Prestressed Concrete Structure

Introduction

Prestressing concrete concerns with a particular method of application of pre-compression in order to control the stresses that results from external loads below the neutral axis of the beam tension, which is greater than the permissible limits of plain concrete. Either axial or eccentric pre-compression can be applied that induces the compressive stress below neutral axis. It results in no tension or compression. The basic concept underlying the development of prestressed concrete is internal stresses of an appropriate distribution and magnitude, which is introduced so as to counteract the stress resulting from the external loads to a desired level (Jiang 2015). It essentially requires concrete with high compressive strength and higher tensile strength instead of ordinary concrete in order to create prestressed concrete (Hassoun & Al-Manaseer 2012, pp. 90). There are a number of classifications of prestressed concrete structures based upon the features and characteristics of design and constructions. There are certain design benefits of prestressed concrete structures as well as specific construction issues (Libby 2012, pp 45). This paper additionally explores the dynamic loading of prestressed along with the behavioral aspects of these concrete structures under certain environmental circumstances such as earthquakes.  

Research Question and Problem Statement

The research questions for conducting this particular research on “Analysis and Design of a Prestressed Concrete Structure” have been formulated as follows:

• What are the basic characteristics and main features of prestressed concrete structures?
• What are the principles and methods of prestressing concrete?
• What are the common properties and behaviors of prestressed concrete under specific circumstances?
• What different prestressing techniques exist in civil engineering?

Therefore, the paper undertakes an investigation to analyze the characteristics, features, principles and methods used for prestressing concretes in a thorough manner (Priestley 1985, pp 12). To be more precise, the research questions hereby formulated helps the researcher in successfully exploring the major aspects to be concerned for the designing of different prestressed concrete structures.

Objective/Aim

The primary objective of this study is to identify the basic features of prestressed concrete and the fundamental principles and methods followed for prestressing based upon the structural behavior of concrete.

Study Hypothesis

The research hypothesis formulated for this study is to identify the primary characteristics of prestressed concrete and the common methods and techniques used for prestressing concrete structures.

Literature Review

The primary materials for prestressed concrete members include either cement, which can be high strength ordinary Portland cement that conforms to IS269 or concrete. As stated by (), the concrete for prestressing should essentially be of high compressive as well as tensile strength, air entrained composed of fine and coarse aggregates, Portland cement, water and admixtures. According to (), prestressed concrete is a highly durable as well as reliable construction material and therefore, it is widely used in building and bridge projects all across the globe.

Analysis and design of circular prestressed concrete storage tanks:

This paper discusses about the aspects of analysis and design of prestressed concrete storage tanks. In addition, it describes a simple analogy for analysing circular prestressed tanks. The method can use either programmable calculators or small microcomputers and is capable of modelling cylindrical tanks and double curvature tanks under a variety of loading such as thermal load, dead load, gas or fluid pressure and prestressing (Taherinezhad et al. 2013, pp 158). The researcher has closely examined the significance and effects of different actions such as swelling and shrinkage of walls, thermal effect and creep redistribution of prestress on the tank design. The paper also includes a detailed comparison between results of frame analogy and analytical methods for a cylindrical reservoir that is ground supported and an elevated double curvature tank. For this purpose, the researcher has performed a thorough discussion on cylindrical tanks with axisymmetric loading. It uses frame analogy for calculating stress in the tank walls for prestressing simulation purpose.

Design of prestressed concrete precast road bridges with hybrid simulated annealing

This paper reflects on a specific technique for the analysis and design of prestressed concrete precast road bridges, typically with isostatic spans and double U shaped cross section. The researcher adopts a process for resolving the combinatorial problem is a typical variant of simulated annealing based on the mutation operator from the generic algorithms (SAMO). The problem includes 59 distinct design variables for the geometry of the slab and beam, materials along with active and passive reinforcement (Rana et al. 2013). The paper involves a parametric study, which demonstrates a correlation for the geometric, cost and reinforcement characteristics with the span of length. It is effective for the design for prestressed concrete precast bridges. Besides, the researchers perform a cost sensitivity analysis that compares how the rise in steel and concrete costs affects in the increase of overall cost (Marti et al. 2013, pp 83). Apart from that, the analysis identified the specific characteristics of cost-optimized bridges and the economic impacts associated with steel and concrete costs. Therefore, the study primarily concerns with structural design with respect to cost minimization of prestressed concrete precast road bridge.

