- Email: [email protected]
Experimental study of epoxy repairing of cracks in concrete
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 157–163
www.elsevier.com/locate/conbuildmat
Experimental study of epoxy repairing of cracks in concrete Camille A. Issa *, Pauls Debs Department of Civil Engineering, Lebanese American University, P.O. Box 36, Byblos, Lebanon Received 24 November 2004; received in revised form 16 June 2005; accepted 30 June 2005 Available online 6 September 2005
Abstract Cracks in concrete structures have always posed a big threat on the durability of concrete. Cracking of concrete is a random process, highly variable and influenced by many factors. Among the crack repair methods is the use of epoxy either by injection or by gravity filling in order to bond the crack and restore its structural integrity. Prior to the use of epoxy compounds and due to their versatility and the wide range of available physical and chemical properties of epoxy resin systems, one has to be completely aware and informed before entering the world of epoxies; other than crack repair, epoxy compounds have found a wide variety of uses in the concrete industry. In this study, 15 concrete cubes, six including cracks without repair, six including cracks bonded with gravity filled epoxy and three with no cracks were crushed and their compressive strengths were obtained. It was found that the cracks caused a reduction in compressive strength up to 40.93% whereas the epoxy system, when properly applied, restored the compressive strength by decreasing the reduction down to 8.23%. 2005 Elsevier Ltd. All rights reserved. Keywords: Concrete; Crack; Repair; Epoxy; Experimental
1. Introduction Cracks in concrete have many causes. They may affect appearance only, or they may indicate significant structural distress or a lack of durability. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude. Their significance depends on the type of structure, as well as the nature of the cracking. For example, cracks that are acceptable for buildings may not be acceptable in water-retaining structures. The proper repair of cracks depends on knowing the causes and selecting the repair procedures that take these causes into account; otherwise, the repair may only be temporary. Successful long-term repair procedures
*
Corresponding author. Tel.: +961 9 944850; fax: +961 9 944851. E-mail address: [email protected] (C.A. Issa).
0950-0618/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.030
must attack the causes of the cracks as well as the cracks themselves. The main purpose of this study is to experiment with the use of epoxy compounds to restore the integrity of a cracked member by gravity filling of the crack, emphasizing on the physical characteristics of epoxies as well as the importance of surface preparation, temperature conditioning of the substrate and epoxy compound, and alterations of the hardening rate of epoxies. Furthermore, all lab tests and results on concrete cubes will be fully documented and interpreted.
2. Causes of cracking in concrete [1] Cracks in concrete are categorized as occurring either in plastic concrete or hardened concrete. There are two kinds of cracks are associated with cracking of plastic concrete:
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C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
Plastic shrinkage cracking. Settlement cracking. Cracking of hardened concrete are caused due to the followings:
Drying shrinkage. Thermal stresses. Chemical reaction. Weathering. Corrosion of reinforcement. Poor construction practices. Construction overloads. Errors in design and detailing.
3. Scope of study Due to the application of repeated loads, a crack may propagate until it reaches a point where fracture occurs and failure results. The longer the crack, the higher the stress concentration induced by it. Due to the presence of a crack, the strength of the structure is decreased: it is lower than the original strength it was designed for. Several methods are used for crack repair among which is the ‘‘Gravity Filling’’ of cracks using epoxy. As a result, the objective of this laboratory work is to restore structural integrity and increase the compressive strength and stiffness of cracked concrete test cubes by gravity filling of cracks using a low viscosity gel-type epoxy resin system.
plaza decks, and similar surfaces. Gravity feed of resin is not effective for repairing moving cracks. This is because the materials are unable to act as a flexible joint material. Likewise, gravity feed of resin should not be viewed as a long-term solution to cracking caused by corrosion, sulfate attack, or AAR. At best, this repair will keep out water, chlorides, and sulfates, which may slow the progress of future damage around the crack, but it certainly will not stop it indefinitely. In these situations, a complete repair and protection approach is required to address the entire area affected, not just the cracks.
5. Description of cube forms Three kinds of cubes were used during the laboratory experiment. The first kind is an ordinary cube (O) (Fig. 1) with the following dimensions: 150 · 150 · 150 mm3. As for the second cube (D1), an artificial crack was installed in it using two steel plates, both having the following dimensions: a height of 150 mm, a width of 35 mm, and a thickness of 2 mm (Fig. 2). The third and final cube (D2) also has an artificial crack made of two steel plates with the following dimensions: a height of 70 mm, a width of 50 mm, and a thickness of 2 mm (Fig. 3). In addition cubes (D1 + E) and (D2 + E) represent the defected cubes (D1) and (D2) with the cracks filled with epoxy, as shown in Fig. 4. The concrete cubes were crushed using a universal testing machine.
