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concrete composite construction
Adhesive-bonded steel/concrete composite construction G.C. Mays and A.E. Vardy
The development of a new form of roadway deck for bridges is described. This consists of a concrete core bonded to a steel soffit plate using epoxy resin adhesive. The durability of structural joints made with a number of adhesives and with steel, concrete, and aluminium adherends is being investigated. There is a marked difference in resistance to damp conditions between different grades of epoxy resin. A search for further potential structural applications continues. Key words: adhesive strength; epoxy resins; bridge decks; composite construction; environmental testing; structural steels; concrete structures.
The Wolfson Bridge Research Unit at the University of Dundee was established in 1973 with the object of developing a new form of roadway deck for bridges which would be lighter in weight than conventional reinforced concrete, yet cheaper than the alternative welded steel deck; This objective has been substantially achieved from a structural point of view by using a deck consisting of a concrete core bonded to a steel soffit plate using epoxy resin adhesive. However, before the technique can be adopted in full-scale bridge construction, the strength of the adhesive bond must be shown to be maintained for many years under extremes of ambient conditions and cyclic loading. This paper describes the development of the 'open sandwich' construction and the investigation of the durability of structural joints which forms a major part of the Unit's activities.
Open sandwich construction The open sandwich is a new type of reinforced concrete member in which an external steel soffit plate has been substituted for steel bars embedded within the concrete. The construction method has been developed with precasting in mind and involves initially thoroughly degreasing and grit blasting the steel plate. The plate is laid horizontal and coated with a layer of epoxy resin adhesive, and wet concrete is immediately poured onto the still soft adhesive. Compaction and curing then proceeds in the normal manner. For applications in bridge decks, the precast slab units are transported to site and the steel plates are connected to the top flanges of the longitudinal girders of the bridge as illustrated in Fig. 1. A small quantity of bar reinforcement and in situ concrete is placed over the joints to provide continuity. The new system is lighter in weight than a normal reinforced concrete slab and therefore, less costly-reduced
weight of the slab considerably reduces the cost of longitudinal plate girders in medium spans, or the cost of the cables in suspension and cable-stayed bridges. The technical reasons for these advantages are: • There is no need for any concrete cover to the underside of the soffit plate. Corrosion protection can be provided by painting. • The steel plate carries biaxial stress with the result that there is often an effective increase in the yield stress. For longitudinal and transverse stresses in the ratio of 2:1, this increase amounts to approximately 15%. Thus the steel plate may be more efficient than a series of orthogonal bars. • The steel plate acts as the soffit form, thus eliminating the need for separate formwork. The structure shown in Fig. 2 represents a 3-span continuous bridge having a 90 m main span with longitudinal girders at 2.6 m centres. For this the required deck thickness is 140 mm compared with the 225 mm required for a conventional deck. The quantities and estimated costs for the two types of decks are given in Table 1. This shows a 10% saving when open sandwich construction is employed.
Sheor COnnecl'orS •
/ /
Concrete .
Adhesive reinforcement
plo~e g,rclers
Fig. 1
ODen
sandwich slab in composite bridge deck
0143-7496/82/020103-05 $03.00 © 1982 Butterworth & Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES APRIL 1982
103
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[--k
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J~
J~
2000
3500
L
14o
2000
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2600
2800
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'
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-1
,
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.a.
70m
30m
780 x 50
Z600 Open sandwich deck
Conventional deck
Mi d sport cross sec+ion Fig. 2
Typical example of open sandwich deck in continuous bridge (dimensions in mm, except where indicated)
Table 1.
