Using Precast Concrete Systems In Construction: A Review

Waste Reduction in Construction

Question:

Discuss About The Systems Offering Little To Waste Control Measures?

The construction scene has been evolving fairly over the years and more methods and materials of use in construction are rapidly coming up. While the business and construction models remain similar, the construction techniques vary significantly necessitating a lot of research and experimentation. Among the systems engineering project cases in civil engineering is the use of precast and in situ concrete together to attain a variety of industry requirements. While the material may be identical, if not fairly similar, the methods of construction vary significantly and therefore a systems engineering conceptual design is necessary to determine how to go about the construction.

Wastes in the construction industry account for around 6% - 9% of total project costs which is a very significant percentage considering the worth of some projects. As construction still remains one of the most profitable industries, most attempts to reduce wastes in the site remain uneconomic leading construction going on without proper preventive mechanisms. These leads to both parties incurring huge costs as the wastes have to be accounted for. In most cases, construction projects utilizing concrete as the main material end up having to include a wastage factor of about 5% - 10% of the total project costs.

The construction industry has also been dogged by a lot of environmental problems and one of the biggest reasons in the waste material disposed every day. A lot of waste material in the form of used formwork, excess concrete and steel is usually a result of human and environmental factors that prohibit environmentally friendliness. While disposal mechanisms also help reduce the environmental impact of this waste, it is largely up to engineers and contractors to find the right quantity margins what would yield least impact. It is therefore necessary to provide for alternatives which not only reduce the amount of wastages but improve the quality of work too.

Precast concrete systems are therefore noteworthy as they successfully address the issue of waste reduction and therefore increase the economies of scare of the overall construction project and provide a green environment. Precast concrete systems can be fully modular or can include in situ concrete at various stages. Examples of fully concrete systems include but are not limited to bridges and harbor structural members e.g. breakwaters, piers and quays. These are able to incorporate precast concrete in every step which includes the foundation, beams, columns and slabs. Other constructions incorporating precast elements in part may do so for specific beam, column and slab components for a variety of reasons including but not limited to, easy demolition, good insulation, cost reduction, high strength of element requirements and personal individual preferences like where hollow slabs are used to reduce noise.

Addressing Environmental Problems with Precast Concrete Systems

This method of construction also easily fits into the construction industry project integration models. This is because the business and legal framework of this type of construction is similar to that or normal concrete structures with the only difference being material acquisition and technical expertise required. Both the Design Build model (DB) and Design Bid Build (DBB) model can apply as it is basically up to the client’s preference. There are also specific contractors who deal primarily with precast constructions making it relatively easy to use the Performance Contracting and Construction Management models which are most popular with this type of construction.

Owing to their significantly different construction approach, the entire system of precast concrete utilization has more operational requirements that the traditional on site construction. The requirements stem from member design and manufacture to the construction process. By design, they are relatively different compared to bricks and other concrete members which necessitates a more technical manufacturing approach. They also require a lot of research and testing in order for a manufacturer to come up with a precast member that satisfies the economic, safe, environmentally friendly and structural soundness requirements. Precast concrete systems also require a higher level of expertise on site as the assembly is relatively different from most other regular brick and mortar, and on site concreting methods. While they only require an individual to place the structural member in the right positions, the connections between members are more complicated than traditional approaches.

It also requires inter-sectorial approaches as technical assistance is needed from other disciplines including ICT, mechanical and manufacturing engineering, electrical engineering, finance and marketing. This construction system however requires far less maintenance than the traditional systems. In addition to this, where maintenance is necessary, the cost is also much lower. This is because it is relatively easier to take apart the precast members individually or in a system and owing to the production and on site assembly costs, member replacement is fairly less costlier. This however applies to members off ground as foundation and slab members are extremely difficult, if not impossible, to remove.

The technical performance measures (TPMs) which are used to measure the effectiveness of this construction engineering system cover the processes of elemental design and manufacture, industrial assembly and onsite construction. Table 1 below illustrates these measures giving their relative priority levels. While the scores shown below are mostly theoretical, they can be verified owing to the amount of documentation present online resulting from tests, simulations and testimonials. The scores indicated below are however meant to be suggestive rather than conclusive as this report is purely theoretical.

Benefits of Precast Concrete Systems

This system is largely preferred due to its wastage reduction which results in good economic value for construction and a higher level of environmental friendliness. With this system, it is possible to incorporate other sustainable solutions so as to reduce negative environmental impact and costs even higher as, following different approaches, one can incorporate insulation techniques and hollow designs. These not only make the structure lighter while preserving structural soundness but costs are also lowered by decreasing volumes. The functional flow block diagram shows how detailed design is accomplished following the identification of system requirements and resources factored in.

                                                    Figure 1: Functional Flow Block Diagram of Precast Concrete Construction

When looking at the various approaches to this system, we can compare 2 main ones whose difference is in the percentage amount of precast elements used. The trade-off analysis below shows how the members vary and perform against each other to ultimately come up with the most recommendable type.

Fully precast concrete systems: these are systems that incorporate precast elements in every member. Where in situ placed concrete is required, it is usually a small percentage of the overall concrete structure. Examples of these systems are artificial harbours and bridges. A particular example that stands out is the Mulberry Harbour constructed by the British during the Second World War to aid in rapid offloading of cargo on the beaches of Normandy. Most bridges are currently being constructed using prefabricated members with the only in situ component being the abutments which in some cases are actually precast.