6. Concrete mix 4. Gravity filling: objectives and purpose According to ACI RAP-2 [2], the primary objective of this repair method is to fill the crack and structurally bond the concrete on both sides of the crack. This repair is to seal cracks that are not moving – for example, shrinkage cracks, and settlement cracks that have stabilized. By penetrating and filling the cracks, the resin is able to form a polymer plug that seals the crack, keeping out water, chlorides, carbon dioxide, sulfates, and other aggressive liquids and gases. This repair method is, therefore, a way to reduce possible future deterioration caused by freeze–thaw cycles, steel corrosion, and chemical attack of the concrete. Finally, it is occasionally an objective to achieve a structural repair of the cracks using this method. While many of these resin materials exceed the strength of the concrete, ensuring a structural repair with full depth penetration by gravity is difficult. If a structural repair of the crack is critical, pressure injection may be a preferred option (ACI RAP-1 [3]). This repair can only be applied to horizontal concrete elements such as bridge and parking decks, floor slabs,
The concrete mix used was a ready mixed concrete with the ingredients for 0.5 m3 as follows: Coarse aggregates: two kinds of coarse aggregates were used in the mix: the first kind: 294 kg of ‘‘normal maximum coarse aggregate size of 19 mm’’ (c =
Fig. 1. O cubes.
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
159
Water: 69 l of mixing water was used alongside the above mentioned ingredients.
7. Epoxy compound
Fig. 2. D1 cube.
Sikadur-52 [4] is a two component, solvent-free, low viscosity liquid, based on high strength epoxy resins. It complies with ASTM C 881-78 Type I, Grade 1 Class B + C. Sikadur-52 is used to fill and seal cavities and cracks in structural components such as bridges, industrial structures, columns, beams, foundations, walls and floors slabs and water-retaining structures. It not only forms an effective barrier against water infiltration of corrosion promoting media, but also structurally bonds the concrete sections together. It has the following advantages:
Fig. 3. D2 cube.
Solvent-free. Suitable in both, dry and damp conditions. Useable at low temperatures. Two grades for different climatic conditions (normal and long pot life). Shrinkage-free hardening. High mechanical and adhesive strengths. Hard but not brittle. Very low viscosity.
The characteristics of the epoxy adhesive are shown in Table 1. Figs. 5–7 display the development of compressive, tensile and shear strengths of the epoxy adhesive, Sikadur-52, with respect to time [4].
8. Crack repair by gravity filling
Fig. 4. D1 + E and D2 + E cubes.
1580 kg/m3); the second kind: 204 kg of ‘‘normal maximum coarse aggregate size of 9.5 mm’’ (c = 1550 kg/ m3). As a result, 498 kg of coarse aggregate was used. Fine aggregates: two kinds of fine aggregates were used in the mix: the first kind: 289 kg of crushed limestone, No. 8 (2.36 mm) size, (c = 1610 kg/m3); the second kind: 209 kg of dune sand, No. 4 (4.75 mm) size, (c = 1550 kg/m3). As a result, a total of 498 kg of fine aggregates was used. Cement: 150 kg of type I cement was used for 0.5 m3 of concrete.
If repair is required to restore structural integrity for cracks with surface widths of 0.001–0.08 in. (0.03–2 mm) and extending downward from nearly horizontal surfaces, they should be repaired by gravity flow using a low viscosity gel-type epoxy resin system incorporating a long pot-life material. But before grouting or sealing structural cracks it should be determined if the crack is active, and if so, what are the causes? However, most cracks are dormant and should be low pressure epoxy injected to fill the entire void and return the concrete to its original monolithic design state. The lower the value of viscosity is the finer the cracks that can be filled. The typical procedure is the following [5–8]: Surface cleaning: Contaminants such as oil, grease, dirt, or fine particles of concrete prevent epoxy penetration and bonding, and reduce the effectiveness of repairs. Preferably, contamination should be removed by vacuuming or flushing with water (water
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Table 1 Characteristics of the epoxy adhesive [4] Characteristics
Guide values
Pot life (2 kg) 5 C 10 C 20 C 30 C 40 C
Long pot life type – – 60 min 30 min 15 min
Viscosity 10 C 20 C 30 C
– 29 cPa s 13 cPa s
Compressive strength (at 20 C) after 10 days Flexural strength (at 20 C) after 10 days Tensile strength (at 20 C) after 10 days Bond strength to concrete (at 20 C) after 10 days Shear strength (at 20 C) Modulus of elasticity (static) Coefficient of thermal expansion (from 20 to +60 C) Heat deflection temperature
53 N/mm2 50 N/mm2 25 N/mm2 4 N/mm2 (concrete failure) 15–20 N/mm2 1060 N/mm2 89 · 106 per C 50–70 C
blasting) or other specially effective cleaning solutions (air blasting). The solution is then flushed out using compressed air and a neutralizing agent or adequate time is provided for air drying. Surface seals: Provide a surface seal on all faces of the crack so that the liquid resin will not leak and flow out of the crack prior to gelling and curing (Fig. 8). A surface can be sealed by applying an epoxy mortar, polyester, or other appropriate sealing material to the surface of the crack and allowing it to harden. Furthermore, highly porous concretes or concrete made of very absorptive aggregate may absorb enough epoxy to starve the glue line. Such concrete should be given a first seal coat of the same epoxy adhesive to penetrate into the absorptive aggregate. Mixing and proportioning: The most accurate method of proportioning is the use of pre-proportioned units supplied by the manufacturer so that the entire contents of both component containers are mixed together. If such packaging is not available, the components may be mixed together in the ratios specified by the manufacturer. Sikadur-52, is consisted of resin (component A) and hardener (component B). Component B is added to component A and the two are
Fig. 5. Development of compressive strength of Sikadur-52 [4].
