Bridge design and construction is a daunting and exciting field that demands imagination and innovation to produce elegant, robust and durable structures (Figure 1). More bridges are constructed around the world using concrete than any other material, also concrete bridges have a consistent record of efficiency and reliability, though at the same time being extremely flexible in both final form and construction methods. Concrete will continue to be used for bridge building because it is inexpensive, robust, flexible and sustainable. When carefully designed, defined and constructed, concrete often offers a high-quality and elegant natural appearance.
This is the first post in a series on concrete bridges, the series will discuss in detail the various aspects of concrete bridge design and construction. This post introduces the concept and the key benefits of using concrete for bridges. The subsequent post will focus on the design and construction of concrete bridges in detail, bridge types and forms, structural layout, pre-stressing, reinforcement details, formwork types and construction methods.
Concrete can be conveniently used in all bridges – high-speed rail, commuter rail, subway networks, roads, aqueducts and footbridges. Recent advancements in both concrete material science and concrete bridge building technology have provided owners, builders and contractors greater importance, durability and protection than ever before. Concrete is used worldwide as a domestically sourced material for much of its parts, making it the natural preference for all bridge structures. In-situ concrete is also readily integrated into all bridge elements, either on its own or as a precast composite. Pre-casting of bridge components under well-controlled factory conditions ensures they are both precision engineered and easy to erect when sent to the site.
Four main advantages – aesthetics, sustainability, durability and construction capability – are listed here. The subsequent post will explain the vast functionality of concrete bridges, with comparisons to the best choice of bridge form for each location; much of which will be decided by the program, the speed of construction and the availability of materials and expertise.
A bridge engineer has dual responsibilities – to use the resources of his client wisely and to create a society’s structure that will enhance the built environment. These two components are the classical equilibrium between form and function. The structural forms that can be done with concrete are constrained only by the ingenuity of the designer, but it obviously takes an experienced engineer with sound judgment and skills to manage this balance between shape and function. Aesthetics is a core aspect of design development, but solutions constructed to suit the movement of forces in the bridge appear to have natural elegance. Bridges also require nothing else to enhance their shape, however, collaboration with professional bridge architects will definitely be a welcome addition, as they can often add a much wider awareness of social impacts. However, a clear appreciation of the context, the scale, the lines and the balance is indeed important for the engineer.
The overall arrangement of the span and depths should be specifically selected to create good proportions, with the relative sizes of the masses and the voids in harmony. The structural form of the bridge is an expression of its strength, durability and economy, and the construction process can also be a defining feature. For eg, a gradual bridge would have a constant depth, whereas a bridge constructed with a balanced cantilever appears to have a variable depth. Concrete bridges can be shaped to suit the flow of forces and the surroundings, giving enormous scope to the engineer and architect to create elegant bridge structures, which are both dynamic and graceful. The overall feature of the scheme will be determined by the nature and context of the bridge. While a bridge only viewed from distance will need an elegant balance of scale and lines, a bridge viewed at close quarters will need its concrete finishes, surface texture and details to be carefully integrat
The design of a bridge has to take a long-term and strategic view. It is essential that design teams develop solutions that minimize the wide range of environmental, economic and social impacts, both during construction and over the whole life of the bridge. Concrete, with its long life and minimum maintenance, is a good solution to address these issues as it is primarily a locally sourced product (Figure 3).
There are two key issues that need to be addressed – the use of finite natural resources and the emissions caused by the consumption of these resources. With a typical design life of at least 100 years, concrete is the most durable material commonly used to build bridges of any form or size. In environmental terms, it is useful to think of concrete as having three phases of life – creation, use in bridges and final recycling. Concrete should be put as close to its point of manufacture as possible in order to reduce the need for shipping to the site, benefit the local economies and avoid exports from having an environmental impact to other locations
Embodied energy and CO2 emissions have been shown to be at their minimum, both during construction and in use, with concrete bridge solutions. Up to 95% of the concrete and steel reinforcement in a concrete bridge can also be recycled once the structure has reached the end of its viable life.
Sustainability is a complex issue, with environmental, economic and social impacts that are inextricably linked, but locally sourced, durable concrete made with many recycled materials continues to demonstrate that it provides the best construction material.
