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Performance of epoxy-repaired concrete under thermal cycling
Cement &Concrete Composites 12 (1990) 47-52
Performance of Epoxy-Repaired Concrete under Thermal Cycling M. Y. Al-Mandil, H. S. Khalil, M. H. Baluch & A. K. Azad Department of Civil Engineering, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia (Received 12 July 1989; accepted 2 January 1990)
Abstract Concrete beams with simulated cracks were epoxyinjected, using three commercially available epoxy compounds. The repaired beam~ were then exposed to a heat-cool cyclic regime. These beams' were tested in flexure, where the epoxy concrete interface was subjected to tensile stresses. In addition, concrete cylinders with embedded inclined cracks were repaired by the same epoxy compounds, exposed to similar heat-cool cyclic regime, and tested in compression, where the epoxy-concrete interface was subjected to combined compressive and shear stresses. Results obtained from these tests indicated that the alternate heating and cooling has a detrimental effect on the performance of bond between epoxy and concrete. This adverse effect on the bond strength is more noticeable when the epoxy-concrete interface is subjected to tensile stresses, as opposed to the case where the interface is subjected to combined compression and shear Keywords; Epoxy resins, bonded joints, thermal cycling tests, flexural strength, compressive strength, adhesive bonding, composite materials, thermal degradation, repairs, tensile stresses, bonding strength, maintenance, strength of materials.
INTRODUCTION Epoxy adhesives represent a wide range of chemical polymers with diverse chemical, mechanical and thermal properties. The following are some of the favourable characteristics of
epoxies which make them desirable for use with concrete: (i) Excellent adhesion and bond to surfaces of nearly all construction materials. (ii) Wide range of favourable physical and chemical properties coupled with a variety of curing agents, extenders, diluents and fillers allow epoxies to be properly formulated for almost any particular application. (iii) The system of a resin and hardener constitutes a thermosetting plastic, which when mixed will change from liquid to a solid state and cannot melt back thereafterJ Hence, they become advantageous in healing cracks and sealing surfaces. However, some of the following precautions need to be considered when using epoxies with concrete: (i) Autogeneous shrinkage which develops in some formulations, as the epoxy hardens, can cause severe strains at the bond interface between epoxy and concrete, and when combined with thermal strains (due to incompatibility of coefficient of thermal expansion), can contribute significantly to delamination and failure of bond between the two materials. (ii) Epoxies are thermosetting plastics, which means that their mechanical properties can change significantly beyond their heat distortion temperature (HDT). The HDT is different for each formulation, and for most epoxies used in the construction industry it ranges between 15"C and 65"C? A number of studies have indicated that temperatures in exposed concrete
47 Cement & Concrete Composites 0958-9465/90/$3.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
48
M.Y. AI-MandU, H. S. Khalil, M. H. Baluch, A. K. Azad
members in the Gulf region could reach up to 70-75°C during the s u m m e r s e a s o n . 2'3 The vast and rapid pace of construction that took place in the Arabian Gulf region over the past two decades offered little or no opportunity for engineers and architects to study and examine the long term effects of the region's harsh environmental conditions on the durability performance of most structures. The marginal quality of the local construction materials coupled with large diurnal and seasonal temperature variations have resulted in rapid deterioration and cracking of many concrete structures. Currently, epoxies are increasingly being used in the repair of cracked concrete elements. They tend to restore the architectural and structural integrity of these elements by bonding cracks and restoring the strength characteristics. Presently, however, there are no data available on the field performance of such epoxy resins in the harsh environmental conditions of the Gulf region. This makes the selection criteria rather difficult in view of the enormous number of resins available on the local market. The present study is part of a multi-faceted programme to address this specific issue.
'
!~2.4mm
Fig. la.
!
Concrete beam specimen for epoxy injection. I 762mm
I
,N # t//
" "
-°--J~- ~" 1t/|6in)
Fig. lb.
Concrete cylindrical specimen for epoxy injection.
EXPERIMENTAL PROGRAMME Test specimens An extensive literature review of the types of specimens that can be utilized for assessing the performance of various epoxy products revealed that pre-cracked beam specimens are most suitable for evaluating the epoxy bonding capacity in tension, while the slant shear test of cylindrical specimens is most widely used for assessing the bonding capacity under combined compressive and shear stresses. 4-8 Figure 1 illustrates the schematic diagram and the dimensions of specimens adopted for this study. The study of thermal cycling on epoxy repaired concrete specimens comprised of the following: (A) A group of unreinforced concrete beams (approximately 150 x 150 x 530 mm in length) with pre-inserted ~- 1.5 mm thick plate at mid length and reaching up to their mid depth ( = 75 mm), simulating a crack (Fig. la). These beams were epoxy injected and were tested in flexure after being subjected to a variable number of heat-cool cycles. This test would subject the epoxy-concrete interface to linearly variable tensile stresses.
