DEGRADATION OF REINFORCED CONCRETE STRUCTURES IN AMMONIUM NITRATE ENVIRONMENTS

Transcription

1 DEGRADATION OF REINFORCED CONCRETE STRUCTURES IN AMMONIUM NITRATE ENVIRONMENTS I. Pepenar Research Centre-CERTINCON, Bucharest, Romania Abstract The paper presents the results of the researches on the service behaviour of reinforced concrete structures subjected to the action of ammonium nitrate environments and the mechanisms of the corrosion processes occurring in the concrete under the action of such specific environments. The researches were performed in situ, on reinforced concrete structures in aggressive environments with ammonium nitrate obtained from the technological process of chemical fertilizers, as well as in laboratory, on concrete samples, extracted from some elements damaged by corrosion. There were also performed researches on cement paste and concrete specimens (reinforced and prestressed concrete specimens) exposed to the action of ammonium nitrate solutions of various concentrations. The conclusions underlined the complexity of the corrosion process on reinforced concrete structures in ammonium nitrate environments, also affecting concrete and steel reinforcement. The considerations on the mechanisms of the corrosion processes of concrete under the action of ammonium nitrate environments are also described. 1. INTRODUCTION The action of ammonium nitrate-based aggressive environments, encounted mainly in the industry of chemical fertilizers, has as an effect various damages upon reinforced concrete structures, regarding the nature and the intensity of damages, and also the mechanism of corrosion processes at construction materials (concrete, reinforcement) [1]. Compared with other aggressive environments, the environments from the industry of chemical fertilizers based on nitrates, present a specific and complex aggressiveness upon reinforced concrete structures, affecting in the same time the concrete and its steel reinforcement. Due to their complexity, the corrosion processes of concrete and its reinforcement under the action of ammonium nitrate-based aggressive environments were little studied. In the specialized literature there are presented data and information, some of them contradictory, regarding especially the concrete and steel corrosion in ammonium nitrate solutions. On the other hand, there is little information regarding the service behaviour of reinforced concrete 442

2 structures in ammonium nitrate-based aggressive environments and the mechanisms of the processes of corrosion of the concrete and its reinforcement in such aggressive environments. The paper presents the results of the researches on the service behaviour of reinforced concrete structures subjected to the action of ammonium nitrate environments and the mechanisms of the corrosion processes occurring in the concrete under the action of such specific environments. 2. SERVICE BEHAVIOUR OF REINFORCED CONCRETE STRUCTURES 2.1 Structures There were investigated two representative types of reinforced concrete structures: - industrial buildings of fabrication, multi-storied, with the bearing structure composed of reinforced concrete frames (columns and beams) and intermediate technological floors disposed on several levels, within their structure occurs the fabrication process of raw materials and of finite products (ammonium nitrate, calcium ammonium nitrate and complex fertilizers NP/NPK); - granulation towers, with tubular, cylindrical reinforced concrete structure and technological floors disposed on several levels, within their structure occurs the granulation of ammonium nitrate, calcium ammonium nitrate and complex fertilizers NP/NPK. 2.2 Aggressive agents The reinforced concrete elements were subjected to a long term action of a strongly aggressive environment, made from various aggressive agents, gaseous, liquid or solid, used or resulted from the technological fabrication installations. The main aggressive agents which affect the elements are, on the one hand, ammonium and calcium nitrates and the compounds with nitrates (NP, NPK), and on the other hand, acid agents (nitric acid, phosphoric acid), in the shape of solutions, melts and pastes (powders + humidity), of various concentrates and temperatures. 2.3 Damages by corrosion The analysis of damage state of reinforced concrete structures, after years of service, based on specific investigation techniques [2], had pointed out severe damages sometimes, generated by the strongly corrosive action of the ammonium nitrate, which are concise presented as it follows. The reinforced concrete columns present visible damages by corrosion (figure 1), which consist mainly in the following: - concrete cracking, along the longitudinal reinforcements, more accentuated in the marginal area, due to corrosion by expansion of the concrete under the action of nitrates, in a further phase, a delamination and spalling of the concrete cover occurred and afterwards the depth corrosion of the concrete; - fragile fractures, without reduction of cross-section (stricture), of the steel stirrups, in some areas, caused by stress corrosion cracking; - superficial corrosion of the longitudinal reinforcements, without a visible sign of their fractures. 443

3 Figure 1: Columns: cracking, delamination and spalling of the concrete cover, due to corrosion by expansion; fragile fractures of the steel stirrups by stress corrosion cracking. The reinforced concrete beams present severe local damages by corrosion (figure 2), which leaded in the end to a significant diminution or loss of bearing capacity of some elements (figure 3). Mainly, these damages consist in: - concrete cracking by corrosion under the action of nitrates, especially at the bottom of the beams, followed by delamination and spalling of the concrete cover and by uncovering of the reinforcements; damage by corrosion of concrete in the depth, leading to the reduction of the elements cross-section; Figure 2: Beams: destruction by corrosion of concrete cover; fragile fractures of the longitudinal and transversal (stirrups) reinforcements by stress corrosion cracking. - fragile fractures, without stricture, of a longitudinal and transversal (stirrups) reinforcements, in some areas, caused by stress cracking corrosion induced by the nitrates; there were also observed local and general corrosion phenomena of the steel; - failures of the beams; 444