Cost ¢omparbon - 3 =pan continuous bridge of
Fig. 2
Item
Conventional
Open sandwich
Quantity
Cost (£)*
Quantity
Cost (£)*
720 t
475 000
640 t
422 4 0 0
50 t
31 000
Steel beams Soffit plate
-
-
Bar reinforcement (MS)
-
(HY) Adhesive
Side form Soffit form Concrete Total
-
98 t -
10 710
21 t
42 140 -
-
-
4 1 6 0 kg
80 m 2
880
1 840 m2
16 560
415 m 3
21 170
for reinforced concrete. Unrestrained open sandwich slabs have been tested to failure under the action of a single patch load whilst supported on all four sides over a span of 0.9 m as shown in Fig. 3. The effect of the reinforcement ratio on the strength and mode of failure of these slabs is given in Table 2. A parallel development to the open sandwich slab is an alternative form of construction termed the inverted catenary. In this, the steel soffit plate is curved convex upwards to form a shallow arch. An inverted catenary bridge deck has the advantage of not requiring bar reinforcement for continuity over girder supports because of the horizontal
2 0 380
290 m 2
3 190
260 m 3
13 260
-
£555 950
Saving
£500 940
Open sandwich s
10%
*Costs f o r complete deck excluding cantilevers
Further studies have shown that the method becomes economic for simply supported spans of 20 m or larger. Greater savings may be expected on longer span bridges as the self weight becomes an increasingly large proportion of the total load to be carried. The development of open sandwich construction has been based on an extensive experimental and theoretical programme of research 1. Open sandwich beams, constructed with a variety of adhesives and adhesive thicknesses, have been tested to failure in four point bending on a span of 1.8 m. The tests have demonstrated that, provided an efficient bond exists between the steel and the concrete, the beams have good ductility and strengths - on average 10% greater than those predicted by normal plastic theory
104
INT.J.ADHESION AND ADHESIVES APRIL 1982
__
II II
II Con=ere ob.i~f~
SuppOr, I ~ m
II
U
Ftoor slab
900
Fig. 3
! 111
II II li
Laboratory test on 1 m square open sandwich slab
Table 2. Strength and failure mode of I m square open sandwich slabs Slab No
Nominal depth (mm)
Reinforcement ratio (%)
Load at:
1 2
100 50
0.62 1.26
120.0 163.5 54.0 84.0
3
50
1.96
78.0 110.0
4
50
3.21
116.0 126.0
5
50
4.40
132.0 156.0
First yield (kN)
Mode of failure
Failure (kN)
Flexural Flexuralshear Flexuralshear Punchingshear Shear
force exerted by the arch. Tests have been carried out on 3 bay models of bridge decks (approximately quarter scale) five of the inverted catenary and one of the open sandwich type. For the inverted catenary decks, illustrated in Fig. 4, it has been found that sufficient horizontal restraint is developed to limit the bending moment at the crown to less than 30% of the bending moment in a simply supported beam. The ultimate strengths obtained for both types of deck show factors of safety in excess of 3 over design load when using the adhesive now favoured for development. Good agreement between finite element calculations and experimental results has been obtained in the working load range. However, due to the complexity of the finite element technique as a design tool for the inverted catenary form of construction, a finite prism method is being developed. Both forms of construction are to be tested under actual traffic conditions by installing 2 m square simply supported slabs in a test site specially constructed in Dundee's northern by-pass. Half-scale replicas of both the open sandwich and inverted catenary forms of deck have withstood at least 7.5 million cycles of the equivalent of an abnormal vehicle (HB) wheel load or greater. The full size open sandwich slab was placed in the road in December 1980 and it is supporting the passage of approximately 0.5 million heavy goods vehicles per annum. Durability of adhesive joints Test procedure
Early in the programme of work aimed at developing adhesive-bonded bridge decks it was appreciated that the durability of the joint would need to be proved. The current design life of bridges in the UK is 120 years 2, during which time a typical detail may be expected to undergo up to 500 million cycles of potentially damaging load. A programme of research was therefore initiated to study the influence of various curing schedules on the structural performance of adhesive-bonded joints by static and fatigue tests after subjecting the joints to ageing, damp, and temperature cycling. This work has been augmented by a theoretical study of failure in bending and shear using the principles of fracture mechanics, and by a study of the form of the fatigue S vs N curve for joints tested over the range of temperature likely to be experienced at bridge sites in the UK. Specimens under test have included steel and aluminium lap specimens, steel butt-joints, concrete prisms with