(B) A group of half concrete cylinders, each with a slant surface at 30 ° from vertical (Fig. lb). Pairs of these halves were joined together by epoxy to form a complete cylinder of ~ 75 mm diameter and ~ 150 mm in height (similar to specimens of the ASTM C882-78). Full cylinders were also subjected to thermal cycling and were tested in direct compression afterwards. The epoxy-concrete interface would be subjected to a stress field comprising of compressive and shear stresses. Materials The concrete proportions used for all the specimens were 2.85:1.39:1.0:0.52 (coarse aggregate: fine aggregate: cement: water). Three types of crack injection epoxy resins were selected for this study. These products will be designated as epoxies A, B and C hereafter. Table 1 indicates some of the physical and mechanical properties of these products. An approved sealant was used to seal all cracks prior to injection. Epoxies were injected by means of a manual pressure gun and were allowed to cure properly.
Performance of epoxy-repaired concrete under thermal cycling
49
Table I. Properties of the three types of epoxy resins used as obtained from manufacturer instruction sheets Property
Epoxy A
Storage conditions and shelf life Working temperature (processing temperature) Pot life Compressive strength Tensile strength Elastic modulus Coefficient of thermal expansion
Epoxy B
Epoxy C
Resin and hardener have a shelf life of one year if stored at 15-25°(2
+ 5°C to + 40°(2. A shelf life of 12 months when
Dry and cool, maximum storage period of 6 months
15°C-30°C (59-86°F) 110 min at 10°C (50°F) 50 min at 230C (73°1:) 25 min at 30°(2 (86°F) (ISO/R 604) 80 N/mm 2 ( 11600 psi) (ISO/R527) = 60 N/mm 2 (8 700 psi) (ISO/R527) = 3 200 N/mm 2 (464 200 psi) 6 0 × 10-6 per °C (33-3 x 10 -6 per °F)
5-10°C
Min 5°(2
50 minat 10°C (50*F) 20 rain at 20°(2 (68°F) 10 rain at 30°C (86°F)
30 min at 20°C (68°F)
unopened and stored correctly
(41-50*F)
(41°F)
Approx. 97-4 N/mm 2 (14 124 psi) 61.9 N/mm 2 (8 976 psi)
25 N/mm 2 (3 625 psi)
N/LP
E-Modulus/bend 2 510 N/mm 2 (363 983 psi) 50 x 10 -6 per °C (27-8 x 10 -6 per °F)
9 0 x 10-6 per °C (50 x 10 -6 per °F)
GROOVED UNREPAIRED (G) REPAIRED BY EPOXY (A)
ileatlCeal Cycles
REPAIRED BY EPOXY (B)
m Btmms
REPAIRED BY EPOXY (C) SOLID UNGROOVED
(S)
i
50 H/CC1 n H/ lO0 H/C 150tW/C
Test be lqeam~
B=per/msntol _ Program
--
neat/cool
~CyHnders
Fig. 2.
o t41c
REPAIRED BY EPOXY (A)
1DO
H/c
Cyelea ..
REPAIRED BY EPOXY (C)
--
SOLID CYLINDERS
(S)
Test in
Ceml~enlm
200 H/C 316 H/C
Schematic diagram for experimental programme.
Thermal cycling programme A unified cycling programme was adopted for the beams and cylinders, each thermal cycle would comprise of subjecting the specimens for 6 h at an environmental chamber set at 70°C and 35% RH, followed by another 6 h at 20°C and 35% RH. Groups of beams were subjected to 0, 50, 100 and 150 heat-cool (H/C) cycles. A group of beams subjected to a specific number of cycles would comprise of two uncracked solid beams (S), two cracked (grooved) but unrepaired beams (G) and two repaired beams by each of the three epoxies (A, B and C) (see experimental flow chart in Fig. 2).
A group of cylinders, on the other hand, would comprise of eight cylinders, two uncracked solid cylinders (S) and two cylinders for each of the repair products (A, B and C). The cyclic regime for cylinders comprised of 0, 100, 200 and 316 H/C cycles (Fig. 2). TEST RESULTS AND DISCUSSION After the specific number of H/C cycles had been completed for a group of beams or cylinders, the specimens were left to cool down to room temperature, and were then tested for strength.