Fig. 6. Development of tensile strength of Sikadur-52 [4].
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161
Fig. 7. Development of shear strength of Sikadur-52 [4].
then mixed with a paddle mixer at low speed. The Sikadur resin is yellow and the Sikadur hardener is brown, so that it is easy to see when the two components are homogeneously mixed. If the mixing speed is too fast, air is entrained into the fresh adhesive mix, which reduces the final strength of the epoxy. Pot life begins when the resin and the hardener are mixed. It is shorter at high ambient temperatures and longer at low temperatures. The quantity mixed also affects pot life: the higher the quantity, the shorter it becomes. In order to obtain a longer workability at high temperatures, the mixed adhesive may be divided into portions. Another method is to chill components A and B before mixing. Application: The monomer or resin can be poured on to the surface and spread with brooms, rollers, or squeegees. The material should be worked back and forth over the cracks to obtain maximum filling since the monomer or resin recedes slowly into the cracks. Excess material should be broomed off the surface to prevent slick, shining areas after curing. If surface friction is important, sand should be broadcast over the surface or mixed into the material before the monomer or resin cures to improve traction, i.e., improve spreading of the epoxy. If the cracks contain
significant amounts of silt, moisture or other contaminants, the sealant cannot fill them. Water blasting followed by a drying time may be effective in cleaning and preparing these cracks. Cores taken at cracks can be used to evaluate the effectiveness of the crack filling. The depth of penetration of the sealant can be measured. Shear (or tension) tests can be performed with the load applied in a direction parallel to the repaired cracks (as long as reinforcing steel is not present in the core in or near the failure area). For some polymers the failure crack will occur outside the repaired crack.
9. Compressive strength of concrete cubes Five types of cubes were tested:
Ordinary type: O. Defect type 1: D1. Defect type 2: D2. Defect type 1 repaired with epoxy: D1 + E. Defect type 2 repaired with epoxy: D2 + E.
For each type of cubes, three cubes were casted for a 28 days period giving a total of 15 cubes. The results of the three cubes crushed at the same period were averaged to get the compressive strength at 28 days. All results are tabulated in Table 2. Figs. 9–11 display the typical crushed concrete cubes of different types.
10. Interpertation of results
Fig. 8. Surface seal on faces of the crack.
‘‘The ultimate strength of concrete or the maximum reliable compression strength of concrete is about 0.85 fC0 ’’ [5]. As shown in Table 3, the compressive strength of types D1 and D2 was decreased to a level where the concrete compressive strength becomes unacceptable since it would be lower than 0.85 fC0 , and thus failure will occur. On the other hand, the use of epoxy in repairing
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Table 2 Compressive strength of concrete cubes Cube # 1 2 3
Type O O O
Days 28 28 28
fC0 (MPa) 31.7 34.4 32.9
% Reduction – – –
Avg: 33.0 4 5 6
7 8 9
10 11 12
13 14 15
D1 D1 D1
28 28 28
D2 D2 D2
28 28 28
D1 + E D1 + E D1 + E
D2 + E D2 + E D2 + E
35a 35 35
35 35 35
18.9 20.4 19.1
42.62 38.16 42.02
Avg: 19.5
40.93
23.5 20.6 225
28.64 37.56 31.91
Avg: 22.2
32.71
18.5b 29.5 29.0
43.81 10.51 11.99
Avg: 29.5c
11.25
30.7 30.2 29.9
6.94 8.42 9.32
Avg: 30.3
8.23
Fig. 10. Crushed D1 concrete cube.
Fig. 11. Crushed D2 + E concrete cube.
a
An extra week was needed to allow curing of epoxy. Intentionally contaminated surface and was disregarded (see interpretation of results). c Average based on two values (excluding the 18.5 value). b
the cracks in cubes (D1 + E) and (D2 + E) (Table 3) has increased the compressive strength to acceptable levels since higher than 0.85 fC0 , decreasing the percent reduction. From Table 3, the percent strength reduction in cubes type (D1) was 40.93 before the use of epoxy and has decreased by a considerable amount to 11.25 after using
the epoxy. Similarly, the percent reduction in strength in cubes type (D2) was 32.71 before the use of epoxy and has decreased by a considerable amount to 8.23 after using the epoxy. The compressive strength of cube #10 (D1 + E) (Table 2) that was treated with epoxy in order to bond the crack is awkward compared with compressive strengths of cubes #11 and 12; however, once the reasons for the discrepancy is known, the result would be acceptable. During this laboratory work, a small test was performed to see whether or not proper surface preparation of the substrate, i.e., crack surface of concrete cube #10, prior to the use of epoxy, plays a very crucial and major part in obtaining a strong bond between epoxy system and concrete thus leading to an increase of the compressive strength of the cracked cube. So, before pouring concrete into the cubes containing the built-in cracks, some Table 3 % Reduction in strength
Fig. 9. Crushed O concrete cube.
