Indian Concrete Journal

Carbonation and its effect on microstructure of concrete with fly ash and ground granulated blast furnace slag B. S. Dhanya, Sundar Rathnarajan, Manu Santhanam, Radhakrishna G. Pillai and Ravindra Gettu

Carbonation induced corrosion of the embedded steel reinforcement is a major deterioration mechanism of reinforced concrete structures, mainly in inland / highway environments of tropical countries. Carbonation leads to uniform reduction in the cross sectional area of steel reinforcement. In general, Supplementary Cementitious Materials (SCMs) can increase durability of concrete. The current study evaluates the influence of three SCMs such as Ground Granulated Blast Furnace Slag (Slag), Class F fly ash and Class C fly ash on the carbonation of concrete, assessed by both macroscopic and micro-analytical investigations. The macroscopic behaviour of concrete was assessed using accelerated carbonation and natural carbonation tests. The micro-analytical studies were conducted to understand the alteration in the micro structure of concrete with SCMs under carbonation exposure. These tests include Scanning Electron Microscopy (SEM), X— Ray Diffraction (XRD) and Thermo Gravimetric / Differential Scanning Calorimetry (TG/ DSC). The results showed that depth of carbonation is more for concretes with SCMs. However, at lower replacement levels, the difference as compared to OPC is not much significant in the case of Slag and Class C fly ash. A relationship between natural carbonation and accelerated carbonation depth was proposed based on CO2 concentration and climatic influence (tropical regions). The microstructural modifications of the paste matrix due to carbonation are explained in terms of calcite formation, decalcification of CSH and change in porosity level.'

The effect of supplementary cementitious materials, w/b ratio and initial moist curing on carbonation of concrete in a semi-arid, urban environment Yunus Bailin, Yunusa Al Hassan

This paper presents the results of a project that was undertaken to consider the effects of FA, GGBS and silica fume as supplementary cementitious materials (SCM’s) in concrete, on the rate of carbonation in Johannesburg, South Africa. Concretes were prepared using CEM-I and blends of CEM-I with a range of commercially available SCM’s as well as a CEM-V. These binders were used to prepare concretes with
four w/b ratios. Concrete cube samples were prepared from each concrete and subjected to different initial moist curing periods before being exposed to carbonation in the natural environment. The samples were then placed in three microclimate exposure conditions. The results indicate that, all else being equal, FA, GGBS and CEM-V concretes show increased rates of carbonation. The paper also proposes a model for predicting rates of carbonation under the conditions of this study.

Carbonation performance of OPC-Slag-Fly Ash binder system Vineet Shah, Shashank Bishnoi

Corrosilon of steel reinforcement in concrete is the leading cause of deterioration of concrete structures. Carbonation induced corrosion is prevalent in cements containing Supplementary Cementitious Materials (SCMs). In this study, the carbonation performance of cement containing fly ash and combination of slag and fly ash is investigated. Concrete specimens cast at two different water to cement ratios were subjected to accelerated and natural carbonation conditions. Effect of carbonation on pore structure properties of concrete like porosity and sorptivity was also investigated. The carbonation resistance of slag-fly ash blend was lower as compared to fly ash for the selected composition. Increase in porosity and rate of water absorption was observed on carbonation in blended cement systems.

Strength and durability parameters of carbonated mineral admixture concrete Shaik Hussain, Dipendu Bhunia and S. B. Singh

The present paper deals with the influence of mineral admixtures such as fly ash and blast furnace slag on the carbonation resistance of concrete when partly replaced by cement. Concrete mixes with water binder ratios of 0.35, 0.50 and 0.65 had been used to investigate the mechanical properties and durability of the concrete specimens exposed to accelerated carbonation of over 1 year. Compressive strength, flexural strength and modulus of elasticity had been studied for strength parameters and for durability tests, depth of carbonation and volume of permeable voids had been carried out. Thermo gravimetric test had also been performed to compare the consumption of calcium hydroxide and the formation of calcium carbonate for the mixes with water binder ratio of 0.50.

Improving the carbonation resistance of self-compacting concrete containing recycled concrete aggregates using blended cements Navdeep Singh, S. P. Singh

The current investigation assess the relative efficacy of Silica Fume (SF) and Metakaolin (MK) blended with Portland Cement (PC) and Fly Ash (FA) in compensating the loss of carbonation resistance due to inclusion of Coarse Recycled Concrete Aggregates (RCA) in Self-Compacting Concrete (SCC). To evaluate the carbonation resistance, different SCC 'mixes at curing age of 28-120 days were tested for accelerated carbonation for 4-16 weeks. The influence of SF and MK has been observed for both low and high volume FA based SCC mixes containing various levels of RCA. As anticipated, the maximum loss of carbonation resistance of RCA based SCC was observed for those mixes in which full replacement of Coarse Natural Aggregates (CNA) with RCA was done. The observed loss was offset to varying degrees by the use of SF and MK. It has been observed from this investigation that inclusion of SF/MK compensates the loss of carbonation resistance of SCC up to 50% replacement of CNA with RCA. Based on the experimental results, it can be seen that the performance of SF blended mixes was better than that of MK blended SCC mixes which lends support to wider use of SF.