Study On High Performance Concrete In Salt Lake: Durability, Mechanism And Service Life Prediction

Thousands of salt lakes are distributed in the four salt lake regions in Northwest of China, among which there are 102 belong to Xinjiang salt lake region, 33 to Qinghai salt lake region, 370 to Neimenggu salt lake region, and 220 to Xizang salt lake region. Corrosion resistance of concrete exposed to salt lakes is of special importance because of the extremely harsh weather in salt lake regions. Aiming at the desiccate environment of salt lake regions, in which the relative humidity ranges within 30%～50%, High Performance Concrete (HPC) accommodated to the circumstance were proportioned on bases of field investigations, incorporated with proportion principals of HPC, cracking prevention effects of fibers and shrinkage compensation functions of expansion agent. C30 ordinary Portland cement concrete (OPC), C25 air-entraining concrete (APC), C70 high strength concrete (HSC) without active additions, C70 HPC with the composites of silica fume, fly ash and slag additions, steel fiber reinforced HPC (SFRHPC), and polyethylene fiber (PF) with high Young's modulus reinforced HPC (PFRHPC), were prepared. For concretes exposed to salt lakes, chloride absorption or binding isotherm, deterioration process and features of concretes subjected to the actions of single, double or multiple factors were investigated. Degradation mechanism of OPC, APC and HSC and the high durable mechanism of HPC were also explored. A service life predicting method based on damage development of concrete structure was proposed, and the service life predicting method based on chloride diffusion was modified and improved. Basic parameters in both models were determined based on a large number of field and laboratorial experiments. Influential factors and the principles on life expectancy were analyzed in specialty. Service life of concretes exposed to four kinds of salt lakes was compared in this paper, and the HPC has been applied to a key project in Qinghai salt lake region. Durability design of concrete structures constructed in salt lake regions was discussed. Results show that high strength HPC without air entraining is of excellent bittern-freezing-thawing durability and perfect bittern attack resistance, result in a long life expectancy. Both of the proposed predicting methods can be generally applied.Main contents of each chapter are mentioned as follows:In Chapter 1, durability of concretes exposed to salt lakes and service life predicting methods were summarized. Problems remaining to be investigated were also proposed, and outline of this contribution was sketched.In Chapter 2, environment and climate in salt lake regions were briefly introduced. Bitterns were reclassified according to concentrations of corrosive ions. Field investigations on reinforced concrete structures in Qinghai salt lake region discovered that corrosions of concrete and steel reinforced concrete were particularly serious and universal.In chapter 3, physical and mechanical properties of HPC exposed to salt lake regions were studied. The hydration products, microstructures and pore structure were investigated by means of XRD, DTA-TG, SEM-EDAX, IR, and MIP. Results show that hydration products of HPC are mainly compact CSH gels with a C/S ratio of 0.97, AFt crystals, and some hexagonal AFm plates, which are formed in the later hydration phase as results of pozzolanic reactions, and the pore structures are mainly gel pores. Essential transformation of microstructures in fiber reinforced HPC exceeding a age of 365 days were observed, namely a kind of highly compact plate-massive CSH gel, so called"ceramic structure similative", formed, which brings favorable effect in concrete durability.In chapter 4, a loading system for 100mm×100mm×400mm prism samples was designed. Damage development and the degradation features of concrete subjected to single or combined action of salt attack, freezing-thawing cycles, wetting-drying cycles, and flexural stresses were investigated. Effect of desiccation on damage development was also discussed. Results show that when exposed to the severe environment of salt lakes, both OPC and APC display poor durability, though HSC shows excellent bittern-freezing-thawing durability, the corrosion resistance against bitterns is not so good, while non-air-entraining HPC shows outstanding performance in both bittern-freezing-thawing durability and corrosion resistance against bitterns, and the performance can be farther improved by adding fibers.In chapter 5, corrosion products and microstructures of concretes were investigated by using XRD, DTA-TG, IR, and SEM-EDAX. Based on damage development process, degradation mechanism of OPC, APC, and HSC subjected to the actions of single, double, or multiple factors were discussed, the third damage mechanism of concrete, viz. salt crystallization induced pressure, was presented. Corrosion mechanism of concrete exposed to bitterns containing Mg2+-Ca2+-Cl--SO42- was extended to bitterns containing Mg2+-Cl--SO42--CO32--HCO3-. Results show that freezing-thawing caused crackles in non-air-entraining HPC subjected to freezing-thawing cycling are resulted from pressure occurred as AFm transforms to AFt. When subjected to combined actions of freezing-thawing cycling and bittern attack, OPC shows poor bittern-freezing-thawing