Corrosion steel in concrete
The term corrosion is used to indicate the conversion of metals by natural agencies into various compounds. The term rusting is used to refer corrosion of ferrous metals.
Theories of Corrosion
1. Chemical Action Theory
- The combined action of oxygen, carbon dioxide and moisture on steel results in soluble ferrous bicarbonate Fe(HCO3)2. This ferrous bicarbonate is then oxidized to basic ferric carbonate 2Fe(OH)CO3. This basic ferric carbonate is converted into hydrated ferric oxide Fe(OH)3 (final product) and carbon dioxide is liberated.
Electrolytic Theory: According to this theory, metal contains anodic and cathodic areas and these areas, when connected by electrolytes such as water, moisture; aqueous solutions, etc. cause corrosion.
Causes of Corrosion
- Congested reinforcement in small concrete sections.
- Excessive water-cement ratio.
- Improper construction methods.
- Inadequate design procedure.
- Insufficient cover to steel from exposed concrete surface.
- Presence of moisture in concrete.
- Presence of salts.
Effect of Corrosion
Important effect of corrosion is the formation of cracks and these cracks usually progress or advance most rapidly where shearing stresses are the greatest and where slipping occurs due to loss of bond.
Important properties of water to be used for cement concrete are:
- Content of organic solids not more than 0.02%
- Content of inorganic solids not more that 0.30%
- Content of sulphates not less than 0.05%
- Content of sulphate alkali chlorides not more than 10%
- Turbidity not more than 2000 ppm.
- Acid not more than 10,000 ppm.
- pH should be between 4.5 to 8.5.
Bleeding of Concrete
If excess water in the mix comes up at the surface causing small pores through the mass of concrete, it is called bleeding. Bleeding is one form of segregation, where water comes out to the surface of the concrete, being lowest specific gravity among all the ingredients of concrete. Bleeding can be easily identified in the field by the appearance of a thin layer of water in the top surface of freshly mixed concrete
Segregation in concrete is a case of particlesegregation in concrete applications, in which particulate solids tend to segregate by virtue of differences in the size, density, shape and other properties of particles of which they are composed.
It is caused when coarse aggregate is separated out from the finer materials resulting in large voids, less durability and less strength.
Some rules-of-thumb are developed for deciding the quantity of water in concrete.
- Weight of water = 28% of the weight of the cement + 4% of the weight of total aggregate.
- Weight of water = 30 % of the weight of the cement + 5% of the weight of total aggregate.
Workability of Concrete
- Workability is the amount of work to produce full compaction.
- The important facts in connection with workability are:
(i) If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.
(ii) If the strength of concrete is not to be affected, the degree of workability can be obtained:
- by slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too we; and
- by adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.
(iii) The workability of concrete is also affected by the maximum size of the coarse aggregates to be used in the mixture.
- Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work.
- It is not a suitable method for very wet or very dry concrete and stiff mix.
- It does not measure all factors contributing to workability.
- The diameter of the rod is 16 mm and its length is 60 cm. The strokes to be given for ramming vary from 20 to 30.
Recommended Slumps of Concrete
Compaction Factor Test
- In the compaction factor test the degree of workability is measured in terms of internal energy required to compact the concrete thoroughly.
- A compaction factor of 0.95 represents flowing concrete having high workability; 0.92 plastic concrete having medium workability; 0.85 stiff plastic concrete having low workability and a compaction factor of 0.75 represents stiff concrete having very low workability.
- The compacting factor test is designed primarily for use in the laboratory but it can also be used in the field.
- The degree of compaction called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted.
Workability, Slump and Compacting Factor of Concrete with 20 mm or 40 mm Maximum Size of Aggregate
- This is carried out in such a manner that the specimen concrete in the test receives more or less same treatment in respect of the method of placing as it would in actual execution of the work. This test is preferred for finding workability of stiff concrete mix having very low workability.
- In this test a Vee-Bee time of 5 to 3 seconds represent stiff plastic concrete having medium workability, 10 to 15 seconds represents stiff concrete of low workability and Vee-Bee time to 18 to 10 seconds represent very stiff concrete having very low workability.
Vee Bee Consistometer
- This is a good laboratory test to measure indirectly the workability of concrete.
- This test consists of a vibrating table, metal pot, a sheet metal concrete a standard iron rod.
- The time required for the shape of concrete to change from slump concrete shape to cylindrical shape in second is known as Vee Bee Degree.
- This method is very suitable for very dry concrete whose slump value cannot be measured by slump test but the vibration is too vigorous for concrete with a slump greater than about 50 mm.
- This is a laboratory test which gives an indication of the quality of concrete with respect to consistency cohesiveness and the proneness to segregation.
- The spread or the flow of the concrete is measured and this flow is related to workability.
The value could range anything from 0-150%.
It can be realized that the compacting factor test measures the inherent characteristics of the concrete which relates very close to the workability requirements of concrete and as such it is one of the good test to depict the workability of concrete.
Estimating Yield of Concrete
- A rule-of-thumb as given below, may be used to find out the approximate yield of concrete from a given concrete mix.
