Production and Documentation of Frost Durable High-Volume Fly Ash Concrete: air entrainment, cracking and scaling in performance testing

Abstract

Concrete with moderate replacement levels of fly ash (FA) has been used for decades and considered sustainable in harsh environments. If the replacement levels become high (FA/C > 40…50%), the range of properties from fresh to hardened, including performance in frost testing give often unfavorable results. To reduce the environmental impact of the cement industry, solutions for sustainable frostresistant concrete with high FA replacements should be developed. Designing such concrete is possible if the exposure is properly characterized, requirements to part materials identified, the guideline for work execution understood and test program agreed upon. Apart from that, the reliable production of frostresistant concrete with FA should be established and controlled. Eventually, frost-testing results of the concrete should be satisfactory and reproducible for the clients. All these aspects of a “life cycle” of concrete were studied in the present Ph.D. project. The study was aimed at: (1) reviewing international requirements and recommendations for frost durable concrete from design to execution and testing; (2) developing robust admixture-binder combination for high-volume FA concrete suitable for both onshore and offshore arctic exposure; (3) understand how freeze-thaw performance testing affects high-volume FA concrete. A series of wet freeze-thaw performance tests in presence of freshwater and 3%NaCl were done on high-volume FA concrete. The aim was to investigate the effect of w/b-ratio, air entrainment, extremely low temperature, curing duration, and FA on resistance to the surface and internal damage and understand how surface, internal damage, and liquid transport interrelate for FA-concrete in such tests. Several pilot studies were also carried out to support the main investigations. Requirements and recommendations for frost durable concrete from standards and specifications in Europe, North-America and Asia, various international organizations and construction projects were reviewed, compared and discussed. This was done based on exposure, material, execution, and tests. Also, some practical examples of the specification together with examples of need of stringency and some occurring peculiarities in testing are given. Finally, the large variation in how frost durability is perceived in different parties of the decision, planning, execution, and commissioning process around the world are discussed and illustrated. Development of a robust admixture-binder combination resulted in a study on the effect of a sequence of addition for air-entraining (AEA) and super plasticizing (SP) admixtures on air entrainment in high-volume FA concrete (≈45%FA⁺C)). The addition of SP before AEA was found to be the most favorable admixture combination for air entrainment in FA concrete, unlike that for OPC (where AEA is added first). Also, Foam Index (common method for evaluation AEA-binder systems) measurements on the same binder materials, admixtures, and dosage sequences were found less useful for studying the effect of admixture combinations. Obtaining a certain air content using the experience with the AEA-SPdosage was found to be an untrivial task if there is a lack of parameter control. Using the experience of the admixture combination seven concrete mixes were produced: six mixes with 0.52 FA/C and w/b ratios 0.293, 0.40 and 0.45 with and without entrained air, and one OPC mix w/b 0.45, all with 0.06 SF/C. Two of the most used methods, ASTM C666, procedure A for rapid freezethaw in water and CEN/TS 12390 for surface scaling in presence of 3 % NaCl solution, were used and extended to investigate how cracking, scaling and saturation progress at standard (-20oC) and arctic (-52oC) temperatures in such severe conditions. The results showed that high-volume FA concrete could be produced frost resistant in standardized testing and in arctic exposure when properly air-entrained. Prolonged water curing was found to have a positive effect, except for salt-scaling resistance of air-entrained FA concrete mixes. Long-term water curing allowed FA concrete with 0.293 w/b without air entrainment to survive a rapid freeze-thaw test in freshwater. Liquid uptake during freeze-thaw was found to be a link, connecting internal and surface frost damage. Air entrainment was found to protect against accelerated liquid uptake during wet freeze/thaw. The work conducted in this thesis contributes to the understanding of how to treat high-volume FA concrete in production and what to expect of the performance in various freeze-thaw environments.