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A pin-loaded strap element may provide a practical means. This element consists of a unidirectional FRP lamina wound around endpins in a racetrack manner. No machining of holes is required. The layers in the composite are cured to produce a solid laminate. Circular pins transfer the tensile load to the components being joined. Such straps have many desirable characteristics, including high tensile load capacity, low weight, low thermal conductivity and low thermal expansion. As a result, laminated pin-loaded straps have been used in many different structural applications, such as temporary bridges. They are ideally suited for bridge repair due to the very simple loading technique with pins. However, both experimental and theoretical studies have revealed “high” stress concentrations next to where the strap leaves the pin. The effect of these concentrations is to considerably reduce the load at failure compared to that of the straight solid laminate, as determined by a standard coupon test.

One means of reducing these undesirable stress concentrations is to replace the solid laminate by the non-laminated equivalent. In the "new" strap, there are a number of non-laminated layers formed from a single, continuous, thin thermoplastic. This type enables the individual layers to move relative to each other. The undesirable stress concentrations are therefore reduced because this structural form has inner shear stress concentrations to be reduced in order to achieve a uniform direct strain distribution in all layers through the thickness.

Apart from improving stress distribution, winding can easily be performed on of the components to be connected. The cost effectiveness of the nonlaminated strap is also superior to the laminated strap because the consolidation process is not required.

This system could have an excellent future in bridge repair. There is a high probability that "non-laminated FRP straps" will be as strong as cables for external post-tensioning, and much cheaper.

IV. Look through the text once again and complete the following sentences.

1.Bridges may need reinforcement because of a change in … and … due to external factor.

2.Another reason making reinforcement necessary is … of a bridge or its parts.

3.There is another severe problem – … of prestressing steel.

4.The way of achieving structural strengthening is using adhesively bonded external steel …

5.From 1982 … have been successfully employed for the post-strengthening of reinforced concrete beams.

6.The classic adhesive for bonding is …

7.Post-strengthening with strips is best suited for …

8.Another important method for rehabilitation is …

9.For external post-tensioning instead of steel … was used.

10.Non-laminated fibre-reinforced plastic … have an excellent future.

V. Write down a plan of the text and let your partner explain each item.

VI. Work in groups. Read the information of how the discussed composed materials are used in bridge repairing. Write out the key phrases on the blackboard and retell your piece of information in Russian (one example is for one group).

1. This bridge, located in the Canton of Lucerne, was completed in 1969. In 1991, it needed repairing. The bridge was designed as a continuous, multispan box beam with a total length of 228 m. The damaged span of the bridge had a length of 39 m. The box section is 16 meters wide, with a central, longitudinal web. During core borings performed to install new traffic signals, a posttensioning tendon in the outer web was accidentally damaged with several of its wires completely severed by an oxygen lance. As a result, the granting of authorisations for special, heavy loads across the bridge was suspended until after completion of the repair work. Since the damaged span crosses Swiss National Highway A2, the traffic lanes in the direction of Lucerne on this highway had to be closed during the repair work. The work could therefore only be conducted at night. Carbon fibre-reinforced plastics (CFRPs) are forty to fifty times more expensive per kilogram than the steel used to this date (Fe 360) for the reinforcement of existing structures. Do the unquestionably superior properties of CFRPs justify their high price? When one considers that, for the repair of the Ibach Bridge, 175 kg of steel could be replaced by a mere 6.2 kg of CFRP, the high price no longer seems so excessive. Furthermore, all the work could be carried out from a mobile platform, thus eliminating the need for expensive scaffolding. The bridge was repaired in 1991 with three CFRP strips of 5000 mm length. The properties of these strips are given in Table I, strip type No. 3. A loading test with an 84-tonne vehicle demonstrated that the

reinstatement work with the CFRP strips was a complete success. The experts working on the repair of the Ibach Bridge were pleasantly surprised at the simplicity of applying the 2 mm thick and 150 mm wide CFRP strips. This was the first repair of a bridge with externally bonded CFRP strips in the world. Since 1991, this application has enjoyed success exceeding all expectations.