scarfed joints, and beams reinforced by extemal steel plates. Five grades of epoxy resin adhesive have been subjected to the full durability programme. These are designated thus: Adhesive No 1; a white thixotropic paste mixed with 10% of its own weight of black liquid hardener of the aliphatic polyamine type. Adhesive No 2; a high viscosity liquid resin mixed with its own weight of amber, high viscosity liquid polyamide hardener. Adhesive No 3; a white, medium viscosity liquid resin mixed with its own weight of liquid polyamide hardener. Adhesive No 4; a white pigmented paste mixed with approximately half its own weight of medium viscosity amber liquid hardener of the aromatic polyamine type. Adhesive No 6; a white paste mixed with one third its own weight of black paste hardener understood to be an aliphatic polyamine adduct. "
The programme with Adhesive No 3 has been curtailed due to the difficulty experienced in manufacturing a suitably ductile externally reinforced beam. This is necessary to ensure that there is adequate visible warning of an impending failure. Other adhesives, including a polyester resin (Adhesive No 5) and a toughened epoxy, have been rejected after preliminary trials. In the former case, this was due to poor durability under damp conditions. In the latter case, it was due to the relatively short pot life of the material. Several other grades of epoxy resin adhesive are currently the subjects of preliminary trials. Test results
Details of earlier studies have been given by Cusens and Smith 3. Conclusions reached more recently are briefly summarized. There is a marked difference in resistance to damp conditions between different grades of epoxy resin. Adhesive No 4 has performed best, sometimes showing no loss in strength after intermittent wetting of open sandwich beams, sealed on all but the top concrete face, for 26 weeks. Adhesive No 2 has performed better than expected by the manufacturer; Adhesive No 1 has performed worse. The performance of Adhesive No 6 is at least as good as that of Adhesives Nos 1 and 2. This conclusion is based both on tests of strength and on observations of rusting of the steel soffit plate in open sandwich specimens. The results are summarized in Table 3. However, over 2 years of continuous immersion of aluminium lap joints made with Adhesives Nos 1, 2 and 4 has resulted in an almost complete breakdown of the bond.
i' i Fig. 4
Load test on 3 bay model of inverted catenary deck
INT.J.ADHESION AND ADHESIVES APRIL 1982
105
Table 3.
The effect of water on the performance of open sandwich beams Change in strength (%)
Surface rusting of steel plate (%)
1
2
4
8 weeks immersion in fresh water
-37
-25
-14
70
10
5
9
26 weeks intermittent wetting with fresh water
-34
-1
+3
-8
100
23
5
2
26 weeks intermittent wetting with salt water
-19
-78
-7
-72
t00
100
7
67
Adhesive number:
6
1
-31
~
2
4
6
Results are based on the mean of at least 3 tests
Table 4. joints
Effect of surface preparation on steel/steel butt
Tensile strength (N/ram2) (and mode of failure)* Adhesive number:
1
4
6
Hand (emery)
21.9 (c/a)
11.1 (c/a)
18.7 (c)
Needle gun
12.3 (a)
7.3 (a)
16.3 (c)
Metal grit blast
34.3 (c)
19.0 (c/a)
16.7 (c)
Results based on a mean of 9 tests *a, adhesion; c, cohesion
Table 5. joints
Effect of surface treatment* on steel/steel lap
Change in strength (%) Adhesive number:
1
15% rust on surface of side laps
-16
Side laps dipped in 5% salt solution and allowed to drain for 20 h
-20
2
4
6
-7
-17
-
-10
-12
-8
There is a considerable reduction in the joint strength with most adhesives when a needle-gun surface is substituted, as can be seen in Table 4. Static and fatigue strengths of steel lap joints in which the side laps have been pre-rusted are not significantly affected. These results are summarized in Table 5. To obtain a reliable and durable steel/concrete specimen it is necessary to use a layer of epoxy resin at least 1 mm thick, or thereabouts. The necessary thickness has not been found to vary greatly with different resins, but those containing relatively large f'tller particles benefit from a greater thickness. However, the action of consolidating the concrete can cause particles of coarse aggregate to puncture the adhesive layer, resulting in subsequent corrosion of the steel plate if the concrete is kept damp for tong. Techniques for preventing this are currently under investigation. They include the use of a double layer of adhesive in which the first layer is allowed to harden before application of the second; the use of epoxy paint primers; delaying the application of the concrete until the adhesive is on the point of setting; the use of super-plasticizers in the concrete to avoid the need for mechanical vibration; and the use of less angular aggregates. The effectiveness of a double layer of Adhesive No 4 in preventing the rusting of the soffit plate is illustrated in Fig. 5. This shows steel plates peeled from beams which have been stored under identical damp conditions. Temperature cycling 1000 times between - 7 ° C and -I35 o C has shown little or no adverse effect on the strength of steel lap specimens.
