International Concrete Abstracts Portal

Corrosion of reinforcement embedded in concrete is the most common cause of failure in concrete structures. The work is being carried out to investigate the rate and amount of corrosion of steel in concrete where cement is replaced by slag in different proportions. This paper reports a detailed corrosion study, carried out on concrete containing blast furnace slag. The study encompasses slag collected from several premier steel plants in India. Corrosion of steel was examined electrochemically through potentiodynamic study and also by accelerated corrosion study. Micrographic study was also carried out to examine the alteration of pore structure of slag concrete. These studies reveal that increase in slag proportion decreases the rate and amount of corrosion of reinforcement embedded in slag concrete.

Marine durability of 15-year old plain and reinforced concrete cylindrical specimens exposed in marine environments for 15 years is presented here. The specimens were made with ordinary Portland, slag (Type A, B and C) and fly ash (Type B) cements. Water-to-cement ratios were 0.45 and 0.55. Compressive strength of concrete, corrosion of steel bars, and chloride-ion concentrations in concrete were evaluated. After 15-year of exposure, compressive strength of concrete increases compared to its 28-day’s strength for the investigated cements, except fly ash cement. Slag cement of Type C shows the best performance against chloride ingress and corrosion of steel bars in concrete. Accumulation of chloride-ion at the surface of concrete made with slag and fly ash cements is observed. The presence of voids at the steel-concrete interface causes the formation of corrosion pits irrespective of the type of cement. The use of seawater as mixing water causes an earlier strength development at the 28-day and does not cause the strength of concrete to regress after 15-year of exposure. However, it causes more corrosion of steel bars at a lower cover depth. At the deeper cover depth, no significant corrosion of steel bars is found irrespective of the type of mixing water.

Mechanism of acid corrosion of geopolymeric cements consists of two subsequent steps. The first step starts by a leaching process in which the soluble contents of the material (i.e. sodium and calcium) are partially depleted and replaced by H+ and H3O+ ions from solution and possibly an attack by acidic protons on polymeric Si-O-Al bonds resulting in the formation of a partially dealkalized and decalcified Si/Al-rich residue. In the second step depending on the type and the concentration of the attacking acid, the cations of sodium and calcium diffusing toward the acidic solution react with the counter-diffusing anions of the attacking acid and result in the formation of sodium and calcium salts. Sodium salts which are usually very soluble are removed by leaching. In the case where the calcium salt is relatively insoluble or less soluble, a deposition forms inside the corroding layer which provides a protective effect inhibiting the process of deterioration.

In developed countries use of mineral admixtures such as fly ash, silica fume has already been adopted in making concrete. This includes commercial application on a large scale either for addition or for replacement of cement. In India too such replacements have been readily accepted. With the introduction of ready mixed concrete the process has been accelerated in recent times. An investigation was undertaken to study the effects of fly ash and silica fume in concrete. Compressive strengths at different levels of replacements were found. Silica fume from a local source and fly ash from Ramagundam thermal power station of the State of Andhra Pradesh were used. Maximum size of coarse aggregate was 12.5 mm. Water to cementious materials ratio was 0.32 and aggregate-cementitious materials ratio was 3.2. Cement replacement levels by fly ash were 0, 10, 20, 30, and 40 percents and by silica fume were 0,4, 8, 12, and 16 percents. Thus a total of 25 mixtures were studied. Strengths at the ages of 28 days and 56 days were found. The results are presented in tables and figures. It was found that the highest replacement level of 40 % by fly ash and 16 % by silica fume, simultaneously, i.e. a total replacement of cement to the extent of 56 % gave a 10 % increase in the 28-day compressive strength compared to that of control concrete. Maximum increase of 43 % in the 28-day compressive strength was observed at a 32 % level of cement replacement ( 20 % by fly ash and 12 % by silica fume).

Good adhesion of a repair material to concrete is of vital importance in the application and performance of underwater concrete repair. This paper reviews techniques and results of bond strength test methods: compressive slant-shear test and tensile bond (load-point) test. The objective of this paper is to evaluate the effects of variation in water-to-binder ratio and silica fume and fly ash replacements on bond strength of underwater concrete repairs. The mixtures were proportioned with a Canadian Type 10 cement and two other binders, one with 10% silica fume, and secondly a ternary cement containing 6% silica fume and 20% Class F fly ash replacements. The water-to-binder ratios tested were 0.41 and 0.47. The mixtures were cast in water, on slabs placed in the bottom of forms, in blocks measuring 0.50 x 0.45 x 1 m with the free fall height in water 0.35 m. Cores were obtained from experimental blocks cast in water to evaluate bond strength test between old and new concretes by tensile load-point. The slant-shear adhesion was determined by casting cylinders above water with consolidation, and in water without consolidation. The samples were composed of old concrete cast in air (sawed and smooth surface) with new concrete above. The incorporation of 10% of silica fume, or 20% of fly ash and 6% of silica fume and the reductions of water-to-binder ratio from 0.47 to 0.41 resulted in significant increases in bond strength.