2. The covered wooden bridge near Sins in Switzerland was built in 1807 to the design of Josef Ritter of Lucerne. The original supporting structure on the western side is almost completely preserved to this day. The eastern side was blown up for strategic reasons on November 10, 1847 during the civil war. In 1852, the destroyed half of the bridge was rebuilt with a modified supporting structure. On the western side, the supporting structure consists of arches strengthened with suspended and trussed members. On the eastern side, the supporting structure is made up of a combination of suspended and trussed members with interlocking tensioning transoms. Originally, the bridge was designed for horse-drawn vehicles. Since the thirties, vehicles with a load of 20 tonnes have been permitted. In 1992, the wooden bridge was in urgent need of repair. It was decided to replace the old wooden pavement with 20 cm thick bonded wooden planks, transversely pre-stressed. The most highly loaded crossbeams were strengthened by EMPA (EMPA is the German acronym for Swiss Federal Laboratories for Materials Testing and Research) using carbon fibre-reinforced epoxy strips. Each of these crossbeams was constructed of two solid oak beams placed one upon the other. In order to increase the depth, wooden blocks were originally inserted between the beams. The lower beams were 37 cm deep and 30 cm wide; the upper beams 30 cm deep and 30 cm wide. The crossbeams were strengthened either with 1.0 mm thick CFRP strips made of high-modulus M46J fibres or with 1.0 mm thick CFRP strips made of highstrength T700 fibres. The M46J strips were 250 mm wide at the top and 200 mm wide at the bottom. The T700 strips were 300 mm wide at the top and 200 mm wide at the bottom. Before bonding the strips, the bonding surface was planed with a portable system. Selected crossbeams are equipped with strain measurement devices, which allow long-time monitoring. Up to now, the results are very satisfactory. After application of the CFRP strips pulse infrared thermography was applied very successfully for the first time for quality assurance of the bonding. The historic wooden bridge in Sins is a valuable structure, both from the aesthetic and from the technical viewpoint. It is also of historic value and under protection as a national monument. The technique using CFRP strips is especially suited for post-strengthening structures such as this since the thin but extremely stiff and strong strips are hardy noticeable and therefore do not detract from the original design of the structure. Since 1992, the strengthened crossbeams of the Sins bridge with CFRP-strip reinforcement have helped to provide practical experience under extremely high loading and built up

confidence in this technique for preserving historic bridges. Meanwhile, many similar structures have been rehabilitated in this manner in Europe and in North America.

3.Rehabilitation of the Oberriet-Meiningen Bridge was planned in late 1996. The bridge, built in 1963, spans the border between Switzerland and Austria, linking Oberriet to Meiningen. It crosses the River Rhine in three spans (35-45-35 m) as a continuous steel/concrete composite girder. Due to increased traffic loads, post-strengthening of the concrete bridge deck became necessary. The application of a total length of 640 m of CFRP strips has proved extremely successful. Thorough investigations have shown that beside routine maintenance the concrete bridge deck was also in need of transversal strengthening. This was obviously due to the fact that the deck was designed in 1963 for the then standard truckload of 14 tons. Today, the standard truckload for this type of bridge is 28 tons.

Because the existing concrete was in good condition and the chloride concentration in the concrete exceeded the critical values only in the outermost 10 mm it was decided not to replace the deck. Simply increasing the depth of the deck by adding concrete to attain the necessary transversal flexural capacity, would, however, have caused inadmissible longitudinal stresses for the superstructure. Bonding of additional reinforcements therefore remained the only solution. Structural components post-strengthened with bonded plates or strips were to have a total residual safety factor of 1.2 after failure of the plates or strips. The fact that the required strengthening factor was 2.15 meant that the sectional area of the deck slab still had to be increased. Bonding transversal CFRP strips on the bottom of the slab and adding 8 cm of concrete on top of the slab made it possible to meet all requirements. Adding new concrete also allowed removal of the top layer of concrete with the high chloride concentration by water blasting. CFRP strips 80 mm wide and 1.2 mm thick (70 vol% T700 fibres, strength 3000 MPa) were chosen for post-strengthening. A total of 160 strips, each 4 m long, were laterally bonded to the bridge deck every 75 cm.

4.This bicycle and pedestrian bridge over the river "Kleine Emme" near Lucerne was post-tensioned with 2 CFRP cables in October 1998. The bridge is

3.8m wide, 47 m long and is designed for the maximum load of emergency vehicles. The superstructure is a space truss of steel pipes in composite action with steel post-tensioned with two CFRP cables inside the tube. Each cable was built up with 91 pultruded CFRP wires of 5 mm diameter. The post-tensioning force of each cable is 2.4 MN. Therefore, the CFRP wires are loaded with a sustained stress of 1350 MPa. Each cable is equipped with three CFRP wires with an integrated the pultrusion process. In the post-tensioning phase it was

possible to calculate the post-tensioning force at all times from the data of the wires with calibrated sensors. Monitoring has continued since then and up to now no relaxation has been observed.

5. The "Verdasio" bridge is a two-lane highway bridge and was built in the seventies. The length of the continuous two-span girder is 69 m. A large internal prestressing steel cable positioned in a concrete web corroded as a result of the use of salt for de-icing. It was replaced in December 1998 by four external CFRP tendons arranged in a polygonal layout at the inner face of the affected web inside of the box. Each cable was made up of 19 pultruded CFRP wires with a diameter of 5 mm. Here too the cables are equipped with sensors to measure the post-tensioning force.

VII. Match the given words with their common and special meanings (consult the dictionary). In what meaning are these words used in Ex. VI?

VIII. Put the following sentences in the correct order to know the process of flexible fabrics application.

Application of sheets (flexible fabrics) would normally require the following steps:

-Remove dirt from concrete surface, round off sharp corners (minimum radius should be in the order of 25-30 mm).