Results based on a mean of 3 tests *Prior to bonding
Future developments
The fatigue strength of adhesive-bonded joints is very high. The results of tests on open sandwich beams have been summarized by Mays and Harvey4 and show that, expressed as a percentage of the static strength, this type of joint performs better than welded steel. The effect of temperature on the fatigue performance at long endurances of steel lap joints is presented as a separate paper in this issue.* The strength of steel lap joints made with Adhesives Nos 1, 2 and 6 is improved by curing at a temperature of 80°C. No adverse effect of mere aseing under ambient laboratory conditions has yet been found. A steel surface which has been thoroughly degreased and metal grit blasted is a suitable p r e p a r a t ~ for making an adhesive joint. Hand roughening using emery paper is an adequate alternative for small scale laboratory specimens. *See page 109
106
INT.J.ADHESION AND ADHESIVES APRIL 1982
In addition to amore detailed investigation into alternative ways of preventing rusting of the soffit plate in open
Single layer of ~dhesive No 4
Double lcyer of odhes~veNo 4
Fig. 5 Steel plates peeled from small open sandwich beams after immersion f o r 8 weeks in tap water indicating the amoun t of corrosion prevention by using a d o ~ e liayer of adhesive
sandwich construction a number of new adhesives are being introduced to the durability programme. The adhesives under investigation are being classified according to both their chemical and physical properties with a view to determining those grades which produce the optimum combination of mechanical performance and durability. At the same time, suitable test methods for both 'all-adhesive' specimens and adhesive joints are being studied to develop a procedure for quality control. Non-destructive test methods for use in detecting flaws in the bond between steel and concrete have been the subject of a preliminary investigation and are being pursued. Further work is also required to determine the load distribution between longitudinal girders in bridge decks of both the open sandwich and inverted catenary form. The relatively good fatigue performance of adhesivebonded joints compared with welded steel suggests that there may be applications for adhesive bonding in fatigueprone locations in steel structures. The fatigue performance of an adhesive-bonded transverse stiffener for a plate girder web, a notionally unstressed joint, is being investigated.
variety of products. A search for further potential structural applications continues.
Acknowledgements
Thanks are due to all past and present members of the Wolfson Bridge Research Unit who have contributed to the programme of research described in this paper, particularly former Directors Professor A.R. Cusens and Mr D.W. Smith. Financial support has been provided by the Wolfson Institute, the Science and Engineering Research Council, the Transport and Road Research Laboratory, and the Scottish Development Department, Manufacturers have supplied adhesives without charge. References 1 2
'Steel, concrete and composite bridges', BS 5400" Part 10: 1980 (British Standards Institution, 1980)
3
Cusens, A.R. end Smith, D.W. 'A study in epoxy resin adhesive joints in shear', The Struct Engnr 58A No 1 (January 1980) pp 13-18
4
Mays, G.C. and Harvey, W.J. 'Fatigue performance of adhesive bonded joints for bridge deck construction', IABSE Colloq-
Conclusions
An extensive experimental and theoretical programme of work carried out within the Wolfson Bridge Research Unit has demonstrated the structural integrity of externally reinforced concrete. Subject to final demonstration of satisfactory long-term durability, the technique applied to bridge decks offers financial savings on main spans exceeding approximately 20 metres. Studies are now being extended to investigate the properties of adhesives and adhesive joints made with a
Ong, K.C.G. 'Open sandwich construction for bridge decks', PhD Thesis (University of Dundee, 1981)
uium "Fatigue of steel and concrete structures" Lausanne, March 1982
Authors
The authors are with The Department of Civil Engineering, The University, Dundee, DD1 4HN, Scotland. Inquiries, in the first instance, should be directed to Mr Mays.
