1) What is bleeding and how you avoid it? 2) Types of cement and their usage. 3)
ID: 1866935 • Letter: 1
Question
1) What is bleeding and how you avoid it? 2) Types of cement and their usage. 3) What is cement water ratio and how it effect and draw it 4) What is segregation and how you avoid it? 5) Concrete allowable temperature and why? 6) Draw BMD and SHD for simple and continues beam 7) Draw simple, continues, cantilever beams reinforcement. 8) Slab reinforcement. 9) Concrete elements allowable cover 10) Types of slabs and their usage. 11) Types of support? 12) Sort of street layer. 13) Horizontal and vertical curve in roads. 14) What are specifications of coarse layer? 15) What is surface water drainage? 16) What is storm water drainage? 17) Types of foundations and uses. 18) Types of settlement 19) How we can determine the soil bearing capacity? 20) What is depth of foundation and how we calculate it? 21) How water table effect foundation depth? How you avoid it? 22) Where is Saudi Arabia benchmark? 23) Water channel cross road?Explanation / Answer
1.
Bleeding can be defined as the tendency of water to rise to the surface of freshly placed concrete. It is another form of segregation where some amount of water comes to the concrete surface after placing and compacting, before setting. The water content carries some particles of sand and cementing materials.
Bleeding cannot be completely prevented as it is an inherent property of concrete. But it can be minimized if following precautions are taken.
Preventing Bleeding
1. By proper proportioning of concrete mix.
2. By mixing the concrete uniformly and completely.
3. By using finely divided pozzolanic materials ( as it creates a longer path for the water to traverse and it has a high surface are)
4. By using air entrain admixtures.
5. By using finer cement or cement with low alkali content.
2.
Types of cement
1. Ordinary Portland Cement (OPC)
2. Portland Pozzolana Cement (PPC)
3. Rapid Hardening Cement
4. Quick setting cement
5. Low Heat Cement
6. Sulphates resisting cement
7.Blast Furnace Slag Cement
8.High Alumina Cement
9.White Cement
10.Coloured cement
11.Air Entraining Cement
12.Expansive cement
13.Hydrographic cement
1. Ordinary Portland Cement (OPC)
Ordinary Portland cement is the most widely used type of cement which is suitable for all general concrete construction. It is most widely produced and used type of cement around the world with annual global production of around 3.8 million cubic meters per year. This cement is suitable for all type of concrete construction.
2. Portland Pozzolana Cement (PPC)
Portland pozzolana cement is prepared by grinding pozzolanic clinker with Portland cement. It is also produced by adding pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending portland cement and fine pozzolana.
This cement has high resistance to various chemical attacks on concrete compared with ordinary portland cement and thus it is widely used. It is used in marine structures, sewage works, sewage works and for laying concrete under water such as bridges, piers, dams and mass concrete works etc.
3. Rapid Hardening Cement
Rapid hardening cement attains high strength in early days it is used in concrete where formworks are removed at an early stage and is similar to ordinary portland cement (OPC). This cement has increased lime content and contains higher c3s content and finer grinding which gives greater strength development than OPC at an early stage.
The strength of rapid hardening cement at the 3 days is similar to 7 days strength of OPC with the same water-cement ratio. Thus, advantage of this cement is that formwork can be removed earlier which increases the rate of construction and decreases cost of construction by saving formwork cost.
Rapid hardening cement is used in prefabricated concrete construction, road works, etc.
4. Quick setting cement
The difference between the quick setting cement and rapid hardening cement is that quick setting cement sets earlier while rate of gain of strength is similar to Ordinary Portland Cement, while rapid hardening cement gains strength quickly. Formworks in both cases can be removed earlier.
Quick setting cement is used where works is to be completed in very short period and for concreting in static or running water.
5. Low Heat Cement
Low heat cement is prepared by maintaining the percentage of tricalcium aluminate below 6% by increasing the proportion of C2S. This makes the concrete to produce low heat of hydration and thus is used in mass concrete construction like gravity dams, as the low heat of hydration prevents the cracking of concrete due to heat.
This cement has increased power against sulphates and is less reactive and initial setting time is greater than OPC.
6. Sulphates Resisting Cement
Sulfate resisting cement is used to reduce the risk of sulphate attack on concrete and thus is used in construction of foundations where soil has high sulphate content. This cement has reduced contents of C3A and C4AF.