During the design of prestressed conctrete structures, Mo (2013) mentioned that higher the grade of concrete, higher bond strength in pretensioned concrete along with a higher bearing strength. In addition to that, the creep and shrinkage losses are also at minimum. According to Clement et al. (2013), the tests on prestressed concrete T beams have successfully implied that the interaction for concave cross sections. Kaewunruen & Remennikov (2013) identified the primary benefits of prestressed concrete, which includes long span structures where it is feasible to save weight and thus develop an economic design. Moreover, McCormac  & Brown (2015) experimented that in case of prestressed members, the fatigue strength is better due to the small variations and therefore, are useful in dynamically loaded structures. On the other hand, there are a number of disadvantages or construction issues related to the design of prestressed concretes. For example, the initial construction equipment cost is significantly high. Apart from that, Kong & Evans (2013) opined that prestressed concrete sections ate brittle as well as less fire resistant.

Researchers Gjorv (2014) closely observed the prestressing system, which involves two major stages such as pretensioning system and post tensioned system. The pretensioned systems concern with tensioning the tendons between the rigid anchor blocks cast. The tendons are stretched with constant or variable eccentricity.

Figure 5.1: Prestressing system

(Source: Chen & Duan 2014, pp 214)

On the other hand, the post tensioning concrete involves incorporation of ducts. Precisely, the high tensile wires are tensioned when the concrete attains sufficient strength through jack bearing anchored by wedge or nuts. Menn (2012) suggested that the patented prestressing systems operate based on certain principle pf anchoring the tendons. It can be performed following different methods such as Freyssinet system, Gifford – Udall system, Magnel blaton system and Lee – McCall system.

Analysis of prestress concrete involves particular design variables such as prestressing force P, which is positive when compressive, eccentricity of prestressing force e, cross sectional area of the concrete member, second modulus of the top and bottom fibers. Researchers Zhang, Huang & Liu (2013) studied non – linear and dynamic responses of reinforced and six prestressed concrete versions. The impacts of different prestressed concrete member properties substantially involve hysteric damping mechanisms. Wilby (2013) identified the ultimate load combinations based on different scenarios. At the designing stage, P₀ (prestressing force at transfer) should ensure the compressive strength of the concrete to a certain allowable limit.

Analysis, Evaluation and Discussion

According to simulation tests carried out by Naaman (1982) in their research studies, the fundamental properties of prestressing strands have been identified. The normal diameter is 12.8 mm; prestressing tendon is seven – wire strand, nominal area is 99.3 mm², modulus of elasticity 195 kN / mm² tensile strength 1860 N / mm². In relation to this context, Sengupta & Menon (2012) said that from structural analysis, the forces due to live load and dead load is provided. Researchers Marti et al. (2013) has calculated in their study the direct stress as well as bending stress due to prestress. However, the different types of losses in prestress can be caused by elastic deformation of concrete, shrinkage or creep of concrete. On the similar note, researchers Rana et al. (2013) has ensured that for prestressed concrete structures, concrete members with high compressive strength should be used that has tensile strength ft’ = 0.615 √ fc’ (fc’≤85 MPa). McCormac & Brown (2015) says that the various types of load actions that are essentially tested during the analysis and design of prestressed concrete structures substantially includes dead loads and imposed loads, dynamic loads, wind loads, seismic loads, earth loads, accidental loads, snow loads and so on.

Researchers Kong & Evans (2013) identified the amount of loss due to creep of concrete, which in turn occurs due to superimposed and permanent lead load that has been added  after the prestress. Loss can also occur due to shrinkage of concrete. In this context, researchers Libby (2012) reported substantial failures of prestressed concrete structures in Germany mostly due to a combination of problems related to prestressing steel and concrete properties, specifically the use of high alumina cement and concrete admixtures that contains thiocyanates or chlorides, as well as due to poor construction and design practices. In addition to that, in the United States, Europe as well as in the UK, the construction designers have potentially encountered durability issues affecting the prestressed concrete bridges.

By means of conducting a number of investigations and studies on prestessed concrete structures, several issues have been identified by assessment. According to previous studies, prestress concrete systems can be easily expanded in future if it requires changing or growing. Furthermore, they effectively provide structural support and for that reason, it can be significantly easy to add second level to the existing roof and an expansion of the structure can be achieved. Furthermore, Taherinezhad et al. (2013) observed that when reinforcement is added to the pestressed concrete specimens, the behavior is potentially similar to that of the unreinforced members until significant cracking starts. A practical example has been considered for discussion and evaluation purpose for demonstrating the two span continuous post tensioned prestressed concrete (PC).