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
107
The development of a new form of roadway deck for bridges is described. This consists of a concrete core bonded to a steel soffit plate using epoxy resin adhesive. The durability of structural joints made with a number of adhesives and with steel, concrete, and aluminium adherends is being investigated. There is a marked difference in resistance to damp conditions between different grades of epoxy resin. A search for further potential structural applications continues. Key words: adhesive strength; epoxy resins; bridge decks; composite construction; environmental testing; structural steels; concrete structures.
The Wolfson Bridge Research Unit at the University of Dundee was established in 1973 with the object of developing a new form of roadway deck for bridges which would be lighter in weight than conventional reinforced concrete, yet cheaper than the alternative welded steel deck; This objective has been substantially achieved from a structural point of view by using a deck consisting of a concrete core bonded to a steel soffit plate using epoxy resin adhesive. However, before the technique can be adopted in full-scale bridge construction, the strength of the adhesive bond must be shown to be maintained for many years under extremes of ambient conditions and cyclic loading. This paper describes the development of the 'open sandwich' construction and the investigation of the durability of structural joints which forms a major part of the Unit's activities.
Open sandwich construction The open sandwich is a new type of reinforced concrete member in which an external steel soffit plate has been substituted for steel bars embedded within the concrete. The construction method has been developed with precasting in mind and involves initially thoroughly degreasing and grit blasting the steel plate. The plate is laid horizontal and coated with a layer of epoxy resin adhesive, and wet concrete is immediately poured onto the still soft adhesive. Compaction and curing then proceeds in the normal manner. For applications in bridge decks, the precast slab units are transported to site and the steel plates are connected to the top flanges of the longitudinal girders of the bridge as illustrated in Fig. 1. A small quantity of bar reinforcement and in situ concrete is placed over the joints to provide continuity. The new system is lighter in weight than a normal reinforced concrete slab and therefore, less costly-reduced
weight of the slab considerably reduces the cost of longitudinal plate girders in medium spans, or the cost of the cables in suspension and cable-stayed bridges. The technical reasons for these advantages are: • There is no need for any concrete cover to the underside of the soffit plate. Corrosion protection can be provided by painting. • The steel plate carries biaxial stress with the result that there is often an effective increase in the yield stress. For longitudinal and transverse stresses in the ratio of 2:1, this increase amounts to approximately 15%. Thus the steel plate may be more efficient than a series of orthogonal bars. • The steel plate acts as the soffit form, thus eliminating the need for separate formwork. The structure shown in Fig. 2 represents a 3-span continuous bridge having a 90 m main span with longitudinal girders at 2.6 m centres. For this the required deck thickness is 140 mm compared with the 225 mm required for a conventional deck. The quantities and estimated costs for the two types of decks are given in Table 1. This shows a 10% saving when open sandwich construction is employed.
Sheor COnnecl'orS •
/ /
Concrete .
Adhesive reinforcement
plo~e g,rclers
Fig. 1
ODen
sandwich slab in composite bridge deck
0143-7496/82/020103-05 $03.00 © 1982 Butterworth & Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES APRIL 1982
103
35m
[--k
-i
J~
J~
2000
3500
L
14o
2000
,
I
t
590 x
I
===
925 x 50
2600
2800
38
2000 X io
i
2000 x I0
LJL 'Sm'-
)
,,oh
660 x 45
'
1
II
220 slob
'
I
II
~
I
-...=
90m El@vqti0n 3500
-1
,
','
.a.
70m
30m
780 x 50
Z600 Open sandwich deck
Conventional deck
Mi d sport cross sec+ion Fig. 2
Typical example of open sandwich deck in continuous bridge (dimensions in mm, except where indicated)
Table 1.
