Effect of Organic and Inorganic Corrosion Inhibitors on Strength Properties of Concrete

In the current study, the M25 grade concrete mixes are admixed with various locally available organic and non-organic corrosion inhibitors such as Calcium Nitrite, Sodium Nitrite, Hexamine and Di-ethanolamine to understand the influence of these organic and non-organic corrosion inhibitors on the strength and corrosion resistance properties of concrete. The percentage dosage of admixed inhibitors vary from 1 to 5% by the weight of cement. For M25 grade concrete the optimum percentages of corrosion inhibitor admixture was found to be 4% for Calcium Nitrite, 3% for Sodium Nitrite, 2% for Hexamine and 3% for Di-ethanolamine. Calcium Nitrite corrosion inhibitor admixture imparts increased compressive, split- tensile and flexural strength than other corrosion inhibitors at 28 days. All the corrosion inhibitors used in the study have enhanced the compressive strength, split tensile strength, and flexural strength of concrete. The initial gain of early strength decreased due to anodic process of inhibitors. Measured electrical resistivity and half-cell potential values of all corrosion inhibitors admixed M25 grade concrete mixes directs that calcium nitrate and Di-ethanolamine have shown high electrical resistance indicating their superior corrosion inhibition ability than sodium nitrite and hexamine. Accelerated corrosion test on reinforced concrete beams admixed with corrosion inhibitors confirmed that possible inception of corrosion in calcium nitrate admixed reinforced concrete beams is very low when compared to other corrosion inhibitors used for the study.


Introduction
The most commonly adopted approaches to enhance the service life of rebars in concrete structures are epoxy or zinc coating on the rebars, applying waterproofing agents, controlling the water/cement ratio, increasing the concrete cover thickness, application of chemical coats, cathodic protection etc [1]. However IS Code propose usage of various supplementary cementitious materials in the form of mineral admixtures provides a good solution for prevention of rebar corrosion, but whenever cracks appear in concrete rebars are exposed to corrosion. The most cost-effective and newly used practices to defer the corrosion of rebars in concrete is the usage of corrosion inhibiting admixtures in concrete [2]. Most sustainable organic inhibitors are green plant extracts which are not toxic, ecologically biodegradable, absence of heavy metals, environmentfriendly and are natural. It is hypothesized that the corrosion inhibiting admixtures not only delays the onset of corrosion but also impacts the other important properties of concrete [3]. There are various corrosion inhibitors that are in usage in the market [4]. The mechanism of corrosion in usually governed by aspects such as ingress of chloride ions and H + ions (in acid) [5]. The corrosion inhibitive properties can be appraised in the laboratory using accelerated corrosion test based on impressed voltage technique in saline medium and supplemented by electrical resistivity and half-cell potential measurements [6].

Mechanism of Inhibitors
Corrosion inhibitors forms shielding hydrophobic film on the rebar surface in high alkalinity concrete environment by adsorbing the ions of inhibitors on to the surface which eventually delays the rate of corrosion by obstructive cathodic /or anodic reactions [7]. More technically, inhibitors forms coordinate bonds by contributing electrons to available d-orbitals of iron atoms and ultimately encouraging the adsorption of anti-oxidant molecules on the surface of rebar developing a protective layer around rebars inhibiting electrochemical process reactions such as anodic process (iron oxidation) and cathodic process(oxygen reduction) [8]. The success of inhibitors is governed by primarily the molarity or the concentration of inhibitor used.
Higher the molarity more will be corrosion inhibiting effect [9,10].

Corrosion Inhibitors used for the present study
For this present study two inorganic and two organic corrosion inhibitors were considered. They are:

Test Results and Discussions
Based on the experimental investigations, the test results are presented as follows-

Compressive Strength
The table 3 and Fig. 8    From the test results, it was observed that out of five different dosages of hexamine by the weight of cement, 2% dosage of hexamine records maximum compressive strength at 28 days. It is very evident that the gain of compressive strength in corrosion inhibitor admixed M25 concrete mix at 3 days is very slow when compared to normal concrete mix due to anodic process. Once the oxide passive layer is formed over the rebar due to anodic process of inhibitor admixture, the gain of strength improved. The improvement of compressive strength at 28 days due to 2% dosage of Hexamine is 3.2 % over conventional concrete's compressive strength  The results evidently showed that there was no suitable correlation between the increase of the compressive strength of the concrete and type of corrosion inhibitor admixed. In inorganic inhibitors calcium nitrite fares better in compressive strength gain. Though initial gain of early strength decreased due to anodic process of inhibitors.

Split Tensile Strength
The table 8displays the split tensile strength of M25 concrete made with calcium nitrite. Nitrite contributes more to split tensile strength due to its more reactivity than other alkali based inhibitors. Alkali nitrates causes alkali aggregate reaction which will affect the integrity of the concrete.

Flexural Strength
The table 9 displays the flexural strength of M25 concrete made with calcium nitrite.

Conclusions
In the present study, four corrosion inhibitors admixtures such as calcium nitrite, sodium nitrite, hexamine, diethanolamine were admixed in M25 grade concrete and studied for their impact on the strength and corrosion inhibiting properties of concrete. From the experimental test results, the following conclusions can be drawn. 1. Optimum dosage of corrosion inhibitor admixture will enhance the integrity and uniformity of concrete. 2. For M25 grade concrete the optimum percentages of corrosion inhibitor admixture was found to be 4% for Calcium Nitrite, 3% for Sodium Nitrite, 2% for Hexamine and 3% for Di-ethanolamine. 3. Calcium Nitrite corrosion inhibitor admixture imparts increased compressive, split-tensile and flexural strength than other corrosion inhibitors at 28 days. 4. All the corrosion inhibitors used in the study have enhanced the compressive strength, split tensile strength, and flexural strength of concrete. 5. Calcium nitrite in concrete has proved superior in enhancing the compressive strength and similar observations are made in split-tensile and flexural strength of calcium nitrite admixed concrete mixes. This is may be due to presence of calcium ions and formation of insoluble C-H bonds. Calcium nitrite corrosion inhibitor forms shielding hydrophobic film on the rebar surface in high alkalinity concrete environment by adsorbing the ions of inhibitors on to the surface which eventually delays the rate of corrosion by obstructive cathodic /or anodic reactions.