The innovation and improvements to the concrete mixer design mean that architecture as a whole has been allowed to evolve. Modern mixers have contributed to the creation of dozens of amazing structures all over the world – structures including your house, town library, and the bridges you use to get to work and school!
Bridges specifically have seen significant improvements thanks to the use of concrete mixers. In this quick read, we’ll be covering some innovative bridge designs made possible by concrete mixers. For your own ingenious construction projects, BS Power offers a vast collection of reliable mixers and other construction equipment.
Ever wondered where Arch bridges get their strength? The answer lies in the vertical loads on the arch which generate compressive forces within the arch ring. The ring itself is made up of materials more than capable of withstanding these forces. The compressive forces within the arch ring make inclined thrusts at the abutments, and the arch abutments must be well buttressed to resist the horizontal and vertical parts found in the thrusts.
Conventionally, arch bridges were constructed using materials such as brick, stone, or mass concrete, mixed within a concrete mixer; this was done as these materials were seen as extremely strong in compression and provided the ability to configure the arches so that tensile stresses would not develop. Modern concrete arch bridges, made in the process using a concrete mixer, use reinforcing or prestressing to resist the tensile stresses that can sometimes develop within the slender arch rings.
The shape of the arch bridge attracted the attention of numerous early pioneers of concrete construction. In the 1930’s a man by the name of Freyssinet was accountable for constructing a monumental arched bridge in France at the Plougastel and after three years Robert Maillart, a Swiss engineer, brought to life the elegant and beautiful Schwandbach bridge. The bridge was created to have slender cross-walls tie the arch onto the horizontally curved roadway.
Reinforced Slab Bridges
For small spans, the simple design is a solid reinforced concrete slab. Generally speaking, cast-in-situ concrete is used as opposed to precast concrete slabs. Concrete mixers prove increidbly useful for cast-in-situ construction projects. These types of bridges are cost-effective due to their flatness. Fortunately, reinforcement is also not a complicated process. However, it is important to note that with larger spans, it is required that the reinforced slab is thicker so that it can carry the extra stresses under load.
This added weight of the slab can itself become a problem, however, this can be solved in one of two ways. The first step is to utilise prestressing techniques and the second step is to reduce the deadweight of the slab. This can be done by including what are called “voids”, often expanded polystyrene cylinders. With up to about 25m span, void slabs such as these are more economical compared to prestressed slabs.
Beam And Slab Bridge
Currently, the most common concrete bridges, made with a concrete mixer of some kind, in the UK are beam and slab bridges. They have the integrity economy, wide availability of the standard sections, speed of erection, and last but not least, simplicity. The reason for the success of standard precast prestressed concrete beams which were originally developed by the Prestressed Concrete Development Group was supplemented at a later stage by alternative designs, culminating in the Y-bean introduced by the Prestressed Concrete Association during the late 1980s.
Precast beams are usually placed on top of the supporting abutments or piers, typically on rubber bearings which are fortunately maintenance-free. After this, an in-situ reinforced concrete deck slab is poured from concrete mixers and when dry and in slab form is cast on permanent shuttering which spans between the beams. These precast beams can be joined together at the supports so that they form continuous beams that will be structurally more efficient.
Concrete beams and slab bridges that are simply supported are currently paving the way to integral bridges that can offer the advantages of lower maintenance and less cost because of the elimination of expansion bearings and joints.
Box Girder Bridge
The most common method of concrete bridge construction is prestressed concrete box girders which have spans greater than roughly 45 meters. The main spans are hollow and, depending on the bridge, the actual shape of the ‘box’ will vary as well as along the span, being deeper in the cross-section at the piers and shallower and abutments at midspan. The techniques utilised in the construction vary depending on the situation and actual design of the bridge. There are three main types which you can find: span-by-span, balanced cantilever, and launched.
When it comes to long multi-span viaducts that have limited access from below, is expensive or practically impossible, then the construction of the deck can be stated from one end to the other one span at a time, where an individual span reaches up to 60m. Bridges such as these are usually built in-situ where the falsework moves forward one span at a time, however, can be constructed from precast sections. In the beginning piers and abutments are constructed.
