New experiences with epoxies for structural applications

New experienoes with epoxies for structural al Oations F. Hugentchmidt

The properties and testing of structural adhesives are reviewed in relation to such applications as segmental concrete bridge construction and the repair and strengthening o f reinforced concrete structures. Criteria for the selection o f epoxy adhesives are discussed with particular emphasis on creep deformation, heat stability, moisture resistance, on site conditions, handling, and field testing. Supportive structural tests for certain large-scale applications are also described. Key words: epoxy resins; concrete structures; construction sites; fatigue testing; environmental testing.

Epoxy resins have been used in bridge construction in the USA since 1955 - for such applications as filling cracks and joints in concrete bridge decks, and for bonding steel beams to concrete slabs. The first concrete bridge built using bonded segmental construction was across the Seine in Paris in 1963. Since then, this method has been used for many bridges in various countries. More recently, concrete structures - both buildings and bridges - have been reinforced with steel plates integrally bonded to the concrete with an epoxybase adhesive. The large number of completed jobs is a fair indication of the experience that has now been accumulated in the handling and processing of epoxy resin adhesives and mortars. However, this does not necessarily apply to the basic data required for the safe application of epoxy resin mortars, such as selection criteria, material property figures, long-term behaviour, etc. A report published in November 19741 by the Transportation Research Board in the US refers to a need for: 'the development of a system for classifying adhesive compounds by use'. The problem is described in this manner: "Considerable effort is being devoted to the writing of guides for the proper use of adhesive compounds in maintenance and construction operations. While such guides provide excellent information on application techniques, they cannot tell the reader which of the myriad of available compounds are suitable for particular jobs. A system through which adhesive compounds can be grouped according to their uses is needed." The point is also made - and justifiably so - that the multitude of products offered obviously makes it very difficult for the construction engineer to make the right choice. Without a doubt, the amount of effort expended on the problem of selecting, testing, and inspecting synthetic resin mortars has increased in recent years. The basis for specifi-

cations have been or are being worked out in various countries and by international professional bodies (eg 'Tentative design and construction specifications for pretest segmental box girder bridges' of the Prestressed Concrete Institute,Chicago; 'Proposal for a standard for acceptance tests and verification of epoxy bonding agents for segmental construction' of the Fdd~ration lnternationale de la Prdcontrainte (FIP)). In Switzerland, the Federal Materials Testing Institute is conducting an extensive research project on: 'Bonded reinforcements' with the goal of preparing dimensionh~g guidelines for the reinforcement of existing structures. However, the laying down of specifications presumes that clarity prevails on the selection criteria and the limits set by the material, ie the epoxy resin mortar.

The selection of appropriate epoxy resin systems Structural applications A structural problem is one whose solution calls not only for information on short-term strengths of epoxy resin mortars, but also on their deformation under static load, behaviour under continuous or dynamic stressing, behaviour at elevated temperature and, in many cases, the effect of moisture on adhesive strength. By this definition, 'structural applications' include the joining of pretest concrete elements, the integral bonding of new concrete to old, of steel to concrete, or of steel to steel, as well as 'straightforward' applications such as the grouting of rails and machinery, repairs to loadbearing members, injections, etc. Applications like industrial flooring, antiskid surface dressing, and bridge nosings are not included here.

Solution criteria Because the properties of epoxy resin systems are highly dependent on the prevailing temperature, it is necessary to exercise considerable care in establishing the temperature

0143-7496/82/020084-13 $03.00 © 1982 Butterworth & Co (Publishers) Ltd 84

INT.J.ADHESION AND ADHESIVES APRIL 1982

limits within which each system can be used, and to employ mortars with different reactivity on jobs that have to be carried out over a broad range of temperatures (eg jobs that take over a half a year to complete). Further consequences of this temperature sensitivity are: that testing individual systems and comparing systems becomes a relatively costly affair; and that quite narrow limits are placed on the selection of appropriate systems, Without any doubt, the construction engineer is interested above all else in the long-term behaviour of a building material - even if it is used as an adhesive with only a thickness of 1-2 mm. Suitable criteria for assessing the long-term behaviour are: determination of deformation under permanent load (creep) at room temperature; behaviour at elevated temperature; and resistance to water and alkalis. Fig. 2 Determination of creep at rising temperature showing specimen (under load) and control (unloaded) in heating chamber with recording apparatus on the left