4 Figure 3: Failure of the beam. The reinforced concrete slabs present severe local damages by corrosion (figure 4), mainly in the perforated areas and in the areas where the anticorrosive floor was damaged. The damages are similar to the ones described at the beams and they leaded to an important reduction and loss of bearing capacity, in certain areas. The reinforced concrete walls of the granulation towers present advanced damages by corrosion (figure 5), visible at the exterior of the towers, in areas disposed on all their heights, in a shape of swellings, cracks, delaminations and local spallings of concrete, with uncovering of reinforcements and fragile fractures of the steel reinforcements by stress corrosion cracking. Taking under consideration the advanced damage state of some structural elements, which affects the resistance, the stability and the durability of the investigated structures, there were proposed intervention measures on them, regarding the repair, strengthening or replacement of the damaged elements. Figure 4: Slab: damages by corrosion of concrete and reinforcements. Figure 5: Wall: destruction by corrosion of concrete cover; fragile fractures of the longitudinal and transversal (stirrups) steel reinforcements by stress corrosion cracking. 3. LABORATORY RESEARCHES ON CONCRETE SAMPLES The results of the laboratory researches performed on a large number of concrete samples extracted from the corrosion affected elements and cement paste and concrete specimens (reinforced and prestressed concrete specimens) exposed to the action of the ammonium nitrate solutions of different concentrations, showed the following: 445

5 The compressive strength resistance to compression of apparently undamaged concrete varies between 16.3 and 29.5 MPa, while the compressive strength of damaged by corrosion concrete is sensitively diminished to zero. This significant difference explains itself through the fact that in the first phase of the corrosion process, the mechanical characteristics of the concrete don t change sensitively, sometimes even increasing temporarily, while in the second phase, the mechanical characteristics gradually reduce, as an effect of corrosion by expansion, until complete destruction of concrete. The concentration of aggressive ions NO 3 - and NH 4 + in the concrete damaged samples varies, depending on the type of the element and of the depth of extraction, between % NO 3 - and respectively between % NH 4 + (by weight of concrete), values to whom they present a strong aggressive effect, both to the concrete and to its steel reinforcement. The ph of aqueous suspension of the concrete damaged samples varies between , values that show the desalkalinization of the concrete subjected to the aggressive environment, respectively the loss of the concrete capacity to assure protection to the reinforcement by passivation phenomenon, fact that confirms the results of the tests performed in situ. The analysis by X-ray diffraction of the cement paste/concrete damaged by corrosion specimens/samples pointed out important changes, structural and compositional, produced in the cement stone under the action of the ammonium nitrate. As shown in figure 6, besides the peaks corresponding to the values of 3.04, 2.50, 2.29, 2.10, 1.91 and 1.87 Å, characteristic of unattacked cement paste compounds, appear new peaks at values of 8.64, 4.30 and 1.60 Å, characteristic of new crystalline compounds based on calcium nitrates-aluminates hydrated, resulted by attack of ammonium nitrate solution. These new compounds, as 3CaO Al 2 O 3 Ca(NO 3 ) 2 10H 2 O, are responsible for damage by expansion (increasing volume) of the concrete. a. b. Figure 6: X-ray diffraction patterns of cement paste hydrated: a) unattacked cement paste; b) cement paste attacked by 5 % ammonium nitrate solution, after 6 months exposure. 4. CORROSION MECHANISM OF CONCRETE The corrosion action of ammonium nitrate-based aggressive environments on concrete consists in two successive stages, characterized by: 446

6 - decalcification phenomena, resulted by ammonium nitrate reaction with calcium hydroxide in the cement stone, with formation of calcium nitrate hydrated and emission of gaseous ammonia which leave the system, having as effect the desalkalinization of concrete: 2 NH 4 NO 3 + Ca(OH) H 2 O Ca(NO 3 ) 2 4 H 2 O + 2 NH 3 ; - expansion phenomena, resulted by hydrated calcium nitrate reaction with calcium hydroaluminate in the cement stone, with formation of some new complex crystalline compounds and with important increasing volume: Ca(NO 3 ) 2 4 H 2 O + 3CaO Al 2 O 3 6 H 2 O 3CaO Al 2 O 3 Ca(NO 3 ) 2 10H 2 O. These decalcification and expansion phenomena lead to desalkalinization, cracking (figure 7.a) and finally to a rapid concrete destruction by corrosion (figure 7.b). (a) (b) Figure 7: Reinforced and prestressed concrete specimens: Concrete cracking due to corrosion by expansion (a); concrete destruction by corrosion (b). 5. CONCLUSIONS The analysis on the service behaviour of reinforced concrete structures, after years of service in ammonium nitrate industrial aggressive environments, has pointed out the existence of a severe corrosion induced damages, which affect the resistance, stability and durability of structures in such aggressive environments. The aggressiveness of ammonium nitrate environments against reinforced concrete elements/structures acts in a complex and specific manner, both upon the concrete and its steel reinforcement, by different mechanisms. The corrosion action on concrete consists in some decalcification phenomena and some expansion phenomena (similar to sulphate corrosion). These corrosion phenomena lead to a cracking and, finally, to a rapid concrete destruction. Non-controlled reduction of physico-mechanical characteristics of concrete by corrosion, associated with the possibility of apparition and development of stress corrosion cracking phenomenon of reinforcements, lead in the end to significant diminution or loss of bearing capacity and sudden break (collapse) of reinforced concrete. In the case of existing structures, along with the intervention measures aiming the rehabilitation of elements damaged by corrosion (repair, strengthening, replacement, anticorrosive protection), there are suggested actions aiming to lower the aggressiveness of 447

7 the environment inside the industrial buildings and measures for maintenance that were recommended as a result of the monitoring of the reinforced concrete structures. REFERENCES [1] Pepenar, I., 'Degradarea constructiilor datorita coroziunii, care pericliteaza siguranta si stabilitatea lor in exploatare', in 'Comportarea in situ a constructiilor', Materialele Conferintei Nationale, Baile Felix, Romania, [2] Pepenar, I., 'Investigation and diagnosis of concrete structures in aggressive environments', in 'Durability of Building Materials and Components 8', Proceedings of an International Conference, Ottawa, Canada,