concrete adhesive joints

Construction and Building

MATERIALS

Construction and Building Materials 20 (2006) 957–970

www.elsevier.com/locate/conbuildmat

Water effects on the bond strength of concrete/concrete adhesive joints M. Frigione b

a,*

, M.A. Aiello a, C. Naddeo

b

a Department of Innovation Engineering, University of Lecce, 73100 Lecce, Italy Department of Chemical and Food Engineering, University of Salerno, 84084 Fisciano (SA), Italy

Received 24 March 2004; received in revised form 22 October 2004; accepted 30 June 2005 Available online 19 September 2005

Abstract The paper discusses the experimental work by the authors investigating bond strength of epoxy adhesives and their efficiency when joining to concrete elements; the epoxies studied were those currently used in the construction industry. Flexural tests were undertaken to determine the mechanical properties of the exposed and the control specimens of three different epoxy adhesives. In addition, the water resistance of concrete/concrete epoxy joints was investigated by comparing bond strength with those of control samples; the maximum period of immersion was one month. A reduction in the glass transition temperature and the stiffness at short immersion time was found for all the adhesives employed, with a subsequent slight increase for prolonged immersion, while the effects on the strengths resulted almost proportional to their initial values. The effect of water on the adhesion of the joints was found to be significant, especially at longer immersion times; the bond strength of concrete–adhesive specimens reduced by 30% after one month of immersion in water.  2005 Elsevier Ltd. All rights reserved. Keywords: Epoxy adhesives; Concrete/concrete joints; Durability in water

1. Introduction In recent years, fiber reinforced composites (FRP), based on polymeric thermosetting resins, have demonstrated to be an attractive alternative for rehabilitation or renewal of civil infrastructures, providing significant advantages to the restoration applications not often attainable with conventional materials. Widespread utilization of FRPs in construction has, however, been hindered by the lack of long term durability and performance data on which to base design calculations, especially when it is realized that FRP composites used in infrastructure applications are intended to have a service life in excess of 50 years.

*

Corresponding author. Fax: +39 0832 297 215. E-mail address: [email protected] (M. Frigione).

0950-0618/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.015

Durability of a structure can be described as the ability of the system to maintain designed performance strength over time under harsh and changing environmental conditions; these durability considerations are generally more important than the materialÕs pristine condition. The adverse conditions that may affect durability of FRPs during their lifetime can be hypothesized to be: repeated loading, aqueous environment (i.e. high atmospheric humidity, seawater, rain water, acid rain), changes in temperatures, exposure to freeze–thaw cycles, deteriorating chemicals and alkaline environment in the proximity of Portland cement concrete. Any material is subjected to microstructural and morphological transformations during its service life, leading to property changes due to physical and chemical aging. Thus, durability of a polymeric reinforced/restored structure deals with the assessment of the initial or design strength of the repaired structure that may have been lost due to

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the harmful physical–chemical attacks during its service life. The presence of moisture in the composite, in particular, can initiate undesirable structural changes within the matrix, in the fiber reinforcement, at their interface and at matrix/concrete interface. In any case, the result is the reduction of the durability of the FRP reinforcement. Hence, problems pertinent to the role of permeability of polymeric matrices for composites and adhesives in conjunction with the concrete adherent are of prime consideration as the limiting factors of their performance in service. The presence of humidity is probably the most harmful environment that can commonly be encountered by epoxies used as adhesives for civil engineering applications. The sorption of water can greatly influence the physical properties of this thermosetting polymer and its composites. Water may enter a resin either by diffusion or by capillary action through cracks and crazes. Once inside, the water may alter the properties of the polymer either in a reversible manner, for example by plasticization, or in an irreversible manner, for example by hydrolyzation, cracking or crazing. In the case of epoxy resins, water molecules bind with resins through hydrogen bonding. In this way, water is able to disrupt the interchain Van der Waals forces inside the network producing an increase of segmental mobility [1]. As a consequence, the absorption of limited amounts of water can be regarded as beneficial in terms of both improved toughness, static fatigue resistance and plastic deformation of the cured resin. On the other hand, an excessive presence of water is generally considered harmful leading to a reduction in modulus and strength with a consequent marked unsuitable decrease of loadbearing capacity through plasticization effects [2,3]. It is well known that the good properties of epoxy resins usually undergo a considerable decay after a long period of immersion in water [4]. Finally, it is not be easy to remove the sorbed water completely. Most of the modern adhesives are not easily hydrolyzed, showing a good chemical resistance to water. However, physical interaction in the form of plasticization is a universal consequence of absorption of water. Plasticization is always accompanied by the lowering of the Tg value of the cured resin. This result is particularly worrying for cold-curing epoxies whose typical glass transition values, when dry, lie in the range 40–55 C, i.e. not much higher than the possible service temperatures. Water absorption, therefore, will generally produce a deterioration in the already poor high temperature load-bearing capacity of epoxy adhesives cured at room temperature. Hence, the need arises to select adhesives whose Tg values do not drop substantially with water sorption or to assure controlled ambient conditions, when possible. However, relatively short term exposure to water lead to more or less reversible

plasticization, and an almost complete recovery of the original Tg value when water is removed [5]. Finally, the presence of water can be particularly dangerous when the adhesive is used to join two dissimilar adherends. Water is a highly polar molecule that is permeating most polymers, and it is practically impossible to prevent water from migrating to the interface where a high-energy surface adherend is present. Although water plasticizes polymers, it is in the interface regions where water is believed to reduce the strength. Mays and Hutchinson [2] reported that water is a harmful factor for epoxy adhesion joints also for its ability to cause displacement of adherents by penetrating the interface of the joint. Moreover, the displacement is even augmented by pre-existing microcracks or debonded areas at the interface, which originate from poor wetting by the adhesives [6]. In the case of FRP composites having an epoxy as matrix, fiber/matrix debonding is among the major reasons for strength decay in samples aged in distilled water [7,8]. In a different study, it was found that the presence of sufficient water at an unsized glass/ epoxy interface causes sudden and catastrophic delamination [9]. The presence of water or moisture accelerates creep phenomena through plasticization of epoxy matrix in FRP composites [7]. It reduces fracture energy and decreases creep-rupture time. Referring to the effect of the presence of water on the performances of concrete, a relevant amount of water can reduce mechanical properties of concrete at the curing stage [10]. Moreover, once the concrete is hardened, the presence of water can represent a harmful agent only for the steel reinforcement, accelerating its electrochemical corrosion [11]. Actual data on durability of cold-curing epoxy adhesives joint to concrete elements related to presence of water is sparse, not always well documented and, when available, not easily accessible to the designer. Few research works published in the last years on this subject indicated a noticeable decrease in the bond strength (50%) after prolonged immersion in water [12]. Water may easily penetrate through a permeable adherend like the concrete, which possesses from 10% to 40% of volumetric fraction of voids and capillary pores [13], and it can diffuse or be transmitted along the interfaces through capillary action. After having accessed the joint, water may cause deterioration of the bond by altering mechanical properties and adhesive displacement at the interface [2]. The objectives of this research were to characterize the degradation behavior of epoxy adhesives in isolation and when bonded with concrete elements if exposed to water, to explain the mechanism involved and, finally, to determine the suitability for utilizing epoxy resins as adhesives to bond concrete elements in such aggressive conditions. To this aim, three different epoxy adhesives, in isolation or in junction with concrete elements, were