Creep deformation

At Ciba-Geigy creep deformation is measured on prisms of 40 x 40 × 160 mm generally at a temperature of 23°C and a pressure of 20 N / m m "z (Fig. 1). Temperature, filling ratio (binder:aggregates), and applied load, are varied for individual mortars. The determination of creep deformation has proved to be one of the most important selection criteria. Of two epoxy-base adhesive mortars quoted for use on the Sallingsund Bridge, in Denmark, one system had 20 times greater creep deformation than the other. Highly cross-linked epoxy resin systems exhibit creep deformation at room temperature approximately 3 to 4 times greater than that of concrete. Temperature

behaviour

At Ciba-Geigy temperature behaviour is determined separately for mortar and adheisve. Mortar prisms measuring

40 x 40 x 160 mm are produced with a constant filling ratio of 1 : 3.5 (binder: aggregate) and stored for 7 days at 23°C. They are then placed under a constant load of 20 N/ram =, the temperature is raised by 0.1°C/rain starting from 23°C, and the deformation is established as a function of time and temperature (Fig. 2). The change in length caused by the heating is measured simultaneously on an unloaded prism and compensated directly by means of a measuring bridge. Fig. 3 shows the deformation curves for three mortars. These deformation curves provide only a vague indication of the temperature limit that can be withstood under the given load, without exceeding a pre-established compressive strain. To establish this temperature limit, the test was modified in that the temperature was raised up to a fixed step and then held constant. The maximum compressive strain was set at 1%. The temperature vs time diagram and resulting strain are given in Fig. 4. Fig. 5 shows the temperature vs deformation diagrams for the same mortars as in Fig. 3. If a structural component, were produced from epoxy resin mortar, it would be expected to behave in the same manner under these temperatures as the tested mortar prisms. 1.5 .....

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INT.J.ADHESlON AND ADHESIVES APRIL 1982

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The behaviour of an epoxy resin system at elevated temperatures is affected by a number of factors, such as binder system, f'filing ratio, shape and preparation of test specimen, loading and temperature rise. The conditions under which the TKel figures (see Fig, 4) were obtained can be regarded as very unfavourable. In practice, components made from epoxy resin mortar will have a more favourable shape and generally be subjected to less loading. Hence the TKel figures can be regarded as a lower limit. The adhesive mortars used to join prefabricated concrete elements in bridge construction and in external reinforcement work will withstand much higher temperatures in the cured state. The TKt]oofigures indicate the maximum temperatures at which bonded surfaces can transmit loads safely. In general it should be noted that the temperatures stated are reached only by highly cross-linked binder systems. When the Rio de Janeiro-Niteroi bridge was built, using precast units, one of the most important criteria in selecting the adhesive was its behaviour in the 20-70"C temperature range. Mortar cylinders 30 mm indiameter and 100 mm long were subjected to torsional stress at rising temperature. The results, which are illustrated in Fig. 8, constitute a clear verdict for the suitability of the different adhesives offeredz' a. Resistance to alkalis and water

The temperature behaviour of adhesive joints is investigated with prisms bonded at 45 ° . Specimens and stress distribution are indicated in Fig. 6. The bonded specimens are exposed to standard conditions (230C, 50-70% RH) for 28 days prior to testing. First the failure temperature is established. The specimen is placed under constant t 0 N/mm 2 loading and heated up by 0.2°C/rain starting from 23°C; this temperature rise is applied until the prisms come apart. The level reached is defined as the short-time failure temperature TKto (mean from 5 readings). The next step is to fred the temperature at which the bonded specimens will bear the constant load for at least 100 hours without visible deformation. This is done by heating identical specimens again by 0.2°C/min under the same conditions, but stopping at a temperature below TKto and maintaining that temperature until the prisms part. Varying this temperature threshold makes it possible to plot a creep diagram as shown in Fig. 7. The curve approaches a limit of TKtlooasymptotically. Below this temperature there will be no failure under the given conditions.

