Aline Bensi Domingues1, Pablo Augusto Krahl2, Mounir Khalil El Debs¹
1: São Carlos School of Engineering, Department of Structural Engineering, University of São Paulo, Brazil 2: State University of Campinas, São Paulo, Brazil
1 Introduction
Concrete structures built in aggressive environments like sea water, sewage or industrial effluents are typically susceptible to attack by inorganic acids. In this context, the use of ultra high performance concrete (UHPC) can increase the structure’s lifetime due to lower porosity compared to conventional concretes due to particle packing. Silica fume and nano silica also influence on performance of cementitious materials under acid action [1-2]. So, due to the lower susceptibility of UHPC, new investigations are required regarding aggressive classes and ambients [3]. Therefore, this paper presents experimental finds on degradation processes of UHPC subjected to acid attack.
2 Experimental program
The UHPC utilized in the present research was developed in Krahl [4]. For characterization, cylindrical specimens (50x100 mm) were molded. After 28 days under moist chamber, six specimens were tested, which resulted in compression strength of 110 MPa. Other six speciments were immersed in hydrochloric acid solution (concentration 3%) and six in sulfuric acid solution (3% concentration) for 90 days. Figure 1 shows the specimens after removal from acid solutions. After drying, the speciments were tested under quasi-static strain rate and analyzed by scanning electron microscopy image (SEM image).
(a) (b)
Figure 1: Laboratory-accelerated degradation tests after 90 days (a) in hydrochloride acid solution (b) in sulfuric acid solution
3 Test results and discussions
The results of the compressive behavior are shown in Fig. 2. The specimens exposed to hydrochloric acid (HCl) presented a small reduction in modulus of elasticity of 6.4% compared to the reference samples. Also, the preapeak nonlinear behavior of the attacked samples was more pronouciated, which resulted in a greates peak strain, as shown in Figure 2(a). The color change in the external layer of the specimens indicates that chemical compounds were precipitated. The attack by chloride ions (Cl-) on harneded cement past results in formation of calcium chloride, which can be easily leached by water. However, analysis of pH variation and microscopy showed that chloride ions attacked only the outer layer. The peak strenght had insignificant variation.
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In contrast, the samples subjected to sulfuric acid (H2SO4) had the strength reduced by 38.6% relative to the referece samples, and a more ductile behavior. This occurs due to reaction between ions sulfate (SO4-2) and the resistant compounds of concrete, such as calcium hydroxide (portlandite), hydrated calcium silicate (C-S-H) and monosulfate hydrate (AFm) [5].
Figure 2(b) shows that specimens after sulfuric acid exposure had hard degradation with formation of expansive compounds and decrease in cross section.
0.0000 -0.002 -0.004 -0.006 -0.008 20
40 60 80 100 120
Compression stress (MPa)
Strain
H2SO4 HCl 28 days
(a) (b)
Figure 2: (a) Stress-strain curves (b) SEM image of gypsum crystals deposited in the corroded layer somewhere close to the unaffected section of the paste specimen after 90 days of exposure to sulfuric acid.
4 Conclusions
From the current research, it was found that the specimens immersed in aqueous solution with 3 % H2SO4 concentration had a reduction of 38.6 % in compressive strength. Such effect can be explained by the reduction in silica content of the cement paste in the outer layer of the specimens.
The samples immersed in water with 3 % of HCl had an insignificant reduction in strength.
The major effect was the decrease in calcium content in a very small outer layer of the specimen with the effect of a slightly more nonlinear prepeak behavior.
References
[1] Poon, C. S.; Kou, S. C.; Lam, L.: Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete. Construction and Building Materials. 20.p 858-865. 2006.
[2] Mahdikhani, M.; Bamshad, O.; Shirvani, M. F.: Mechanical properties and durability of concrete specimens containing nano silica in sulfuric acid rain condition, Construction and Building Materials.167. p 929-935. 2018.
[3] An, M. Z.; Wang, Y., Yu, Z. R.; Ji, W. Y.; Han, S.: Durability of uhpc under complex environments.
RILEM Proceedings. 1st International conference on uhpc materials and structures. V 105. P 380-394. Changsha. 2016.
[4] Krahl, P. A.: Lateral stability of ultra-high performance fiber-reinforced concrete beams with emphasis in transitory phases. Tesis (Doutorado em Estruturas) - São Carlos School of Engineering, University of São Paulo. São Carlos. 2018.
[5] Mehta, P.K.; Monteiro, P. J. M.: Concrete: Microstructure, Properties and Materials. 4. Ed. McGraw Hill Professional, 2013.
Session B8: Durability II
149
Investigation on the resistance of UHPFRC-RC composite beams to chloride ingress under mechanical loading
Toni Pollner, Christoph Dauberschmidt, Andrea Kustermann
Institute for Material and Building Research, Munich University of Applied Science (MUAS), Germany
1 Introduction
Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) has exceptional mechanical properties and a binder matrix that is virtually capillary pore-free. It is therefore particularly suitable for the repair and strengthening of reinforced and pre-stressed concrete structures that are exposed to both chlorides and high mechanical loads. Due to the high strength, RC-structures can be strengthened with very thin layers of UHPFRC in regard to their load-bearing capacity and serviceability. In combination with additional reinforcement, this effect is further enhanced. Furthermore, UHPFRC can serve as a protective layer against aggressive media such as chloride ions.