M. Frigione et al. / Construction and Building Materials 20 (2006) 957–970

immersed in distilled water for different time spans. After the scheduled time, each sample was mechanically tested in order to evaluate the effect of exposure to water on the mechanical resistance of each adhesive and on its adhesion strength with concrete.

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Table 2 Mechanical properties of the concrete Concrete type

fc (MPa)

sfc (MPa)

fct (MPa)

sfct (MPa)

fck 25 fck 50

30.20 66.23

0.75 1.59

2.52 4.46

0.07 0.90

fc = mean compressive strength; sfc ¼ standard deviation of fc ; fct = mean tensile strength; sfct ¼ standard deviation of fct .

2. Materials is mainly used to bond fresh concrete to hardened concrete and to bond concrete and steel structures.

2.1. Concrete Different concrete mixes (furnished by FICES S.p.a., Lecce) were used, with a target compressive strength of 25 MPa (fck 25) and 50 MPa (fck 50), respectively, where fck indicates the characteristic compressive strength of concrete. Details of compositions of each mix are reported in Table 1. The mechanical properties (compression and tension strengths) of the different concretes were evaluated by means of standard tests, i.e. UNI 6132-72 and UNI 6135-72, respectively. Their average compressive and tension strengths and the corresponding standard deviations are reported in Table 2. 2.2. Adhesives Different commercial cold-curing epoxy adhesives, supplied by SIKA Italia S.p.A. and MAC S.p.A., were selected in this study and they are indicated as S50, M16 and M20. S50 is a bisphenolic epoxy resin having a low molecular weight (MW < 700) and a low viscosity (viscosity = 290 MPa s at 20 C). It is used in the restoration of concrete to fill and repair cracks of small width and to join concrete to concrete and also to different materials. M16 is a bisphenolic epoxy resin with the addition of 66% of an inorganic filler. It is mainly used to bond concrete and steel structures and to fix steel reinforcements within damaged concrete elements. The filler is largely composed of quartz. Fillers are commonly added to structural adhesives to improve their mechanical properties, reduce costs and, possibly, sensitivity to moisture. Silicates and silica are added to formulations as either hydrophobic, usually non reinforcing, particles or as hydrophilic reinforcing filler particles [5]. M20 is a bisphenolic epoxy resin with the addition of 49% of an inorganic filler, i.e. calcium oxide. The viscosity of M20 is lower than that of M16 adhesive and M20

3. Experimental investigation 3.1. Characterization of adhesives Thermal and mechanical properties of epoxy adhesives were investigated analyzing samples of S50, M16 and M20 previously cured for 20 days at room temperature. Two differential scanning calorimeters (DSC) were used to perform the thermal analysis, i.e. a thermoanalyzer Mettler – TA 4000 equipped with DSC 30 cell and a thermoanalyzer Perkin–Elmer DSC-7. All the thermal scans were carried out between 50 and 250 C with a heating rate of 10 C/min, under nitrogen atmosphere. The glass transition temperature (Tg) of each adhesive was calculated as the mean value of four experiments. Flexural characteristics (Young modulus, E; yield strength, ry; and strain at break, eb) were measured using an Instron tensile testing machine (Series 4300), fitted with a three-point bending fixture at a cross-head speed of 2 mm/min, following the standard ASTM D 790-92 [14]. The dimensions of the specimens were 80 · 10 · 4 mm and the span to thickness ratio was set at 16:1. Five samples were tested to determine the repeatability of the results. 3.2. Tests of water absorption on the adhesives Tests of water absorption were performed on the epoxy adhesives cured at ambient temperature for 20 days, following the standard ASTM D 570-81 [15]. Before the test, the samples were subjected to a conditioning procedure, reported in the code, as follows: the samples were dried in an oven for 24 h at 50 C and then cooled in a desiccator. Thermal and mechanical tests were performed on conditioned samples in order to

Table 1 Details of concretes composition Concrete type

Sand (kg/m3)

Gravel (kg/m3)

Cement (kg/m3)

Water (kg/m3)

Filler (kg/m3)

Additive (%)

fck 25 fck 50

1009 930

1747 1610

250 360

217 214

330 480

1.24 0.6

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evaluate the effects of this treatment on the properties of the three adhesives. The percentage of water absorbed after immersion of 24 h and the percentage of water absorbed by samples substantially saturated, both normalized to the effective resin content of each adhesive, were determined gravimetrically. Epoxy adhesives are prone to water absorption, because they possess polar sites that attract water molecules. Water molecules are typically hydrogen bonded to hydrophilic groups of the cured resin, mainly hydroxyl and amine groups [3]. Both the amount of absorbed water and the rate of absorption depend on formulation variables, such as the epoxy resin and curing agent types employed, together with environmental variables, such as temperature and relative humidity, as well as curing conditions. A wide range of equilibrium water concentration values and diffusion coefficients have been quoted in literature for an equally wide range of formulations and absorption conditions [16–18]. In particular, equilibrium concentrations from 0.25% to 10% by weight have been estimated. The glass transition temperature and the flexural characteristics of samples immersed for 14 and 28 days in distilled water and substantially saturated were calculated with the same procedure used for dry samples. Before any test, the samples were left for 2 days in air at ambient temperature. Each measure was performed on five samples and the results averaged.

as the ratio between the load carried by the specimen at failure (Fu) and the effective area of the bonded surface (Ab). Three different thicknesses of each adhesive (0.5, 2, 5 mm) were employed to study their possible influence on the bond strength. Each measure was performed at least on three samples and the results averaged.

