An attempt is made to develop a systematic approach for estimating the quantity of concrete and optimisation of concrete cost based on a case study. The article will be published in two parts. The present article (Part 1) discusses about the Metro Projects in brief, construction methodology adopted, estimation of concrete quantity and associated cost. The next article (Part 2) will be discussing about the methodologies used for optimising the cost associated with production, transportation and placement of concrete.
In India, for infrastructure projects, the concrete is used as one of the essential construction materials. Appropriate selection of concrete (type/grade) and precise estimation of concrete quantity are essential to achieve optimised cost of concrete. Further, optimisation of construction cost related to concrete could also be achieved by appropriate selection of construction methodology along with equipment selection (rental/purchase/outsourcing) suitable for site conditions. various equipment are required for the production, transportation and placing of concrete. The selection and usage of these equipment are crucial for the successful completion of project work. Generally, the selected equipment shall fulfill project requirements within the timeframe.
In this regard, comprehensive analysis on selection of equipment and their usage in construction site needs to be carried out by the contractor well in advance (i.e bidding stage). If equipment planned to use in construction site is inadequate, it will be difficult to follow the schedule of fast track infrastructure project. On the other hand, to reduce the time period of construction, the plan for excess usage of equipment, may not be financially beneficial for the project. Hence, there is need to develop a systematic method for for optimising the cost of concreting by accurate estimation of concrete quantity and effective decision on selection of appropriate construction systems. The method was developed based on a case study (construction of metro project in Mumbai).
Mumbai Metro project
The master plan for metro project in Mumbai includes nine corridors covering a length of 172 km, out of which 32.50 km is proposed underground and the rest is elevated. First 11.40 km long elevated metro corridor between Versova – Andheri – Ghatkopar (Line 1) is commissioned in June 2014. Elevated corridors between Andheri (East) to Dahisar (East) – 16.475 km (Line 7), Dahisar to DN Nagar – 18.589 km (Line 2A) and underground metro between Colaba – Bandra – SEEPZ is under construction. Construction of elevated corridor between DN Nagar to Mandale (Line 2B) is about to start. After implementation of the Mumbai metro master plan, 70 lakh commuters are expected to get benefit, in turn, will reduce the traffic on roads and congestion in suburban rails.
1.1. The metro line 2A is selected as a case study for the present article. Brief scope of elevated metro project is as follows:
Viaduct: Design and construction of around 18 km elevated viaduct including viaduct and ramp for depot entry.
Stations: Design of 16 elevated stations (excluding architectural finishing and pre-engineered steel roof
structure).
Construction methodology: Construction methodology of a metro project is explained in this section. Civil construction activities includes mainly two pases, i.e., construction along the alignment and construction of precast elements at casting yard.
Construction along the alignment (route of metro line): Key construction activities along the alignment are as follows:
Barricading is provided to enclose the construction area to ensure safety and coordinated movement of vehicular and pedestrian traffic (Fig. 1).
Pile locations are accurately marked on ground by using the total station. Pilot trenches are made to check for any infringing underground utilities. If utilities are found, they are shifted for hassle free construction. To initiate the piling work, hydraulic rotary piling rig is positioned at piling location for boring of the pile (Fig. 2). While boring, temporary casing is provided up to required depth depending on the ground strata. After completion of boring, reinforcement cage is lowered. For casting of pile, the tremie pipe is lowered and concrete is poured up to required level through the tremie pipe.
After completion of group of piles for one pier, excavation of the pile cap is taken up. After laying PCC, the reinforcement cage is tied and concreting of pile cap is done.
After tying reinforcement for pier, the starter is cast. After casting the starter, balance formwork is erected for casting pier. Concrete will be placed up to bottom of pier cap using the truck mounted boom placer (Fig. 3 and Fig. 4). At the end, anti-crash barrier is cast.
Precast pier cap from casting yard is transported to the site and erected on pier by using the crane (Fig. 5 and Fig. 6).
"Stitch concrete" (Fig. 7) of pier cap (junction of pier reinforcement and pier cap reinforcement) is done by using the crane and bucket.
First stage stressing of pier cap is done. Bearing pedestals are cast using crane and bucket. For curing of bearing pedestals, curing compound can be used.
Precast "U" girder from the casting yard is transported to the site using multi-axle hydraulic trailer. The girder is placed on bearing pedestal using high capacity cranes (Fig. 8).
After erection of girders in the adjoining spans, second stage stressing of pier cap is done.
Construction of station building by combination of precast and cast in-situ concrete is done at each station location (Fig. 9).
