The Durability of Reinforced Concrete Bridges

Abstract

Bridge materials durability is a key issue to be considered for reinforced concrete bridges. The maintenance and the repair are critical and costly. These issues may exhibit significant implications for the safety and functionality of these transportation infrastructures. Reinforced concrete bridges must withstand atmospheric environmental and artificial service conditions. They have to resist the adverse effects of cracking, carbonation, chloride migration, steel rebar corrosion, as well as wind, snow, rain, and freezing actions. The cyclic variation of these conditions has a relevant consequence on the long-term resistance. The durability of reinforced concrete bridges is also influenced by the geographic orientation and the local environmental exposition of the several bridge elements. The upper and lower structures of the bridges are linked through special bearings, which exhibit degradation over time. Foundation piles must be monitored with respect to the conservation state. Therefore, all durability aspects must be considered prior to planning and repairing the bridges. In this manner, it is possible to design bridges with specific needs and ensure the service endurance and safety.

1. Introduction

The reinforced concrete bridges are widely widespread in the world. They are mainly composed of three fundaments materials: rock aggregates, cementitious binders, and steel rebars. The rock aggregates and the cement-based components form the main ingredients for the concrete. The concrete is mainly responsible for the compressive strength solicitation of the infrastructure. The addition of steel rebars mainly enables to whole structure to withstand the tensile stresses. From a material point of view, the combination of steel rebars and concrete forms a composite material capable of facing some of the main solicitation stresses that arise during the service life of bridges. This composite material, namely the reinforced concrete, must give a fundamental contribution to the quality and durability of the infrastructure. Each type of component needs to be chosen with attention. The rock aggregates must exhibit a high quality, adequate lithology, hardness, mineralogy, resistance to fragmentation, shape, and form, and the combination of the aggregate size must follow well-defined granulometric curve ranges [1] in order to be used for the production of concrete. On the other hand, the choice of the cement type also needs to accomplish clear parameters to satisfy the strength development requirement, the bonding capability, and the durability of the concrete [2, 3]. Today, also the sustainability plays a major role. The steel rebars are also required to fit within specified requirements concerning the mechanical properties, fatigue, weldability, and the geometrical factors [4]. The adhesion bond between the steel rebars and the concrete is a major concern with respect to the durability of the entire construction system. The deterioration of this latter building system is affected by the mechanical solicitations and from the environmental exposure condition. The atmospheric interaction causes carbonation, chloride migration, freezing action, cracking and scaling of the concrete, and corrosion of the steel rebars [3].

The conventional use of type of cement, in particular, CEM I-based Ordinary Portland cement, is a common practice. Various strength class cements are used, ranging from 32.5 to 52.5 Mpa with a normal (N) or rapid (R) development of the strength. Nevertheless, the CO₂ development associated with the production of types of cement and the general sustainability target to reduce such emission, triggered the use of alternative binders or recycled components. The addition of supplementary cementitious materials with filler or hydraulic properties or a combination of them is largely investigated [5], while the substitution or addition of recycled components to the Ordinary Portland cement component is sometimes largely disregarded. The long-term service life requirements of bridges up to 80–100 years, still arise some questions related to the low material quality of recycled concrete for infrastructures.

It must be stated that nowadays, the compressive strength target up to 35 MPa can be easily reached. Nevertheless, the durability still remains an issue to be solved. The high durability requirements asked through laboratory tests [3], concerning the freeze/thaw resistance, the depth of carbonation, and the chloride diffusion tend to decrease the permeability of the material at relatively early age up to 28 days. Consequently, high modulus of elasticity and stiffness are seen. The structure may become more rigid and more prone to micro cracking. The latter feature significantly lowers the service life. That means high strength does not mean necessarily longer service life. Furthermore, a higher durability may be achieved with more sustainable materials and combination of them to produce safe and more durable reinforced concrete bridges.

