

Geometry

Geometry of the Piperack structure is mentioned in Table 1 and illustrated in Fig. 5 and Fig. 6.

 

 Fig. 5: Typical Cross Section                     Fig. 6: Longitudinal Elevation of Piperack
of Piperack                   

 

In longitudinal direction, two braced bays were considered, as shown in Fig. 6. Piperack was designed as a moment resisting frame/rigid frame in transverse direction and as a braced frame in the longitudinal direction.

 

Design Concept

Design of the above precast concrete piperack was envisaged and planned such that the construction, erection and assembling of the precast elements would be facile. The concept adopted for precast piperack, with this purpose in mind is described in this section.

 

Precast pipe rack construction methodology used in the project involved casting frames in a construction yard equipped with latest machinery under strict quality controlled conditions, transporting them to site, followed by assembly of components and erection of the frames thereafter with anchoring arrangement.

 

However, to enhance productivity, activities such as excavation, levelling, etc., including cast-in-situ foundations with columns (up to 2 m height above HPP level) were executed simultaneously, which brought down the construction time for the entire structure monumentally.

 

Main pipe tier beams of portal frame were planned in precast. So, precast H-sections were adopted as shown in the Fig. 7, with column, portal beam and corbels (in the longitudinal direction for supporting longitudinal beam). Further, the joints between column and beam were of rigid type (Fig. 8).

 

Longitudinal beams were also considered in precast and were supported on the corbels extended from the columns of precast H-section. The supporting conditions of these beams were designed as simply supported.

 

Vertical bracings of structural steel were provided in the longitudinal direction (above first level of longitudinal beam), as shown in Fig. 9, to ensure stability of the structure against lateral loading, such as wind, seismic forces, etc. Even though, the first level longitudinal beams were supported on columns through steel brackets, rigidity of the frame at this level was ensured through additional anchorage of reinforcing bars (Fig. 10). This arrangement of frame at first level of longitudinal beams was adopted to facilitate the proper clearance of pumps, along with its piping underneath.

 

Fig. 7: 3D View of Precast                         Fig. 8: Elevation View of 
Piperack                                                      H-Section/Precast Frame      

 

                            

Fig. 9: Typical View Showing Braced Bay                 Fig. 10: Detail of Connection between Column       
Arrangement in Precast Piperack                                  and Longitudinal Beam at First Level      

 

Structural steel sections were used for supporting grating, instrumentation duct, electrical tree and walkway, which were then connected to the RCC beams by means of insert plates.

 

Joint Detailing

Joint detailing of concrete members is a key concern in precast construction, as the stability of the entire structure majorly depends on the integrity of the connections, especially those between beams and columns. The design and planning of these joints warrant careful attention, so that loads are transferred safely, ensuring the stability and integrity of the structure. It is also important to bear in mind the envisaged erection and construction methodology while developing and designing the joint details.

 

Majorly, there are four types of joints in a precast piperack. Following are the details of these joints:

  • The first type of joint is the one between the cast-in-situ column and precast frame, as shown in Fig. 11. In the main portal frame, joints are planned in the columns at the points of contraflexure, i.e., the points where bending moment is zero. The joints, thus planned, are designed for transferring shear forces and axial forces only. In this type of joint, a shear key is provided on top of cast-in-situ column for transferring the shear forces from precast portal frame to cast-in-situ column. Thus, 6 nos. of 32 mm diameter dowel bars were provided in the cast-in-situ column to transfer axial forces and were inserted in the grout holes provided in the precast frame. This was followed by filling the holes with free-flow non-shrink grout. Grouting needs to be done with care to ensure that there is no air entrapment inside the grout.

 

 

        

Fig. 11: Detail of Connection between  Cast-in-situ Column and Precast Frame

 

  • The second type of joint is the one between two precast frames (Fig. 12). In this joint also, shear key is provided on top of the lower precast frame for transferring the shear forces from one portal frame to the other. Also, 6 nos. of grout holes were provided in the precast frame. During erection, 28 mm diameter rebars were inserted in these grout holes and these were then fixed to the precast frame by the means of grout.

 

Fig. 12: Detail of Connection between two Precast Frames

  • The third type of joint is the one between the precast longitudinal beam and the corbel provided in the column of precast frame. Precast longitudinal beams were designed as simply supported beams resting on the corbels. These were fixed to the corbels by the means of 24 mm diameter bars (Fig. 13). Only axial forces from longitudinal beam were transferred to the column through these bars.

