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Durability of precast concrete tunnel segments
Published in Daniele Peila, Giulia Viggiani, Tarcisio Celestino, Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art, 2019
Acid attack is a chemical attack that can be a major durability issue when concrete structure is exposed to high concentrations of aggressive acids with high degrees of dissociation. The deterioration of concrete by acids is primarily the result of decomposition of the hydration products of the cementitious paste (ACI 201.2R 2016). Sulfuric and hydrochloric nitric acids are main inorganic (mineral) acids, and acetic, formic and lactic acid are main organic acids with rapid rate of attack on concrete at ambient temperature. Acids reduce the pH or alkalinity of the concrete, and once the pH reduces to less than 5.5 to 4.5, severe damages are imminent as cement hydration products such as Portlandite (CH, Ca(OH)2) and C-S-H starts to decompose when pH drops to around 12 and 10, respectively (ACI 201.2R 2016). This is the main reason that no concrete materials have a good resistance to acids.
Optimization and Performance of Nanomaterials in Cement Concrete
Published in V. R. Remya, H. Akhina, Oluwatobi Samuel Oluwafemi, Nandakumar Kalarikkal, Sabu Thomas, Nanostructured Smart Materials, 2022
Mainak Ghosal, Arun Kumar Chakraborty
Although mostly the test procedure itself is similar, differences—for instance, in the specimen size, the attack cycle (length of submersion and drying intervals), the way of maintaining the pH within certain limits, the chosen measure for deterioration, the fact whether loose material is detached or not before measuring, etc., may result in different or even contradictory results. No codes and standards are available which holistically addresses the testing for acid attack. Hence, the development of a sound test method is the pressing demand of today. There is also a need for standardization and automation of accelerated test procedures [29]. Also critically reviewed various test methods to evaluate the performance of cementitious materials in aggressive aqueous environments. According to the author, the choice of a particular test method will have a major influence on the usefulness of the test results. The parameters such as scale of the test method, physical state of the attacking medium, pH, and concentration of the solution, temperature, rate of replenishment, mechanical action, alternate wet and dry cycles, pressure, etc., must be given utmost importance while selecting a test method. The author stressed on the need for using a combination of multiple relevant indicators to arrive at conclusions [29]. Suggested the use of various degradation measures for assessing acid attack which include thickness change, mass loss, length or volume change, residual strength, pH change of liquid, calcium ions released in liquid, hydrogen ion consumption, loss of elastic modulus, etc. The parameters such as mass change, thickness change and strength changes are subjected to large variability on smaller sized specimens when accelerated testing has been carried out. Hence, additional indicators correlated with the microstructural deterioration should be used. An attempt has been made to evaluate the dynamic modulus of elasticity of cylindrical mortar samples exposed to acid solutions. The ultrasonic pulse velocity (UPV) test is carried out, and dynamic elasticity modulus is estimated based on the UPV and the bulk density of the specimens in saturated condition. The loss in dynamic modulus of elasticity is attributed to the weakening of micro-structure due to the decalcification. These results could be supplemented with scanning electron microscopy (SEM) or x-ray diffraction (XRD) analysis to examine the microstructure. Additionally, x-ray fluorescence (XRF) can be done on samples to determine the changes in oxide composition of the degraded layers.
Bioconcrete based on sulfate-reducing bacteria granules: cultivation, mechanical properties, and self-healing performance
Published in Journal of Sustainable Cement-Based Materials, 2023
Kirthi Chetty, Ulf Garbe, Zhiyang Wang, Shuxin Zhang, Timothy McCarthy, Faisal Hai, Guangming Jiang
The mass loss of the samples because of 5% sulfuric acid attack is given in Figure 3. The samples sprayed using real wastewater were employed for this acid immersion test. The mass loss after 120 d in control was 7.5% and the 1% and 2% healed samples showed 3.8% and 3.2% weight loss, respectively. The 1% and 2% fresh bioconcrete samples had a mass loss of 5.9% and 5.4%, respectively. It can be observed that the control underwent mass loss rapidly during the 4 months time and the rate of mass loss doubled for every month. For the 1% and 2% healed bioconcrete samples showed identical amounts of mass loss until 90 d and after 120 d the mass loss in 2% healed samples reduced by 15%. The 1% and 2% fresh bioconcrete samples showed similar mass loss rate as that of control for the first 3 months. The control lost nearly 2.7% and 4.2% mass during the 60- and 90-d analysis, respectively. During the same period, the fresh 1% bioconcrete sample had 2.2% and 3.8% mass loss respectively and 2% samples had 2.1% and 3.4% mass loss, respectively. Later during the 4 months, the mass loss rate in control was 8.8% and the 1% and 2% fresh bioconcrete samples showed 5.9% and 5.4% mass loss, respectively. The bioconcrete samples irrespective of the calcite deposition on the surface showed similar mass loss independent of the percentage of SRB granules in the bioconcrete. To conclude, the mass loss in the control was more followed by the fresh and healed bioconcrete samples.
Effects of sulfuric acid attack on hydrated calcined clay–limestone cement mortars
Published in Journal of Sustainable Cement-Based Materials, 2021
The following conclusions were made based on the study: LC3 mortars exhibited higher compressive strength than OPC at 90 days of curing. This shows that the compressive strength of LC3 mortars increased with curing as a result of increased pozzolanic activity of FRCB.Water absorptivity was found to increase with increase w/c. This implied that additional water increases the permeability of the mortars.LC3 and PPC exhibited lower water absorptivity than OPC. This indicated that blended cements are less porous compared to OPC. They are hence less prone to degradation due to the penetration of aggressive media.LC3 and PPC exhibited the higher resistance to sulfuric acid attack than OPC. This demonstrated that they are potential materials for more durable structures.In terms of strength loss, at w/c = 0.50 and w/c = 0.60, LC3 exhibited 12.71% and 14.06%, respectively, higher resistance to sulfuric acid attack than neat OPC.Decrease in compressive strength as a result of sulfuric acid attack on the cement hydration products.
Strength properties of nano-MgO and cement stabilized coastal silty clay subjected to sulfuric acid attack
Published in Marine Georesources & Geotechnology, 2020
Wei Wang, Yuan Li, Kai Yao, Na Li, Aizhao Zhou, Chen Zhang
After curing, samples of cement-admixed clay with nano-MgO content of 10‰ were submerged inside 0.06 mol/L sulfuric acid solution, for 7, 14, 21, 28 days, respectively, to investigate the effect of the sulfuric acid exposure period. The relevant shear stress displacement curves are shown in Figure 10. It can be deduced from Figure 11(a) that the shear strength of the nano-MgO cement-admixed clay increased continuously during the soaking period even with the sulfuric acid environment. This may be attributable to the continuous hydration and pozzolanic reaction of the cementitious materials. During the first 14 days of immersion in the sulfuric acid solution, the shear strength of the mixture showed a significant increase. However, for longer soaking periods, such as 21 days, the strength did not change a great deal more. This may due to near-completion of the cementation in the earlier soaking period, while the negative effects of sulfuric erosion tend to be more obvious. For some normal stresses, such as 100 and 400 kPa, the shear strength showed a slight decrease after soaking in sulfuric acid solution for 28 days. This reflected the influence of sulfuric acid attack, which could have a significant long-term deleterious effect. Similar trends were observed for the normalized shear strength curves, as shown in Figure 11(b). In terms of the shear strength parameters, Figure 12 indicates that both the cohesion value and the friction angle of the mixture increased up to an exposure period of 21 days, beyond which a slight decrease was observed.