2. Concrete degradation

2.1 General deterioration

The concrete is a very versatile building material. The technology development of the last decades allowed to obtain complex structures and to speed up the casting procedure. On the other hand, cementitious materials need a special attention. High compressive strength and modulus of elasticity, as well as the load intensity and fatigue, may cause crack formation (Figure 1 left). The cracks can be seen with the bare eye, by spraying water on the surface and detecting the darker areas along the cracks. These latter may exhibit a fine widespread pattern or a sequence with variable opening up to a few millimeters. The multiple material composition causes a relatively wide susceptibility to several environmental actions, while chemical interactions between the hydrated cement pastes with the aggregates or the physical/chemical reaction between the cementitious matrix and the external elements such as CO₂, chloride, acids, water leaching of the cement paste due to the mechanical droplet actions transported by the vehicles, the windy conditions, and the temperature variation all contribute to damage. Alkali-silica reaction between the amorphous Silicon-rich aggregates and the cement alkalis form an expansive gel and lead to multiple chaotic net-like cracking [6]. The contact between sulfate-rich waters and the aluminate content of the cement leads to the formation of expansive hexagonal-shaped ettringite crystals, which lead to expansion and cracking. This is particularly observed for Na₂SO₄, while with MgSO₄ a disgregation is seen [7]. Acids strongly attack the basicity of the concrete and cause a relevant dissolution of the material and in some cases directly the steel rebars [8]. Nonetheless, the degrading phenomena occurring along bridges are load-related microcrack formation, temperature-induced thermal expansion, water leaching, and freezing, [3, 9] and the main focus must be driven to carbonation and chloride contamination.

Figure 1.

Bridges across a highly trafficked highway in service for over 40 years. Cracking (left), coloring effects (center-left), dirt accumulation (center-right), and segregation (right).

2.2 The concrete carbonation

The cement hydration produces portlandite Ca(OH)₂, which is responsible for the high alkalinity of the concrete pore solution. The basicity ranges at pH between 13 and 13.5 [10]. The concrete of bridges is constantly in contact with the CO₂ of the atmosphere. The CO₂ diffusion within the concrete causes the reaction with the portlandite to form calcium carbonate. The reaction depends on the humidity [11], the Ca(OH)₂ availability, the porosity, and the microstructure [12, 13]. The carbonation takes place at a relatively constant rate [14]. The process does not directly damage the concrete. On the contrary, the precipitation of carbonate may be seen as a partial carbon sink for the environment, and the CaCO₃ formation may, in some cases, close the porosity at an initial stage. This may lower the further CO₂ penetration within the concrete. Nevertheless, in the field along bridges, this does not take place as expected. Additional detrimental atmospheric effects allow the CO₂ to continuously enter the microstructure. The carbonation of the concrete may have a slight coloring effect on the concrete surface. It becomes whitish as compared to the intense gray color of the as-cast in place concrete (Figure 1 left-center). In some areas, especially if the bridges cross highly trafficked highways, a significant deposition of dust and dirt is seen along many parts of the structures, and the surfaces become covered with a dark-colored deposition (Figure 1 center-right). It appears that such a natural layer of deposits does not significantly influence the CO₂ penetration. In spite of the high-quality concrete that is transported on-site, the cast and placing, play a relevant role in the long-term duration of the infrastructure. This is not always an easy issue and locally, segregations (Figure 1 right), delamination, or surface detachments caused by an inadequate formwork placing, and treatment may accelerate the entrance of the CO₂. This may be accounted for one of the main partial lack of direct correlation between laboratory accelerated carbonation tests [3] and real field behavior.

3. Steel rebar corrosion

The corrosion of steel rebars takes place within concrete when the alkalinity is lowered from pH 13.5 to about pH 9. Under these conditions, the passive protective layer on the rebars is no longer stable and corrosion initiates [14, 15, 16]. The iron hydroxide corrosion products exhibit a higher volume as compared to the original iron in steel. An expansion action occurs. This leads to high local stresses along the rebars and cracks form. The formation of cracks further accelerates the penetration of water, and the corrosion is promoted. The carbonation solely does not adversely affect the steel rebars or the concrete durability. On the other hand, the combination of carbonation, the lowering of the pH, and the presence of water leads to the steel rebar corrosion. This type of corrosion is called uniform corrosion and is uniformly widespread along the rebar surface, thus causing spalling of the concrete cover. This is observed at advanced corrosion stage. It is important to notice that the cracking does not occur only along the steel rebars but also on the sides up to 10–15 cm away from the rebar (Figure 2 left). The tensions that arise during the corrosion process are quite widespread around the rebar regions.

