Inhibitors added cement is used for construction of durable concrete structures for longer life.
The crack formation in concrete structure occurs due to the corrosion of rebar inside the concrete. This occurs because of the surface oxidization of iron rod into iron oxide due to diffused oxygen and moisture in the concrete.
The places, where the chloride content in the groundwater is more, carbonation reaction occurs which leads to the decrease in alkalinity. Because of this, the rate of corrosion of steel rebar increases and eventually the cracks are formed. By adding the inhibitors admixture to the concrete mix, the corrosion of steel rebar can be prevented.
Two Major Causes of Corrosion in Concrete
Concrete is highly alkaline material when first produced (pH range 12-13). The embedded steel is protected by a passive oxide layer which is maintained by high alkalinity at the surface of steel.
Under certain exposures and conditions the natural passivating protection of steel breaks down. In the presence of moisture and oxygen, the corrosion then occurs.
Carbonation of Concrete:
The most common cause of loss of passivating alkalinity is carbonation-a process where by atmospheric carbon-di-oxide and sulphur-di-oxide present in lime reacts with the soluble alkaline calcium hydroxide in concrete and it is converted into soluble calcium carbonate and calcium sulphate.
The alkalinity of the cement matrix is reduced and its passivating ability is lost progressively from the surface inward.
Once the concrete in contact with reinforced steel was carbonated the steel is no longer protected. In the presence of moisture and oxygen corrosion is inevitable.
Chlorides in Concrete:
The concentration of chlorides require to promote corrosion of embedded reinforcement is affected by the pH of the concrete. In alkaline fresh concrete higher chloride concentration is needed to promote corrosion. When the alkalinity is reduced due to carbonation process even small chloride concentration may induce the corrosion of rebar in concrete.
Process of Corrosion in Concrete Structures
The corrosion product of steel (iron oxides or hydroxides) occupies a much greater volume than the steel (4-12 times the volume). This increase in volume exerts a great expansive pressure within the concrete, leading to cracking.
Being an electrochemical process, corrosion of steel in concrete requires an electrolyte. Concrete is full of small pores which contains moisture, and so, is an effective electrolyte. A small, electrical current flows between the anode and the cathode with corrosion activity (rust formation) taking place at the anode. When INHIBITORS reaches reinforcing steel, it forms a protective layer that protects the steel in both anode and cathode areas.
Corrosion Rate of Steel:
|PROPERTY||CEMENT||CEMENT + INHIBITOR|
|Corrosion rate (mmpy)||0.0191||0.0029|
Corrosion Prevention in Concrete by Use of Inhibitors
When inhibitor approaches the steel rebar surface, it forms a protective layer which prevents the corrosion of iron by moisture and other ions such as chloride. The steel rebar’s life is increased by five times, which is proved by the weight loss experiment.
The inhibitor does not alter the properties of concrete such as strength and setting time but increases the life of the concrete. The chemical nature of the inhibitor does not change with time. This property helps in increasing the life of the concrete.
COMPARISON OF PHYSICAL CHARACTERISTICS:
|S.No.||CHARACTERISTICS||CEMENT||CEMENT + INHIBITORS|
|1.||Compressive Strength (N/mm2)|
On mortar cubes as per
IS 1489 (Part I):1991
|2.||28 days compressive strength|
on concrete cubes (N/mm2)
|3.||Tensile strength (N/mm2}|
(As per IS 269: 195)
|4.||Consistency for 33 mm penetration|
(As per IS269:1952)
|5.||Setting time (minutes)|
[As per IS 1489 (Part I): 1991]
Initial setting time (mts)
Final setting time (mts)
CORROSION RESISTANT PROPERTY OF CONCRETE (AFTER 3 YEARS):
|TECHNIQUE||CORROSION PARAMETER||ADDED CHLORIDE IN PPM||CEMENT||CEMENT + INHIBITOR||DURABILITY|
|Impedance||Charge transfer resistance (Rct) (K-ohm)||10,000||5.2||37||7|
We infer from the tabular column, that the concrete’s corrosion resistant property is very much less when compared to that of INHIBITOR ADMIXED CONCRETE.
The same is shown in the graphical representation as below;
COMPARISON OF CORROSION RESISTANT PROPERTY: