Slip-Form Application to Concrete Structures

Transcription

1 Slip-Form Application to Concrete Structures Tarek Zayed 1 ; M. Reza Sharifi 2 ; Sandel Baciu 3 ; and Mohamed Amer 4 Abstract: Because of superior speed and productivity, slip forms were extensively utilized as a potential formwork candidate in constructing concrete structures for the past few decades. Typical projects that employ this formwork technique are: Core of high-rise buildings, silos, telecommunication towers, cooling towers, heavy concrete offshore platforms, etc. The research presented in this paper aims at studying slip-form application to cores and silos, assessing its productivity, and determining its appropriate speed as well as auxiliary resource combinations. Simulation models are developed in which the potential control units in a slip-form system are described for cores and silos. Data are collected from several case study projects. A set of charts has been developed to predict productivity considering different stoppages, core cross section area, slipping jacking rate, and concrete placing methods. These charts play an essential role in managing slip-form application to cores and silos. Results show that the developed simulation models predict the productivity of case study projects with and 99.30% accuracy for cores and silos, respectively. The presented research is relevant to both researchers and practitioners. It provides practitioners with charts that assist in scheduling and managing the required resources for slip-form application. In addition, it provides researchers with simulation models and framework for implementing slip forms to core and silo construction. DOI: / ASCE :3 157 CE Database subject headings: Construction management; Productivity; Simulation models; Concrete structures. Introduction 1 Associate Professor, Building, Civil, and Environmental Engineering Dept., Concordia Univ., Montreal PQ, Canada H3G 1M7. 2 Former Graduate Student, Building, Civil, and Environmental Engineering Dept., Concordia Univ., Montreal PQ, Canada H3G 1M7. 3 Graduate Student, Building, Civil, and Environmental Engineering Dept., Concordia Univ., Montreal PQ, Canada H3G 1M7. 4 Assistant Professor, Dept. of Construction Engineering and Utilities, Zagazig Univ., Zagazig, Egypt. Note. Discussion open until August 1, Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on February 17, 2006; approved on June 8, This paper is part of the Journal of Construction Engineering and Management, Vol. 134, No. 3, March 1, ASCE, ISSN /2008/ /$ Building a high rise, or other large buildings, is typical for urban areas. High-rise buildings are rapidly becoming a developing nature of urban zones due to increased population and businesses. The core of high-rise buildings is one of the most important elements in such gigantic structures. Fast and efficient construction of the concrete core of a high-rise building is essential to maintain phased progress on other parts of the building. Some contractors have developed expertise in building cores using slip forms, while others maintain satisfactory progress by jumping gang forms from one floor to the other Hurd Jaafari et al reported that many formwork methods could be used to construct a highrise building core, such as slip-form, climb-form, jump-form, super-shafter, and conventional forms. It has been shown that, for building cores of less than 15 stories high, none of the alternative methods can compete with the conventional forms. Jaafari et al further stated that for tall structures greater than 30 stories, the alternative methods could potentially reduce the cost of conventional method by up to 30 40%. In addition, slip forms showed cost advantages for more than 20 stories and larger than 600 m 2 formed area per floor Jaafari et al Industrial silos, such as coal or production line storage silos, are typically used to store grains as well as construction and industrial materials Peurifoy and Oberlander For silos higher than 15 m, the slip-form method is the most economical and time saving technique Jaafari et al Consequently, slip forms were extensively used in the core of high-rise buildings and silo construction in the past few decades Risser 1995; Hurd 1990; Jaafari et al. 1989; Anon Slip form is a sliding-form construction method, which is used to place vertical concrete structures. Slip-form construction technology has become important in high-rise concrete structures Risser 1995, and the most common methods of construction in concrete silos. It differs from conventional concrete forms because it moves semicontinuously with respect to the concrete surface in which form ties are not used Risser Recent improvements in larger yoke capacities and better laser guidance result in more efficient and faster slipping rates Risser After studying 42 combinations of floor and wind framing systems by Pruitt 1987, it was concluded that slip forming might cost somewhat more than other methods, but would shorten the total construction schedule by at least 3 months. Due to lack of research in modeling slip-form productivity, the presented research assists in designing simulation productivity models for core and silo construction using slip forms. These models help determine the best slipping jacking rate and resource combination of slip form. They further accommodate limitations of concrete properties and construction difficulties, such as stoppages, technological problems, concrete setting time, and their effect on productivity. 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2 Fig. 1. Construction steps for concrete core using slip forms Slip-Form Background Slip-form work begins with the construction of the form on top of building foundation utilizing a backup support and bracing system in order to ensure that the form maintains its required shape during vertical movement. The inner and external forms create the void thickness of the wall. In this void, reinforcing steel is tied together vertically and horizontally to reinforce the concrete wall. The form is then connected to jack rods with hydraulic jacks, which automatically move the form in a vertical direction. Once placing concrete begins, it continues until the top of the structure is reached. A few decades ago, for the first time, a wooden rising form using wooden screw jacks and wooden yokes was operated, and six years later, in Tennessee, the first apartment building was constructed using slip forming Ratary In 1974, the slip-form method was used to construct the concrete shaft of the CN tower in Toronto, with 345 m height, in 8 months. Slip forming is a technique used to build high-rise structures quickly, in which the wet concrete is extruded, rather than retained in forms until it has hardened Anon 1978 and In slip forming, the concrete is placed at a predetermined rate on top of a traveling form, which emerges in a hardened state from the bottom. Concrete is shaped to the desired profile during the travel of the form. Slip form is most economical for structures with a uniform cross section, but it is adopted for structures that vary in cross section and shape through their height as well. Slipping jacking speed has a fixed rate; therefore, the form will leave the concrete after being strong enough to retain its shape while it can carry the load of its weight. Therefore, careful planning is essential to establish a suitable concrete mix well before starting the work. With the reductions in construction time and labor cost, slip forms prove more economical in the long run in spite of their high initial investment Anon 1978 and Slip forms have many advantages over other techniques: High operational speed, economical, accurate operations, high quality finished surfaces, and a continuous moving monolithic structure. It can be used to construct special structures, such as telecommunication towers, cooling towers, silos, heavy concrete offshore platforms, etc. Many challenges face slip-form usage in the construction of a high-rise building core. Continuous work needs high-level management of resources and a convenient work environment. In addition, weather conditions and labor union restrictions might add to these challenges. Many accompanied equipment have to work continuously parallel to the slip-form work. This will increase the initial investment as well as assembling and disassembling expenses of such form and equipment Betterham In addition, any changes in the operational information during construction cost a lot of time and money. The number of boxes and embedded parts in each floor of the structure influences greatly the cost and time as well. There are several basic criteria for selecting slip-form method, such as project time, required speed, structure cross section and height, number of openings, and necessary stoppages along the height of the structure. Likewise, the risk of modifying operational information and mistakes during construction must be mitigated in order to enhance productivity and quality of work performed Hanna Speed of slip form is a function of concrete properties, number of stoppages, weather conditions, and management capabilities. The slipping jacking rate depends on concrete setting time and how fast horizontal reinforcements and anchor plates are placed. Slip form is able to move vertically whenever concrete can carry the load of its weight in the lowest part of the form sheet; therefore, slipping rate is directly related to concrete setting time. However, setting time is influenced by weather conditions temperature, humidity, etc., cement ratio, type of cement, slump, and admixtures Hurd The best slipping rate will be chosen based upon job conditions. It should not be too fast to cause collapses, where soft concrete falls out from under the form. It should also not be too slow to cause stick, where concrete sticks to the slip form and concrete parts might rip away from the wall. Slip-Form Construction Algorithm Simulation technique can be applied to the modeling of slip-form operations in order to study different combinations of resources. MicroCYCLONE modeling and programming technique are used to simulate this process. The elements of MicroCYCLONE, originally developed by Halpin in 1973, are used to model and simulate slip-form operations. MicroCYCLONE is a simple and a good tool for construction process planning, as demonstrated by many researchers Zayed and Halpin To build the simulation model of constructing a high-rise building core or concrete silo using slip-form technique, the construction phases algorithm have to be identified. A slip-form passes through several steps in order to build one floor of the core or 1 m of silo height as follows Fig. 1 : Step 1: Slip form starts from the bottom level where the building structure connects to the concrete core. If the rest of the building is made of steel typical high-rise structures, steel beams have to be welded to embedded plates that should be installed during slip-form operation in the core. Specific formworks for the 158 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