---~'--

to

=-

The determination of resistance to alkalis and water is important for two reasons. First, concrete always contains moisture, and joint surfaces on a construction site will show a high moisture content after rain. Second, one of the main reasons for using an epoxy resin adhesive to join precast concrete bridge segments is to seal the joint permanently and protect the post-tensioning tendons against corrosion. The test is relatively simple. Dry or damp cement/mortar prisms measuring 40 x 40 x 80 mm ate bonded, cured for a specified length of time under specified humidity conditions, and finally immersed in water. Such prisms that were bonded dry, cured for 7 days at room temperature and then immersed in water for up to 2 years showed no loss of F

t Cement

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86

INT.J.ADHESION

A N D A D H E S I V E S A P R I L 1982

Fig. 6 Test ~aecimen for determining the temperatur ~ .l~=haviour. of an adhesi~ joint (8tl dimensions in ~ . ) : applied, tollO, P = I~.KN; axial ¢oml~msive =trm=. G - 10 N/ram ; ¢ompreulve stress on Iomt, Gj = 10 N/mmZ; shear stress, ~j = 10 N/ram z

,20

segmental bridge job for instance, the adhesive should be applied only when the new element has been brought close enough to the existing bridge superstructure to allow the application to be made from there.

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flexural strength when highly cross-linked epoxy resin adhesives were used. Experience at Ciba-Geigy indicates that the long-term tests described are highly appropriate for rating the suitability of epoxy resin systems for structural applications. The best results have been obtained with highly crosslinked binders based on selected medium- to low-viscosity epoxy resins cured with modified polyamines. These comprise only a small group of binders, which simplifies matters for the user. These highly cross-linked binders and mortars are also highly reactive. Therefore, the other properties such as pot life, open or contact time, and curing rate that are of interest to users and contractors have also to be examined. As already mentioned, systems having different reactivity should be chosen for use at different temperature ranges. A useful breakdown, and one postulated by the FIP, employs three temperature intervals: + 5°C to 20°C (fast-reacting systems), and 15°C to 30°C (medium-fast-reacting systems), and 25°C to 40°C (slow-reacting systems). Pot life and contact time at the upper temperature limit have to be sufficiently long to suit the particular working conditions, yet the reactivity at the lowest temperature must still be high enough to ensure adequate cross-linking and reasonable progress for the given construction method. For mortars with a low aggregate ratio, the systems which have been found-to be suitable have minimum pot lives of about 30 minutes, and their contact times are between 45 and 90 minutes. Their reactivity at the lower temperature limits suffices to ensure that bonded cement/mortar prisms (compressive strength 60 N/mm 2) subjected to a flexural test will be certain to exhibit fracture in the cement mortar only after about 16 hours. In the majority of cases, work can be organized on the construction site to conform with the above times. On a 2

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One of the major prerequisites for obtaining the data necessary to permit safe application of a construction material is the running of tests on large specimens. In particular, such specimens lend themselves to the investigation of behaviour under continuous and dynamic loading. Sound conclusions on strength changes as a function of time can also be drawn from the results of tests under dynamic loading. Furthermore, long-term tests under static load can be run out-of-doors, so that the results can include the effect of weathering - which can be quite meaningful, especially in the case of thin bonding layers. It is very important that outdoor testing includes the recording of temperature - daily maximums and minimums - and perhaps the humidity as well. From mid-1972 to the end of 1973, the Swiss Federal Institute of Materials Testing at Duebendorf carried out tests on bonded reinforced concrete beams on behalf of Ciba-Geigy4. The object was to test two adhesives - one a fast reacting 'winter formulation', the other a slower reacting 'summer formulation' - under both staUc load and dynamic fatigue stressing. A monolithic concrete beam of identical dimensions was used as a reference. The dimensions of six test beams and the configuartion of the reinforcement and the bonded joint are given in Fig. 9. The shear reinforcing consisted of closed half-stirrups that did not penetrate the bonded surface. 28 days after the bottom half of each beam had been cast, the adhesive was applied in a layer approximately 1 mm thick, the reinforcing cage was laid on, and the rest of the concrete was poured. The results of the creep test on three beams are shown in Fig. 10, the reference beam is No. 7. All three specimens exhibit smaller deformation behaviour but the bonded beams show a creep that was some 10-20% higher. One beam was subjected to a pure fatigue test; three other beams were first subjected to a static sustained-load test. All of the beams withstood at least 4.1 x 106 load cycles at a maximum load o f F o = 2.5 Fadmissin the ultimate load stage. No relative movement of the two beam halves was observed. Up to the moment immediately before failure, the bonded concrete beams behaved like monolithic beams. Failure occurred because the strength of the concrete was exceeded. Fig. 11 shows the load v s deflection diagram of a beam. The tests demonstrated that the two epoxy resin adhesives used, and therefore the binders used to formulate them, are suitable for integrally bonding new concrete to old. But the same binders can be used to formulate adhesives for bonding precast concrete units, steel to concrete, or steel to steel. Tests on large-scale specimens are often carried out to establish the effect of conditions peculiar to the particular job. Such conditions may result from factors such as the type of construction, shape of a cross-section, or specific material requirements. A case in point is the large-scale test conducted in connection with reinforcing work on a Zurich telephone exchange (described below) where one of the floors was strengthened by bonding steel plates to it and a