3.3. Adhesion tests

4.1. Properties of adhesives

The strength of the bond between each epoxy adhesive and the different concretes was studied in accordance with ASTM C 882-91 [19]. Each adhesive was used to bond together two equal sections (76.2 mm · 152.4 mm) of concrete cut at a 30 angle from vertical of a concrete cylinder (see Fig. 1). Before the application of the adhesive, any concrete surface was carefully dried and cleaned. After 20 days, which was considered the time required to reach the complete setting of the resin, adhesion tests were performed using a compression testing machine. The bond strength (rb) of the composite cylinder was determined

The main physical (thermal and mechanical) properties of the cross-linked (cured) resins S50, M16 and M20 are reported in Table 3. It is confirmed that the epoxy based adhesives cured at ambient temperature possess a relatively low Tg, never exceeding 60 C. Referring to the effect of fillers on the mechanical characteristics of M16 and M20 adhesives, higher stiffness values for both resins were registered with respect to that found for the unfilled one. On the other hand, the inclusion of fillers in the epoxy adhesives did not show a definite influence on their maximum strength.

3.4. Tests after immersion in water on concrete–adhesive– concrete samples The physical effects of water exposure on the bond developed between any adhesive and concrete was, finally, studied. The samples of concrete bonded with different adhesives were immersed in distilled water maintained at a temperature of 23 ± 1 C for different periods of time: 2, 7, 14 and 28 days. After the different immersion periods, the samples were left for 2 days in air at ambient temperature and they were subjected to compression tests. A total of three specimens for each test condition were examined. Due to the lack of standards on this kind of test, the authors chose the described test procedure trying to simulate some real service conditions and following the indications of other researchers [12].

4. Results and discussion

4.2. Water absorption properties of adhesives

30˚

10.2 mm

76.2 mm

Fig. 1. Concrete/concrete adhesive joint specimen.

142.2 mm

15 2.4

mm

Cutting surface

The water absorption test on adhesive S50 was performed in a previous study [20] and the results are summarized in Table 4. After a 24 h immersion the cured samples gained 0.62% in weight. The total water absorbed by the samples substantially saturated was about 1.56% in weight and was reached after 19 weeks of immersion in water. Table 4 reports water absorption results also for M16 and M20 adhesives. The percentage of water absorbed by the epoxy adhesives containing inorganic fillers, normalized to the effective resin content, after one day

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conditions similar to that found for S50 resin. For M16 adhesive, on the other hand, a much lower water content at saturation, i.e. 0.42% wt, was found, possibly due to the hydrophobic nature of its filler. The time required for these adhesives to reach saturation conditions, finally, was substantially higher (38 weeks) than that observed for the unfilled S50 adhesive. In Tables 5–7 the results of the thermal and mechanical tests performed on immersed samples S50, M16 and M20, respectively, after different time spans, are reported. For comparison purposes, the properties calculated on un-conditioned samples as well as on samples conditioned before the immersion (i.e. 24 h at 50 C) are reported, in order to assess the influence of the thermal treatment on the final properties of the three adhesives. The conditioning procedure performed at 50 C on samples before the immersion in water can be regarded as a thermal treatment that can influence the properties of the adhesives. The effects of a thermal treatment on an epoxy resin depend on the initial structure and the thermal properties of the resin. In particular, the heating of a cross-linked epoxy for prolonged time at a temperature close or higher than the Tg of the resin can cause one or both of the following: (a) the erasing of physical aging and (b) the post-curing of the resin (in addition to the removal of the water eventually contained in the samples). These effects have important influence on the

Table 3 Thermal and flexural mechanical properties of epoxy adhesives S50, M16 and M20 Adhesive

Tg (C)

E (GPa)

ry (MPa)

eb (mm/mm)

S50 M16 M20

46 ± 2 58 ± 2 51 ± 2

0.830 ± 0.110 5.795 ± 0.805 4.487 ± 0.487

27.1 ± 4.5 21.2 ± 1.5 51.0 ± 6.1

0.130 ± 0.020 0.005 ± 0.001 0.011 ± 0.001

Tg = glass transition temperature; E = Young flexural modulus; ry = yield flexural strength; eb = strain at break.

Table 4 Water absorption characteristics of cured adhesives S50, M16 and M20 Adhesive

% Water (24 h)

% Water (saturation)

Saturation time (weeks)

S50 M16 M20

0.62 ± 0.07 0.25 ± 0.02 0.08 ± 0.00

1.56 ± 0.17 0.42 ± 0.09 1.37 ± 0.11

19 38 38

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% Water (24 h) = percentage of water absorbed after immersion of 24 h, normalized to the effective resin content; % water (saturation) = percentage of water absorbed by samples substantially saturated, normalized to the effective resin content; saturation time = time for water saturation; Tg = glass transition temperature, measured on samples substantially saturated.

immersion was in both cases lower than that calculated for the unfilled resin, i.e. S50. Referring to the water content values found for substantially saturated samples, M20 adhesive showed a water content at saturation

Table 5 Thermal and flexural mechanical properties of epoxy adhesive S50 as a function of immersion time in distilled water Adhesive S50

Tg (C)

E (GPa)

sE (GPa)

DE (%)

ry (MPa)

sry (MPa)

Dry (%)

eb (mm/mm)

seb  102 (mm/mm)

Un-conditioned Conditioned 14 days immersion 28 days immersion Saturation (19 weeks)

46 ± 2 46 ± 0 38 ± 0 41 ± 0 43 ± 2

0.830 0.800 0.615 0.969 0.663

0.17 0.07 0.14 0.08 0.14

– 23 +21 17

27.1 24.8 25.3 28.1 22.5

7.91 1.55 0.41 1.61 2.60

– +2 +13 9

0.130 0.080 0.047 0.037 0.064

2.0 4.0 0.55 0.41 0.77

Table 6 Thermal and flexural mechanical properties of epoxy adhesive M16 as a function of immersion time in distilled water Adhesive M16

Tg (C)

E (GPa)

sE (GPa)

DE (%)

ry (MPa)

sry (MPa)

Dry (%)

eb (mm/mm)

seb  102 (mm/mm)

Un-conditioned Conditioned 14 days immersion 28 days immersion Saturation (38 weeks)