Concrete has been in use since 7,000BC and there is significant evidence to show that it is a very durable construction material. Most bridges are specified to have an intended working life of at least 100 years and so durability is a primary objective. It is therefore essential to consider the overall layout of the bridge, the detailing of the elements and the specification of the component materials. A significant reduction in the number of joints and bearings, or their elimination, will be a major feature of such considerations, as will the careful specification of concrete types and cover to reinforcement. All external areas of the concrete surface need to be carefully detailed with dedicated routes away from the structure (for the flow of salt-laden water, and the incorporation of suitable drips, channels and pipes) where necessary.
Waterproofing of bridge decks is recognised in the UK as a necessary operation to enhance the longevity and durability of the structure. The most common form of waterproofing is a liquid sprayed system applied in several coats
Well designed and constructed concrete bridges require only minimum maintenance to keep them in good working condition. As a result, their competitive initial construction costs coupled with reduced inspection and maintenance, ensure a very attractive whole life cost. High quality, low permeability concretes with the correct covers, low water/cement ratios and the appropriate cementitious the material will provide durable bridge structures in all environments.
Construction capability relates to buildability with respect to the various method of bridge construction available. This will be considered extensively in a later post, however, the particular issues as it relates to precast and in-situ construction are explained here. The general parameters of the scheme (such as the typical spans, the overall deck area, width and length, the clearance requirements, alignment and the overall aesthetic) will start to suggest which bridge types and which construction methods might be appropriate. However, the final choice will then depend on many other particular parameters such as:
- site access, layout and availability
- construction programme and phasings
- geotechnical issues
- environmental issues
- material quantities and costs
- traffic management
- labour rates
- resource requirements
- plant supply
- temporary works layouts
as well as the overall casting, transportation and craneage or erection issues. The owner should try to leave as many of these parameters as flexible as possible, such that the designer and contractor can consider as many of the available construction methods as possible – this strategy is likely to lead to the optimum
solution for the owner.
Many of these parameters are controlled by the need to increase the speed and ease of the construction process, and while fast construction is not necessarily an objective in itself, it can generally be seen that the best construction methods are indeed driven by speed. Although precasting is often the preferred method of
achieving these speed objectives, in-situ concrete can also deliver the same results (Figure 4). In practice, a combination of the two (in-situ piers and precast decks) is typical. The appropriate use of labour, materials and plant to create the best construction method can assure the successful delivery of a wide range of bridge deck structures within tight programmes and across the spectrum of
environmental and site conditions.
Precasting and factory production methods can significantly improve the safety regime by shifting the works to a more regular and controlled series of operations, with a workforce who have become familiar with the production process. Precasting of elements canal so significantly reduce risks and improve rates of production. The repetition created by standardisation of details and dimensions
helps to reduce construction time (Figure 5).
Bridge products that are produced away from the construction site (e.g. Figure 6), in an effective factory setting, can be produced to very high quality and without any adverse weather problems. It is possible to closely regulate the consistency of the concrete and plan and place the formwork, reinforcement and pre-stressing to exceptionally high tolerances. After it has been poured, to improve its performance, durability and aesthetics, the concrete can be cured efficiently. Importantly, precast concrete can be stored and delivered to t
Precast elements, which can range from several tons to several thousand tons, are much faster and easier to erect, while more advanced erection methods would usually be required. These techniques can be used by large mobile or site cranes (Figure 7), lifting frames or shear legs, gantries (some of which may be self-launching and very advanced mechanical/electrical engineering parts), falsework towers and a number of girders.
As such, pre-casting requires a higher level of capital spending, with not only more complicated erection processes, but also casting storage areas, transportation, and more equipment than is needed for in-situ works. Therefore, it tends to need greater planning and more care in its execution. Nevertheless, there is a significant benefit to precasting, but in-situ construction is still also very valid, in the right circumstances.
Much of this post as dwelled on the benefits of using concrete for constructing briges, it should be considered an introductory article. Future articles will consider the other aspects in detail.
The Institution of Structural Engineers (2014)- Technical Guidance Note CBDC Series (part 1)
Concrete Bridge Development Group (2000) Technical Guide No. 4: The aesthetics of concrete bridges, CBDG and The ConcreteSociety: Camberley, UK
Concrete Bridge Development Group (2014) Technical Guide No.14: Best construction methods for concrete bridge decks, CBDG: Camberley, UK (publication in 2014)
The Concrete Society (2010) Technical Report 72: Durable Posttensioned
Concrete Structures, The Concrete Society: Camberley, UK