INT.J.ADHESION AND ADHESIVES APRIL 1982

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and was then loaded statically to failure. At 2.5 times the design load, three shear angle plates parted with a 'bang', and at 2.8 times the design load the ends of the tensile steel reinforcement plates parted from the concrete beam; failure followed. The test demonstrated the excellent adhesion between the steel reinforcing plate and the concrete to which it was bonded. The improved rigidity of the reinforced beam is clearly shown by the much Steeper slope of the load v s deflection curve.

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large-scale test was set up on a T-beam simulating those in the existing floor s. The T-beam with its dimensions, the instrumentation, and the new tension and shear reinforcement is shown in Fig. 12. Fig. 13 shows the load v s deflection diagram for the centre of the beam. First the test beam was loaded statically - with a load corresponding to the dead and live loads existing in the building - and then the reinforcement plates were bonded to it. After the adhesive has cured, the beam was subjected to 2 x 10 6 load cycles,

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INT.J.ADHESION

v$

deformation diagram for beams 1 , 4 and 7

A N D A D H E S I V E S A P R I L 1982

P/2

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Fig. 15 the test set-up, and Fig. 16 the crack pattern in the area of rupture of the prestressed beam. The failure load was 2.3 times the design load. Kupfer points out that the diagnoal compressive stresses are easily transmitted across the bond; that is, adhesion and friction of the adhesive mortar transmit compressive forces acting at an angle of about 45 ° in the surface between eRoxy resin and concrete. Her.ce one need have no qualms in applying the shear theory of reinforced and prestressed concrete bonded constructions, provided that adequate precautions have been taken to assure a good bond. The three types of test described above indicate the sort of performance to be expected from epoxy resin adhesives. Because such tests are very costly to run, only binders that have been conscientiously developed to deliver the properties required and have been carefully tested should be considered; the same applies to the adhesive mortars formulated from them.

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In 1973 tests were started at .the Swiss Federal Materials Testing Institute on bonded reinforcements with the aim of working out dimensioning data for the external reinforcement of reinforced concrete structures. The project was divided into preliminary testing, main testing and long-term testing. The preliminary tests dealt with the effect of the width, thickness, and length of the reinforcing plates, and with the problem of the butt joints between the plates. Of special interest are the main tests completed in 1978

15

Deflecl"~on, 8 (mm) Fig. 1 1 New-to-old concrete bonding: load v$ deflection diagram

for beam No 4 under fatigue loading

Before the final selection of the adhesive mortar for the Rio de Janeiro-Niteroi bridge, a test was likewise arranged on a large specimen:' a. Fig. 14 shows the test specimen as a section of the bridge in the neighbourhood of the piers,

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I N T . J . A D H E S I O N A N D A D H E S I V E S A P R I L 1982

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After sandblasting, the steel reinforcing plates were given two coats with a solvent-type epoxy resin primer. In the static failure test the yield point of the steel plate was reached under a load of 517 kN, and at a load of 596 kN classic bending failure took place with spreading of the concrete failure zone. When failure occurred, the deflection was about 230 mm or about 1/25 of the beam span. The shear plate design selected was able to absorb the loads up to the point of concrete failure (Fig. 18). A fatigue test on an identical beam was carried out in the steps shown in Table 1. Table 1.