58 ± 2 73 ± 4 56 ± 1 57 ± 2 61 ± 0

5.795 5.000 2.980 3.348 3.208

0.66 1.73 0.32 0.46 0.25

– 40 33 36

21.2 25.0 24.7 23.8 22.0

1.30 3.20 1.87 1.24 1.97

– 1 5 12

0.005 0.009 0.008 0.010 0.012

0.06 0.20 0.13 0.08 0.09

Table 7 Thermal and flexural mechanical properties of epoxy adhesive M20 as a function of immersion time in distilled water Adhesive M20

Tg (C)

E (GPa)

sE (GPa)

DE (%)

ry (MPa)

sry

Dry (%)

eb (mm/mm)

seb  102 (mm/mm)

Un-conditioned Conditioned 14 days immersion 28 days immersion Saturation (38 weeks)

51 ± 2 53 ± 2 47 ± 0 49 ± 1 53 ± 0

4.487 5.395 3.930 4.275 4.244

0.40 0.73 0.12 0.31 0.63

– 27 21 21

51.0 55.0 55.8 46.0 45.1

5.18 4.14 2.42 3.30 4.98

– +1 17 18

0.011 0.014 0.014 0.011 0.011

0.11 0.06 0.08 0.10 0.17

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properties of the cured products and must be taken into account. Physical aging is a universal phenomenon that occurs in all the amorphous polymers below the glass transition temperature (Tg) and that leads to a reduction in the polymerÕs free volume over time, i.e. in a ‘‘densification’’ [21]. The reduction in free volume reduces the polymer mobility and increases the relaxation time. Structural relaxation in the glassy state is a very slow process, while it is a quicker process at and above the glass transition temperature [22,23]. The effects of physical aging are generally manifest in a reduction of creep compliance, in a stiffening, in a reduction of ultimate elongation, in an increase in yield strength [22,24–28]. However, physical aging is a thermoreversible phenomenon that can be erased by heating the polymer above its glass transition temperature [21,22]. As mentioned, a conditioning temperature of 50 C is very close to or higher than the Tg of the adhesive resins employed in civil engineering applications. As a consequence, the heating of the aged adhesives at temperatures higher than their Tg will cause their de-aging and the reestablishment of their initial properties. The erasing of physical aging does not affect the Tg of a cured epoxy resin. Eventual variations in Tg, that can be observed in conditioned samples, can be attributed to a post-curing process which takes place on samples which are not fully cured. It has been observed that the thermal treatment at 50 C produces an increase in Tg of cold-curing epoxy adhesives; a higher value is obtained by increasing the time of heating [29]. Although after curing times (four months) are well above those suggested by suppliers, i.e. 15 days, the Tg of the resin reaches a constant value, the resin system may not be fully cross-linked. The curing (ambient) temperature, i.e. around 23 C, is about 30 C lower than the final Tg of system and any further cross-linking reaction may be slowed by kinetic restraints [30]. Hence, if the resin is heated at a temperature higher than the ambient temperature, i.e. 50 C, but still lower than its Tg, a post-curing process takes place. In this condition, the cross-linking reactions start again and the Tg increases by increasing the post-curing time. The amount of post-curing depends on the initial Tg of the system when compared with the conditioning temperature. The thermal treatments used to condition the adhesive samples before the immersion in water can, therefore, produce different effects on mechanical properties in relation to the different extents of proceeding of de-aging (erasure of physical aging) and/or post-curing processes. When analyzing a fully cross-linked adhesive, possessing a Tg lower than the conditioning temperature, the thermal treatment performed on this adhesive will erase the physical aging, while it does not produce any

post-cure. This is eventually the case of S50 adhesive, which Tg does not change as a consequence of the conditioning procedure. Though it is not completely crosslinked, in fact, the completion of curing reactions begins at higher temperatures, i.e. above 90–100 C. Since the major effects of physical aging on mechanical properties of a thermosetting resin are: (a) the stiffening of the glassy material and (b) the reduction of ultimate elongation and the increase in yield strength, the remove of physical aging should produce reductions in modulus and maximum strength and an increased ultimate strain. Thus, small reductions in flexural modulus and maximum stress are found for S50 adhesive, as the result of de-aging process. However, the noticeable reduction in ultimate strain found for the same resin does not match with the expectations. Referring to M20 adhesive, the conditioning procedure removed most of the effects of physical aging, while the post-cure process again did not take place, since the Tg increases by only 2 C. Only the slight increase of ultimate strain, as a consequence of the de-aging procedure, was in line with the expectations. A less clear situation is the behavior of this adhesive concerning its stiffness and strength, both of which increase slightly. When the adhesive sample possesses a higher Tg than the conditioning temperature but is not fully cured, as in the case of M16 adhesive, the de-aging procedure does not take place but the thermal treatment, on the other hand, is able to partly post-cure the adhesive, with the result that the Tg is increased by 15 C. Consequently, the maximum strength is increased after the thermal treatment. However, a decrease in flexural modulus is also observed as a consequence of the conditioning procedure, this effect being due to post-curing, as reported by other researchers [30]. An increase in rupture strain was also observed. It must be emphasized, however, that the flexural test employed does not allow an accurate definition of the stiffness and ultimate strength of the thermosetting materials, since it is based on the hypothesis of elasticlinear behavior of the samples up to the collapse. A thermosetting resin, in fact, will present such a behavior only within the first stage of loading. Therefore, a further analysis of the mechanical properties of the adhesives by means of tensile tests, accurately measuring the deformation by electrical resistance strain-gauges during the test, has been considered. The aim of the authors was to report the preliminary results in order to qualitatively compare the properties of the materials after different exposure conditions, even if the measured properties should not be considered as reference values without any further confirmation. With respect to the analysis of the results of thermal characterization of the epoxy adhesives immersed for a prolonged time in water, the comparison must be performed with the values obtained for each adhesive on