Cost ¢omparbon - 3 =pan continuous bridge of
Fig. 2
Item
Conventional
Open sandwich
Quantity
Cost (£)*
Quantity
Cost (£)*
720 t
475 000
640 t
422 4 0 0
50 t
31 000
Steel beams Soffit plate
-
-
Bar reinforcement (MS)
-
(HY) Adhesive
Side form Soffit form Concrete Total
-
98 t -
10 710
21 t
42 140 -
-
-
4 1 6 0 kg
80 m 2
880
1 840 m2
16 560
415 m 3
21 170
for reinforced concrete. Unrestrained open sandwich slabs have been tested to failure under the action of a single patch load whilst supported on all four sides over a span of 0.9 m as shown in Fig. 3. The effect of the reinforcement ratio on the strength and mode of failure of these slabs is given in Table 2. A parallel development to the open sandwich slab is an alternative form of construction termed the inverted catenary. In this, the steel soffit plate is curved convex upwards to form a shallow arch. An inverted catenary bridge deck has the advantage of not requiring bar reinforcement for continuity over girder supports because of the horizontal
2 0 380
290 m 2
3 190
260 m 3
13 260
-
£555 950
Saving
£500 940
Open sandwich s
10%
*Costs f o r complete deck excluding cantilevers
Further studies have shown that the method becomes economic for simply supported spans of 20 m or larger. Greater savings may be expected on longer span bridges as the self weight becomes an increasingly large proportion of the total load to be carried. The development of open sandwich construction has been based on an extensive experimental and theoretical programme of research 1. Open sandwich beams, constructed with a variety of adhesives and adhesive thicknesses, have been tested to failure in four point bending on a span of 1.8 m. The tests have demonstrated that, provided an efficient bond exists between the steel and the concrete, the beams have good ductility and strengths - on average 10% greater than those predicted by normal plastic theory
104
INT.J.ADHESION AND ADHESIVES APRIL 1982
__
II II
II Con=ere ob.i~f~
SuppOr, I ~ m
II
U
Ftoor slab
900
Fig. 3
! 111
II II li
Laboratory test on 1 m square open sandwich slab
Table 2. Strength and failure mode of I m square open sandwich slabs Slab No
Nominal depth (mm)
Reinforcement ratio (%)
Load at:
1 2
100 50
0.62 1.26
120.0 163.5 54.0 84.0
3
50
1.96
78.0 110.0
4
50
3.21
116.0 126.0
5
50
4.40
132.0 156.0
First yield (kN)
Mode of failure
Failure (kN)
Flexural Flexuralshear Flexuralshear Punchingshear Shear
force exerted by the arch. Tests have been carried out on 3 bay models of bridge decks (approximately quarter scale) five of the inverted catenary and one of the open sandwich type. For the inverted catenary decks, illustrated in Fig. 4, it has been found that sufficient horizontal restraint is developed to limit the bending moment at the crown to less than 30% of the bending moment in a simply supported beam. The ultimate strengths obtained for both types of deck show factors of safety in excess of 3 over design load when using the adhesive now favoured for development. Good agreement between finite element calculations and experimental results has been obtained in the working load range. However, due to the complexity of the finite element technique as a design tool for the inverted catenary form of construction, a finite prism method is being developed. Both forms of construction are to be tested under actual traffic conditions by installing 2 m square simply supported slabs in a test site specially constructed in Dundee's northern by-pass. Half-scale replicas of both the open sandwich and inverted catenary forms of deck have withstood at least 7.5 million cycles of the equivalent of an abnormal vehicle (HB) wheel load or greater. The full size open sandwich slab was placed in the road in December 1980 and it is supporting the passage of approximately 0.5 million heavy goods vehicles per annum. Durability of adhesive joints Test procedure
Early in the programme of work aimed at developing adhesive-bonded bridge decks it was appreciated that the durability of the joint would need to be proved. The current design life of bridges in the UK is 120 years 2, during which time a typical detail may be expected to undergo up to 500 million cycles of potentially damaging load. A programme of research was therefore initiated to study the influence of various curing schedules on the structural performance of adhesive-bonded joints by static and fatigue tests after subjecting the joints to ageing, damp, and temperature cycling. This work has been augmented by a theoretical study of failure in bending and shear using the principles of fracture mechanics, and by a study of the form of the fatigue S vs N curve for joints tested over the range of temperature likely to be experienced at bridge sites in the UK. Specimens under test have included steel and aluminium lap specimens, steel butt-joints, concrete prisms with
