Concrete

Concrete:

Concrete is a composite material composed of aggregate bonded together with a fluid cement that cures over time. Concrete is the second-most-used substance in the world after water, and is the most widely used building material.

Water-Cement Ratio:  

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 than 0.3%.
• 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 10000 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.

Segregation: 

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:

1. If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability. 
2. The workability of concrete is also affected by the maximum size of the coarse aggregates to be used in the mixture.

Note:  In order to measure the workability of concrete mixture, various tests are developed. Tests such as flow test, Vee-Bee test and compaction factor test are used in great extent in laboratory. Slump test, which is commonly used in the field, is briefly described below. test for workability

1. Slump Test
2. Compaction factor test 
3. Vee-Bee Test
4. Vee- Bee Consistometer
5. Flow Test 

MIX DESIGN: 

• 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. 

AGGREGATES SIZE:

• For RCC work the maximum size of aggregates is limited to 20-25mm.
• 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 aggregate > 4.75mm size.

          Fine aggregates < 4.75mm size. 

Fineness modulus:

• 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 coarse 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 80mm to 150 µm and dividing it by 100.

VIBRATORS:

Following are the four types of vibrators:

1. Internal Vibrators:   These vibrators consist of a metal rod which is inserted in fresh concrete. Skilled and experienced men should handle internal vibrators. These vibrators are more efficient the other types of vibrators. 
2. Surface Vibrators:   These vibrators are mounted on platform or screeds. They are used to finish concrete surface such as bridge floors road slabs, station platform, etc.
3. Form Vibrators:    These vibrators are attached to the formwork and external centering of walls, column etc. The vibrating action is conveyed to concrete through the formwork during transmission of vibrations. Hence the are not generally used. But they are very much helpful for concrete sections which are too thin for use of internal vibrators. 
4. Vibrating Tables:  These vibrators are widely used for making precast products.

Water-Proofing Cement Concrete:

Cement concrete to a certain extend may be made impermeable to the water by using hydrophobic cement.

Following are the three methods adopted for water proofing of RCC flat roofs:

1. 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 outbin cement mortar of proportion 1:4, i.e., one part of cement to four parts of sand by volume. 

      2. 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) or brickbats cement concrete (1:8:14). The thickness of the concrete layer is about 10cm. 

      3. 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. 

Lightweight Concrete:

The bulk density of ordinary concrete is about 2300kg/m3. Concrete having bulk density between 500 to 1800 kg/m3 is known as lightweight concrete and it is prepared from the following materials:

1.  Binding material: Ordinary Portland cement and its varieties can be used as binding material. 

2.  Aggregates: For lightweight concrete, loose porous materials are used as aggregates. 

3.  Steel:  Lightweight concrete is highly porous and hence, it leads to corrosion of reinforcement. 

Corrosion of steel in concrete:

The term corrosion is used to indicate the conversion of metals by nature agencies into various compounds. The term rusting is used to refer corrosion of ferrous metals. 

Causes of Corrosion:

1. Congested reinforcement in small concrete sections.
2. Excessive water-cement ratio.
3. Improper construction methods.
4. Inadequate design procedure. 
5. insufficient cover to steel from exposed concrete surface.
6. Presence of moisture in concrete. 
7. 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.