-Apply putty after the primer becomes tack free. After putty application, allowable unevenness must be in the order of 1 mm.

-Primer coating, putty application (optional).

-Mix the epoxy resin and apply it on the concrete surface (undercoating).

-Adhere the fabric.

-Blow or sweep dust off the concrete surface after sandblasting or grinding, dry the concrete (if wet).

-Press using a roller, allow for complete fabric impregnation and apply another coating.

-Protect reinforcement from rain, sand and dust; apply paint (if needed) once the resin is tack free.

-Remove residual resin using a rubber scraper.

Post-strengthening with sheets is best suited for wrapping of columns with a rectangular cross-section. A minimum radius of curvature of approx. 25 mm is required.

Home Exercises

I.Memorize the words from Ex. I page 111.

II. Advertise any composite material you have read about.

Text 31 

I. Try to translate the following word combinations which are made according to N + N model.

world obstacles; sea currents; boundary dimensions; seabed silt; maintenance costs; bridge supports; weather conditions; railway tunnel; travel time

II. Look at the title of the text. What do you think it is about? Say words you expect to find in the text.

III. Now read the text, check your guesses.

Bridge or Tunnel

The vast water areas all over the world represent the world obstacles for people’s communication. A man has been striving to subdue water spaces by means of the main road connecting continents, islands, etc. because the economy becomes globolised.

The English Channel separating Great Britain and Europe the Straits of Gibraltar between Europe and Africa, the Bosporus between Europe and Asia, the Bering Strait connecting the Eurasian and American continents, Japaneseislands might offer the missing link for rapid increase land-based transportation to promote improved trade and commerce to facilitate greater economic integration.

Builders always have the choice between bridging and tunneling in crossing over or under a large waterway. Each sort of a structure offers its advantages

2.Bridges require lower maintenance … than tunnels. a) corrosion b) corrugations c) costs

3.Tunnels are much … as compared to the bridge crossing. a) saving b) saver c) safer

4.Weather conditions cannot influence the … through tunnels. a) traffic b) transfer c) transit

5.The cost of the tunnel under the English … is enormous.

a) Cheddar b) Channel c) Canal

V. Read the text once again and find the word that means the opposite of

VI. Mark these sentences true, false or not given.

1.______ The Straits of Gibraltar separates Europe from Africa.

2.______ The under clearance of the bridges must be over 60 m high not to prevent shipping.

3.______ For the first time the Bosporus was bridged in 1973.

4.______ Tunnels require lower maintenance costs.

5.______ Vessels can navigate much easier if there are no bridges.

6.______ The Bosporus bridge managed to withstand the wind blows up to 162 km per hour.

VII. Copy the table and complete it using the information from the text and your own knowledge.

bridge tunnel

advantages

disadvantages

passages, sewerage, water supply, water power, public utility and canal. There are also immersed tubes. The pipes of great length are laid into a trench in the sea or riverbed and jointed under water. The tunnel length considerably exceeds its cross section. Tunnels are the most complicated engineering works and call for great expenses.

Tunnels classification as well as bridges classification involves several indications.

Indication 1 – by the tunnel function.

1.1.Traffic tunnels. (Much attention will be paid to this kind of tunnel works).

1.2.Mine tunnels. (For mining mineral resources).

1.3.Public utility tunnels. (For public utilities in large cities – water supply and sewerage, electricand telecables).

1.4.Water power or hydraulic tunnels. (For water supply and water discharge).

1.5.Special-purpose tunnels. (For increasing the country defensive capacity).

Let’s pay due attention to the traffic tunnels because the students of the «Bridges and Transport Tunnels» faculty deal with these type of tunnel works. The traffic tunnels may be classified as following:

Indication 1.1 – by the tunnel function.

1.1.1.Railway tunnels.

1.1.2.Motor way tunnels.

1.1.3.Pedestrian tunnels.

1.1.4.Metro tunnels.

1.1.5.Shipping tunnels.

Indication 2 – by the tunnel location.

2.1.Plain tunnels.

2.2.Mountain tunnels.

2.3.Submarine tunnels.

The Mersey Tunnel in Great Britain belongs to the longest submarine tunnels and links Liverpool and Birkenhead. The workers began tunneling from the both banks of the river and the breakthrough took place in the middle of the river beneath the riverbed.

Submarine tunnels can be bored through the rock or sometimes they appear to be huge metal tubes resting on the riverbed and having their portals on the opposite banks of the water obstacle.

There are several tunnels beneath the Thames in London which provide railway and foot passages under water. The twin tunnels the Blackwall Tunnel (Southbound) and the Blackwall Tunnel (Northbound), the Greenwich Foot Tunnel and the Rotherhithe Tunnel link the motor ways of the both banks.

Indication 3 – by the construction method.

3.1.Tunnels built by the cut-and-cover method.

3.2.Tunnels built by cutting technique or rock tunnels.

3.3.Shield driven tunnels.