Sequence of loading

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Static test

Fatigue test Pu Isating stress, o I/o 2, steel (N/mm 2)

Number of load cycles (x 106)

I II II I IV

x x x x

240/120 300/150 360/180 400/200

2.31 1.64 2;00 2.00

V*

x

---

7,95 *In step V the beam failed under static loading when the steel plate began to yield at a stress of 414 N/ram2.

The third part of the test programme involved investigation of the long-term behaviour o f concrete beams with bonded external steel plate reinforcement. Test parameters chosen were tlie type of loading, type of rust preventive applied to the external plates, and outdoor weathering. A total of 66 reinforced concrete beams measuring 0.15 x 0.25 x 2.40 m were produced, The conventional reinforcement comprised two rods 8 mm in diameter, and the steel plate had a cross-section of 3 x I ~ mm and a length Of 1950 ram. In all, the reinforcement added up to: internal rod reinforcement external reinforcement Fig. 15

90

Test beam under load

INT.J.ADHESION A N D ADHESIVES APRIL 1982

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The programme was started in November 1977 and is scheduled to cover a 10-15 year period. The scope of testing is shown in Table 2. The bonding surface of the concrete beams was roughened with a pneumatic granulating hammer until the grain structure became visible. Then the beams were preloaded in such a way that at least four bending cracks formed per beam. After removal of the load, the beams were placed in storage until the time came to bond the steel plates to them. The steel plates were first shotblasted. Following degreasing they were given two primer coats, some all over, and the rest on the bonding surface merely 10 mm inward from the edge. The primer selected was an epoxy/polyurethane combination that is claimed to provide excellent adhesion, rust prevention, and thermal resistance. After the steel plates had been attached and the adhesive cured, an initial 6 beams (batch zero) were subjected to loading up to failure to determine the starting strength. No difference was observed between the beams with plates coated all over with primer and those primed merely at the edge. Some of the beams were loaded only with their own weight (10 outdoors, 9 indoors), the remaining 41 beams were subjected to permanent loading (a concrete weight of T a bl e 2.

about 20 kN hung at the one-third points along the 2.0 m span) 30 of them outdoors and 11 indoors (Fig. 19). After one year of exposure, static load tests were carried out to failure on 12 reinforced beams selected as shown in Table 3. Table 3.

Selection of beams f o r static load tests

No of beams

Conditions

3

loaded

weathered

plate with edge primer coat

3

loaded

weathered

plate primed all over

2

unloaded

weathered

plate primed all over

2

unloaded

unweathered

plate primed all over

2

loaded

unweathered

plate primed all over

S c o p e o f testing

Environmental condition

Number of beams Open air

Indoors

Loading*

g

g

Overall primer coat Edge primer coat, 10 mm

-

Total

10 30

g+2F

10 15

*g = dead weight; 2F = applied load

15

g+2F 9

-

14 3

9

17

Fig. 18 T-beam after rupture in the compression zone: the steel plate had exceeded t h e yield point at the centre and separated from the concrete when the load was removed

INT.J.ADHESION AND ADHESIVES A P R I L 1982

91

Fig. 19 Long-term open-air test on concrete specimens: under dead weight (foreground); and live load (background)

Fig. 21 rupture

Results

mixed thoroughly and in the proper ratio simply by looking at the mixture.

The failure loads of the 1-year-old specimens do not show any significant differences from batch zero, nor do the deflections at the centre of the beam. The load v s deflection diagrams are virtually identical. Failure occurred consistently as a result of compressive strain in the concrete compression zone approximately at the centre of the field, with simultaneous yielding of the steel. In the area of constant bending moment, the steel plates parted from the adhesive, while failure occurred in the concrete outside of that area. (Figs 20, 21). Investigation of the steel plates for traces of rust showed that there was no rust on the plates primed all over, but a slight amount of rust had formed inside the edge on one primed merely at the edges. The creep measurements revealed that the indoor beams creep more initially than do those exposed to weather. However, the latter exhibit greater permanent deformation (deflection) than the ones kept indoors.