Figure 2.

Uniform corrosion induced by carbonation (left) of a bridge in service for over 40 years in a south alpine region and localized pits initiation during chloride attack (right) on a steel rebar.

The corrosion induced by chlorides that comes from the NaCl salt spreading during the winter time to avoid the water freezing along the roadways is locally more intense. It is generally enhanced by alternating wetting and drying cycles [17, 18]. The localized corrosion starts with the formation of local pits (Figure 2 right). This form of localized corrosion of the steel rebar can lead to a reduction of the rebar diameter. This fact may cause structural problems. This form of corrosion is mainly present along the alpine spaces and in weather conditions, where an alternating seasonal temperature range between 10 and 30°C is present. In very hot climas and in the absence of freezing or in very cold regions, where the temperature may well be below −15°C, this issue is less present.

4. The environmental exposure

The several parts of bridges are differently exposed to environmental conditions. Therefore, it is necessary to clearly identify the sampling areas in order to get a complete a reliable conservation state of the infrastructure. In this concern, adequate, stable, and safe equipment needs to be used. Bridges are subjected to the atmospheric conditions: wind, rain, sun irradiation, and snow. All these factors contribute to a slow but continuous interaction with the structure.

Around the bridges, several micro clima conditions may form. Some parts are directly and continuously exposed to rain such as the curbs. Other parts of the infrastructure are more indirectly exposed such as the lower and the lateral parts that are partially sheltered by the curbs (Figure 3 left). The bridge girders (Figure 1 center-left) are almost completely protected by the structure itself by the direct interaction with the weather parameters such as rain and the direct sun irradiation, while the lower section of the piles is occasionally directly exposed.

Figure 3.

Corrosion induced by carbonation along the lateral parts of an arch bridge in service for over 40 years. Partial direct atmospheric sheltering (left), concrete spalling (center), and rebar diameter reduction (right).

In this concern, the type and the extent of the degradation may be significantly different. Generally, the South side of the bridges is subjected to the cyclic exposure to rain and sun. The relatively higher presence of sun irradiation lowers the frequent presence humidity. On the other hand, the cyclic temperature variation and the subsequent relative humidity variation cause an increased carbonation of the side elements, partially covered by the curbs (Figure 3 left). The high penetration depth of the carbonation decreases the pH down to the rebar and causes the rebar corrosion. This results in a clear and visible cracking located along the vertical rebars (Figure 3 center). The spalling of the concrete cover and the resulting direct contact of the rebars to the atmosphere pushes the corrosion to an advanced stage. The corrosion products detach from the rebars. The steel rebars may start to exhibit a diameter loss (Figure 3 right).

In the North side, the curbs are constantly exposed to rainfall. and only an indirect sun irradiation is present. Therefore, a more constant presence of humidity and a growth of organic deposits is seen. Consequently, the depth of carbonation, even after decades, can be until 10 orders of magnitude lower as compared to the South side (Figure 4 left).

Figure 4.

Low carbonation (left) of the north side curb (center-left) detected with the phenolphthalein method directly on-site [19] and lower elements of an arch bridge with the presence of carbonation, humidity, and water infiltration (center-right) from the upper parts of the infrastructure and corrosion (right).

Generally, it can be stated that no direct constant correlation exists between the concrete cover and the carbonation depth. As stated earlier, the concrete quality and type of cyclic exposition play a relevant role. Shoulders, piles, beams, and shelves exhibit a similar carbonation depth, while the struts and especially the curbs show the lowest carbonation. This latter behavior is due to the direct atmospheric rainfall and splashing exposition of these elements. It must also be said that the carbonation conditions and the direct exposition to the atmospheric elements may vary the extent of the degradation and frequent windy conditions, and cold climate may largely influence the type and extent of the degradation due to carbonation.