M.Y. AI-Mandii, H. S. Khalil, M. H. Baluch, A. K. Azad
50
Beams were tested in flexure with third point loading in accordance with ASTM C78-84. The ultimate flexural strength (Ou~,)was recorded for each specimen (Table 2). Cylinders were tested in direct compression in accordance to ASTM C3983b. The ultimate compressive strength (o,,i,) is likewise recorded for each of the tested cylinders (Table 3). Discussion of H/C cycling for beams Graphical presentation of the data in Table 2 is shown in Fig. 3. This figure indicates that the grooved unrepaired beams (G) represent about 25% of the ultimate load capacity of the solid beams (S). This is to be expected, since the section modulus for the G beams equals one-quarter of that for the S beams. It is noted that both the S and the G groups of beams have gained some strength as the number of H/C cycles increases. Total gain in strength, after 150 H/C cycles, is estimated around 20% over that at the 0 H/C cycles level. The following factors may have contributed to this gain in strength: (i) Gain in strength with age of concrete as the specimens tested at 0 H/C cycles were about 40 days old while those tested after 150 H/C cycles were over 120 days old. (ii) The H/C cycling regime may have contributed to the hydration of the remaining
LEGEND 0 Solid S z Epoxy A 0 Epoxy D O Epoxy C • Crackf.d G
0
5.49 3.32 2.35 3.92 1.48
5.72 1.88 1.66 2.56 1.40
5.80 1.96 1.52 2.41 1.36
3. Effect of heat-cool cycling on the compressive strength of epoxy-repaired cylinders
Table
Specimen type
Ultimate compressivestrength (MPa) o H/c /oo H/c 200 H/c 316 H/C
Solid uncracked (S) Repaired by epoxy (A) Repaired by epoxy (B) Repaired by epoxy (C)
cycles cycles
cycles cycles
50.38 39"09 22.86 41.08
54.55 50'33 48.44 52.24
51.50 50.30 33.16 49.12
49.41 46.85 39.27 47.15
100
120
|40
1110
(A) At 0 H/C cycles all repaired beams failed
(B)
o H/C 5OH/C IO0H/C /50 H/C 4.57 3.52 4.55 5.13 1.25
80
The following observations are made on the effects of H/C cycling on beams:
Uitimateflexural strength (MPa)
Solid ungrooved (S) Repaired by epoxy (A) Repaired by epoxy (B) Repaired by epoxy (C) Grooved unrepaired (G)
aO
unhydrated cement, especially since the hydration process is accelerated by the rise in ambient temperature. 9
of epoxy-repaired beams
cycles cycles cycles cycles
40
No. oF H/C CYCLES Fig. 3. Effect of heat-cool (H/C) cycling on the flexural strength of repaired beams.
Table 2. Effect of heat-cool cycling on the flexural strength
Specimen type
20
(c)
through a fracture in the concrete material (tensile failure), away from the epoxy bond line. This indicates that the bonding strengths for the three epoxies exceeded the tensile strength of concrete. A debonding failure was observed for all the repaired specimens with some traces of concrete attached to the epoxy surface at 50 H/C cycles, and with no such traces at 100 and 150 H/C cycles. This is a clear manifestation of the reduction in the bonding capacities of epoxies as they are subjected to an increasing number of H/C cycles. As the number of H/C cycles increased, a marked reduction in the flexural strength for the repaired beams (A, B and C) was observed. This systematic decrease brought the strength of these specimens to an asymptotic level towards the grooved unrepaired beams (G). This means that as the number of H/C cycles increases, the epoxy products tend to lose their bonding strength and may eventually become structurally ineffective.
The degradation in bond between epoxy and concrete as the number of H/C cycles increases is believed to be caused by the difference in the coefficients of thermal expansion of each type of epoxy and that of concrete, which led to the development of repetitive thermal stresses
Performance of epoxy-repaired concrete under thermal cycling
51
(thermal fatigue). The reduction in the tensile bonding capacities of the three epoxies has been found to be directly proportional to the differences in the coefficient of thermal expansion between these epoxies and that of concrete.m°
that epoxy B failed mostly by the sliding of the two half cylinders against each other (debonding failure), except at 200 H/C cycles where the crushing of concrete resulting in a conic pattern of failure.