Conclusions

It can be concluded that with adequate ductility and reduction of excessive loss of stiffness, the calculation of the ultimate limit load of prestressed concrete members should be performed. With sufficient information available in the Internet, it is clear that the durability behavior of prestressed or reinforced concrete significantly depends on the impact of ingress or moisture. Furthermore, the design of this type of concrete structures can be primarily based on the service loads. In other words, the ultimate strength is checked so as to be greater than the demand. The bending resistance is calculated by means of different strength values. The various methods for designing prestrssed members have also been discussed in this paper. The researcher has minutely elaborated on the concept of load balancing along with the major factors that influences deflection such as imposed load and self – load, magnitude of prestressing force, modulus of elasticity of concrete, second moment of area od cross section as well as the shrinkages and creeps. The design variables are mainly geometric values including the depth, thickness and width of the particular structure along with material strength. The variables (such as compressive stress, prestressing force, tendons eccentricity, section modulus) play an important role in determining the behavior of the prestrssed concrete structures.

References

Chen, W.F. and Duan, L. eds., 2014. Bridge Engineering Handbook: Construction and Maintenance. CRC press.

Clement, T., Ramos, A.P., Fernández Ruiz, M. and Muttoni, A., 2013. Design for punching of prestressed concrete slabs. Structural Concrete,14(2), pp.157-167.

Gjorv, O.E., 2014. Durability design of concrete structures in severe environments. CRC Press.

Hassoun, M.N. and Al-Manaseer, A., 2012. Structural concrete: theory and design. John Wiley & Sons.

Jiang, D., 2015. Experiments, Analysis and Design Models for Slab on Prestressed Concrete Girder Bridge Structures (Doctoral dissertation).

Kaewunruen, S. and Remennikov, A., 2013. On the residual energy toughness of prestressed concrete sleepers in railway track structures subjected to repeated impact loads.

Kong, F.K. and Evans, R.H., 2013. Reinforced and prestressed concrete. Springer.

Libby, J.R., 2012. Modern prestressed concrete: design principles and construction methods. Springer Science & Business Media.

Martí, J.V., Gonzalez-Vidosa, F., Yepes, V. and Alcalá, J., 2013. Design of prestressed concrete precast road bridges with hybrid simulated annealing.Engineering Structures, 48, pp.342-352.

Martí, J.V., Gonzalez-Vidosa, F., Yepes, V. and Alcalá, J., 2013. Design of prestressed concrete precast road bridges with hybrid simulated annealing.Engineering Structures, 48, pp.342-352.

McCormac, J.C. and Brown, R.H., 2015. Design of reinforced concrete. John Wiley & Sons.

Menn, C., 2012. Prestressed concrete bridges. Birkhäuser.

Mo, Y.L., 2013. Dynamic behavior of concrete structures (Vol. 44). Elsevier.

Naaman, A.E., 1982. Prestressed concrete analysis and design: fundamentals (pp. 360-364). New York: McGraw-Hill.

Priestley, M.J.N., 1985. Analysis and design of circular prestressed concrete storage tanks. PCI Journal, 30(4), pp.64-85.

Rana, S., Islam, N., Ahsan, R. and Ghani, S.N., 2013. Application of evolutionary operation to the minimum cost design of continuous prestressed concrete bridge structure. Engineering Structures, 46, pp.38-48.

Sengupta, A.K. and Menon, D., 2012. Prestressed concrete structures.nptel. iitm. ac. in.

Taherinezhad, J., Sofi, M., Mendis, P.A. and Ngo, T., 2013. A review of behaviour of prestressed concrete sleepers. Electronic Journal of Structural Engineering, 13(1), pp.1-16.

Wilby, C.B., 2013. Structural Concrete: Materials; Mix Design; Plain, Reinforced and Prestressed Concrete; Design Tables. Elsevier.

Zhang, W., Huang, J. and Liu, H., 2013. Design of Temporary Fixing Structure for Construction of Prestressed Concrete Continuous Box Girder Bridge Cantilever [J]. Journal of Highway and Transportation Research and Development, 2, p.012.

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