The compressive strength is related to the total porosity of concrete. This latter parameter influences the entrance of pollutants within the cementitious materials. Hence, a high porosity allows more easily the CO₂ to enter within the microstructure, although the presence of a porosity and a capillary porosity up to the surface of the concrete are not the only factors that affect the carbonation depth but also as previously stated. In fact, during that time, the aging of the infrastructure causes a general trend toward slightly higher compressive strength, but also a tendency to form microcracks. Consequently, within a compressive range between 40 and 120 Mpa, we may find total porosities variation from 10 to 20%. This is a quite wide range. The carbonation is only indirectly correlated with the total porosity because the capillary pores play a more significant role in the CO₂ penetration. The shoulders may exhibit a relevant variation in the carbonation for strengths up to 65 MPa. This variation is seen for in service concrete bridge over the decades within compressive strength range from 45 to 65 MPa. On the other hand, at higher strengths, the relationship between strength and mean carbonation tends to be more evident [20]. On the contrary, a too high rigidity of the material may enhance the formation of cracks and in turn, increase the penetration of CO₂ and the carbonation.

The splashing of chloride-rich waters and the infiltration from the upper to the lower parts on an infrastructure are the main concern for the rebar localized corrosion. The shoulders, heads, and curbs are subjected to these contaminations and often exhibit high chloride content within the concrete with values up to 0.400% referred to the concrete mass. The struts, piles, and beams generally exhibit lower values. As for the carbonation, the many parts of a bridge show different chloride contamination. Shoulders and heads may exhibit lower concentration with depth due to the washing effects. Although a high concentration is found at the surface. A limit of 0.025% by concrete mass is often set to initiate corrosion processes along the rebars [21]. Nevertheless, many laboratories test set-ups are not realistic enough to determine the threshold values and the corrosion behavior in the field [22]. Therefore, critical chloride content data were gathered from real infrastructures [23]. In addition, the exposition to chloride splashing and the alternate wetting and drying cycles may partially evaporate the water and lead to intermittent increased chloride concentrations well above the theoretical limits. This may happen on the surface several times during the service life of bridge elements. These cyclic processes and the combined actions between carbonation, water leaching, freeze, and thaw, as well as alternate chloride penetration steps and salt crystallization, may accelerate the degradation [9]. Chloride penetration and carbonation are some of the main adverse actions that can occur in reinforced concrete. It is also important to know that in service bridges elements, such as shelves, shoulders, and curbs, exist a low correlation between a low mean carbonation and a high chloride content. The shelves and the shoulders are exposed to wetting/drying cycles. Therefore, carbonation and chloride contamination may be both present. The curbs are highly exposed to chloride-enriched water splashing, and the chloride content is high down to 40 mm depth. This may also be caused by the mechanical leaching action, the water droplet, and the enhancement of the surface porosity. On the contrary, struts, beams, and piles are sheltered from rainfall or splashing and the corrosion induced by carbonation takes the lead as a main degrading action [24]. In very cold climas, usually outside the European alpine regions, the freeze/thaw and the alkali-aggregate damage may often be considered as main deterioration mechanisms [9].

5. The durability of the components

The environmental exposition can range in different time slots: yearly, seasonally, monthly, weekly, daily, and within hours. Rapid and relevant short-time atmospheric changes are particularly seen along the alpine region, starting from 1000 meters above sea level. The cyclic exposure conditions are often partially disregarded with respect to the durability. Consequently, to ease the interpretation of the environmental interactions, such as carbonation, chloride migration, freeze, and thaw resistance, these long-term degradation mechanisms are separately tested within the laboratories [3, 21]. In this manner, the link with the real field behavior is often incomplete with respect to the onsite durability of bridges.