Discussion of H/C cycfing for cylinders Data of Table 3 is graphically presented in Fig. 4. As can readily be seen from the behaviour of the solid unrepaired cylinders (S), the H/C cycling has very limited effect on the compressive strength of concrete, the reduction in strength between the cylinders tested at 200 H/C cycles and those tested at 316 H/C cycles (about 9% reduction) may have been caused by out-of-group variations in the concrete mixes, rather than by an actual reduction in strength. The following observations are deduced from Fig. 4:
Although test results of beam specimens clearly portrayed the negative effect of repetitive changes in temperature on the bond between epoxy and concrete, test results of cylindrical specimens did not reveal such an effect. It is believed that the heat-cool cyclic regime applied to cylinders could not degrade the bond between epoxy and concrete, where the strong shear resistance of the bond itself enhanced by the frictional forces resulting from the compression component of the applied load could withstand the sliding force component of the applied load. Thus, no shear failure through the bond line could occur before the crushing of concrete.
(A) Ultimate strengths of epoxy repaired cylinders in groups A and C are very compatible to each other and closely followed the behaviour of uncracked cylinders (S). ; h e failure pattern for these cylinders also resembled that of the S cylinders, i.e. it was characterized by the cracking and the crushing of concrete in the middle part, leaving the well formed end cones. (B) Cylinders repaired with epoxy B showed a reduction in strength of about 40% from that of the other repaired cylinders (A and C) at 0 and 100 H/C cycles, while they were of comparable strength to the other repaired cylinders at the 200 and 316 H/C cycles. This erratic behaviour of cylinders repaired by epoxy B may have been caused by the misalignment of the top parts of cylinders during the repair process. It is worth mentioning at this point
z
~o', g~o LEGEND 0 A 0 n
hi
,° o
I Ioo
I 1so
No. oF,
I 2o0
.rc
Solid Epoxy Epoxy Epoxy
I 2so
S A B C
I 3oo
I
SUMMARY AND CONCLUSIONS The following are the main conclusions obtained from data presented in this paper: 1. Epoxies A, B and C perform adequately under favourable environmental conditions (20°(2 and 0 H/C). Bond strength always exceeded the tensile and shear strengths of concrete.
2. Heat-cool cycling of repaired beams resulted in considerable reduction in the bonding strength of epoxies. An average reduction of 80% in the epoxy bonding capacity was obtained after beams were subjected to 150 H/C cycles and tested in flexure. 3. Repaired cylinders did not appear to be sensitive to the adopted H/C cycling programme, since a failure was mostly by the crushing of concrete instead of the sliding of repaired cylinders assc,ciated with the bond failure of epoxies. 4. Testing cylinders in direct compression creates a favourable combined stress field (shear + compression) on the epoxy bond line; contrary to the debonding tensile field created by testing beams in flexure. ACKNOWLEDGEMENTS
CYCLES
Fig. 4. Effect of heat-cool (H/C) cyclingon the compressive strength of repaired cylinders.
The authors wish to thank the suppliers of the epoxy products used in this study; namely Saad
52
M.Y. AI-Mandil, H. S. Khalil, M. H. Baluch, A. K. Azad
Abu-Khadra and Co., Anjali Trading and Zschokka Steidle Establishments in the Eastern Province. Thanks also to KFUPM for the technical work involved in this study. REFERENCES 1. ACI Committee 503, Use of epoxy compounds with concrete. J. Am. Concrete Inst., 70 (9) (1973) 614-45. 2. Mahmoud, M., Mohamed, M., Durability and thermal incompatibility of concrete constituents made from local materials in the Arabian Gulf countries. MSc thesis, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, 1988. 3. CIRIA Guide to Concrete Construction in the Gulf Region. CIRIA Special Publication 31, CIRIA, London, 1984. 4. Ciba-Geigy, Guidelines for testing araldite epoxy resinbased structural adhesives and mortars. Publication No. 24648/e, Ciba-Geigy, Switzerland, 1983.
5. Fattuhi, N. I., Two simple techniques for testing the performance of repair materials for concrete cracks. Magazine of Concrete Research, 35 (124) 1983, 170-4. 6. Fattuhi, N. I., Techniques for testing repair adhesives for concrete cracks. In Proceedings of the 2rid International Conference on Deterioration and Repair of Reinforced Concrete in the Arabian Gulf, Vol. 1. The Bahrain Society of Engineers, Bahrain, 1987, pp. 227-34. 7. BS 6319: Part 4: Testing of resin compositions for use in construction. Method for the measurement of bond strength (Slant shear method), British Standards Institution, London, 1984. 8. ASTM C882-78: Standard test method for bond strength of epoxy-resin systems used with concrete. In Annual Book of ASTM Standards, Vol. 4.02, 1985. 9. Neville, A. M., Properties of Concrete. John Wiley & Sons, 1973, New York. 10. Khalil, H. S., Performance of epoxy-repaired concrete in hot weather conditions. MSc thesis King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia, 1989.