As in reinforced concrete, the question of the durability in the areas of cracks (or stress induced micro-cracks) in the UHPFRC layer also arises. Aggressive media can penetrate into the cracked concrete (or UHPFRC) much faster than into the non-cracked concrete. Many investigations show that the penetration resistance to aggressive media decreases with increasing crack width, for example [1]. Even in the non-cracked state, however, there is a possibility that high mechanical stresses may have a negative effect on the penetration resistance [2]. This circumstance has not yet been taken into account in durability design.
Therefore, investigations have been carried out at the Institute for Material and Building Research at the Munich University of Applied Science (MUAS), to determine the material properties of the UHPFRC used and the durability of UHPFRC-RC composite beams with simultaneous flexural tension and chloride exposure. The presentation focuses on the combined loading tests.
2 Materials and methods
A UHPFRC ready-mix was used as repair and strengthening material. The mixture consists of a premix with a maximum grain size of 0.6 mm, a superplasticizer and 2.11 Vol.-% steel wire fibres with a length of 14 mm and a diameter of 0.2 mm. The material has a compressive strength of 187 MPa after 28 days of water storage. The tensile strength has been determined with bending tests according to SIA 2052 and amounts to 13.1 MPa. There was no strain hardening effect assessed under axial tensile stress.
For the experimental investigations on combined loading, roughened and notched reinforced concrete RC-beams were produced in advance, loaded to initiate a crack width of 0.2 mm in four-point flexural tensile tests and then strengthened with a 30 mm thick UHPFRC layer. The substrate concrete was prepared by exposing the coarse aggregate with a high-pressure cleaner shortly before the concrete solidified. After sufficient hardening of the UHPFRC, the composite beams were subjected to a combined load of simultaneous bending tensile stress and chloride exposure with a 10 % NaCl solution. During the test procedure, the electrolyte resistance Z of the composite beam was recorded using Electrochemical Impedance Spectroscopy (EIS) between a titanium strip anode in the chloride container and the electric conductively connected reinforcement. The electrolyte resistance provides information on the charge transport in the pore solution of the concrete and can be used to estimate the chloride penetration resistance. The test setup is shown in Figure 1.
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Subsequently, two samples were taken from the crack area of each composite beam and prepared for the following investigations. Half of the samples were examined at HafenCity University Hamburg (HCU) for their chloride distribution using Laser Induced Breakdown Spectroscopy (LIBS). The procedure is described in [1].
The other half of the samples was prepared according to [3] and examined under a digital reflected light microscope for crack formation.
3 Results and Discussion
The results show a decrease of the electrolytic resistance both under continuous static and cyclic mechanical stress. Depending on the type, level and duration of the load, the electrolyte resistance dropped by up to 73 % of the initial value. It is assumed that the loss of resistance is caused by microcracks in the UHPFRC layer, through which chloride ions dissolved in water can penetrate into the concrete. As a result, the conductivity of the electrolyte increased and the electrolytic resistance decreased accordingly.
The investigations on chloride distribution in the crack area revealed a high chloride content of up to 2 wt.% by cement directly in the crack area of the UHPFRC as well as in the substrate concrete and thus confirms the results of the electrolytic resistance measurements.
With the help of digital reflected light microscopy, it could also be shown that several micro cracks with remaining crack widths of about 30 µm were formed in the UHPFRC by applying increasing load and that the wide crack in the reinforced concrete was thus divided into several small cracks within the UHPFRC.
4 Conclusions and Outlook
As the investigation results show, the penetration of chloride ions under simultaneous mechanical stress and the resulting crack formation could not be prevented by the thin protective layer of UHPFRC. It can be stated, however, that the large crack widths or the crack width changes in the RC-beams are divided up into several small, presumably less harmful cracks in UHPFRC, even if the material is not strain-hardening.
The future investigation in the framework of a new research project on retrofitting with sprayed UHPFRC (i-SCUP) at MUAS will focus on the question: which modification of material, load and specimen layout can reduce the chloride ingress to a harmless value? The aim is to determine critical crack width, below which a chloride ingress can be significantly reduced.
Furthermore, the investigations will be carried out with UHPFRC with strain hardening effect.
References
[1] Šavija, B., Schlangen, E., Pacheco, et al.: Chloride ingress in cracked concrete: a laser induced breakdown spectroscopy (LIBS) study. Journal of Advanced Concrete Technology 12 (2014), S.
425–442.
[2] Yao, Y., Wang, L., Wittmann, F. H., et al.: Test methods to determine durability of concrete under combined environmental actions and mechanical load: final report of RILEM TC 246-TDC. Materials and Structures 50 (2017), S. 105.
[3] Kustermann, A.: Einflüsse auf die Bildung von Mikrorissen im Betongefüge. Dissertation. München 2005.
Figure 1: Experimental set-up for combined loading (dimensions in millimetres)
Poster Presentations
151