3 openings of elevators, mechanical, and electrical should be constructed during stoppage duration. Usually at this stage, vertical reinforcements are installed with the appropriate lengths to cover one floor. For silos, slip form starts from a specific level in its height based on foundation design. In this step, a tower crane lifts reinforcements and embedded plates to the platform. Step 2: One-step jacking is done where the slip form is driven up one jacking step =2 in.=5.08 cm. Step 3: A horizontal rebar will then be installed and concrete will be pumped to fill the empty form. Concrete will be cured and finished in order to be ready for the next step. Step 4: Repeat Step 2 four times where 20 cm of form is raised. Horizontal rebar will be installed for the next 20 cm of floor or silo height. Step 5: Repeat the above four steps until the completion of one floor bottom of the slip form reaches the following upper floor level or 1 m of silo height. Step 6: Repeat the above five steps until the completion of the building core or silo. The aforementioned steps are better explained through the schematic diagram in Fig. 2, which shows the detailed construction process of slip forming in silo construction. Fig. 3 shows the details of the above construction algorithm of slip-form application to both core and silo construction. The difference between core and silo algorithm is mainly in the idea of stoppages to install vertical steel, embedded parts, and opining forms in core construction. However, slip forms work continuously without stoppages in silo construction. Therefore, it is expected to have higher productivity in silos than core construction. After identifying the construction algorithm, a simulation productivity model is designed. The presented research introduces this model using MicroCYCLONE simulation package Halpin and Riggs 1992, as shown later in the paper. Case Studies Several case studies are used to test the assumptions and accuracy of the developed models. These models are designed utilizing the parameters of case studies i.e., physical characteristics, activities durations, etc.. Hence, the outputs of the developed models are compared to the actual outputs of case studies in order to test models accuracy in predicting productivity. The details of these case studies are explained below. Data were collected from two high-rise building projects: One in Chicago, Ill. and the other in Seoul, South Korea. The elevator core of an apartment building in Chicago 21 floors was constructed using slip-form lifting system. Scanada s two-deck slipform lifting system was used to construct this project. One floor per day was typical productivity for elevator core construction. Reinforcing steel and openings for the next floor were placed using a small night crew 8 h. Actual slip forming took place during the day shift 16 h. The Samsung building in Seoul, South Korea had a composite structure of steel beams and columns as well as a concrete core. The core was used for elevators, mechanical, and electrical ducts. The building was 73 stories high 251 m and concrete was placed using a pump with a rate of approximately 20 m 3 /h. During stoppages, embedded steel parts were installed in order to connect steel structure to the core. Table 1 shows general project information that is considered in the simulation productivity model. On the other hand, the duration of simulation activities are shown in Table 2. Silo construction data were collected from the Hormozghan cement factory project in Bandar Abbas, Iran. It included raw meal silos and towers with 6,000 t cement production per day. All silos and towers of the cement factory were constructed using a slip-form lifting system, as shown in Fig. 2. Samarah Construction Co. general civil contractor performed the slip forming part of this project. Each silo had 16 m inner diam., 50 cm thickness, and 50 m height. Concrete was placed using bucket and crane with a rate of approximately 8 m 3 /h; however, rebar and materials were lifted to the platform by the same crane. Slip forming took place in 24 working h per day through three shifts. Table 1 shows general project information that is considered in the simulation productivity model. On the other hand, the duration of simulation activities are shown in Table 2. Simulation Models Development Activity durations for core construction are estimated based upon: Jacking rate, concrete placing method, horizontal rebar installation, and stoppage times. Horizontal rebar installation time is determined based on a crew of four rebar workers rodmen. Various stoppage times are used in the designed simulation model: 4, 6, or 8 h, based on night crew productivity. Concrete placing time is determined based on pumping rates. For silo construction, the required data include: Maximum possible jacking rate according to the mechanical capability of the form, mix design and concrete properties, concrete placing method, rebar installation, and material lifting technique. Rebar installation time is determined based on a crew of eight rebar workers rodmen. Concrete placing time is determined based on crane and bucket rates. Material lifting time is based on average crane lifting speed. Two MicroCYCLONE models are developed to represent core and silo construction as shown in Figs. 4 and 5, respectively. Both models accommodate the differences between core and silo construction characteristics, as discussed early in the paper. Triangular distributions are mainly used to represent simulation activities duration in both models, as shown in Table 2. Triangular distribution has been chosen because its required data are easy to estimate and collect from practitioners. In addition, the collected data set was not enough to generate a probability distribution for each activity. Therefore, each practitioner provides three-duration format for simulation activities of case study projects. Three values have been developed to represent each activity data point : Minimum, most probable, and maximum. These values constituted the triangular distribution lower value, mode value, and higher value that was used in simulating activities duration. In other words, because data were not enough to generate probability distribution for each simulation activity, data were collected from practitioners in triangular distribution form based upon their experience. Results and Sensitivity Analysis The MicroCYCLONE package is utilized to simulate with the developed models, shown in Figs. 4 and 5, in order to manage both core and silo construction resources. Using core simulation model, a sensitivity analysis is used to estimate productivity based upon various slipping jacking rates, core areas, concrete placing method, and stoppage times. Jacking rates depend mainly on various concrete setting times. Based on common practice, jacking rates change from 10 to 60 cm/h because jacking rates JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008 / 159