Cube type
fC0 @ 28 (MPa)
% Reduction
Compressive strength status of cube
O D1 D2 D1 + E D2 + E
33.0 19.1 22.2 29.3 30.3
– 40.93 32.71 11.25 8.23
– Not acceptable Not acceptable Acceptable Acceptable
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
grease was introduced on the steel plates that formed the artificial cracks in order to make easier the process of pulling the concrete out of the metal cubes without any risk of damaging the concrete cubes since the concrete will stick on the steel plates. As a result, the crack surface of one of the cubes, i.e., cube #10, was not flushed by water as the rest of the cubes containing cracks prior to applying the epoxy, but was left dirty, contaminated with grease. Bond failure was the result thus leading to a very notable lowering in the compressive strength of the contaminated cube as compared with the other repaired ones; it was even lower than the compressive strength of the cracked cubes not treated with epoxy. Conclusions can be drawn from this experiment, that proper preparation of any surface to receive an epoxy application is of primary importance no matter how carefully other phases of the application procedure have been performed.
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percent reduction in compressive strength of cube D1 + E (i.e., cube D1 filled with epoxy) was 11.25% compared with 40.93% reduction for cube D1, whereas the percent reduction in compressive strength of cube D2 + E (i.e., cube D2 repaired with epoxy) was 8.23% compared with 32.71% reduction for cube D2. Thus, the use of epoxy can increase tensile and compressive strengths across a crack, if further cracking is not anticipated. However, the use of epoxy alone might not be sufficient sometimes to restore the damage in a structure, but it will be necessary to use other repair methods in addition such as additional reinforcement or using post-tensioning. Finally, proper preparation of a given surface is an art and a science and must be given careful attention because irreversible damage can be done to the bond with the substrate or the epoxy itself, resulting in lower strength.
Acknowledgements 11. Observations and conclusions Cracking of concrete is a random process, highly variable and influenced by many factors. However, one process is insured: the longer the crack, the higher the stress concentrations induced by it. Due to the presence of a crack in a structure, the strength of the structure will decrease progressively with the increase of the crackÕs size. As a result, the structure will be subject to failure when its strength becomes so low that fracture occurs under normal loading. From the experimental results, it was proved that a crack reduces strength; a reduction in strength of 40.93% was obtained due to D1 cracks and 32.71% due to D2 cracks. Consequently, cracks in concrete structures should not be taken for granted, but on the contrary, they must be inspected and repaired. Several methods are used in crack repair. Based on the careful evaluation of the extent and cause of cracking and depending on the nature of the damage, one or more repair methods may be selected. If repair is required to restore structural integrity of a crack, it should be repaired with epoxy. Epoxy adhesives are the most common adhesives used for crack either by injection (Epoxy injection method) or by gravity flow (Gravity filling method). The use of epoxy in repairing the cracks of cubes D1 and D2 by the gravity filling method was a success. The
Special thanks to Mr. Farid Zgheib, manager of Nakhle´ Zgheib & Co. for providing the ready mix concrete and Mr. Wassim Mahfouz, sales engineer at SIKA NEAR EAST S.A.L for providing the epoxy product.
References [1] ACI Committee 224. Causes, evaluation, and repairs of cracks in concrete structures (224.1R-93). Farmington Hills (MI): American Concrete Institute; 1993. 22pp. [2] ACI Committee E706. Crack repair by gravity feed with resin (ACI RAP Bulletin 2). Farmington Hills (MI): American Concrete Institute; 2003. 4pp. [3] ACI Committee E706. Structural crack repair by epoxy injection (ACI RAP Bulletin 1). Farmington Hills (MI): American Concrete Institute; 2003. 5pp. [4] SIKA. Sika data book 2002, Sika Near East, S.A.L., Beirut, Lebanon; 2002. 319pp. [5] ACI Committee 503. Use of epoxy compounds with concrete (ACI 503R-93 (Reapproved 1998)). Farmington Hills (MI): American Concrete Institute; 1998. 28pp. [6] ACI Committee 546. Guide for repair of concrete bridge structures (546.1R-80 (Reapproved 1988)). Farmington Hills (MI): American Concrete Institute; 1988. 20pp. [7] ACI Committee 546. Concrete repair guide (546R-96). Farmington Hills (MI): American Concrete Institute; 1996. 41pp. [8] ASTM C 881-90. Standard specification for epoxy-resin based bonding systems for concrete. West Conshohocken (PA): ASTM International; 1990. 5pp.