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conditioned specimens. In all cases, at short immersion time an initial decrease in Tg was measured; this was probably due to plasticization effects. Following this, a new increase in glass transition at longer immersion time was noted. A maximum decrease in Tg was measured for the adhesive with the highest initial Tg, i.e. M16 (DTg = 17 C). In a saturation condition, moreover, the adhesives M20 and S50 almost recover the initial Tg value, while M16 adhesive showed a decrease in Tg by about 12 C. The results obtained in this study seem to confirm those of several authors. Referring to the effect of an immersion in water on the thermal properties of epoxy adhesives, relatively short time of exposure lead to more or less reversible plasticization, producing a lowering of the Tg [31]. The decrease of Tg as a consequence of immersion in water is a physical change that can partially be reversed upon drying. The glass transition temperature is a very important parameter of epoxy resin and epoxy matrix composites because the Tg establishes the service environment for the materialÕs usage. Usually, when the material is exposed to a hygrothermal environment the Tg decreases and, as a consequence, the service temperature of the material changes. This modification in Tg reflects the degree of resin plasticization and water/resin interactions occurring in the material. As already pointed out, this effect is of particular concern for cold-curing epoxies whose typical glass transition temperatures, when dry, are not much higher than the possible service temperature. Hence, the need arises to select adhesives whose TgÕs do not drop substantially with water sorption or to assure controlled ambient conditions, when possible. On the other hand, the increase in Tg after a longer immersion time is most likely due to additional cross-linking during exposure to water. Additional cross-linking can take place, as the epoxy samples would not be fully cured at room temperature and immersion in water can cause plasticization of the resin with a consequent reduction in Tg of the cured wet resin [32]. The lowering in Tg upon moisture ingress allows the polymer chains to become mobile; this allows a limited displacement of polymeric segments promoting post-curing. To confirm this hypothesis, it was reported that higher values of Tg resulted for longer immersion time and higher exposure temperature [33]. The greatest depression in Tg as consequence of immersion in water seems to be related to: (a) the initial Tg values, (b) the higher the initial Tg value and (c) the greater the reduction in Tg. Moreover, the initial Tg values influence also: (a) the new increase in Tg after the first immersion period, (b) the higher the initial Tg values and (c) the greater the increase in TgÕs after the maximum reduction. As reported from other studies, the effect of an immersion in water for prolonged time on the mechanical

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properties of mild-cured bisphenolic epoxy adhesives (Tg = 76 C) can be summarized in an initial increase in ultimate tensile strength (up to 21% after a threemonthÕs immersion), followed at longer immersion times (i.e. five months) by a decrease to values similar to that of the unaged polymer [7]. The initial increase in the ultimate tensile strength was again explained in terms of an increase in cross-link density [34]. Later reductions in strength, finally, were the result of degradation due to the presence of water. From the same study, the YoungÕs modulus of the aged epoxy resulted marginally lower than that of the control samples, increasing the reduction in modulus by increasing the immersion time (after a five months immersion the reduction approached 18%). The elongation at break, moreover, tended to increase initially (up to 76% after a three-monthÕs immersion), but, at later times, the material became brittle (with a final increase of about 30% with respect to unaged samples after a five-monthÕs immersion). It has been reported that a reduction in the failure strain can be regarded as a clear and sensitive indicator of polymer degradation [35]. All the adhesives analyzed in the present study showed values of flexural modulus that decrease at the initial stages of immersion and then slightly increase at longer immersion times, reaching a constant value after about one month. The effects of post-curing during immersion could lead to a higher modulus in the epoxy systems. The differences are very small for the resins S50 and M20; and, in fact, their values of flexural modulus at saturation are reduced by about 17% and 21%, respectively, with respect to the initial value. In the case of M16, the reduction in flexural modulus in saturation conditions is more marked, i.e. about 36% with respect to the initial value. The influence of water at short exposition times on the strength of S50 adhesive leads to a slight increase of this property, while on M16 adhesive it is rather insignificant. At longer exposition times, on the other hand, the reductions of maximum stress for both resins are around 10% of the initial value of the pristine condition of samples. These results are in accordance with the mentioned literature. For M20 adhesive, on the other hand, the immersion in water causes a limited decrease of this property. In particular, after one month of immersion in water the reduction in maximum stress is by about 17% and it retains this value up to saturation condition. The effect of water on ultimate strain is almost always a decrease of deformability. For M20 adhesive, containing 49% wt of filler, the already low value of strain at break is reduced by about 22% after one month of immersion. For the unfilled adhesive, i.e. S50, the reduction in deformability is even more severe. An immersion period of one month reduces the ultimate strain of S50 by about 54%. At saturation, however, this reduction

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is partly recovered, i.e. becoming 20% of initial value. The only exception is M16 (filler content = 66% wt). For this adhesive, in fact, the initial low value of strain is improved by about 33% when saturation is reached. It must be emphasized, however, that M16 presented a different behavior in terms of increase in Tg and water absorbed at saturation. In conclusion, the effect of permanence in water on Tg of the cured adhesives is a reduction in Tg at short immersion time, by about 3–12 C; this can be explained by the plasticization phenomena. Then, this property increases slightly for a prolonged immersion. It is also confirmed that the system with a higher initial Tg shows a greater reduction in Tg at the first stages of immersion and subsequently a greater new increase in Tg at longer immersion times. The effect of water on stiffness of these three adhesives seems to be very similar to that of water on their TgÕs. It was noted that the effect of water on the strength of epoxy adhesives is proportional to the initial value of this property. 4.3. Adhesion tests The main results of adhesion tests previously determined are given in Table 8. The epoxy adhesives, which are used in concreteto-concrete bonding, often possess mechanical strengths greater than those of Portland cement concrete [12]. In such cases, the fracture takes place within the concrete, when its tensile strength is achieved. As a confirmation, all the samples prepared with fck 25 concrete, i.e. the concrete with a low resistance comparable to those of M16 and M20 adhesives, showed a collapse typical of the entire concrete specimens under compression load irrespective of the adhesive employed, i.e. vertical cracks within the whole samples. In these cases, the kind of the adhesive as well as the thickness of the adhesive layer do

not influence the bond strength of the joint. For both M16 and M20 resins, in fact, values of bond strength around 15–16 MPa were generally measured. Considering the analysis of the joined samples made with fck 50, and since this concrete possesses a strength appreciably higher than those of each adhesive, the bond strength, as well as the kind of failure observed in the specimens, is mainly influenced by the strength of the specific adhesive. A higher bond strength was achieved by using the more resistant adhesive (M20), even when compared to specimens produced with M16 adhesive, that possesses similar modulus but different strength value with respect to M20. When adhesives, having similar resistance but different stiffness (i.e. M16 and S50) were used to bond specimens of fck 50, lower bond strength values were recorded with the resin possessing the lowest modulus (i.e. S50). For each of the adhesives employed, by increasing the thickness of the adhesive layer a lower bond strength was recorded. This is explained by a higher deformation in the adhesive joint, resulting in an early failure. As expected from a concrete with a high strength, the failure mechanism, generally, is of a mixed type, with simultaneously crushing within the concrete and inside the adhesive resin and the interface debonding. Failure at interface was more frequently observed in samples prepared with M20 resin, i.e. by using a more resistant and stiff adhesive. On the other hand, when M16 and S50 resins were used, fracture inside the adhesive layer was often recorded, because these resins have strength values almost halve with respect to M20. 4.4. Adhesion tests after immersion in water Referring to the samples made with the fck 25 concrete, reported in Tables 9 and 10, the bond strength reduced by increasing the time of permanence in water