4 Fig. 2. Complete construction process of silo construction using slip forms 160 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

5 Fig. 3. Construction algorithm of slip-form application to core and silo construction Table 1. General Project Data Item Information a Core construction Floor height 3.0 m Wall thickness 0.5 m Core concrete area 32.0 m 2 b Silo construction Silo height 1.0 m Wall thickness 0.5 m Silo inner diameter 16.0 m Table 2. Activities Duration Triangular Distribution Duration Activity min a Core construction Jacking 5 Concrete placing 1, 3, 4 Rebar installation 16, 20, 23 Embedded parts installation 240, 360, 480 b Silo construction Jacking 5 Concrete placing 4, 4.8, 5 Rebar installation 7, 8, 9 Material lifting 5, 6, 7 JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008 / 161

6 Fig. 4. MicroCYCLONE model for slip-form application to high-rise building core lower than 10 cm/ h lead to concrete setting before completing the work, which reduce productivity and cause caving problems. However, the rate can be increased beyond 60 cm/h, but concrete requires extra admixtures, such as setting time accelerators. The concrete placing method is also altered between two systems: Pump system average placing rate 3 min/m 3 and cranebuckets system average placing rate 10 min/m 3. The results of simulation as well as sensitivity analysis are shown in Tables 3 and 4. Table 3 shows jacking rates and their associated productivity in floors/h and floors/day at various stoppage times. It further shows that productivity at 4 h stoppages with 30 cm/h jacking rate using pump to place concrete is floors/h and 1.2 floors/day assuming 24 working h per day. On the other hand, if the slip form is working at 8 h stoppages with 60 cm/h jacking rate using pump to place concrete, then, productivity will be floors/h and 1.3 floors/day. Fig. 6 is developed to show the relation among productivity floor/h and floor/day, stoppage times, and jacking rates using pump to place concrete. 162 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

7 Fig. 5. MicroCYCLONE model for slip-form application to concrete silo Table 3. Different Productivity Values Using Pump for Concrete Placing Jacking rate cm/h Time min Productivity floors/h at stoppages Productivity floors/day at stoppages 4h 6h 8h 4h 6h 8h JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008 / 163