MATERIALS
Construction and Building Materials 21 (2007) 157–163
www.elsevier.com/locate/conbuildmat
Experimental study of epoxy repairing of cracks in concrete Camille A. Issa *, Pauls Debs Department of Civil Engineering, Lebanese American University, P.O. Box 36, Byblos, Lebanon Received 24 November 2004; received in revised form 16 June 2005; accepted 30 June 2005 Available online 6 September 2005
Abstract Cracks in concrete structures have always posed a big threat on the durability of concrete. Cracking of concrete is a random process, highly variable and influenced by many factors. Among the crack repair methods is the use of epoxy either by injection or by gravity filling in order to bond the crack and restore its structural integrity. Prior to the use of epoxy compounds and due to their versatility and the wide range of available physical and chemical properties of epoxy resin systems, one has to be completely aware and informed before entering the world of epoxies; other than crack repair, epoxy compounds have found a wide variety of uses in the concrete industry. In this study, 15 concrete cubes, six including cracks without repair, six including cracks bonded with gravity filled epoxy and three with no cracks were crushed and their compressive strengths were obtained. It was found that the cracks caused a reduction in compressive strength up to 40.93% whereas the epoxy system, when properly applied, restored the compressive strength by decreasing the reduction down to 8.23%. 2005 Elsevier Ltd. All rights reserved. Keywords: Concrete; Crack; Repair; Epoxy; Experimental
1. Introduction Cracks in concrete have many causes. They may affect appearance only, or they may indicate significant structural distress or a lack of durability. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude. Their significance depends on the type of structure, as well as the nature of the cracking. For example, cracks that are acceptable for buildings may not be acceptable in water-retaining structures. The proper repair of cracks depends on knowing the causes and selecting the repair procedures that take these causes into account; otherwise, the repair may only be temporary. Successful long-term repair procedures
*
Corresponding author. Tel.: +961 9 944850; fax: +961 9 944851. E-mail address: [email protected] (C.A. Issa).
0950-0618/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.030
must attack the causes of the cracks as well as the cracks themselves. The main purpose of this study is to experiment with the use of epoxy compounds to restore the integrity of a cracked member by gravity filling of the crack, emphasizing on the physical characteristics of epoxies as well as the importance of surface preparation, temperature conditioning of the substrate and epoxy compound, and alterations of the hardening rate of epoxies. Furthermore, all lab tests and results on concrete cubes will be fully documented and interpreted.
2. Causes of cracking in concrete [1] Cracks in concrete are categorized as occurring either in plastic concrete or hardened concrete. There are two kinds of cracks are associated with cracking of plastic concrete:
158
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
Plastic shrinkage cracking. Settlement cracking. Cracking of hardened concrete are caused due to the followings:
Drying shrinkage. Thermal stresses. Chemical reaction. Weathering. Corrosion of reinforcement. Poor construction practices. Construction overloads. Errors in design and detailing.
3. Scope of study Due to the application of repeated loads, a crack may propagate until it reaches a point where fracture occurs and failure results. The longer the crack, the higher the stress concentration induced by it. Due to the presence of a crack, the strength of the structure is decreased: it is lower than the original strength it was designed for. Several methods are used for crack repair among which is the ‘‘Gravity Filling’’ of cracks using epoxy. As a result, the objective of this laboratory work is to restore structural integrity and increase the compressive strength and stiffness of cracked concrete test cubes by gravity filling of cracks using a low viscosity gel-type epoxy resin system.
plaza decks, and similar surfaces. Gravity feed of resin is not effective for repairing moving cracks. This is because the materials are unable to act as a flexible joint material. Likewise, gravity feed of resin should not be viewed as a long-term solution to cracking caused by corrosion, sulfate attack, or AAR. At best, this repair will keep out water, chlorides, and sulfates, which may slow the progress of future damage around the crack, but it certainly will not stop it indefinitely. In these situations, a complete repair and protection approach is required to address the entire area affected, not just the cracks.
5. Description of cube forms Three kinds of cubes were used during the laboratory experiment. The first kind is an ordinary cube (O) (Fig. 1) with the following dimensions: 150 · 150 · 150 mm3. As for the second cube (D1), an artificial crack was installed in it using two steel plates, both having the following dimensions: a height of 150 mm, a width of 35 mm, and a thickness of 2 mm (Fig. 2). The third and final cube (D2) also has an artificial crack made of two steel plates with the following dimensions: a height of 70 mm, a width of 50 mm, and a thickness of 2 mm (Fig. 3). In addition cubes (D1 + E) and (D2 + E) represent the defected cubes (D1) and (D2) with the cracks filled with epoxy, as shown in Fig. 4. The concrete cubes were crushed using a universal testing machine.