- If the proportion of concrete is a: b: c, i.e., if a parts of cement, b parts of sand and c parts of coarse aggregates are mixed by volume, the resulting concrete will have a volume of 2/3 (a + b + c).
- Let w, a, b and c be absolute volumes of water, cement, fine aggregate and coarse aggregate respectively. Then, w + a + b + c = 1.
Methods for Proportioning Concrete Mixes
1. Minimum voids method
The quantity of sand used should be such that it completely fills the voids of the coarse aggregate and similarly the quantity of cement used should be such that it fills the voids of sand. However in actual practices the quantity of sand used in the mix is kept 10% more than the voids in the coarse aggregate and the quantity of cement is taken 15% more than the voids in the sand.
2. Maximum density method
Method of minimum voids was later improved by Fuller. For maximum density of mix.
He gave following expression.
D = Maximum size of aggregate.
P = % by weight of matter finer than diameter d.
3. Abram's water-cement ratio law
- This law states that for any given conditions of test the strength of workable concrete mix is dependent only on the water cement ratio. It means that if the concrete is fully compacted, the strength is not affected by aggregate shape, type or surface texture or the aggregate grading. According to this law, the strength of mix increases with decrease in water content.
- In terms of crushing strength after 7 days curing where P7 is cylinder crushing strength in kg/cm2 and x is water cement ratio by volume.
- In terms of crushing strength after 28 days curing where P25 is cylinder crush strength after 28 days curing.
- Strength of concrete increases with age.
- When the task of deciding the proportion of the constituents of concrete is accomplished by use of certain established relationships (which are based on inferences drawn from large number of experiments) the concrete thus produced is termed as Design mix concrete.
- When the proportions of cement, aggregate and water are adopted based on arbitrary standard the concrete produced is termed as Nominal mix concrete.
- Nominal mix concrete is used in works where the quality control requirement for design mixes are difficult to be implemented. Nominal mix concrete can be produced by taking cement, fine and coarse aggregate in the ratio of 1: n: 2n for normal work. However, the ratio of the coarse aggregate to fine aggregate can vary from 1.5: 2.5: 1 in situations where denser or more workable concrete is to be produced.
- For RCC work the maximum size of aggregates is limited to 20-25 mm.
- For a concrete of given workability rounded aggregates require least water cement ratio. Particle shape is very important since the water cement ratio governs greatly the strength of concrete.
Coarse aggregates > 4.75 mm size.
Fine aggregates < 4.75 mm size.
- The fineness modulus of an aggregate is an index number which is roughly proportional to the average size of the particles in the aggregate.
The coarser the aggregate, the higher the fineness modulus.
- Fineness modulus is obtained by adding the % of the weight of the material retained on the total 10 number of IS sieves (between 80 mm to 150 μm) and dividing it by 100.
Vibrators: Following are the four types of vibrators:
- Internal Vibrators: These vibrators consist of a metal rod which is inserted in fresh concrete. Skilled and experience men should handle internal vibrators. These vibrators are more efficient than other types of vibrators.
- Surface Vibrators: These vibrators are mounted on platform or screeds. They are used to finish concrete surfaces such as bridge floors, road slabs, station platform, etc.
- Form Vibrators: These vibrators are attached to the formwork and external centering of walls, columns, etc. The vibrating actions is conveyed to concrete through the formwork during transmission of vibrations. Hence they are not generally used. But they are very much helpful for concrete sections which are too thin for the use of internal vibrations.
- Vibrating Tables: These vibrators are widely used for making precast products.
- Period of Curing: The curing period is about 7 to 14 days.
Water-proofing Cement Concrete
- Cement concrete to a certain extent may be made impermeable to water by using hydrophobic cement.
Following are the three methods adopted for water-proofing of RCC flat roofs:
- Finishing: For ordinary building of cheap construction, finishing of roof surface is done at the time of laying cement concrete. The finishing of flat roof is carried out in cement mortar of proportion 1: 4, i.e., one part of cement to four parts of sand by volume.
- Bedding Concrete and Flooring: In this method, the surface of RCC slab is kept rough and on this surface, a layer of concrete is laid. The concrete may be brickbats lime concrete (1:2:4) or brickbats cement concrete (1:8:14). The thickness of the concrete layer is about 10 cm.
- Mastic Asphalt and Jute Cloth: In this method, a layer of hot mastic asphalt is laid on the roof surface. Jute cloth is spread over this year.
The bulk density of ordinary concrete is about 2300 kg/m3. Concrete having bulk density between 500 to 1800 kg/m3 is known as lightweight concrete and it is prepared from the following materials:
- Binding material: Ordinary Portland cement and its varieties can be used as binding material.
- Aggregates: For lightweight concrete, loose porous materials are used as aggregates.
- Steel: Lightweight concrete is highly porous and hence, it leads to corrosion of reinforcement.
IS 456 Guidelines
For Min. Cement Content, Min./Max. Grade of Concrete & Max. W/C Ratio for Plain and Reinforced Cement Concrete for Different Exposures with Normal Weight Aggregates of 20 mm Nominal Maximum Size
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