8 Table 4. Different Productivity Values Using Crane Buckets for Concrete Placing Jacking rate cm/h Time min Productivity floors/h at stoppages Productivity floors/day at stoppages 4h 6h 8h 4h 6h 8h Similarly, Table 4 shows productivity analysis using cranebuckets system to place concrete. It also shows that productivity at 4 h stoppages with 30 cm/h jacking rate using crane buckets to place concrete is floors/h and 0.7 floors/day assuming 24 working h per day. On the other hand, if the slip form is working at 8 h stoppages with 60 cm/h jacking rate using crane buckets to place concrete, then, productivity will be floors/ h and 0.7 floors/day. Fig. 7 is developed to show the relation among productivity floor/h and floor/day, stoppage times, and jacking rates using crane buckets to place concrete. Using a silo simulation model, a sensitivity analysis is used to estimate productivity based upon various potential maximum jacking rates, silo diameter and thickness, and concrete placing method. Jacking rates depend mainly on various concrete setting times and power of jacks. The results of simulation, as well as sensitivity analysis, are shown in Tables 5 7. Table 5 and Fig. 8 show jacking rates and their associated productivity in a case study project silo. It further shows that productivity is m/h assuming 24 working h per day using 30 cm/h jacking rate and crane-buckets system to place concrete. The cross section area of the core is a factor that affects productivity of flip-form system. Based on collected data from experts, who were working as engineers in the case study projects, charts were developed considering various core cross section areas using the developed simulation models. These data were collected in triangular distribution format. Three charts have been developed to predict productivity floor/day considering core area, jacking rate, and stoppage time as shown in Figs For example, Fig. 9 shows that a project of 20 m 2 core area using 10 cm/ h jacking rate produces 0.6 floor/day at 8 h stoppages. Fig. 6. Slip-forming productivity using pump in concrete placing 24 working h/day Fig. 7. Slip-forming productivity using bucket and crane in concrete placing 24 working h/day 164 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

9 Table 5. Different Productivity Values Using Crane Buckets and Pump for Concrete Placing in Silo 16 m Diameter and 0.5 m Thickness Jacking rate cm/h Productivity using bucket m/h Productivity using pump m/h Similarly, the productivity of 6 and 4 h stoppages are predicted using Figs. 10 and 11. It is obvious that the developed curves for pump and crane-buckets systems are deemed beneficial to practitioners in the slip-form industry. These curves can be used further to plan slip-form projects efficiently. They enable experts to optimally schedule slip-form operation within a specific project and among various projects. The cross section area, represented by diam, of the silo and thickness of walls are the factors that affect productivity of slipform system. Therefore, two charts have been developed to predict productivity m/h considering silo diam and jacking rate, as shown in Figs. 12 and 13. Fig. 12 shows that slip-form productivity is 0.2 m/h for silo diameter of 12 m, 40 cm/h jacking rate, and crane-buckets system to place concrete. Similarly, the productivity of different silo projects, using pump to place concrete, Fig. 8. Slip-forming productivity using crane buckets versus pump system to place concrete for case study silo 24 working h/day can be predicted using Fig. 13. It is obvious that the developed curves for pump and crane-buckets systems are deemed beneficial to practitioners who are using slip forms in silo projects. These curves can further plan slip-form work in silo projects efficiently within a specific project and among different projects. Table 6. Different Productivity Values Using Crane Buckets for Concrete Placing Silo: diam thickness m Jacking rate cm/h Table 7. Different Productivity Values Using Pump for Concrete Placing Silo: diam thickness m Jacking rate cm/h JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008 / 165