6. Concrete mix 4. Gravity filling: objectives and purpose According to ACI RAP-2 [2], the primary objective of this repair method is to fill the crack and structurally bond the concrete on both sides of the crack. This repair is to seal cracks that are not moving – for example, shrinkage cracks, and settlement cracks that have stabilized. By penetrating and filling the cracks, the resin is able to form a polymer plug that seals the crack, keeping out water, chlorides, carbon dioxide, sulfates, and other aggressive liquids and gases. This repair method is, therefore, a way to reduce possible future deterioration caused by freeze–thaw cycles, steel corrosion, and chemical attack of the concrete. Finally, it is occasionally an objective to achieve a structural repair of the cracks using this method. While many of these resin materials exceed the strength of the concrete, ensuring a structural repair with full depth penetration by gravity is difficult. If a structural repair of the crack is critical, pressure injection may be a preferred option (ACI RAP-1 [3]). This repair can only be applied to horizontal concrete elements such as bridge and parking decks, floor slabs,
The concrete mix used was a ready mixed concrete with the ingredients for 0.5 m3 as follows: Coarse aggregates: two kinds of coarse aggregates were used in the mix: the first kind: 294 kg of ‘‘normal maximum coarse aggregate size of 19 mm’’ (c =
Fig. 1. O cubes.
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
159
Water: 69 l of mixing water was used alongside the above mentioned ingredients.
7. Epoxy compound
Fig. 2. D1 cube.
Sikadur-52 [4] is a two component, solvent-free, low viscosity liquid, based on high strength epoxy resins. It complies with ASTM C 881-78 Type I, Grade 1 Class B + C. Sikadur-52 is used to fill and seal cavities and cracks in structural components such as bridges, industrial structures, columns, beams, foundations, walls and floors slabs and water-retaining structures. It not only forms an effective barrier against water infiltration of corrosion promoting media, but also structurally bonds the concrete sections together. It has the following advantages:
Fig. 3. D2 cube.
Solvent-free. Suitable in both, dry and damp conditions. Useable at low temperatures. Two grades for different climatic conditions (normal and long pot life). Shrinkage-free hardening. High mechanical and adhesive strengths. Hard but not brittle. Very low viscosity.
The characteristics of the epoxy adhesive are shown in Table 1. Figs. 5–7 display the development of compressive, tensile and shear strengths of the epoxy adhesive, Sikadur-52, with respect to time [4].
8. Crack repair by gravity filling
Fig. 4. D1 + E and D2 + E cubes.
1580 kg/m3); the second kind: 204 kg of ‘‘normal maximum coarse aggregate size of 9.5 mm’’ (c = 1550 kg/ m3). As a result, 498 kg of coarse aggregate was used. Fine aggregates: two kinds of fine aggregates were used in the mix: the first kind: 289 kg of crushed limestone, No. 8 (2.36 mm) size, (c = 1610 kg/m3); the second kind: 209 kg of dune sand, No. 4 (4.75 mm) size, (c = 1550 kg/m3). As a result, a total of 498 kg of fine aggregates was used. Cement: 150 kg of type I cement was used for 0.5 m3 of concrete.
If repair is required to restore structural integrity for cracks with surface widths of 0.001–0.08 in. (0.03–2 mm) and extending downward from nearly horizontal surfaces, they should be repaired by gravity flow using a low viscosity gel-type epoxy resin system incorporating a long pot-life material. But before grouting or sealing structural cracks it should be determined if the crack is active, and if so, what are the causes? However, most cracks are dormant and should be low pressure epoxy injected to fill the entire void and return the concrete to its original monolithic design state. The lower the value of viscosity is the finer the cracks that can be filled. The typical procedure is the following [5–8]: Surface cleaning: Contaminants such as oil, grease, dirt, or fine particles of concrete prevent epoxy penetration and bonding, and reduce the effectiveness of repairs. Preferably, contamination should be removed by vacuuming or flushing with water (water
160
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
Table 1 Characteristics of the epoxy adhesive [4] Characteristics
Guide values
Pot life (2 kg) 5 C 10 C 20 C 30 C 40 C
Long pot life type – – 60 min 30 min 15 min
Viscosity 10 C 20 C 30 C
– 29 cPa s 13 cPa s
Compressive strength (at 20 C) after 10 days Flexural strength (at 20 C) after 10 days Tensile strength (at 20 C) after 10 days Bond strength to concrete (at 20 C) after 10 days Shear strength (at 20 C) Modulus of elasticity (static) Coefficient of thermal expansion (from 20 to +60 C) Heat deflection temperature
53 N/mm2 50 N/mm2 25 N/mm2 4 N/mm2 (concrete failure) 15–20 N/mm2 1060 N/mm2 89 · 106 per C 50–70 C
blasting) or other specially effective cleaning solutions (air blasting). The solution is then flushed out using compressed air and a neutralizing agent or adequate time is provided for air drying. Surface seals: Provide a surface seal on all faces of the crack so that the liquid resin will not leak and flow out of the crack prior to gelling and curing (Fig. 8). A surface can be sealed by applying an epoxy mortar, polyester, or other appropriate sealing material to the surface of the crack and allowing it to harden. Furthermore, highly porous concretes or concrete made of very absorptive aggregate may absorb enough epoxy to starve the glue line. Such concrete should be given a first seal coat of the same epoxy adhesive to penetrate into the absorptive aggregate. Mixing and proportioning: The most accurate method of proportioning is the use of pre-proportioned units supplied by the manufacturer so that the entire contents of both component containers are mixed together. If such packaging is not available, the components may be mixed together in the ratios specified by the manufacturer. Sikadur-52, is consisted of resin (component A) and hardener (component B). Component B is added to component A and the two are
Fig. 5. Development of compressive strength of Sikadur-52 [4].