 

   Fig. 13: Detail of Joint between Longitudinal Beam and Corbel

 

The fourth type of joint is the connection of vertical steel bracing member to precast longitudinal beams and columns, as shown in Fig. 14. Refer detail-Cof Fig. 15; bracing members were connected to the precast beams through gusset plates and 30 mm diameter through-type bolts provided on side faces of the beam. At the other end, bracing members were connected to the precast beam and column/corbel through gusset plate and bolts, as shown in detail-Bof Fig. 14. With this detailing in place, the axial force from the bracing member was transferred to the column (through the corbel) and precast longitudinal beam through the bolts.

 

Fig. 14: Elevation of Bracing Bay of Piperack

 

Lifting Analysis

A proper lifting scheme and its analysis are required before implementation at site to ensure quality and safe installation of precast elements. These elements need to be checked for their adequacy in handling stresses and impact loads encountered while lifting.

Lifting of precast frame may be of various types, such as:

  • Through column
  • Through beam without additional reinforcement
  • Through beam with additional reinforcement

 

             

Fig. 15: Detail of Connection between Logitudinal Precast Beam and Steel Bracing Member

Fig. 16: Detail of Connection between Steel Bracing Member and Corbel/Logitudinal Precast Beam

 

Through Column 

While lifting is done through column, additional force generated due to lifting is primarily the axial forces, which is in sync with the intended force to be carried by the column, and is only a fraction of the design capacity. Therefore, additional reinforcement is not required for the purpose of lifting. Hence, this is one of the preferable methods of lifting. However, when the span of the beam between two columns of H-section becomes large, this method is not recommended, so as to avoid substantial bending of the beam during lifting, which, in turn, demands intermediate lifting points in the beam.

 

Through Beam Without Additional Reinforcement

In this method of lifting, sleeves are provided at the centre of the beam cross section, where slings are attached for lifting. Proper planning of lifting/sleeve location is important to ensure the behaviour of the member in bending during lifting is a match with bending pattern under operational cases, and moments generated in a beam during lifting are less than the design bending moment capacity of beam. Due care has to be taken to avoid local break out of concrete in shear arising from forces generated through slings. This method of lifting is also one of the preferable methods and was adopted in the present case study (Fig. 17).

 

    Fig. 17: Erection Scheme of Precast Beam

 

Through Beam With Additional Reinforcement

Extra reinforcement in the form of lifting hooks is provided in this method. Additional reinforcement for local shear and bending due to lifting needs to be provided. This

method is highly recommended when the behaviour of the beam during lifting is predicted to be different from the beam in normal operating conditions (Fig. 18).

 

Fig. 18: Erection Scheme of Precast Beam with Lifting Hooks

 

Erection

Erection of precast elements is a stupendous event, requiring a very careful choice of the appropriate lifting devices, equipment, personnel, safety measures and rigorous planning to ensure an efficient and good-quality installation, and to fully reap the benefits of precast construction. The following are the key aspects to be taken care of while planning the erection of a precast structure:

  • erection of precast elements at the exact locations, with the correct levels
  • ensuring alignment with necessary grouting/castings at all the intersections of the precast members
  • a clear plan in place, with details about the exact sequence of construction and the methodology of integration of elements, supporting systems, lifting arrangements, etc.

 

Schedule Impact

Precast construction methodology results in faster construction due to reduced in-situ shuttering, reinforcement binding and concreting activities. Furthermore, simultaneous construction activities can be planned; for example, casting of foundation and precast column/beam elements can be done at the same time.

Detailed data regarding construction time needed in case of cast-in-situ construction vis-à-vis precast construction are presented in various technical papers available in public domain. One such set of data has been reproduced in Table 2 for a better understanding of the subject.

Calculation Of Construction Time Taken For Single Bay Of  Cast-in-situ Piperack

Cast-in-situ construction is carried out in the following stages as mentioned in Table 2 (Alam, Choudhary and Sehdev, 2015).

 

Hence, as observed from the table, 48-50 days are required to execute the casting of one bay of cast-in-situ piperack, and standard construction practices are to be followed.