Table 8 Results of adhesion tests performed on joints obtained with different concretes and epoxy adhesives System

Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

fck 25 + M16

0.5 2.0 5.0

120.00 138.52 124.45

19.00 22.25 27.70

16 16 22

14.64 16.85 15.28

2.44 2.64 3.46

17 16 23

fck 25 + M20

2.0 5.0

131.03 129.79

21.30 6.75

16 5

16.01 15.94

2.52 0.93

16 6

fck 50 + S50

0.5 2.0 5.0

164.96 152.20 146.32

1.61 0.97 17.98

1 1 12

19.56 17.62 16.51

0.12 0.03 2.00

1 1 12

fck 50 + M16

0.5 2.0 5.0

215.28 205.81 190.82

34.02 18.37 17.12

16 9 9

25.98 24.53 22.83

4.29 1.99 2.10

16 8 9

fck 50 + M20

2.0 5.0

274.22 243.88

74.16 48.36

27 20

32.82 29.23

8.56 5.80

26 20

M. Frigione et al. / Construction and Building Materials 20 (2006) 957–970

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Table 9 Results of adhesion tests performed on joints obtained with fck 25 concrete and M16 epoxy adhesive after different immersion time in distilled water Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

Drb (%)

0

0.5 2.0 5.0

120.00 138.52 124.45

19.00 22.25 27.70

16 16 22

14.64 16.85 15.28

2.44 2.64 3.46

17 16 23

– – –

2

0.5 2.0 5.0

132.61 127.38 125.49

21.28 19.52 4.04

16 15 3

16.28 15.60 15.37

2.50 2.35 0.42

15 15 3

+11 7 0

7

0.5 2.0 5.0

117.81 122.03 116.42

2.90 7.44 3.34

2 6 3

14.39 14.87 14.21

0.32 0.98 0.44

2 7 3

2 12 7

14

0.5 2.0 5.0

89.48 123.35 114.48

5.19 11.44 8.05

6 9 7

10.87 15.15 14.02

0.67 1.35 0.99

6 9 7

26 10 8

28

0.5 2.0 5.0

107.24 124.06 142.72

14.20 1.33 7.96

13 1 6

13.02 15.01 17.32

1.80 0.12 1.00

14 1 6

11 11 +13

Days of immersion

Table 10 Results of adhesion tests performed on joints obtained with fck 25 concrete and M20 epoxy adhesive after different immersion time in distilled water Days of immersion

Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

Drb (%)

0

2.0 5.0

131.03 129.79

21.30 6.75

16 5

16.01 15.94

2.52 0.93

16 6

– –

2

2.0 5.0

158.03 142.82

15.53 3.07

10 2

19.37 17.48

1.92 0.61

10 3

+21 +10

7

2.0 5.0

144.20 157.66

16.36 20.40

11 13

17.69 19.37

2.03 2.54

11 13

+10 +21

14

2.0 5.0

130.16 111.24

22.68 7.64

17 7

15.88 13.60

2.87 0.92

18 7

1 15

28

2.0 5.0

127.94 92.70

6.96 25.40

5 27

15.63 11.36

0.81 3.09

5 27

2 29

and it is only in some extent influenced by the behavior of the adhesive resin when immersed in water. As can be observed in Table 9, the specimens bonded with M16 resin at short immersion times retain the initial values of bond strength, while, at longer immersion times, the decay of the bond strength reaches values around 10–20%. The results, however, were very scattered and the influence of the adhesive thickness on the bond strength degradation is not very clear. As can be seen in Table 10, the specimens prepared with M20 resin showed a decrease in bond strength at longer exposure times to water (i.e. when exceeding 2 weeks of immersion), increasing the reduction in bond strength with immersion time. However, at shorter immersion times, an increase in the bond strength was, actually, recorded. Moreover, a more relevant degradation of bond properties is observed by using the highest resin thickness (i.e. 5 mm). The adhesive joint reflects the behavior of the resin M20 when immersed in water. In fact, after one month of immersion in water, a decrease of its strength by about 17% is observed.

An initial slight increase in joint strength with time was found for various heat-cured epoxy adhesive bonded metal joints exposed to wet environment [36– 38], attributed to the relief of shrinkage stresses in the adhesive due to the presence of water or moisture [39]. At longer exposure time, however, the average shear strength was found to decrease with time. The visual inspection of failure surfaces, moreover, revealed that the failure mode becomes increasingly interfacial as the exposure time was increased. The kind of collapse observed in specimens immersed in water depends on several parameters, i.e. the period of immersion, the kind of concrete and resin employed and, in a few cases, the thickness of the adhesive layer. Under dry conditions, failure of structural joints normally occurs by cohesive failure of the adhesive layer or within the concrete, depending on the resistance of the single components. Prolonged exposure to a wet environment, however, shifts the failure mode to adhesive failure through the polymer–substrate interface [6,40]. This trend is favored by increasing exposure time.