Performance of Epoxy-Repaired Concrete under Thermal Cycling M. Y. Al-Mandil, H. S. Khalil, M. H. Baluch & A. K. Azad Department of Civil Engineering, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia (Received 12 July 1989; accepted 2 January 1990)
Abstract Concrete beams with simulated cracks were epoxyinjected, using three commercially available epoxy compounds. The repaired beam~ were then exposed to a heat-cool cyclic regime. These beams' were tested in flexure, where the epoxy concrete interface was subjected to tensile stresses. In addition, concrete cylinders with embedded inclined cracks were repaired by the same epoxy compounds, exposed to similar heat-cool cyclic regime, and tested in compression, where the epoxy-concrete interface was subjected to combined compressive and shear stresses. Results obtained from these tests indicated that the alternate heating and cooling has a detrimental effect on the performance of bond between epoxy and concrete. This adverse effect on the bond strength is more noticeable when the epoxy-concrete interface is subjected to tensile stresses, as opposed to the case where the interface is subjected to combined compression and shear Keywords; Epoxy resins, bonded joints, thermal cycling tests, flexural strength, compressive strength, adhesive bonding, composite materials, thermal degradation, repairs, tensile stresses, bonding strength, maintenance, strength of materials.
INTRODUCTION Epoxy adhesives represent a wide range of chemical polymers with diverse chemical, mechanical and thermal properties. The following are some of the favourable characteristics of
epoxies which make them desirable for use with concrete: (i) Excellent adhesion and bond to surfaces of nearly all construction materials. (ii) Wide range of favourable physical and chemical properties coupled with a variety of curing agents, extenders, diluents and fillers allow epoxies to be properly formulated for almost any particular application. (iii) The system of a resin and hardener constitutes a thermosetting plastic, which when mixed will change from liquid to a solid state and cannot melt back thereafterJ Hence, they become advantageous in healing cracks and sealing surfaces. However, some of the following precautions need to be considered when using epoxies with concrete: (i) Autogeneous shrinkage which develops in some formulations, as the epoxy hardens, can cause severe strains at the bond interface between epoxy and concrete, and when combined with thermal strains (due to incompatibility of coefficient of thermal expansion), can contribute significantly to delamination and failure of bond between the two materials. (ii) Epoxies are thermosetting plastics, which means that their mechanical properties can change significantly beyond their heat distortion temperature (HDT). The HDT is different for each formulation, and for most epoxies used in the construction industry it ranges between 15"C and 65"C? A number of studies have indicated that temperatures in exposed concrete
47 Cement & Concrete Composites 0958-9465/90/$3.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
48
M.Y. AI-MandU, H. S. Khalil, M. H. Baluch, A. K. Azad
members in the Gulf region could reach up to 70-75°C during the s u m m e r s e a s o n . 2'3 The vast and rapid pace of construction that took place in the Arabian Gulf region over the past two decades offered little or no opportunity for engineers and architects to study and examine the long term effects of the region's harsh environmental conditions on the durability performance of most structures. The marginal quality of the local construction materials coupled with large diurnal and seasonal temperature variations have resulted in rapid deterioration and cracking of many concrete structures. Currently, epoxies are increasingly being used in the repair of cracked concrete elements. They tend to restore the architectural and structural integrity of these elements by bonding cracks and restoring the strength characteristics. Presently, however, there are no data available on the field performance of such epoxy resins in the harsh environmental conditions of the Gulf region. This makes the selection criteria rather difficult in view of the enormous number of resins available on the local market. The present study is part of a multi-faceted programme to address this specific issue.
'
!~2.4mm
Fig. la.
!
Concrete beam specimen for epoxy injection. I 762mm
I
,N # t//
" "
-°--J~- ~" 1t/|6in)
Fig. lb.
Concrete cylindrical specimen for epoxy injection.
EXPERIMENTAL PROGRAMME Test specimens An extensive literature review of the types of specimens that can be utilized for assessing the performance of various epoxy products revealed that pre-cracked beam specimens are most suitable for evaluating the epoxy bonding capacity in tension, while the slant shear test of cylindrical specimens is most widely used for assessing the bonding capacity under combined compressive and shear stresses. 4-8 Figure 1 illustrates the schematic diagram and the dimensions of specimens adopted for this study. The study of thermal cycling on epoxy repaired concrete specimens comprised of the following: (A) A group of unreinforced concrete beams (approximately 150 x 150 x 530 mm in length) with pre-inserted ~- 1.5 mm thick plate at mid length and reaching up to their mid depth ( = 75 mm), simulating a crack (Fig. la). These beams were epoxy injected and were tested in flexure after being subjected to a variable number of heat-cool cycles. This test would subject the epoxy-concrete interface to linearly variable tensile stresses.