10 Fig. 9. Slip-form productivity versus various core areas using 8 h stoppage Fig. 11. Slip-form productivity versus various core areas using 4 h stoppage To explain the usage of the abovementioned figures charts, which are generated from simulation models and sensitivity analysis, an example is introduced later in the paper. Simulation Models Accuracy Data were collected from several case study projects: Two core construction and eight silos. The average jacking rate of core construction projects was reported as 30 cm/ h and average productivity was 1.0 floor/day considering 8 h stoppage and concrete placing using pump. By consulting Table 3 and Fig. 6, model productivity for such case was floors/day. Therefore, based upon the above case study, the model predicts the project productivity with 1.0*100/ = 99.70% accuracy. The Hormozghan Cement Project had more than 20 towers and silos that were constructed using slip-forming technique. Out of the 20, four row meal and four cement silos were identical; therefore, the slip-forming system that was used in these eight silos was identical as well. Data were collected from these eight silos considering jacking rates and productivity in m/h. The average jacking rate was 30 cm/ h and average productivity was 0.15 m/h for the eight silos. Concrete was placed using crane and bucket. Applying the developed model to this case study eight silos results in productivity of m/h as shown in Table 5 and Fig. 8. Based on this result, the developed model predicts project productivity with 0.15*100/ 0.151= 99.30% accuracy. According to case studies two core construction and eight silos, the developed models and charts produce robust results. However, the developed models and charts will be better tested using a larger data set in order to check their accuracy. Further case studies are targeted to perform these tests. Examples Part A Two core projects for office buildings in a city downtown are proposed to be constructed using slip forms. Building K has 63 floors and Building J has 49 floors. Building K has the following given information: A core area of 59 m 2, wall thickness of 1.0 m, jacking rate is expected to be 40 cm/h, and the pump can be used to place concrete. In addition, stoppages of 6 h per day are planned based upon the nature of the project. On the other hand, the available information for Building J are as follows: Core area is 80 m 2, wall thickness is 1.0 m, jacking rate is expected to be 30 cm/h, the concrete will be placed using crane and bucket because of site limitations, and stoppages of 8 h per day are planned. How many floors/day can the contractor do in each project? How many working days does the contractor need the slip forms in each project? Part B Fig. 10. Slip-form productivity versus various core areas using 6 h stoppage The company has three silo projects of 20, 15, and 9 silos will be constructed using slip forms with heights of 55, 65, and 60 m, respectively. The first project has eight silos with inner diam 22 m and 0.7 m thickness as well as 12 silos of 12 m inner diam and 0.5 m thickness. The second project has 15 silos with inner diam of 18 m and thickness of 0.6 m. On the other hand, the third project has nine silos of 28 m inner diam and 0.8 m thickness. Crane and bucket will be used to place concrete in the first and third projects; however, pump will be used in the second project. Jacking rates are expected to be 30 cm/h for the second and third projects; however, it is 50 cm/h for the first project. How many m/day can the contractor do in each project? What will be the project duration? 166 / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

11 Fig. 12. Slip-forming productivity versus silo diameter using bucket and crane in concrete placing 24 working h/day Solution of Part A Based on productivity figures for core construction Figs. 6, 7, and 9 11, the slip-form productivity can be calculated as follows: Building K. It has a core area of 59 m 2, wall thickness of 1.0 m. By consulting Fig. 10, productivity equals 1.06 floors/day when jacking rate is 40 cm/h using pump and 6 h per day stoppages. However, to construct 63 floors, the contractor needs a slip form for 60 days 63 floors/1.06 floors/day. The contractor has to add contingency duration to cover the risk of any delays. Building J. It has a core area of 80 m 2 and wall thickness of 1.0 m. By consulting Fig. 9, productivity equals 0.80 floors/day for jacking rate of 30 cm/h using pump and 8 h per day stoppages. However, to construct 49 floors, the contractor needs a slip form for 62 days 49 floors/0.80 floors/day. The contractor has to add contingency duration to cover the risk of any delays. Fig. 13. Slip-forming productivity versus silo diameter using pump in concrete placing 24 working h/day JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008 / 167

