Freeze-Thaw Resistance and Microstructural Characteristics of Concretes Containing Pozzolans

  • George Christodoulou

    Student thesis: Doctoral Thesis


    The thesis describes a study undertaken to determine the effect of air entrainment on workability and air content of fresh concrete incorporating silica fume (SF), metakaolin (MK), fly ash (FA) and blends of FA and MK and to assess the effects of such pozzolans on freeze-thaw durability, air void system and microstructure of hardened concrete. Cement was partially replaced by various quantities of the pozzolanic materials. The results demonstrated that the increase in workability attributed to the air-entraining admixture was greater in MK concrete than in SF concrete and occurred for a greater range of dosages of the admixture. Improvements in workability due to the air-entraining admixture were also obtained in concretes with low levels (20%) of FA. Concretes with 30 and 40% FA although more workable, accrued no such benefit. In addition, the workability of FA-MK concrete was substantially reduced with increasing MK level at all total replacement levels, i.e. 20, 30 and 40%. Furthermore the air content tests indicated that up to 0.24% air entraining admixture resulted in steady increase in the air content of MK concrete, compared to a limit of 0.12% for SF concrete. Alternatively, FA caused large reductions in the air content of fresh concrete, irrespective of the dosage of the airentraining agent. The freeze-thaw durability was determined on both air-entrained and non air-entrained concretes. Based on a criterion that unsatisfactory resistance to freezing and thawing corresponds to a durability factor (DF) less than 60% or a change in length greater than 200 um/m, all the air-entrained concretes exhibitedexcellent performance under freeze-thaw conditions irrespective of the MK or FAcontent. On the other hand the non air-entrained concretes performed poorly under freezing and thawing. Thus, it would appear that air entrainment is the controlling factor for good freeze-thaw performance and the material effects are less important. However there were indications to suggest that non air-entrained concretes containing MK at low replacement levels (2.5 and 7.5%) could be frost resistant (DF > 80%). This was attributed to the increased spacing factor effected by the presence of fine particles of MK. Air entrainment was also a key factor for good scaling resistance. For instance, non air-entrained concretes with high replacement levels of MK (7.5 and 10% MK) exhibited more scaling than concretes with low replacement levels (2.5% MK), whereas scaling of air-entrained MK concretes was independent of the replacement level. The concrete containing high amounts of FA (30%) exhibited more scaling than the control and 10% FA concrete. However, blending FA with MK (MK/FA = 1/3) at total replacement levels of 10 and 30% improves the scaling resistance of the resulting concrete as compared to the FA only concrete. Non air-entrained concretes containing FA or blends of FA with MK showed an increase in weight at the beginning of freezing and thawing, an indication of uptake of water. Water absorption results confirmed that this was a result of a more open porosity.There is a strong correlation between sorptivity and pore refinement. Increasingamounts of the MK appear to cause refinemenent of the concrete's pore structure. As a result of this pore refinement sorptivity decreases with increasing amounts of MK. In addition blending FA with MK causes pore refinement. Irrespective of the pozzolanic material or blends of materials used the presence of entrained air appears to have a negative effect on pore refinement.
    Date of AwardDec 2001
    Original languageEnglish


    • Concrete

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