(B) A group of half concrete cylinders, each with a slant surface at 30 ° from vertical (Fig. lb). Pairs of these halves were joined together by epoxy to form a complete cylinder of ~ 75 mm diameter and ~ 150 mm in height (similar to specimens of the ASTM C882-78). Full cylinders were also subjected to thermal cycling and were tested in direct compression afterwards. The epoxy-concrete interface would be subjected to a stress field comprising of compressive and shear stresses. Materials The concrete proportions used for all the specimens were 2.85:1.39:1.0:0.52 (coarse aggregate: fine aggregate: cement: water). Three types of crack injection epoxy resins were selected for this study. These products will be designated as epoxies A, B and C hereafter. Table 1 indicates some of the physical and mechanical properties of these products. An approved sealant was used to seal all cracks prior to injection. Epoxies were injected by means of a manual pressure gun and were allowed to cure properly.
Performance of epoxy-repaired concrete under thermal cycling
49
Table I. Properties of the three types of epoxy resins used as obtained from manufacturer instruction sheets Property
Epoxy A
Storage conditions and shelf life Working temperature (processing temperature) Pot life Compressive strength Tensile strength Elastic modulus Coefficient of thermal expansion
Epoxy B
Epoxy C
Resin and hardener have a shelf life of one year if stored at 15-25°(2
+ 5°C to + 40°(2. A shelf life of 12 months when
Dry and cool, maximum storage period of 6 months
15°C-30°C (59-86°F) 110 min at 10°C (50°F) 50 min at 230C (73°1:) 25 min at 30°(2 (86°F) (ISO/R 604) 80 N/mm 2 ( 11600 psi) (ISO/R527) = 60 N/mm 2 (8 700 psi) (ISO/R527) = 3 200 N/mm 2 (464 200 psi) 6 0 × 10-6 per °C (33-3 x 10 -6 per °F)
5-10°C
Min 5°(2
50 minat 10°C (50*F) 20 rain at 20°(2 (68°F) 10 rain at 30°C (86°F)
30 min at 20°C (68°F)
unopened and stored correctly
(41-50*F)
(41°F)
Approx. 97-4 N/mm 2 (14 124 psi) 61.9 N/mm 2 (8 976 psi)
25 N/mm 2 (3 625 psi)
N/LP
E-Modulus/bend 2 510 N/mm 2 (363 983 psi) 50 x 10 -6 per °C (27-8 x 10 -6 per °F)
9 0 x 10-6 per °C (50 x 10 -6 per °F)
GROOVED UNREPAIRED (G) REPAIRED BY EPOXY (A)
ileatlCeal Cycles
REPAIRED BY EPOXY (B)
m Btmms
REPAIRED BY EPOXY (C) SOLID UNGROOVED
(S)
i
50 H/CC1 n H/ lO0 H/C 150tW/C
Test be lqeam~
B=per/msntol _ Program
--
neat/cool
~CyHnders
Fig. 2.
o t41c
REPAIRED BY EPOXY (A)
1DO
H/c
Cyelea ..
REPAIRED BY EPOXY (C)
--
SOLID CYLINDERS
(S)
Test in
Ceml~enlm
200 H/C 316 H/C
Schematic diagram for experimental programme.
Thermal cycling programme A unified cycling programme was adopted for the beams and cylinders, each thermal cycle would comprise of subjecting the specimens for 6 h at an environmental chamber set at 70°C and 35% RH, followed by another 6 h at 20°C and 35% RH. Groups of beams were subjected to 0, 50, 100 and 150 heat-cool (H/C) cycles. A group of beams subjected to a specific number of cycles would comprise of two uncracked solid beams (S), two cracked (grooved) but unrepaired beams (G) and two repaired beams by each of the three epoxies (A, B and C) (see experimental flow chart in Fig. 2).
A group of cylinders, on the other hand, would comprise of eight cylinders, two uncracked solid cylinders (S) and two cylinders for each of the repair products (A, B and C). The cyclic regime for cylinders comprised of 0, 100, 200 and 316 H/C cycles (Fig. 2). TEST RESULTS AND DISCUSSION After the specific number of H/C cycles had been completed for a group of beams or cylinders, the specimens were left to cool down to room temperature, and were then tested for strength.