Precast concrete maintain a high level structural integrity with members having better compressive and tensile strengths compared to in situ members. It affords the client and contractor a rapid construction time and waste saving system. When looking at the economies of scale, larger projects incorporating this construction system are also much cheaper than their in situ concrete counterparts. This is because construction time is significantly reduced by total removal of curing and formwork assembly activities. The wastes which are proportional to the construction size, by virtue of not being present also see to the reduction of costs.

These systems can however be similarly priced to traditional systems for smaller structures as, again, the economies of scale apply. Where constructions are smaller, time spent and wastes recorded are usually less leaving very little room for marginal savings. With their relatively higher production costs, they could end up being more expensive than normal concrete systems. Their seismic performance is also debatable owing to their joint connections which some researchers suggest could be weak.

Technical Performance Measures for Precast Concrete Systems

Partial precast concrete systems: these are usually a mix of both technologies. This is where some structural members like foundations, ground slabs and/or columns are cast in situ while the other members cast off site. These have been incorporated in high-rise structures, warehouses, bridges, harbours and residential units. While not as strong as precast structures, they also maintain a high level of structural integrity. It is also possible to engineer portal frames and space frames using this technology making its seismic resistance very good because of the joint rigidity. Owing to the limited use of precast concrete, they are also a lot cheaper than their counterparts when constructing relatively small units e.g. those with three storeys or less.

These systems are however prone to wastages unlike their counterparts. While the waste is significantly reduced compared to fully in situ cast concrete structures, they are however open to some degree of wastage as the in situ cast concrete still leaves a mess. All the same, the incorporation of precast members still significantly reduces waste. While also being time friendly in comparison to fully in situ cast concrete, they cannot be compared to fully precast systems whose construction time is usually dependent the availability of skilled personnel and materials.

Conclusion

The trade-off analysis proves that both systems are fairly applicable with their buildability being subject to qualified personnel and materials. They both also offer reduced wastages but with varying levels which, in partial precast concrete systems, is dependent on the percentage use of precast members. Their economic value however varies with sizes of the projects as bigger projects could be more economically sound if they used precast concrete while smaller projects would be more profitable if partial precast systems were used.

In relation to the cost of construction, it is recommendable to only use precast systems where constructions projects are large and have high strength requirements while green building is paramount. They will save costs lost due to wastage by a very large margin. In smaller structures, partial precast structures would be more affordable. Percentage wastes occurring would be higher than fully precast systems but would ultimately be insignificant depending on the project size.

References

Anderson M. and Anderson, P., 2006. Prefab Prototypes: Site-Specific Design for Offsite Construction, Princeton Architectural Press: 16–17.

Bergdoll, B., and Christensen, P., 2009. Home Delivery: Fabricating the Modern Dwelling. New York: Museum of Modern Art: 224–227.

Blismas, N. G., 2007. Off-site Manufacture in Australia: Current State and Future Directions. Brisbane: CRC for Construction Innovation.

Blismas, N. and Wakefield, R., Hauser, B., 2010. Concrete prefabricated housing via advances in systems technologies: business Development of a technology roadmap, Engineering, Construction and Architectural Management, [17], 99-110.

Dozier, B., 2014. Technical and analysis advantages of precast concrete in (Saudi market). [Online]
Available at: https://barbradozier.wordpress.com/2014/03/17/technical-and-analysis-advantages-of-precast-concrete-in-saudi-market/
Retrieved 24 Aug 2017.

Dym, Clive L., and Little, Patrick, 2009. Engineering Design: A Project-Based Introduction, 3rd Ed, John Wiley & Sons, New York.

Haiken, M., 2015. 7 Prefab Eco-Houses You Can Order Today. Retrieved 24 Aug 2017. https://www.takepart.com/article/2015/09/14/7-prefab-eco-houses-you-can-order-today

Mashable, 2012. 5 Companies on the Cutting Edge of Sustainable Prefab Housing. RetrieveTradeoff Analysisd 24 Aug 2017. https://mashable.com/2012/07/25/sustainable-prefab-housing/#fvfvrGKm3Zq3

Naum, C., 2011. Modular Home and Construction Operations. Retrieved 24 Aug 2017. https://www.commandsafety.com/2011/01/16/modular-home-construction-and-operations/

Ochia, T., 2011. Cement and Concrete Composites. July, 29(6), pp. 448-455.

Smith, R. E., 2010. Prefab Architecture: A Guide to Modular Design and Construction. 1 ed. New Jersey: John Wiley and Sons.

Staib, G., Dorrhofer, A., and Rosenthal, M., 2008. Components and Systems: Modular Construction, Design, Structure, New Technologies. Birkhauser: 59.

Tayabji, S. D., & Ye, D. (2013). Precast concrete pavement technology management. Washington D.C.: Transportation Research Board.

US Department of Transportation, 2017. Prefabricated Bridge Elements and Systems Cost Study: Accelerated Bridge Construction Success Stories. Retrieved 24 Aug 2017. https://www.fhwa.dot.gov/bridge/prefab/successstories/091104/index.cfm

Wang, Y., Wu & Li, V., 2010. Concrete Reinforcement with Recycled Fibres.. Journal of Materials in Civil Engineering., November.

Windle, J., Chapman, T. & Anderson, S., 2007. Reuse of Foundations, London: CIRIA.