Test beams after steel plate had been removed following

Rio de Janeiro - N i t e r o i bridge

On the Rio de Janeiro-Niteroi bridge the adhesive was delivered in 5 kg cans. The mixing ratio of the two components was 1 : 1 and the hardener can was sufficiently large for the two components to be mixed in it. One large and one small can belong together and the labels were printed in red and blue, respectively. These measures prevented any mixing mistakes even though the work was done by untrained personnel. The adhesive was applied to both of the surfaces being joined, which improved the wetting o f the concrete and enhanced adhesion (Fig. 22). A company quite independent of the building consortium was entrusted with checking the adhesive - and all the other materials used - during

Job-site practice Bonding of prefabricated concrete units

The delivery of adhesive mortar to site in working packs consisting of resin and hardener components seems to have been adopted everywhere. To avoid mixing errors, it is important to specify that the individual components be pigmented in sharply contrasting colours. This is the only easy way to check whether the resin and hardener have been

Fig. 20

92

Test beam under load

INT.J.ADHESION AND ADHESIVES APRIL 1982

Fig: 22 Rio de Janeiro-Niteroi bridge, s ~ w i n g ~ x y adhesive being applied to both surfaces

the entire operation. For each joint, three prisms measuring 40 x 40 x 160 mm were prepared. In practice, two new concrete elements (one at either end of the bridge) were added at 6-hourly intervals, and the first prism was tested after this same interval. If it did not reach the prescribed flexural strength of 20 N/mm 2, the second and third prisms were tested after a period of time determined by the construction engineer on site. If it was found that a drop in temperature was preventing the prescribed strength from being reached, the splitting tensile strength of the 40 x 40 x 80 mm prism halves was determined. The relationship between flexural and splitting tensile strength had been established beforehand. Thus the relatively low number of test specimens provided enough readings t ° follow the strength development of the mortar at a joint. Working temperatures were usually above 20°C but at the centre of the bridge the wind lowered the temperature to about 12°C. Consequently, a faster reacting adhesive had to be used on this part of the bridge. It is essential to check the adhesive right on site. The cost is minimal. The checks do not guarantee prevention of every single mistake, but those occurring are at least discovered very quickly. This also helps train the application crew to work very carefully. Moreover, achievement of the prescribed strength figures means that the supplier of the adhesive mortar and the application contractor have already met a substantial portioia of their guarantee. The procedures just described represent only part of the checking and inspection system.

after bonding and curing at 5°C; damp bonding of cement mortar prisms with subsequent water immersion;and determination of flexural strength on these 40 x 40 x 160 mm prisms, as well as the compressive strength of prisms of the same size bonded diagonally. A great deal of importance was attached to testing on the site. A site-log was kept with data on the weather, the temperature during and after bonding, and humidity. A second section of the log covered the concrete segment, including data on shotblasting, moisture content of the concrete, and concrete temperature during and after bonding. Each batch of adhesive arriving at the site was tested, and tests were also made when the adhesive was applied to the concrete segments. For each batch, pot life, open time, and sag flow were tested and three bonded concrete prisms used to determine the flexural strength. On the bridge, only the flexural strength was determined. However, the concrete prisms were first immersed in water, then withdrawn and partly dried until a maximum moisture content of 20% remained, bonded together, and then reimmersed in water for seven days following a 24-hour cure period. Spot checks were made on modulus of elasticity and creep deformation over a short period of time and bonded reference prisms were saved for subsequent control tests.

Olympic Stadium, Montreal

The Sallingsund Bridge in Denmark (Fig. 23) is approximately 1730 m long, is divided into 17 standard spans of 93 m each and two end spans of 51 m each, and has two 25 m-long abutments. Segmental construction with precast concrete units was used for the bridge superstructure. Segment length is 3.57 m, and the heaviest segment weighed about 120 tons. Tests which were carried out at Copenhagen Technical University on the two adhesives quoted - each in a 'summer' and 'winter' version - revealed a quite clear difference in quality between the two products, based on: modulus of elasticity; creep deformation (60 days); shear strengths

The problems of casting thick joints was presented in the construction of the Olympic Stadium in Montreal. In the upper part of the precast concrete piUars, two joints with a thickness of approximately 80-100 mm were called for, which provided latitude for making up variations in the geometry of the structure. Each such joint had an area of about 9 m 2. This presented the problem of developing a casting mortar with a sufficiently long pot life, good flow properties, high strength and low creep deformation. The aggregates had to be selected to avoid any sedimentation before the binder gelled. It was also necessary to ensure production of a homogenous cross-section, if possible without any cavities. A suitable mortar was developed with four components: resin; hardener; and two types of fillers. Fig. 24 shows a typical joint. In this case, the creep reading gave an indication of deformations occurring and of their effect on the structure.