The different elements of bridges are differently exposed to the atmospheric conditions [25]. Therefore, the carbonation and the chloride penetration may vary. Additional investigations on over 35 years old highway and road bridges indicate a variable extent of the carbonation. The shelves exhibit changing values from very low up to 20 mm (Figure 5 upper row left) and the beams a slightly higher trend (Figure 5 upper row center). The curbs indicate the lowest values among all the bridge elements, with values that only locally reach 10 mm (Figure 5 upper row right). In most of the cases, the carbonation of these elements remains well below 5 mm during the decades, especially along the North-East exposed bridge sides, where the humidity is more often present. These elements are directly exposed to rainfall and the atmospheric condition, but above all, they are continuously splashed by the traffic vehicles during the rainy days. The presence of water and humidity largely hinders the CO₂ penetration, necessary to cause the carbonation process. The decks also exhibit relatively low values (Figure 5 lower row left). This may be accounted for the protection of these elements by the polymer-based coating water sealing material and mastic asphalts, which also serve as a partial CO₂ barrier. On the contrary, the unsheltered and direct exposition to the atmosphere of the shoulders and the piles causes an increased carbonation up to 35 mm, although with variable values depending on the bridge concrete quality and sampling location (Figure 5 lower row, center and right). In this concern, only a slight correlation in the field exists between the carbonation and the compressive strength for values up to 65 MPa, depending on the bridge element considered. Only compressive strength values above 65 Mpa with a significant reduction in the porosity may significantly hinder the carbonation [20]. On the other hand, it must also be stated that the carbonation largely depends on the Ca(OH)₂ availability and the cyclic relative humidity ranges, particularly present in the field.

Figure 5.

Carbonation depth profile of bridge elements along a highway in service for over 40 years in a south alpine region. Upper row: (left)-shelves, (center)-beams, (right)-curbs. Lower row: (left)-decks, (center)-shoulders, (right)-piles.

The chloride content is relatively low for the shelves (Figure 6 upper row left) and the beams (Figure 6 upper row center), which are partially sheltered by the direct splashing and exposition to the chloride-rich water. They generally show values below the conventional chloride critical value of 0.025% referred to the concrete mass [21]. The curbs exhibit the highest chloride content with values above 0.1% (Figure 6 upper row right). This is due to more frequent contact of chloride-rich splashing water. The water leaching effects that may be seen in the shoulder and heads with chloride enrichments with the depth and a decrease in the surface [24] are not seen in the curbs as horizontal elements. However, high chloride content up to 0.1% down to a depth of 40 mm is quite a constant observation and was observed also in these bridges. As mentioned, the decks are protected by the water sealing coating and the mastic asphalt or asphalt concrete. Nonetheless, local chloride enrichments above 0.1% may also be intense (Figure 6 lower row left). This may be due to local defects of the protection polymer membrane junctions or bituminous layer cracking that allows the contaminated water to penetrate and reach the concrete deck. The shoulders also belong to the bridge elements with a variable but relatively intense chloride concentration. This is often due to unsealed junctions (Figure 6 lower row center). While the piles exhibit a lowering of the values due to the partially sheltered conditions so that the chloride-rich waters cannot often come directly in contact with these latter elements (Figure 6 lower row right).

Figure 6.

Chloride depth profile of bridge elements of a highway in service for over 40 years in a south alpine region at a depth 0–10 mm. Upper row: (left)-shelves, (center)-beams, (right)-curbs. Lower row: (left)-decks, (center)-shoulders, (right)-piles.

With depth, the chloride concentration may decrease for all elements as it can be seen for the depth 10–20 mm (Figure 7).

Figure 7.

Chloride depth profile of bridge elements at a depth 10–20 mm. Upper row: (left)-shelves, (center)-beams, (right)-curbs. Lower row: (left)-decks, (center)-shoulders, (right)-piles.

A further decrease is seen for the depth 20–30 mm (Figure 8). Nevertheless, chloride enrichments with depth may be partially found along support walls and underpasses due to the water leaching action along the surface [25].

Figure 8.

Chloride depth profile of bridge elements at a depth 20–30 mm. Upper row: (left)-shelves, (center)-beams, (right)-curbs. Lower row: (left)-decks, (center)-shoulders, (right)-piles.

The carbonation and the chloride penetration are processes that lead to rebar corrosion. The former decreases the pH and causes uniform corrosion. This can be more easily monitored also from a visual point of view. The pressure arised from the formation of rebar corrosion products leads to internal tension and cracking within the concrete cover and spalling is observed. At that point, the water and the CO₂ ingress further accelerate the process. On the other hand, a similar basic tensional state is observed by chloride-induced attack of the steel rebars. However, the localized form of corrosion promoted by the chlorides leads to premature diameter reduction of the rebar. This process may enhance the speed of deterioration and lead to structural adverse issues. It must also be emphasized that in an alpine space, increased carbonation and chloride penetration are often both observed. Consequently, the combined action of these degrading mechanisms on reinforced bridge structures decreases their durability at a faster speed as often wrongly detected by the actual laboratory tests [3, 21]. In this regard, the increased detrimental effects triggered by the environmental combined actions are higher than the single action considered separately [9, 26, 27, 28, 29].