scarfed joints, and beams reinforced by extemal steel plates. Five grades of epoxy resin adhesive have been subjected to the full durability programme. These are designated thus: Adhesive No 1; a white thixotropic paste mixed with 10% of its own weight of black liquid hardener of the aliphatic polyamine type. Adhesive No 2; a high viscosity liquid resin mixed with its own weight of amber, high viscosity liquid polyamide hardener. Adhesive No 3; a white, medium viscosity liquid resin mixed with its own weight of liquid polyamide hardener. Adhesive No 4; a white pigmented paste mixed with approximately half its own weight of medium viscosity amber liquid hardener of the aromatic polyamine type. Adhesive No 6; a white paste mixed with one third its own weight of black paste hardener understood to be an aliphatic polyamine adduct. "
The programme with Adhesive No 3 has been curtailed due to the difficulty experienced in manufacturing a suitably ductile externally reinforced beam. This is necessary to ensure that there is adequate visible warning of an impending failure. Other adhesives, including a polyester resin (Adhesive No 5) and a toughened epoxy, have been rejected after preliminary trials. In the former case, this was due to poor durability under damp conditions. In the latter case, it was due to the relatively short pot life of the material. Several other grades of epoxy resin adhesive are currently the subjects of preliminary trials. Test results
Details of earlier studies have been given by Cusens and Smith 3. Conclusions reached more recently are briefly summarized. There is a marked difference in resistance to damp conditions between different grades of epoxy resin. Adhesive No 4 has performed best, sometimes showing no loss in strength after intermittent wetting of open sandwich beams, sealed on all but the top concrete face, for 26 weeks. Adhesive No 2 has performed better than expected by the manufacturer; Adhesive No 1 has performed worse. The performance of Adhesive No 6 is at least as good as that of Adhesives Nos 1 and 2. This conclusion is based both on tests of strength and on observations of rusting of the steel soffit plate in open sandwich specimens. The results are summarized in Table 3. However, over 2 years of continuous immersion of aluminium lap joints made with Adhesives Nos 1, 2 and 4 has resulted in an almost complete breakdown of the bond.
i' i Fig. 4
Load test on 3 bay model of inverted catenary deck
INT.J.ADHESION AND ADHESIVES APRIL 1982
105
Table 3.
The effect of water on the performance of open sandwich beams Change in strength (%)
Surface rusting of steel plate (%)
1
2
4
8 weeks immersion in fresh water
-37
-25
-14
70
10
5
9
26 weeks intermittent wetting with fresh water
-34
-1
+3
-8
100
23
5
2
26 weeks intermittent wetting with salt water
-19
-78
-7
-72
t00
100
7
67
Adhesive number:
6
1
-31
~
2
4
6
Results are based on the mean of at least 3 tests
Table 4. joints
Effect of surface preparation on steel/steel butt
Tensile strength (N/ram2) (and mode of failure)* Adhesive number:
1
4
6
Hand (emery)
21.9 (c/a)
11.1 (c/a)
18.7 (c)
Needle gun
12.3 (a)
7.3 (a)
16.3 (c)
Metal grit blast
34.3 (c)
19.0 (c/a)
16.7 (c)
Results based on a mean of 9 tests *a, adhesion; c, cohesion
Table 5. joints
Effect of surface treatment* on steel/steel lap
Change in strength (%) Adhesive number:
1
15% rust on surface of side laps
-16
Side laps dipped in 5% salt solution and allowed to drain for 20 h
-20
2
4
6
-7
-17
-
-10
-12
-8
There is a considerable reduction in the joint strength with most adhesives when a needle-gun surface is substituted, as can be seen in Table 4. Static and fatigue strengths of steel lap joints in which the side laps have been pre-rusted are not significantly affected. These results are summarized in Table 5. To obtain a reliable and durable steel/concrete specimen it is necessary to use a layer of epoxy resin at least 1 mm thick, or thereabouts. The necessary thickness has not been found to vary greatly with different resins, but those containing relatively large f'tller particles benefit from a greater thickness. However, the action of consolidating the concrete can cause particles of coarse aggregate to puncture the adhesive layer, resulting in subsequent corrosion of the steel plate if the concrete is kept damp for tong. Techniques for preventing this are currently under investigation. They include the use of a double layer of adhesive in which the first layer is allowed to harden before application of the second; the use of epoxy paint primers; delaying the application of the concrete until the adhesive is on the point of setting; the use of super-plasticizers in the concrete to avoid the need for mechanical vibration; and the use of less angular aggregates. The effectiveness of a double layer of Adhesive No 4 in preventing the rusting of the soffit plate is illustrated in Fig. 5. This shows steel plates peeled from beams which have been stored under identical damp conditions. Temperature cycling 1000 times between - 7 ° C and -I35 o C has shown little or no adverse effect on the strength of steel lap specimens.