Fig. 23 The bridge deck of the Sailingsund Bridge under construction in early August 1976

Fig. 24 The Sports Stadium, Montreal, showing a"thick' joint -used to adjust the geometry of the structure

Sallingsund Bridge, Denmark

INT.J.ADHESION AND ADHESIVES APRIL 1982

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External reinforcement of bridges and buildings The need for reinforcement of existing structures of reinforced concrete is arising with increasing frequency, both for buildings, and in other structures such as bridges. Reasons for reinforcing a given structure include: inadequate cross-section of the existing reinforcement as a result of faulty static calculations, or poor execution; weakening of existing reinforcement through corrosion; increased load on a structural member because the structure is now being used for a different purpose; and modification of the static system. The process of reinforcing an existing structure with bonded external reinforcement has the following positive features: • easily adaptable to existing geometry; • no significant reduction of room height or overhead height; • monolithic bond between new reinforcement and existing load-beating structure; and • economically attractive with regard to the actual reinforcement work and total costs. The prerequisites for carrying out such jobs are: an engineer familiar with this technique (calculation, design, job supervision); availability of suitable binder systems and adhesives; and an experienced application company. Tests to determine the load-beating capacity of concrete elements reinforced externally have been described above. Some practical applications are cited below.

Bridges Recent examples of bridge strengthening have included several motorway bridges in Englands with supporting testss's being carried out by the Transport and Road Research Laboratory, Department of the Environment. In Tokyo, some 10% of the 100 km or so of elevated motorways with concrete surface slabs have been reinforced by bonding on additional steel plates and structural steel sections. Similar schemes have been used in other countries, including South Africa, France and Belgium.

Buildings Large-scale reinforcements have also been carried out in many buildings. One noteworthy example was the reinforcement of two floors in the Ftissli telephone exchange building in Zurich. The first step was to investigate the structural condition of the building and its suitability for modem requirements. Before new switching equipment was installed, two concrete floors had to be strengthened to raise their load-bearing capacity to a live load of 7 kN/m 2. An important constraint was that no substantial reduction of the second storey room height could be tolerated, and there had to be a minimum delay caused by the reconstruction. The first floor above ground - with a span of 2.20 m was reinforced by pouring a 120 mm thick, mesh-reinforced slab of lightweight concrete on to the existing 110 mm thick slab. The shearproof bond with this reinforced concrete slab was provided by all-over application of an Araldite adhesion layer. For the second floor, bonded external reinforcement was applied to strengthen the load.bearing structure. To investigate certain problems not treated adequately in the existing literature, eg admissible shear load in the mortar joint, endanchoring of the tension plate, joining of the shear reinforce-

94

INT.J.ADHESION AND ADHESIVES APRIL 1982

~" Fig. 25 Reinforcement work being completed on the FCissli Telephone Exchange

ment of the T-beams with the compression stab, etc, the client ordered a test with a life-size reinforced beam by the Federal Materials Testing Institute (described above). The test results showed substantially higher failure resistance than expected from calculations. Reinforcement work on relatively complicated structural elements calls for tailor-made reinforcement plates, brackets and end-anchors. In this particular case the three-dimension. al reinforcement plates were welded together at site. The concrete structure had to be made visible onthe surfaces to be bonded, and the steel surfaces needed shotblasting, if an extended period of time is to pass between shotblast~ and application of the adhesive, the freshly blasted plates should be coated with a primer having an epoxy resin base to protect them against the slightest rusting. Fig. 25 provides a general view of the reinforcement work on floors and girders. The Federal Materials Testing Institute also assumed responsibility for inspection of materials during the bonding work and for checking the structural behaviour of the newly reinforced components for 2 years following the new exchange equipment becoming operational in the building (Fig. 26). The necessary fire protection is provided by newly applied plaster lath ceilings and the existing ftre alarm system. Similar strengthening was undertaken at the Hottinger. strasse telephone exchange building in Zurich. Here again, expansion of the exchange made =it necessary to raise the load-bearing capacity of the second-floor rooms from 2 kN/m 2 to 7 kN/m 2. Fig, 27 shows:the reinforced floor with supplementary tension anchoring at the point where