durability, due to pressure resulted from crystallization of Na2SO4·10H2O. Effects of bittern on concretes undergoing bittern-freezing-thawing cycling comprise both positive ones, including lowering ice point and abating freezing-thawing damage, and negative ones, such as accelerating salt crystallization and inducing concrete expansion cracks. APC exhibits poor bittern-freezing-thawing durability in bittern from Xizang salt lakes, and collapses while undergoing freeing-thawing cycles in Neimenggu bittern. When subjected to single action of bittern attack or combined actions of bittern attack and wetting-drying cycles, damage of OPC, APC, and HSC are mainly due to chemical corrosions, resulting in several attack products, and also due to crystallization of NaCl-KCl. Two attack products were firstly reported, viz. spherical crystals of (C1-xMx)0.94(S1-yAy)H (x=0.4,y=0.13) and CaCO3·CaSiO3·CaSO4·15H2O.In chapter 6, corrosion products, microstructures and pore structures of HSC-HPC were analysised by XRD, DTA-TG, IR, SEM-EDAX, and MIP methods. Long durability mechanism of high strength non-air-entrained HPC exposed to salt lakes under the actions of single, double or multiple factors, was investigated, and a corrosion induced strengthening mechanism of HPC was proposed. The experimental results show that the excellent bittern-freezing-thawing durability of high strength non-air-entrained HPC benefits from the isolated tiny crackles resulted from wetting swelling or autogenous shrinkage, the pore structures containing transitional pores and gel pores. So the interfacial zone was strengthened, and gel was compacted. When HPC and fiber reinforced HPC are exposed to salt lakes, 4 advantageous effects, viz. mild corrosion of hydration products, transformation of CSH gel subjected to bittern attack, corrosion induced hydration of the remaining active additives, and close of tiny crackles, will occur.In chapter 7, chloride absorption or binding isotherm of concretes was investigated, linear chloride binding capacity and a non-linear coefficient were firstly proposed, based on a large amount of experimental data of concretes under the actions of single, double, or multiple factors in four typical bitterns. Experimental results show that linear absorption of chloride is dominating while the free chloride concentration is in a low range. However, non-linear Langmuir isotherm shows itself when the free chloride concentration is in a high range. Linear and non-linear coefficients of chloride binding capacity were determined from a large number of experimental data. Chloride absorption or binding isotherm of concretes were discussed based on physical absorption or chemical binding respectively, among which the chemical binding includes mainly chemical reactions of AFt-AFm and CH reacting with NaCl, KCl, CaCl2, or MgCl2, resulting in Friedels salt and complex salt with MgCl2, and the physical absorption comprises of absorption of chloride ions by CSH gel surfaces and pore surfaces in cement pastes.In chapter 8, damage and degradation of concretes subjected to single factor of bittern attack, or to double factors of combined bittern attack and freezing-thawing cycles were investigated. A universal damage developing equation, which including initial damage velocity and damage acceleration was deduced. Results show that when concrete subjected to corrosion or freeing-thawing related durability factors, the damage initials at a certain initial damage velocity and then accelerates at damage acceleration. Both the initial damage velocity and damage acceleration are determined by the environment, climate and the loading state. Based on the damage developing equation, a service life predicting method was established. This equation had been applied to concrete structures of a fertilize plant located in Qinghai salt lake, subway of Nanjing city, and the Runyang Bridge across Yangtse River. It also provided a method for service life predicting of key concrete projects constructed in environments without chloride attack.In chapter 9, the 8 problems remained to be resolved on chloride diffusion of concretes was discussed, and theoretical modifications on Fick's second law was proposed. A diffusion equation, which took account of effects of concrete chloride binding capacity, time dependence on diffusion coefficients, defects in microstructures and loading, was proposed. For both linear and non-linear chloride binding, analytic solutions for one, two, or three-dimensional diffusion in finite or infinite space with different boundary conditions either independent or dependent on time change, were deduced. A theoretical model of chloride diffusion was also presented. Parameters of the model could be determined by certain procedures, and the key parameters were evaluated, a basic database was established also. Effects of theoretical conditions and experimental factors on concrete service life were analyzed. Durability design of HPC exposed to salt lake regions was carried out, and determination of design parameters was discussed. Results show that concrete structures constructed by non air-entrained HPC, with sufficient strength and reasonable protective covers, could attain a service life of 100 years.A summarization of this contribution was presented in chapter 10. Propositions for further researches and schemes for ChinaDuraCrete were also listed.