Sulfate resisting cement is used in construction exposed to severe sulphate action by water and soil in places like canals linings, culverts, retaining walls, siphons etc.
7. Blast Furnace Slag Cement
Blast furnace slag cement is obtained by grinding the clinkers with about 60% slag and resembles more or less in properties of Portland cement. It can be used for works economic considerations is predominant.
8. High Alumina Cement
High alumina cement is obtained by melting mixture of bauxite and lime and grinding with the clinker. It is a rapid hardening cement with initial and final setting time of about 3.5 and 5 hours respectively.
The compressive strength of this cement is very high and more workable than ordinary portland cement and is used in works where concrete is subjected to high temperatures, frost, and acidic action.
9. White Cement
It is prepared from raw materials free from Iron oxide and is a type of ordinary portland cement which is white in color. It is costlier and is used for architectural purposes such as precast curtain wall and facing panels, terrazzo surface etc. and for interior and exterior decorative work like external renderings of buildings, facing slabs, floorings, ornamental concrete products, paths of gardens, swimming pools etc.
10. Colored cement
It is produced by mixing 5- 10% mineral pigments with ordinary cement. They are widely used for decorative works in floors.
11. Air Entraining Cement
Air entraining cement is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of sulphates etc. during the grinding of clinker.
This type of cement is especially suited to improve the workability with smaller water cement ratio and to improve frost resistance of concrete.
12. Expansive Cement
Expansive cement expands slightly with time and does not shrink during and after the time of hardening . This cement is mainly used for grouting anchor bolts and prestressed concrete ducts.
13. Hydrographic cement
Hydrographic cement is prepared by mixing water repelling chemicals and has high workability and strength. It has the property of repelling water and is unaffected during monsoon or rains. Hydrophobic cement is mainly used for the construction of water structures such dams, water tanks, spillways, water retaining structures etc.
3.
Water Cement Ratio means the ratio between the weight of water to the weight of cement used in concrete mix.
water cement ratio will directly affect the strength of concrete. Either it increases the strength if used in correct proportion or decrease it.
How it affects?
Concrete is a macro content. It contains micro ingredients such as cement, sand, fine aggregate & Coarse aggregate. In order to acquire high strength concrete which withstands up to our desired compressive strength, We need correct proportion of admixture to combine these materials.
Here comes the Water which will initiate this chemical process by adding 23%-25% of the cement volume. This initiates the chemical process and makes 15% of water cement paste also known as a gel to fill the voids in the concrete.
Effects of too much water in concrete
As stated above we need 23% of water to initiate the chemical process on cement.
Adding more water than this allowable Water cement limit will actually affect the strength.
If we keep on adding water to increase the workability then the concrete has lots of fluid materials where the aggregates will settle down. Once the water evaporated it leaves lots of voids in concrete which affects the concrete strength.
But if we follow the guidelines in order to maintain the strength of the concrete then it will affect the concrete workability and make us hard to handle and place them. That’s why we are using plasticisers & superplasticizers which will increase the workability without affecting the W/C Ratio.
4.
Segregation of concrete is separation of ingredients of concrete from each other. In good concrete all concrete aggregates are evenly coated with sand and cement paste and forms a homogeneous mass.
During handling, transporting and placing, due to jerks and vibrations the paste of cement and sands gets separated from coarse aggregate. If concrete segregates during transit it should be remixed properly before depositing. However a concrete where initial setting time is over, should not be used.
Prevention
Wherever depth of concreting is more than 1.5 meters it should be placed through temporary inclined chutes. The angle of inclination may be kept between 1:3 and 1:2 so that concrete from top of chutes travels smoothly to bottom, use of small quantity of free water from top at intervals helps in lubricating the path of flow of concrete to bottom smoothly. The delivery end of chute should be as close as possible to the point of deposit.
Segregation in deep foundations and rafts of thickness more than 1 meter, there is every possibility of presence of segregated concrete near bottom or in center if proper supervision is not there. Such segregation can be detected by advanced method of testing like ultrasonic testing. In case of doubt random ultrasonic testing should be conducted and if it is present, designer’s opinion should be taken. This type of segregation can be rectified by pressure grounding with special chemical compounds.
After any defect rectified by pressure grouting core test has to be performed to ensure that the strength of concrete has reached to the desired level.
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