M.Y. AI-Mandii, H. S. Khalil, M. H. Baluch, A. K. Azad
50
Beams were tested in flexure with third point loading in accordance with ASTM C78-84. The ultimate flexural strength (Ou~,)was recorded for each specimen (Table 2). Cylinders were tested in direct compression in accordance to ASTM C3983b. The ultimate compressive strength (o,,i,) is likewise recorded for each of the tested cylinders (Table 3). Discussion of H/C cycling for beams Graphical presentation of the data in Table 2 is shown in Fig. 3. This figure indicates that the grooved unrepaired beams (G) represent about 25% of the ultimate load capacity of the solid beams (S). This is to be expected, since the section modulus for the G beams equals one-quarter of that for the S beams. It is noted that both the S and the G groups of beams have gained some strength as the number of H/C cycles increases. Total gain in strength, after 150 H/C cycles, is estimated around 20% over that at the 0 H/C cycles level. The following factors may have contributed to this gain in strength: (i) Gain in strength with age of concrete as the specimens tested at 0 H/C cycles were about 40 days old while those tested after 150 H/C cycles were over 120 days old. (ii) The H/C cycling regime may have contributed to the hydration of the remaining
LEGEND 0 Solid S z Epoxy A 0 Epoxy D O Epoxy C • Crackf.d G
0
5.49 3.32 2.35 3.92 1.48
5.72 1.88 1.66 2.56 1.40
5.80 1.96 1.52 2.41 1.36
3. Effect of heat-cool cycling on the compressive strength of epoxy-repaired cylinders
Table
Specimen type
Ultimate compressivestrength (MPa) o H/c /oo H/c 200 H/c 316 H/C
Solid uncracked (S) Repaired by epoxy (A) Repaired by epoxy (B) Repaired by epoxy (C)
cycles cycles
cycles cycles
50.38 39"09 22.86 41.08
54.55 50'33 48.44 52.24
51.50 50.30 33.16 49.12
49.41 46.85 39.27 47.15
100
120
|40
1110
(A) At 0 H/C cycles all repaired beams failed
(B)
o H/C 5OH/C IO0H/C /50 H/C 4.57 3.52 4.55 5.13 1.25
80
The following observations are made on the effects of H/C cycling on beams:
Uitimateflexural strength (MPa)
Solid ungrooved (S) Repaired by epoxy (A) Repaired by epoxy (B) Repaired by epoxy (C) Grooved unrepaired (G)
aO
unhydrated cement, especially since the hydration process is accelerated by the rise in ambient temperature. 9
of epoxy-repaired beams
cycles cycles cycles cycles
40
No. oF H/C CYCLES Fig. 3. Effect of heat-cool (H/C) cycling on the flexural strength of repaired beams.
Table 2. Effect of heat-cool cycling on the flexural strength
Specimen type
20
(c)
through a fracture in the concrete material (tensile failure), away from the epoxy bond line. This indicates that the bonding strengths for the three epoxies exceeded the tensile strength of concrete. A debonding failure was observed for all the repaired specimens with some traces of concrete attached to the epoxy surface at 50 H/C cycles, and with no such traces at 100 and 150 H/C cycles. This is a clear manifestation of the reduction in the bonding capacities of epoxies as they are subjected to an increasing number of H/C cycles. As the number of H/C cycles increased, a marked reduction in the flexural strength for the repaired beams (A, B and C) was observed. This systematic decrease brought the strength of these specimens to an asymptotic level towards the grooved unrepaired beams (G). This means that as the number of H/C cycles increases, the epoxy products tend to lose their bonding strength and may eventually become structurally ineffective.
The degradation in bond between epoxy and concrete as the number of H/C cycles increases is believed to be caused by the difference in the coefficients of thermal expansion of each type of epoxy and that of concrete, which led to the development of repetitive thermal stresses
Performance of epoxy-repaired concrete under thermal cycling
51
(thermal fatigue). The reduction in the tensile bonding capacities of the three epoxies has been found to be directly proportional to the differences in the coefficient of thermal expansion between these epoxies and that of concrete.m°
that epoxy B failed mostly by the sliding of the two half cylinders against each other (debonding failure), except at 200 H/C cycles where the crushing of concrete resulting in a conic pattern of failure.