Fig. 6. Development of tensile strength of Sikadur-52 [4].
C.A. Issa, P. Debs / Construction and Building Materials 21 (2007) 157–163
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Fig. 7. Development of shear strength of Sikadur-52 [4].
then mixed with a paddle mixer at low speed. The Sikadur resin is yellow and the Sikadur hardener is brown, so that it is easy to see when the two components are homogeneously mixed. If the mixing speed is too fast, air is entrained into the fresh adhesive mix, which reduces the final strength of the epoxy. Pot life begins when the resin and the hardener are mixed. It is shorter at high ambient temperatures and longer at low temperatures. The quantity mixed also affects pot life: the higher the quantity, the shorter it becomes. In order to obtain a longer workability at high temperatures, the mixed adhesive may be divided into portions. Another method is to chill components A and B before mixing. Application: The monomer or resin can be poured on to the surface and spread with brooms, rollers, or squeegees. The material should be worked back and forth over the cracks to obtain maximum filling since the monomer or resin recedes slowly into the cracks. Excess material should be broomed off the surface to prevent slick, shining areas after curing. If surface friction is important, sand should be broadcast over the surface or mixed into the material before the monomer or resin cures to improve traction, i.e., improve spreading of the epoxy. If the cracks contain
significant amounts of silt, moisture or other contaminants, the sealant cannot fill them. Water blasting followed by a drying time may be effective in cleaning and preparing these cracks. Cores taken at cracks can be used to evaluate the effectiveness of the crack filling. The depth of penetration of the sealant can be measured. Shear (or tension) tests can be performed with the load applied in a direction parallel to the repaired cracks (as long as reinforcing steel is not present in the core in or near the failure area). For some polymers the failure crack will occur outside the repaired crack.
9. Compressive strength of concrete cubes Five types of cubes were tested:
Ordinary type: O. Defect type 1: D1. Defect type 2: D2. Defect type 1 repaired with epoxy: D1 + E. Defect type 2 repaired with epoxy: D2 + E.
For each type of cubes, three cubes were casted for a 28 days period giving a total of 15 cubes. The results of the three cubes crushed at the same period were averaged to get the compressive strength at 28 days. All results are tabulated in Table 2. Figs. 9–11 display the typical crushed concrete cubes of different types.
10. Interpertation of results
Fig. 8. Surface seal on faces of the crack.
‘‘The ultimate strength of concrete or the maximum reliable compression strength of concrete is about 0.85 fC0 ’’ [5]. As shown in Table 3, the compressive strength of types D1 and D2 was decreased to a level where the concrete compressive strength becomes unacceptable since it would be lower than 0.85 fC0 , and thus failure will occur. On the other hand, the use of epoxy in repairing
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Table 2 Compressive strength of concrete cubes Cube # 1 2 3
Type O O O
Days 28 28 28
fC0 (MPa) 31.7 34.4 32.9
% Reduction – – –
Avg: 33.0 4 5 6
7 8 9
10 11 12
13 14 15
D1 D1 D1
28 28 28
D2 D2 D2
28 28 28
D1 + E D1 + E D1 + E
D2 + E D2 + E D2 + E
35a 35 35
35 35 35
18.9 20.4 19.1
42.62 38.16 42.02
Avg: 19.5
40.93
23.5 20.6 225
28.64 37.56 31.91
Avg: 22.2
32.71
18.5b 29.5 29.0
43.81 10.51 11.99
Avg: 29.5c
11.25
30.7 30.2 29.9
6.94 8.42 9.32
Avg: 30.3
8.23
Fig. 10. Crushed D1 concrete cube.
Fig. 11. Crushed D2 + E concrete cube.
a
An extra week was needed to allow curing of epoxy. Intentionally contaminated surface and was disregarded (see interpretation of results). c Average based on two values (excluding the 18.5 value). b
the cracks in cubes (D1 + E) and (D2 + E) (Table 3) has increased the compressive strength to acceptable levels since higher than 0.85 fC0 , decreasing the percent reduction. From Table 3, the percent strength reduction in cubes type (D1) was 40.93 before the use of epoxy and has decreased by a considerable amount to 11.25 after using
the epoxy. Similarly, the percent reduction in strength in cubes type (D2) was 32.71 before the use of epoxy and has decreased by a considerable amount to 8.23 after using the epoxy. The compressive strength of cube #10 (D1 + E) (Table 2) that was treated with epoxy in order to bond the crack is awkward compared with compressive strengths of cubes #11 and 12; however, once the reasons for the discrepancy is known, the result would be acceptable. During this laboratory work, a small test was performed to see whether or not proper surface preparation of the substrate, i.e., crack surface of concrete cube #10, prior to the use of epoxy, plays a very crucial and major part in obtaining a strong bond between epoxy system and concrete thus leading to an increase of the compressive strength of the cracked cube. So, before pouring concrete into the cubes containing the built-in cracks, some Table 3 % Reduction in strength
Fig. 9. Crushed O concrete cube.