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the floor starts to thicken. Fig. 28 shows the shear reinforcement and end-anchoring ot~the tension plate at a girder bearing. Summary

Epoxy resin systems have been in use in civil engineering for about 20 years. In most cases, the applications have proven successful and the materials employed have met requirements. Incorrect selection of epoxy resin systems and insufficient knowledge about them have also resulted in setbacks over the years. In other cases, the handling of the material has been adjudged too costly and complicated in comparison with concrete, the most common building material. The two parties involved in this whole development have tended to view the application of epoxy resins in civil engineering from diametrically opposed standpoints. As manufacturers of resins and hardeners, the synthetic resin industry has tried to utilize all of the possible variations offered by organic chemistry to put the broadest possible selection of products (and grades) on the market; formulating companies have taken the same attitude. The construction

industry, on the other hand, looked to the new materials as a possibility for overcoming the weaknesses of existing materials, eg in concrete, and for obtaining better-adapted auxiliary products to meet the requirements of conventional and new construction methods, Most of these hopes assumed that long-term performance would be adequate. In this situation, the dynamic approach of the epoxy resin producers and formulators clashed with the rather conservative attitudes prevailing in the construction industry. The intent of this article was to review the selection criteria and material limits primarily from the standpoint of the civil engineer. Firstly, to demonstrate just what can be expected of epoxy resin mortars in civil engineering design - provided the materials are used properly - and secondly, to point out when and where benefits can be gained from modifying a site procedure to suit the special material properties of epoxy resins. The ultimate goal being to produce designs more in conformance with the nature of the material and, therefore, safer. Though particular emphasis is placed on the determination of the long-term performance of the epoxy resin mortar - under continuous loading, under load at rising temperature and when exposed to moisture - it was not the intention to imply that every single product has to undergo testing on this score before being employed. What is important, however, is that these long-term figures be established for the basic formulations produced with the respective binders. Once this has been obtained, a basis is established for formulating ready-to-use products varying only negligibly from the basic formulations as far as the main properties go. Specifications aimed in the last analysis at providing the client and building contractor with reliable data on the selection of materials available, properties and test procedures should certainly include information on long-term behaviour. This is the only way of obtaining assurance that the material chosen will in fact perform as intended over the life of the structure.

References 1

Transportation Research Circular No 160 (The Transportation Research Board, USA, 1974)

2

Ernani Diez, B. 'The technique of gluing precast elements of bridges built by the cantilever method', Pub/ Tec No 52 (Antonio A. Noronha, Servi(;osde Enginharia SA, Rio de

Janeiro, 1974) 3

Kupfar, H. 'Kontaktfugen mit Kunststoffverklebung von Stahlbeton- und Spannbetonfertigteilen, Rationalisierung yon Bauverfahren, Tragverhalten und Bemessung', VDI Reports 225 (VDl-Verlag GmbH, Dusseldorf, 1975)

4

'Biegeversuchean Stahlbetonbalken mit Hafbrucken', EMPA No 24"171/2 (Federal Materials Testing Institute, 1974) (unpublished report)

5

'Verstarkung von Tragkonstruktionen mit getlebter Armierung', Schwweiz Bauzrg 92 No 19 (1974) pp 457-474

Fig. 28 Reinforced beams with end-anchoring of the tensile plate and special shear reinforcement

6

Raithby, K.D. 'External strengthening of concrete bridges with bonded steel plates', TRRL Report SR 612 (Department of the Environment, Department of Transport, Crowthorne, 1980)

7

Macdonald, M.D. 'The flexural behaviour of concrete beams with bonded external reinforcement', TRRL Report SR 415 (Department of the Enviroment, Department of Transport, Crowthorne, 1978)

8

Calder, A.J.J. 'Exposure tests on externally reinforced concrete beams -- first two years', TRRL Report SR 529 (Department of the Environment, Department of Transport,

Crowthorne, 1979)

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