Casting yard: Casting yard is mainly utilised for casting of the precast elements (pier caps, "U" girder, "I" girders, etc.), which are transported to the desired location along alignment for erection. Key construction activities at the casting yard are as follows:
Development of infrastructure at casting yard is very important activity. This includes the RMC plant installation, stacking of materials for RMC plant, construction of casting beds for precast piers, "U: girders and "I" girders, arrangement for stressing activity at each casting bed, stacking beds for casted precast elements and shed / gantry for handling formwork and casted precast elements (Fig. 10 and Fig. 11).
For casting of "U" girders, first cleaned bottom and outside shutters are placed in positioned and aligned properly. Thereafter, the reinforcement cage is placed in position. Inside shutters are placed after placing reinforcement and high tensile steel wires. Stressing of strands is done before casting "U" girders (Fig. 12 and Fig. 13).
Concreting of girder is done by using the placer boom. After achieving desired concrete strength, girders are shifted on the stacking beds (Fig. 14).
Casting of precast pier cap is done at the casting beds of pier cap. Truck mounted boom placer is used for placing concrete. After achieving desired concrete strength, piercaps are shifted on stacking beds (Fig. 15 and Fig. 16).
Estimation of concrete quantity
Based on the project, the required concrete quantity for viaduct and station needs to be calculated.. The estimated concrete quantity forms the base for calculating the cost of concrete and associated optimisation. Total concrete quantity at a glance for the selected project.
The cost associated with the concrete production and placement shall be estimated based on the following categories. Materials cost – depends on various grades of the concrete and the required quantity in each grade.
Plant and machineries cost – depends on the duration of project and the total concrete quantity required.
Transportation cost – depends on the location wise concrete requirement (at casting yard and/or along alignment).
Placement of concrete cost – Based on the site requirement, the concrete placement method (pumping/placer boom/bucket) needs to be planned for various concrete structures.
Details about planning and costing of above points are explained in subsequent points.
Costing of concrete
Concrete using fly ash/ground granulated blast furnace slag (GGBS) along with 53 Grade Ordinary Portland Cement (OPC) has been considered in calculation of cost.
Following are the possible technical advantages of using fly ash/GGBS along with OPC :
Diameter of viaduct pier is 1.8 m. In case of usage of only OPC, the core temperature of concrete may be higher and there are chances of thermal gradient leading to cracks within mass of concrete piers. Usage of fly ash will reduce the heat evolution during hydration, leading to reduced core temperature.
If high grade concrete is produced using only OPC, cement content in concrete increases. This may lead to shrinkage of concrete. Use of fly ash may minimise the shrinkage cracks.
In case of use of fly ash/GGBS, there will be secondary hydration which will make concrete more impermeable and greatly improve the durability of concrete structure.
Cohesive concrete can be achieved. Further, surface finish of the concrete structure can be improved.
For piles and pile caps, concrete can be produced using 53 grade OPC and GGBS.
For mass concrete, up to 70 per cent GGBS of total cementitious materials can also be used. This will be helpful in reducing core temperature of concrete. However, the limits on the percentage replacement shall be specified in the case of pumping.
Average material cost (as received from ready-mix concrete suppliers in Mumbai) for different grades of concrete required in metro construction using OPC and fly ash is given in Table 1. The fly ash is used up to 30 per cent (by mass) in concrete and the percentage replacement varies depending on the grades of concrete.
In case of GGBS usage, there will be further reduction of material cost in the range of Rs 200 to 400 per m3, depending on the grade of concrete and percentage of GGBS used.
Summary
The present article discussed about the metro projects in brief along with construction methodology of an elevated viaduct. Further, the article discussed about the method of estimation of concrete quantity and associated costs for various grades of concrete.
Acknowledgment
Mumbai metro rail projects
Schwing Stetter (India) for information on equipment required for RMC plant
AIMIL Ltd. for providing information on laboratory equipment
RMC suppliers in Mumbai for providing rates of RMC and raw materials
Authors
Mahesh Tendulkar
M.Tech Student
Construction Technology and Management
Department of Civil Engineering
Indian Institute of Technology Bombay
Powai, Mumbai – 400 076.
tendulkar_mahesh@yahoo.com
Basavaraj M B
Chief Engineer (Civil) – Metro
Mumbai Metropolitan Region Development Authority
Old Administrative Building, 6th Floor
Bandra – Kurla Complex, Bandra (East)
Mumbai – 400 051.
basavaraj.mb@mailmmrda.maharashtra.gov.in
Prakash Nanthagopalan
Assistant Professor
Construction Technology and Management
Department of Civil Engineering
Indian Institute of Technology Bombay
Powai, Mumbai – 400 076.
prakashn@civil.iitb.ac.in
Table 1 :Material cost for various grades of concrete
| Concrete |
Material
cost/m3 (in Rs) |
| M15 |
3,800 |
| M35 |
4,550 |
| M40 |
4,650 |
| M45 |
5,200 |
| M55 |
5,600 |