 

Calculation Of Construction Time To Be Taken For Single Bay Of Precast Piperack

Precast concrete construction is carried out in two separate stages. In the first stage, casting of frames is done at the yard, followed by the erection of those frames at site.

 

Fig. 19: 3D View of Single Bay of Precast Piperack


As observed, on an average, it takes 13 days to complete the first stage successfully.

 

 

As observed, on an average, 10 days are required to complete the second stage.

 

Totally, on an average, 23 days are taken to complete the construction of precast anchor bay. It is therefore, evident from the above data that approximately, 50% time can be saved by adopting precast construction methodology, in general.

 

Cost Impact

Economic factors play an important role in the choice of a suitable construction methodology. As the precast elements were self-supporting, shuttering and scaffolding were not needed, which on a large scale, resulted in a reduced shuttering cost. It was also observed that the labour required to carry out the erection was much less. Apart from direct cost saving, there was a considerable saving on indirect costs as well, owing to results being achieved within schedule. This was because of the following:

  • as concrete is already set at the time of erection, activities such as pipe erection, etc. can be simultaneously carried out by other contractors.
  • delays are eliminated in activities down the line, which avoid any extra claim
  • scheduled commissioning of plant and ensuring timely revenue generation is a great incentive

 

It can be deduced from the data of cost savings, taken from various technical papers on cost analysis of cast-in-situ vs. precast concrete available in public domain, that approximately around
20% cost reduction is achieved by adopting precast technology (More and Patil, 2017).

 

Sustainability Aspects

A big part of overall sustainability includes the long-established principles of reduce, reuse and recycle. Precast concrete technology perfectly encapsulates sustainability. It is environmentally sound, economical to use and contributes to social responsibility.

 

The few reasons for which precast technology can be defined as a sustainable practice include - in cast-in-situ construction as lot of timber was required for preparing the shuttering of the beams and columns; whereas in precast construction, steel moulds were used for casting of precast elements at construction yard. This saved a lot wood work and indirectly trees, which are much essential for maintaining a safe environment. Further, as the construction of precast elements is done in a controlled environment, water consumption for curing of precast elements reduces.

 

Conclusion

This case study is a comparison of the savings in direct and indirect costs, as well as the time required for execution using precast piperack construction vs. those required for conventional cast in-situ construction. The practical aspects, for example, joint detailing, lifting methodology, etc., presented herein can form a basis and guide for practicing structural engineers who intend to choose precast construction over cast-in-situ construction of structures. Major findings may be summarised as follows:

  1. Sustainability of precast concrete method lies in clean and environment friendly construction. Further, due to the absence of batching plant and concreting work in the vicinity of the site, pollution and health hazards of the site personnel can be avoided.
  2. Erection of precast concrete structures facilitates simultaneous mobilisation of various contractors, otherwise not possible in the case of cast-in-situ construction, owing to the time taken by concrete to harden and the time required for associated activities, such as removing of shuttering, etc.
  3. Quality is assured due to execution of concreting work in controlled environment using latest machineries and under supervision of qualified personnel.
  4. Substantial reduction in time related to site execution, leading to accomplishment of activities well ahead of schedule. As per the current case study, around 50% reduction in construction work schedule has been achieved.
  5. Direct cost saving on account of labour, shuttering, fireproofing, protective coating and painting, etc., is of considerable amount. Also, there is a huge amount of indirect cost saving by eliminating any sort of slippage in the schedule and timely revenue generation thereof. Approximately, 20% cost saving has been attained as per the current case study.

 

Although the study limits itself to discussing the precast concrete practices adopted in the construction of only one type of structure, viz. piperack structure, the practices seem to be immensely promising and thus, can be easily extrapolated to other areas of structures, such as pipe sleepers, trenches, box culverts, manholes, etc., as well as both inside and outside the Hydrocarbon sector. However, the specific requirements and demands of various industries need to be kept in mind while adopting these precast concrete construction practices.

 

References

  1. Alam, K. M., Choudhary, V., & Sehdev, S. (2015). A Case Study on Acceptability of Precast Concrete. Journal of Civil Engineering and Environmental Technology, 2(15), 16-19.
  2. More, S. A. & Patil, A. V. (2017). Time, Cost, Productivity and Quality Analysis of Precast Buildings. International Research Journal of Engineering and Technology, 4(11).