Results based on a mean of 3 tests *Prior to bonding
Future developments
The fatigue strength of adhesive-bonded joints is very high. The results of tests on open sandwich beams have been summarized by Mays and Harvey4 and show that, expressed as a percentage of the static strength, this type of joint performs better than welded steel. The effect of temperature on the fatigue performance at long endurances of steel lap joints is presented as a separate paper in this issue.* The strength of steel lap joints made with Adhesives Nos 1, 2 and 6 is improved by curing at a temperature of 80°C. No adverse effect of mere aseing under ambient laboratory conditions has yet been found. A steel surface which has been thoroughly degreased and metal grit blasted is a suitable p r e p a r a t ~ for making an adhesive joint. Hand roughening using emery paper is an adequate alternative for small scale laboratory specimens. *See page 109
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INT.J.ADHESION AND ADHESIVES APRIL 1982
In addition to amore detailed investigation into alternative ways of preventing rusting of the soffit plate in open
Single layer of ~dhesive No 4
Double lcyer of odhes~veNo 4
Fig. 5 Steel plates peeled from small open sandwich beams after immersion f o r 8 weeks in tap water indicating the amoun t of corrosion prevention by using a d o ~ e liayer of adhesive
sandwich construction a number of new adhesives are being introduced to the durability programme. The adhesives under investigation are being classified according to both their chemical and physical properties with a view to determining those grades which produce the optimum combination of mechanical performance and durability. At the same time, suitable test methods for both 'all-adhesive' specimens and adhesive joints are being studied to develop a procedure for quality control. Non-destructive test methods for use in detecting flaws in the bond between steel and concrete have been the subject of a preliminary investigation and are being pursued. Further work is also required to determine the load distribution between longitudinal girders in bridge decks of both the open sandwich and inverted catenary form. The relatively good fatigue performance of adhesivebonded joints compared with welded steel suggests that there may be applications for adhesive bonding in fatigueprone locations in steel structures. The fatigue performance of an adhesive-bonded transverse stiffener for a plate girder web, a notionally unstressed joint, is being investigated.
variety of products. A search for further potential structural applications continues.
Acknowledgements
Thanks are due to all past and present members of the Wolfson Bridge Research Unit who have contributed to the programme of research described in this paper, particularly former Directors Professor A.R. Cusens and Mr D.W. Smith. Financial support has been provided by the Wolfson Institute, the Science and Engineering Research Council, the Transport and Road Research Laboratory, and the Scottish Development Department, Manufacturers have supplied adhesives without charge. References 1 2
'Steel, concrete and composite bridges', BS 5400" Part 10: 1980 (British Standards Institution, 1980)
3
Cusens, A.R. end Smith, D.W. 'A study in epoxy resin adhesive joints in shear', The Struct Engnr 58A No 1 (January 1980) pp 13-18
4
Mays, G.C. and Harvey, W.J. 'Fatigue performance of adhesive bonded joints for bridge deck construction', IABSE Colloq-
Conclusions
An extensive experimental and theoretical programme of work carried out within the Wolfson Bridge Research Unit has demonstrated the structural integrity of externally reinforced concrete. Subject to final demonstration of satisfactory long-term durability, the technique applied to bridge decks offers financial savings on main spans exceeding approximately 20 metres. Studies are now being extended to investigate the properties of adhesives and adhesive joints made with a
Ong, K.C.G. 'Open sandwich construction for bridge decks', PhD Thesis (University of Dundee, 1981)
uium "Fatigue of steel and concrete structures" Lausanne, March 1982
Authors
The authors are with The Department of Civil Engineering, The University, Dundee, DD1 4HN, Scotland. Inquiries, in the first instance, should be directed to Mr Mays.
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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