Cube type
fC0 @ 28 (MPa)
% Reduction
Compressive strength status of cube
O D1 D2 D1 + E D2 + E
33.0 19.1 22.2 29.3 30.3
– 40.93 32.71 11.25 8.23
– Not acceptable Not acceptable Acceptable Acceptable
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grease was introduced on the steel plates that formed the artificial cracks in order to make easier the process of pulling the concrete out of the metal cubes without any risk of damaging the concrete cubes since the concrete will stick on the steel plates. As a result, the crack surface of one of the cubes, i.e., cube #10, was not flushed by water as the rest of the cubes containing cracks prior to applying the epoxy, but was left dirty, contaminated with grease. Bond failure was the result thus leading to a very notable lowering in the compressive strength of the contaminated cube as compared with the other repaired ones; it was even lower than the compressive strength of the cracked cubes not treated with epoxy. Conclusions can be drawn from this experiment, that proper preparation of any surface to receive an epoxy application is of primary importance no matter how carefully other phases of the application procedure have been performed.
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percent reduction in compressive strength of cube D1 + E (i.e., cube D1 filled with epoxy) was 11.25% compared with 40.93% reduction for cube D1, whereas the percent reduction in compressive strength of cube D2 + E (i.e., cube D2 repaired with epoxy) was 8.23% compared with 32.71% reduction for cube D2. Thus, the use of epoxy can increase tensile and compressive strengths across a crack, if further cracking is not anticipated. However, the use of epoxy alone might not be sufficient sometimes to restore the damage in a structure, but it will be necessary to use other repair methods in addition such as additional reinforcement or using post-tensioning. Finally, proper preparation of a given surface is an art and a science and must be given careful attention because irreversible damage can be done to the bond with the substrate or the epoxy itself, resulting in lower strength.
Acknowledgements 11. Observations and conclusions Cracking of concrete is a random process, highly variable and influenced by many factors. However, one process is insured: the longer the crack, the higher the stress concentrations induced by it. Due to the presence of a crack in a structure, the strength of the structure will decrease progressively with the increase of the crackÕs size. As a result, the structure will be subject to failure when its strength becomes so low that fracture occurs under normal loading. From the experimental results, it was proved that a crack reduces strength; a reduction in strength of 40.93% was obtained due to D1 cracks and 32.71% due to D2 cracks. Consequently, cracks in concrete structures should not be taken for granted, but on the contrary, they must be inspected and repaired. Several methods are used in crack repair. Based on the careful evaluation of the extent and cause of cracking and depending on the nature of the damage, one or more repair methods may be selected. If repair is required to restore structural integrity of a crack, it should be repaired with epoxy. Epoxy adhesives are the most common adhesives used for crack either by injection (Epoxy injection method) or by gravity flow (Gravity filling method). The use of epoxy in repairing the cracks of cubes D1 and D2 by the gravity filling method was a success. The
Special thanks to Mr. Farid Zgheib, manager of Nakhle´ Zgheib & Co. for providing the ready mix concrete and Mr. Wassim Mahfouz, sales engineer at SIKA NEAR EAST S.A.L for providing the epoxy product.
References [1] ACI Committee 224. Causes, evaluation, and repairs of cracks in concrete structures (224.1R-93). Farmington Hills (MI): American Concrete Institute; 1993. 22pp. [2] ACI Committee E706. Crack repair by gravity feed with resin (ACI RAP Bulletin 2). Farmington Hills (MI): American Concrete Institute; 2003. 4pp. [3] ACI Committee E706. Structural crack repair by epoxy injection (ACI RAP Bulletin 1). Farmington Hills (MI): American Concrete Institute; 2003. 5pp. [4] SIKA. Sika data book 2002, Sika Near East, S.A.L., Beirut, Lebanon; 2002. 319pp. [5] ACI Committee 503. Use of epoxy compounds with concrete (ACI 503R-93 (Reapproved 1998)). Farmington Hills (MI): American Concrete Institute; 1998. 28pp. [6] ACI Committee 546. Guide for repair of concrete bridge structures (546.1R-80 (Reapproved 1988)). Farmington Hills (MI): American Concrete Institute; 1988. 20pp. [7] ACI Committee 546. Concrete repair guide (546R-96). Farmington Hills (MI): American Concrete Institute; 1996. 41pp. [8] ASTM C 881-90. Standard specification for epoxy-resin based bonding systems for concrete. West Conshohocken (PA): ASTM International; 1990. 5pp.