12 Solution of Part B Based on productivity figures for silo construction Figs. 8, 12, and 13, the slip-form productivity can be calculated as follows: First Project. For silos with inner diam of 22 m, 0.7 m wall thickness, and jacking rate of 50 cm/h, by consulting Fig. 12, productivity is 0.12 m/h. Eight silos with 55 m height will then take 8 silos*55 m*0.12 m/h=53 days. However, the other 11 silos are constructed with a rate of m/h for 12 m inner diam, 0.5 m thickness using crane to place concrete. The 12 silos will then take 12 silos*55 m*0.225 m/h=149 days. Hence, the total project takes 60 days+ 149 days+ contingency, if one slipform crew is used in both types of silo. But it will be 149 days + contingency, if two concurrent slip-form crews are used in each type of silo. Second Project. For silos with 18 m inner diam, 0.6 m wall thickness, 60 m height, pump to place concrete, and 30 cm/h jacking rate, by consulting Fig. 13, productivity is 0.19 m/ h. Fifteen silos will then take 15 silos*65 m*0.19 m/h=186 days. Hence, the total project will take 186 days+ contingency. Third Project. For silos with 28 m inner diam, 0.8 m wall thickness, 60 m height, crane and bucket to place concrete, and 30 cm/ h jacking rate, by consulting Fig. 12, productivity is 0.08 m/ h. Nine silos will then take 9 silos*60 m*0.08 m/ h = 44 days. Hence, the total project will take 44 days + contingency. Note. Calculations assume that only one slip-form machine and crew is used for the whole project. However, the relation between the number of machine crews and project duration are inversely related. In other words, if two machine crews are used, the project duration will be 50% of that of one machine-crew project duration. If three machine-crews are used, then duration will be 33% of that of one machine-crew project duration and so on. Conclusions Productivity models for the application of slip forms to the core of high-rise buildings and silos are designed using simulation. These models consider several factors that affect productivity, such as stoppage times, jacking rates, silo diameter, placing method, cross section area, and concrete setting time. Several charts are developed to determine productivity of slip forms considering different stoppages, core cross section area, and concrete placing methods. The models are tested and show high accuracy in predicting slip-form productivity. They are essential to practitioners and researchers because they provide practitioners with a planning and scheduling tool for their slip-form operation in core and silo construction. They can further be used in bid estimating and project planning processes. On the other hand, they provide researchers with simulation models that are flexible enough to modify and add more features in order to enhance their capabilities in predicting productivity of slip-form work in core and silo construction. Acknowledgments The writers would like to extend their appreciation to Scanada Slip-Form Company, particularly Mr. Hans Vikstrom, the director of Montréal branch for his great help and professional advice in the presented research. In addition, special appreciation would be extended to Samarah Construction Co., particularly Mr. Ahmad Keshavarz project engineer, project manager, and construction manager of Hormozghan cement factory project for their great cooperation in the presented research. References Anon Concrete. Indian Concr. J., 61 4, Anon Key to courthouse puzzle. Eng. News-Rec., , Betterham, R. G Slip-form concrete, Longman, New York. Halpin, D. W., and Riggs, L. S Planning and analysis of construction operations, Wiley, New York. Hanna, A. S Concrete formwork systems, Marcel Dekker, New York. Hurd, M. K Self-lifting forms shape building cores. Concr. Constr., 35 2, Hurd, M. K Formwork for concrete, 7th Ed., American Concrete Institute, New York. Jaafari, A., Kew, Y. C., and Yeoh, C. K Alternative methods for construction of vertically-formed concrete structures. Institution of Engineers, Australia, Civil Engineering Transactions, CE31 1, Peurifoy, R. L., and Oberlander, G. D Formwork for concrete structures, 3rd Ed., McGraw-Hill, New York. Pruitt, J. D Slip-forming of Atlanta s IBM tower. Concr. Constr., 32 4, Ratary, R. T Handbook of temporary structures in construction, McGraw-Hill, New York. Risser, B Advances in vertical slip-form construction. Conc. Constr., ID 718&articleID Zayed, T. M., and Halpin, D. W Simulation as a tool for resource management. J. Constr. Eng. Manage., 127 2, / JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT ASCE / MARCH 2008

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