Hence, a new method of approaching the durability of bridges has to be developed in order to consider the system as a whole. To implement the laboratory-field relationship, it is necessary for a more realistic simulation of the field conditions. This can be achieved solely by the combination of the several factors occurring on side. On the other side, the combination of the single actions, such as carbonation, chloride penetration, freeze and thaw, wetting/drying cycles, and water leaching, must be tested in a reasonable sequence. In addition, the process cannot be accelerated above some clear defined limits to avoid distortion of the real conditions on-site. Temperature and cyclic exposure frequency must be set at an adequate level. Artificial cracking may be introduced by loading the samples. Nevertheless, firstly, it would be wise to gain a deep knowledge of the surface micro-cracks or cracks pattern present along the infrastructures. This will allow us to better simulate the load-crack-creep relation. The relatively high compressive strength often attained in real structures, and the resulting high modulus of elasticity may often promote premature cracking and reduce the durability. Modulus of elasticities up to 60 MPa was measured. These values need to be avoided in the initial cement hydration phase. A task not carefully considered in the last two decades. The premature high stiffness may be related to the low porosity needed to attain the durability test requirements of the environmental exposition classes [21]. The bridges reported in this work, in service for more than 3 decades exhibit a mean compressive strength value of 65 Mpa, and a mean modulus of elasticity below 40 GPa (Figure 9). Quite acceptable values with respect to the ductility and capability of the materials to partially adapt to the stresses, thus reducing the cracks and increasing the durability.

Figure 9.

Compressive strength and modulus of elasticity of the bridge elements investigated.

6. Junctions

The junctions are a relevant part of a bridge infrastructure. At both ends of the bridges, they have an important function in the dynamic response and dilatation behavior. On the other side, the slow but relatively frequent movement is associated with a strong solicitation of the built system in these zones. These regions may represent a weak point concerning the durability of bridges. A particular attention must be devoted to these regions. Most of the junctions are also planned in order to avoid the infiltration of pollutants and water from the above road surface. Despite the precautions, on a long-term basis, the sealing material of these junctions, such as bituminous course layers and extruded polystyrene foam XPS, are no longer able to withstand the water infiltration. Along these critical zones and beneath the sealing systems, it is necessary to foresee a water collection or conveying system that reduces the uncontrolled direct infiltration to the below reinforced concrete elements of the bridges such as the shoulders and the bridge supports (Figure 10 left). In fact, the cracking of the bituminous sealing material (Figure 10 left-center) and the chloride-contaminated waters from the upper parts, infiltrate along the junctions, and induce the rebar corrosion, spalling, and partial detachment of the concrete (Figure 10 center right). The XPS material placed along some junctions (Figure 10 right) only partially avoids a partial dirt accumulation within the gap.

Figure 10.

Junction zone along a bridge across a highway in service for over 40 years (left). Cracking (center-left), chloride-rich water infiltration (center-right), and junction filling material (right).

7. The bridge roller bearings

The bridge bearings are a fundamental part of a bridge structure and need to be panned, built, maintained, and replaced according to specific and clear guidelines. Type of bearing, service life, movement allowed, alignments, and structural, as well as seismic safety, have to be carefully evaluated [30]. This topic needs a wide discussion, but it is not the aim of the present chapter. This subchapter only describes some major issues concerning the stainless steel roller bearings. A current topic related to the durability of reinforced concrete bridges. These latter are still widely present. The planning of such roller bearings is usually done taking into account the Hertzian contact theory, although it appears that the fractures observed in some case studies indicate a partial deviation from the theory concerning some tensions arised in some failed bearings [31].