Discussion of H/C cycfing for cylinders Data of Table 3 is graphically presented in Fig. 4. As can readily be seen from the behaviour of the solid unrepaired cylinders (S), the H/C cycling has very limited effect on the compressive strength of concrete, the reduction in strength between the cylinders tested at 200 H/C cycles and those tested at 316 H/C cycles (about 9% reduction) may have been caused by out-of-group variations in the concrete mixes, rather than by an actual reduction in strength. The following observations are deduced from Fig. 4:
Although test results of beam specimens clearly portrayed the negative effect of repetitive changes in temperature on the bond between epoxy and concrete, test results of cylindrical specimens did not reveal such an effect. It is believed that the heat-cool cyclic regime applied to cylinders could not degrade the bond between epoxy and concrete, where the strong shear resistance of the bond itself enhanced by the frictional forces resulting from the compression component of the applied load could withstand the sliding force component of the applied load. Thus, no shear failure through the bond line could occur before the crushing of concrete.
(A) Ultimate strengths of epoxy repaired cylinders in groups A and C are very compatible to each other and closely followed the behaviour of uncracked cylinders (S). ; h e failure pattern for these cylinders also resembled that of the S cylinders, i.e. it was characterized by the cracking and the crushing of concrete in the middle part, leaving the well formed end cones. (B) Cylinders repaired with epoxy B showed a reduction in strength of about 40% from that of the other repaired cylinders (A and C) at 0 and 100 H/C cycles, while they were of comparable strength to the other repaired cylinders at the 200 and 316 H/C cycles. This erratic behaviour of cylinders repaired by epoxy B may have been caused by the misalignment of the top parts of cylinders during the repair process. It is worth mentioning at this point
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SUMMARY AND CONCLUSIONS The following are the main conclusions obtained from data presented in this paper: 1. Epoxies A, B and C perform adequately under favourable environmental conditions (20°(2 and 0 H/C). Bond strength always exceeded the tensile and shear strengths of concrete.
2. Heat-cool cycling of repaired beams resulted in considerable reduction in the bonding strength of epoxies. An average reduction of 80% in the epoxy bonding capacity was obtained after beams were subjected to 150 H/C cycles and tested in flexure. 3. Repaired cylinders did not appear to be sensitive to the adopted H/C cycling programme, since a failure was mostly by the crushing of concrete instead of the sliding of repaired cylinders assc,ciated with the bond failure of epoxies. 4. Testing cylinders in direct compression creates a favourable combined stress field (shear + compression) on the epoxy bond line; contrary to the debonding tensile field created by testing beams in flexure. ACKNOWLEDGEMENTS
CYCLES
Fig. 4. Effect of heat-cool (H/C) cyclingon the compressive strength of repaired cylinders.
The authors wish to thank the suppliers of the epoxy products used in this study; namely Saad
52
M.Y. AI-Mandil, H. S. Khalil, M. H. Baluch, A. K. Azad
Abu-Khadra and Co., Anjali Trading and Zschokka Steidle Establishments in the Eastern Province. Thanks also to KFUPM for the technical work involved in this study. REFERENCES 1. ACI Committee 503, Use of epoxy compounds with concrete. J. Am. Concrete Inst., 70 (9) (1973) 614-45. 2. Mahmoud, M., Mohamed, M., Durability and thermal incompatibility of concrete constituents made from local materials in the Arabian Gulf countries. MSc thesis, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, 1988. 3. CIRIA Guide to Concrete Construction in the Gulf Region. CIRIA Special Publication 31, CIRIA, London, 1984. 4. Ciba-Geigy, Guidelines for testing araldite epoxy resinbased structural adhesives and mortars. Publication No. 24648/e, Ciba-Geigy, Switzerland, 1983.
5. Fattuhi, N. I., Two simple techniques for testing the performance of repair materials for concrete cracks. Magazine of Concrete Research, 35 (124) 1983, 170-4. 6. Fattuhi, N. I., Techniques for testing repair adhesives for concrete cracks. In Proceedings of the 2rid International Conference on Deterioration and Repair of Reinforced Concrete in the Arabian Gulf, Vol. 1. The Bahrain Society of Engineers, Bahrain, 1987, pp. 227-34. 7. BS 6319: Part 4: Testing of resin compositions for use in construction. Method for the measurement of bond strength (Slant shear method), British Standards Institution, London, 1984. 8. ASTM C882-78: Standard test method for bond strength of epoxy-resin systems used with concrete. In Annual Book of ASTM Standards, Vol. 4.02, 1985. 9. Neville, A. M., Properties of Concrete. John Wiley & Sons, 1973, New York. 10. Khalil, H. S., Performance of epoxy-repaired concrete in hot weather conditions. MSc thesis King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia, 1989.