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The loss of joint strength due to water is, therefore, believed to be due to degradation of the interface rather than weakening of the bulk adhesive when low resistant concrete is employed. Referring to the kind of failure observed in the samples made with fck 25 concrete, the influence of the permanence in water was noticed only after 7 days treatment. In this case, the analysis of the tested specimens evidenced the presence of slip at the interface in addition to the vertical cracks within the concrete, as in the case of the control specimens. An explanation for the more frequent occurrence of slip at the interface could be the weakened adhesion between concrete and adhesive due to the presence of water at the interface. It must be emphasized, however, that such a phenomenon prevailed in samples prepared with M20 adhesive, in agreement with the greater water content absorbed by this adhesive at long immersion times. After 29 days of immersion in water, interface debonding was the dominant mechanism of fracture which occurred in samples prepared with M20 adhesive, particularly when the highest adhesive thickness was employed. In this case, the longer the period of exposure the more appreciable diffusion of water took place toward the concrete/resin interface and, in addition, a significant decay of the mechanical properties of the adhesive; both phenomena growing with the amount of resin used in the specimens. After the same period of immersion in water, the samples made with M16 adhesive were affected to a lesser extent by the presence of water. They showed slip at the interface in several cases, even though the decisive collapse was always caused by fracture inside both the resin and the concrete. With fck 50 concrete a greater influence of the resistance of the adhesives on the resistance of the whole sys-

tem was expected, since the resistance of the concrete in this case is appreciably higher than that of each adhesive. The results of bond strength tests performed on specimens made with fck 50 and S50, M16 and M20 adhesives, reported in Tables 11–13, and in Figs. 2–4, respectively, confirmed this assumption. The presence of water, in fact, influences the bond strength of the specimens to a larger extent with respect to the samples produced with fck 25 concrete, especially employing the adhesives with a higher water uptake at saturation (i.e. S50 and M20). It has been reported in the literature that a critical water concentration exists below which waterinduced damage of the joint may occur to a minor extent. For any epoxy system, it is estimated to be 1.35% wt [40]. Any loss in the joint strength by the absorbed water can be restored upon re-drying if the equilibrium moisture uptake is below the critical water concentration [6]. The low amount of water uptake at saturation of M16 adhesive, therefore, could explain the limited effect of water on the bond strength of specimens joined with this resin. In addition, the bond strength at which failure occurs generally falls progressively with time of exposure to water. There is an indication for S50 and M20 adhesives that the strength values decay to a minimum level after 14 days of exposure to water, although there is some scatter in the data. Analyzing in detail the results for S50 adhesive, reported in Table 11, it is observed that after a 2 days immersion similar results to control specimens were found, but by increasing the exposition time to 7 days, a general decrease of bond strength around 20% was registered. As mentioned, after two weeks of immersion in water, the decrease in bond strength reached an almost constant value of about 35%, confirmed also for 28 days of exposition to water. All the results seemed

Table 11 Results of adhesion tests performed on joints obtained with fck 50 concrete and S50 epoxy adhesive after different immersion time in distilled water Days of immersion

Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

Drb (%)

0

0.5 2.0 5.0

164.96 152.20 146.32

1.61 0.97 17.98

1 1 12

19.56 17.62 16.51

0.12 0.03 2.00

1 0 12

– – –

2

0.5 2.0 5.0

153.34 169.20 135.82

0.53 7.51 22.78

1 4 17

18.02 19.68 15.65

0.02 1.01 2.30

1 5 15

8 +12 5

7

0.5 2.0 5.0

138.33 118.59 –

3.06 5.12

2 4

15.78 14.04 –

0.33 0.62

2 4

19 20

14

0.5 2.0 5.0

105.59 96.09 94.83

11.38 11.22 7.1

11 12 7

12.52 11.49 11.15

1.30 1.34 0.75

10 12 7

36 35 32

28

0.5 2.0 5.0

104.38 92.37 92.65

15.65 15.87 9.71

15 17 10

12.41 10.81 10.73

1.76 1.90 1.26

14 18 12

36 39 35

M. Frigione et al. / Construction and Building Materials 20 (2006) 957–970

967

Table 12 Results of adhesion tests performed on joints obtained with fck 50 concrete and M16 epoxy adhesive after different immersion time in distilled water Days of immersion

Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

Drb (%)

0

0.5 2.0 5.0

215.28 205.81 190.82

34.02 18.37 17.12

16 9 9

25.98 24.53 22.83

4.29 1.99 2.10

16 8 9

– – –

2

0.5 2.0 5.0

153.77 228.02 175.85

46.80 27.26 27.45

30 12 16

18.87 27.83 21.67

5.74 3.10 3.44

30 11 16

27 +13 5

7

0.5 2.0 5.0

156.26 199.30 206.07

22.75 20.41 18.40

14 10 9

19.21 24.38 25.26

2.93 2.71 2.17

15 11 6

26 1 +11

14

0.5 2.0 5.0

135.68 213.65 195.13

27.36 30.25 6.21

20 14 3

16.68 25.93 24.12

3.42 3.44 0.90

20 13 4

36 +6 +6

28

0.5 2.0 5.0

162.13 197.20 196.12

36.47 35.85 43.70

22 18 22

19.23 23.64 23.63

4.15 4.04 5.03

22 17 21

26 4 +4

Table 13 Results of adhesion tests performed on joints obtained with fck 50 concrete and M20 epoxy adhesive after different immersion time in distilled water Days of immersion

Adhesive thickness (mm)

Fu (kN)

sF u (kN)

COV (%)

rb (MPa)

srb (MPa)

COV (%)

Drb (%)

0

2.0 5.0

274.22 243.88

74.16 48.36

27 20

32.82 29.23

8.56 5.80

26 20

– –

2

2.0 5.0

257.04 210.80

36.04 50.04

14 24

30.79 25.15

3.98 5.70

13 23

6 14

7

2.0 5.0

227.83 238.43

6.16 20.39

3 8

27.97 28.42

0.81 2.48

3 9

15 3

14

2.0 5.0

197.82 190.78

72.38 64.84

37 34

23.63 22.75

8.84 7.57

37 33

28 22

28

2.0 5.0

166.81 196.81

20.22 8.03

12 4

20.30 24.02

2.54 0.97

13 4

38 18

to show that the adhesive thickness has the same influence on the bond strength values as that registered on dry samples (i.e. it slightly decreases passing from

the lowest thickness to the highest). This could again be explained by the mechanical properties of the concrete which are higher than those of the adhesive resin. 30

22

Bond strength (MPa)

Bond strength (MPa)

28 20

t=2.0

18 16 14

t=5.0

t=0.5

12

t=2.0

26 24 t=5.0

22 20 18

t=0.5

16

10

14 0

10

20

30

Immersion time (days) Fig. 2. Bond strength vs. immersion time for joints obtained with fck 50 concrete and S50 epoxy adhesive (t = thickness of the adhesive layer).

0

10

20

30

Immersion time (days) Fig. 3. Bond strength vs. immersion time for joints obtained with fck 50 concrete and M16 epoxy adhesive (t = thickness of the adhesive layer).

