Durability Studies of Concrete by Accelerated Carbonation Test & DIN Impermeability Test

Abstract

Concrete usage around the globe is second only to water. Concrete degrades over time; one process which is responsible for this is the absorption of carbon dioxide, known as carbonation of concrete. The alkaline conditions of hydrated cement paste are neutral- ized by carbonation. This neutralization, by dropping the pH of concrete, affects the protection of reinforcing steel from corrosion. Thus, if the entire concrete cover to steel were carbonated, corrosion of steel would occur in the presence of moisture and oxygen. In natural exposure conditions, carbonation is a very slow process. The content of CO2 in the atmosphere varies around 400 ppm (0.04%) by volume, and it is possible to ob- tain results for analysis of reinforced concrete structures for carbonation only after some years of exposure of it. In accelerated carbonation test different grades of concrete (M20, M25, M30, M35 & M40) are exposed to different carbon dioxide concentration of 2% & 4% respectively. All the five grades of concrete are exposed to two different temperature levels of 28C & 60C level during Accelerated Carbonation Test. All the five grade of concrete mixes are exposed to two different relative humidity content of 60% & 80% in Accelerated Carbonation Chamber. Accelerated carbonation test with different environ- mental conditions are carried out for 28 days and 180 days of curing period. In this test cylinders of size 100 mm diameter and 200 mm length are casted and they are exposed to 2% and 4% carbon dioxide concentration in accelerated carbonation chamber. After 7 days of exposure, the cylinders are taken out and with phenolphthalein method, the depth of carbonation is measured. From results of Accelerated Carbonation Test, it is observed that increment in CO2 concentration and temperature increases carbonation of concrete. Increment in relative humidity results into decrease in carbonation depth for all the five grade of concrete mixes. Carbonation of concrete is found to be decreasing with the increase in compressive strength. Age of concrete is found to have a very little impact on carbonation of concrete for all the concrete mixes. Carbonation coefficient of beam and column is found higher compared to that of slab in indoor and outdoor exposure of an existing structure in Adalaj, Gandhinagar. Time required to initiate the corrosion process by full carbonation of concrete cover is 15.31 years for slab in indoor exposure and 28.09 years in outdoor exposure.Impermeability of concrete is an ability of concrete to resist pressurized water pene- tration. Corrosion of the reinforcing steel is one of the most critical problem faced in RCC structures. The more porous or permeable the concrete, the higher the rate at which contaminants can penetrate and degrade the concrete. Therefore, assessment of impermeability of concrete is necessary. Assessment of penetration depth of M25 & M40 grade of concrete is carried out during this investigation. Mineral admixtures i.e. Fly ash, GGBFS, Rice husk ah, Silica Fume, Metakaoline are used as replacement of cement with their three different dosages for each mineral admixtures for M25 and M40 mixes to evaluate the effect of replacement to Impermeability of concrete. From the results of DIN Impermeability Test, it is observed that replacement of cement by mineral admixtures helps to improve impermeability of concrete. Silica fume with 15% replacement gives least penetration depth with increase in compressive strength.