The material used for such stainless steel roller bearings is mainly martensitic stainless steel. A high-strength material with a high hardness that allows the rolling with very minimal abrasion. On the other hand, such material seems to be prone to corrosion and hydrogen embrittlement [32]. The high-strength materials, in particular steels, and high-strength steels may be particularly susceptible to surface defects, which, in turn, create initiation zone for fractures [33]. Most of the stainless steel bridge roller bearings were designed and produced during the 70’s years. The rollers are in direct contact with stainless steel plates, which, in turn, are supported by large low alloyed steels S235 and S355 [34]. The stainless steel often contains a 13% weight chromium, which corresponds to a X39Cr13, 1.4031 stainless steel [35]. A different extent of degradation is seen on the rollers. Some craters may form on the surface due to humidity, contact with dirt particles, or iron powder. Aligned localized corrosion craters may be found in regions, where an infiltration of chloride-rich waters from the upper bridge junctions is seen. Corrosion may be observed when the face of the rollers remains in contact with the plates and lately, scratches or surface defects are attacked by atmospheric and pollutant agents. In this case, a sharp line of surface corrosion is seen (Figure 11 left). Coating detachment from the lower plates can also be seen due to the movement of the roller (Figure 11, second from left). Corrosion and abrasion issues can be found along the central guide of the rollers (Figure 11 center), while to reduce these phenomena gliding polymer, materials are inserted to reduce the degradation (Figure 11 center-right). The type of damage and the extent of corrosion also depend on the service condition of the bridge, and in extreme cases, vertical cracks may be found across a roller (Figure 11 right).

Figure 11.

Contact corrosion (left), plate coating detachment (second from left), corrosion and coating detachment along the central guide (center), Teflon sliding bands (center-right), and vertical crack (right-picture M. Paderi) on bridge roller bearings in service for over 40 years.

The detection and controlling of such damages can mainly be carried out with regular visual inspection, liquid penetration [36, 37], and ultrasound techniques [38, 39]. The former technique enables to localize surface to reach defects not visible with the bare eye, and the latter allows the identification of internal inhomogeneities, usually larger than 3 mm. This defect size may be a partial limiting factor of technique.

A constant maintenance is necessary for such types of high-strength martensitic stainless steels, regardless of their chemical composition and type or the presence of a low alloyed steels in the core of the cylinders. All surfaces need to be protected from corrosion, especially the contact with aggressive NaCl-rich waters must be avoided. Sealed junctions, adequate oil, or grease may help the protection. The alignments of the roller bearing systems must be constantly verified in order to avoid the degradation and the resulting structural-related issues. Tensions and abrasion along the central guide elements must be eliminated by appropriate alignments and by inserting sliding materials along the contact zone rollers-central metallic guide. It is also necessary to limit the accumulation of dirt and sand particles, particularly along the central zones to avoid oxygen depletion and instability of the protective oxide coating. These regions are particularly subjected to rupture, as well as the lateral cylindric parts, where fracture may arise and propagate longitudinally along the bearings [31]. It is necessary to avoid the cleaning of the rollers with abrasive papers. The abrasion is detrimental since it forms scratches on the bearing surface. High-strength metals may be particularly susceptible to surface defects. They may act as initiation sites for localized corrosion and fracture. Therefore, any type of surface anomaly must be carefully evaluated [40], by also taking into account the loading conditions.

8. The ground embedded infrastructure

The foundation piles are a part of a bridge completely embedded into the ground. The basement of pylons is also stuck into the ground. The pylons foundation piles penetrate down to variable depths. In the case presented here, 10 to 15 foundation piles were present for each highway viaduct pylon and reached a depth ranging from 10 to 15 meters. The foundation piles are made of rebar-reinforced concrete or steel cylinders filled with cementitious material and covered with concrete or special injection mortars on the external side. The two types of cementitious materials may exhibit significant differences for the internal and external applications with respect to the water permeability (from 2 g/m²*h to 17 g/m²*h, respectively) and compression strength (from 70 to 170 MPa, respectively). Other types of piles are also possible. The concrete carbonation is largely lowered within the ground. Hence, the corrosion induced by carbonation is barely an issue. The chloride attack is also scarcely present unless the chloride-contaminated water from the bridge superstructure is reaching the ground due to inadequate water drainage and conveying.