A gantry, that has the length of at least two successive spans, is positioned there initially between one abutment and the first inner support. In cementitious materials or epoxy, which can be mixed within a concrete mixer, the joints between the segments are cast and formed. When every single segment is installed at the appropriate positions and their joints are cured, then post-tensioning cables are passed through each segment and stressed. This creates the span between the two supports. At this stage, the gantry is then moved forward to the next span.
In instances where the deck is completely straight, or with constant curvature, and it has been stipulated to avoid giving falsework in the area, which is spanned, incremental launching may be suitable. Segments are constructed at the end of the preceding segments and are pushed into their correct placement. This process continues up until the entire bridge has been constructed.
Usually, either a steel beam or a truss element is connected to the leading edge of the bridge. This is done so that the movement of the cantilever on the main deck is reduced. Many individuals refer to this steel element as the nosing. To help and facilitate the movement of the deck, sliding bearings are implemented over the intermediate supports.
Ulrich Finsterwalder was a German engineer who in the early 1950s developed a particular manner of erecting prestressed concrete cantilevers each segment at a time with each additional unit being prestressed to those that have already been in position. This way avoids the requirement for falsework and since then the system has been developed.
Regardless if it has been created in situ or by using precast segments, the balanced cantilever is one of the most dramatic ways of constructing a bridge. The work starts with the building of piers and abutments. After that, from each pier, the bridge is built in both directions at the same time.
Constructing in this manner ensures that each pier will remain stable- hence “balanced”- until finally the structural elements individually meet and are joined together. In every single case, the segments are tied back progressively to the piers utilising prestressing bars or tendons threaded through each unit.
It can be debated what are some of the most difficulties in designing any structure, but ultimately it lies in deciding where the joints should be put.
These joints are necessary as they allow movement as the structure expands under the heat which occurs during summer or contracts as a result of the cold during winter. The expansions that occur in bridges are notorious for making bridges more susceptible and prone to leakages. Water burdened with road salts can reach the top of the abutments and the piers, and this can end up in the corrosion of all reinforcements. This can be very serious and dangerous, and the effects of the expansion can cause rust which can split the concrete apart.
On top of that, bearings and expansion joints are an additional cost so more and more bridges are now being constructed without either. Structures such as these are known as ‘integral bridges’, and can be built with all types of concrete decks and concrete mixers. They have been built with their decks joining directly to the supporting abutments and piers and without any distribution in the form of expansions or bearing joints for thermal movement. Thermal movement of the deck is supported by flexure of supporting the horizontal movements of the abutments and piers, this is done with elastic compression of the soil surrounding the area.
This bridge is already used for lengths which reach up to 60m, as time goes on the integral bridge is becoming more and more popular as designers and engineers find more ways of dealing with thermal movement.
One solution for really large spans is the cable-stayed bridge. This bridge has been typified by the Dee Crossing where all the elements involved happen to be concrete (usually mixed within a concrete mixer) the design consists of supporting towers that carry cables which support the bridge from each side of the tower.
The diagonal cables are used to transfer the vertical loads from the deck directly towards the tower. Consequently, the main deck of a cable-stayed bridge works just as if were a continuous beam on cable supports; these tend to be more flexible compared to pier supports, with the added compression force throughout the whole deck. The majority of cable-stayed bridges are built utilising a form of cantilever construction which can either be precast concrete, usually mixed with a concrete mixer or in-situ.
When it comes to the construction of suspension bridges concrete and a concrete mixer play an important role. Typically embedded into the ground are massive foundations which are used to support cable anchorages and the weight. Additionally, there are also abutments, once again probably in the form of mass concrete, which is once again mixed within a concrete mixer. The mass concrete provides vital strength as well as the ability to resist massive forces, and to add to this, the slender superstructures carrying the upper ends of the supporting cables are generally also constructed out of reinforced concrete.
About BS Power
BS Power manufactures a variety of high-performance construction equipment suitable for anyone who feels the urge to get involved in the agricultural, forestry, and construction industries as well as any individual who would like to start their journey in the world of construction. If you would like to know more about our construction products as well as how these materials and tools are used visit our website today!