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M. Frigione et al. / Construction and Building Materials 20 (2006) 957–970

Bond strength (MPa)

34 30 26

t=5.0

22 t=2.0

18 0

5

10

15

20

25

30

Immersion time (days) Fig. 4. Bond strength vs. immersion time for joints obtained with fck 50 concrete and M20 epoxy adhesive (t = thickness of the adhesive layer).

Therefore, by increasing the thickness of the adhesive layer, the influence of the strength of the resin becomes more relevant. In Table 12 the results of the tests of the adhesion strength performed on specimens of fck 50 bonded with M16, are presented. As already stated, the effect of immersion time on the resistance of the joint is rather negligible. This result is in good accordance with the trend of the strength of the pure resin when immersed in water, since it shows only a slight decrease even after 28 days of immersion. On the other hand, the influence of the adhesive thickness on the bond strength is significant, irrespective of the immersion time. When using the lowest thickness, a higher degradation of the bond strength was constantly registered, around 30%. On the other hand, with higher thicknesses, the bond strength values remain roughly unchanged even after one month of exposition to water. These results appear quite different to those observed for the same resin bonded with fck 25. However, as can be seen in Table 12, the results of the tests performed on the specimens, using the higher resistance concrete, were even more scattered than those obtained with fck 25 concrete, particularly in the case of specimens prepared with the lowest thickness, as confirmed by the higher values of the covariance. As already observed, a continuous decrease in bond strength by increasing the immersion time up to 14 days was registered for the specimens prepared with M20 resin, as can be seen in Table 13. Also in this case, the effect of the presence of water on the mechanical strength of the adhesive reflects more severely on the bond strength of the joint than on the specimens produced with M20 adhesive and the less resistant concrete (Table 10). This confirms the critical role of the adhesive when using high strength concretes. The thickness of the adhesive layer has a certain effect on the degradation of the resistance of the joint, increasing this degradation by using the lowest thickness. It must be emphasized, however, that also in this case very

high values of covariance for the results were registered. The presence of water influences in a similar way the fracture behavior of the specimens made with fck 50 concrete and S50 adhesive. In fact, after 14 days of permanence in water a noticeable decrease in bond strength was registered and it remained constant with increasing immersion time, irrespective of the thickness of the adhesive layer. Comparing these results with those found for the neat resin immersed for prolonged period of time, it seems that the ultimate strain of the adhesive is the most influential parameter for the strength of the joint, that reduces even for short immersion times, as both the modulus and the maximum strength remained almost constant. Considering the samples prepared with M16 adhesive, they frequently showed slip at the interface after 2 and 7 days of immersion. This tendency was even accentuated at longer exposition times. After 14 and 29 days of immersion in water, the collapse was characterized in most cases by interface debonding. Analyzing the samples produced with M20 and S50 adhesives, the kind of collapse was always of mixed type, involving either the concrete, the resin and, possibly, an interface failure. A sudden and explosive collapse was sometimes observed for samples prepared with M20. The obtained results showed that the amount of degradation of all the adhesive resins in the presence of water reflects on their joint performance, in terms of bond strength as well as the kind of failure mechanism, particularly when concretes with high strength are employed. Similar behavior was observed by employing adhesives with comparable water uptake at saturation. However, water has a slightly higher detrimental effect on the joint produced with the unfilled adhesive (S50) compared with the filled one (M20); this compares with other researchers. It has been reported that, after a 135 days immersion in distilled water, a heavily filled epoxy resin presents a lower bond strength loss (21%) compared with the un-immersed samples than an unfilled epoxy adhesive (strength loss 50%), in both cases using the same kind of concrete [12].

5. Conclusions Rather than being the universal solution for any kind of deterioration in civil infrastructure, thermosetting materials and their composites could provide alternatives for rehabilitation and renewal not possible with conventional materials. However, these materials can degrade when water is present. In particular, the resin matrix allows moisture adsorption and this can lead to a variety of mechanisms, some of which result in deterioration of the polymer and, in turn, to a

M. Frigione et al. / Construction and Building Materials 20 (2006) 957–970

decay of the effectiveness of the performance of the restoration. The reported study, in particular, investigated the mechanical performances of concrete structures repaired by using epoxy adhesives, as for injecting cracks, for anchoring steel reinforcements and finally for the strengthening of concrete structures using FRPs. The most critical aspect when bonding different materials is the interface behavior, that influences in to a great extent the effectiveness of the bonded system both under service and ultimate stages. Some key parameters, involved in the interface behavior, have been investigated in this paper, such as: (a) the properties of adhesives and concrete, (b) the thickness of the adhesive layer and (c) the presence of environmental agents, in particular the presence of water. Epoxy adhesives-concrete joints, in fact, can be considered, only to a certain degree, suitable for load bearing applications when the requirement is a constant exposure to water. On the basis of obtained results, the following considerations can be made:  The performances of the system concrete-resin is significantly influenced by the mechanical and physical properties of both materials. An important aspect to be considered is the strength and the modulus of the concrete substrate in relation to that of the adhesive.  When increasing the thickness of the adhesive layer, the joint effectiveness generally decreases. On the other hand, a very thin layer of adhesive involves a carefully manufactory procedure to prevent problems relating to the joint quality. Therefore, an appropriate range of adhesive thicknesses should be defined from a design point of view.  Environmental agents, in particular water, influence the joint performances causing a decay of mechanical properties. Similar studies supported the idea that the loss in joint strength is primarily due to degradation of interfacial region through water–substrate interaction. As a consequence, service conditions in terms of both applied load and environmental agents have to be considered when adhesives are used for repairing concrete structures.  The lack of standard tests for durability investigation of adhesive–concrete joints makes the assessment of reliable theoretical models difficult, due to the difficulty of comparing results obtained from different test procedures and apparatus. Referring to adhesives, the available standard tests generally refer to resins cured at elevated temperatures, and do not consider the specific properties of adhesives cured at low temperatures. Durability in severe environments is one of the key issues in broadening the application of structural joints beyond the aerospace industry.

969

 Accelerated durability tests are generally accepted to provide an indication of the long-term behavior of such systems. However, a deeper insight into the behavior of materials and structures exposed to environmental agents would require a proper experimental investigation, in particular under real conditions, in order to properly define the relationships between results obtained under accelerated and long-term exposure tests.

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