The sampling and quality control of such ground embedded elements are not an easy task. During the placing but also for the inspections, safety precautions must be taken to avoid ground subsidence into the excavated places around the piles. Furthermore, during rainy days the presence of high-level groundwaters may hinder the work, and some of the water have to be pumped out of the excavation (Figure 12 left). The groundwater level variation and flux are responsible for relevant leaching effects along the pylon basements. The water level variation can also be seen directly on foundation piles, where some slight horizontal marks of the water level can be observed. Nonetheless, the low permeability of the external covering mortars largely reduces the water leaching phenomena. The compressive strength class for pylons and foundation piles may be relatively high ranging from a class C 70/85 to a class C 40/50. Some pylon basements may exhibit a clear leaching due to the groundwater level change. In these cases, horizontal leaching lines are clearly visible and cause the disgregation of the cementitious binders, thus exposing the stone aggregate (Figure 12 center-left). Consequently, the surface compressive strength class of the cementitious materials may drop to C 25/30. Groundwater flux velocity, the frequency of level change, and the chemistry parameters, such as carbonate hardness, free CO₂ content, chloride, sulfate, and pH, may significantly affect the leaching resistance.

Figure 12.

Safety concern for the inspection of the upper parts of foundation piles (left), pylon groundwater leaching phenomena (left-center), rebars (center-right), and steel conservation state of the piles (right). Highway bridges in service for over 40 years, still in service.

The corrosion behavior of the buried metallic parts is largely influenced from their embedment into the cementitious materials. The cover depth may range from 30 mm to 80 mm. It is worth emphasizing the changes that may occur in the injection of materials with depth. Down to 10–15 meters, the injection conditions may vary and the cementitious cover of the steels may not be homogeneous. In addition, a change in the ground and rock composition may expose the foundation piles to different conditions. Local depletion or enrichments of oxygen may create macro-element cells that increase the corrosion of the oxygen depleted zones. This fact needs to be particularly considered for clay-rich and the progressive change to sand-rich soils must be carefully taken into account. A variable extent of the concrete leaching may be present depending on the groundwater situation, but both types of foundation piles may not exhibit a significant corrosion of the rebar or steel piles if they are embedded in an adequate quality of cement-based materials (Figure 12 center-right; right). All parts of an infrastructure buried into the ground and with a relative extension in size may be susceptible of stray current corrosion. This may happen in the vicinity of direct current railway lines. Therefore, the electrochemical potential of the buried reinforced concrete steel rebar or metallic elements needs to be monitored [41].

9. Conclusions and future outlook

The reinforced concrete bridges belong to the most relevant infrastructures in the construction field. The mechanical performance of such structures remains a key issue. Nonetheless, the durability increased its importance with time. In fact, such structures are widely exposed to variable loading and environmental conditions. These latter largely affect the durability. The degradation of bridge elements is strongly asymmetrical. The different parts of a bridge are differently exposed to the aggressive actions. Direct and indirect exposition to the atmosphere, cyclic exposure, rain, wind, sun, freeze and thaw, chloride contamination, and carbonation are among the main degrading actions taking place around a bridge. In this concern, different microclimas arise along such infrastructures. This fact needs to be considered in the planning phase.

The higher requirements set by some concrete mix design concepts and requirements such as the water/cement ratio and the minimum cement content, as well as the laboratory tests and the normative concerning the durability, tend to decrease the general porosity of the concrete. This results in an implementation of the compressive strength of the materials, especially at early age. The modulus of elasticity and the stiffness also increase. Consequently, premature cracking may often occur in the field. This fact significantly reduces the durability. Therefore, a variable relationship between laboratory tests and real field durability is often ascertained.

In this concern, it is necessary to rethink some concepts about the concrete mix design. It is necessary to lower the clinker cement component in the cementitious binders and to increase the use of composite cement. In addition, the use of recycled concrete aggregates also needs to be considered for infrastructure by limiting the percentage of addition to reasonable values. This enables to attain the required durability. Furthermore, it is worth to implement the relationship between laboratory durability tests and real field behavior. This may be partially accomplished by exposing the laboratory samples to combined degradation actions as it is in the field, in order to obtain more reliable laboratory results and more durable bridges in the future.

Acknowledgments

The author would like to thank the collaborators and technicians of the institute of materials and construction for the field interactions, sampling, and testing of the materials.

Conflict of interest

The author declares no conflict of interest.