Que3.1. What do you mean by ‘ blend design ’ in concrete? Explain its types and objects.
Answer Concrete Mix Design Mix Design is the wisdom of determining the relative proportions of the constituents of concrete to achieve the asked parcels in the most provident way.
Types of composites Following are the types of mixes
1. Nominal composites In the specifications for concrete specified the proportions of cement, fine and coarse summations. These composites of fixed cement total rate which ensures acceptable strength are nominated nominal mixes
2. Standard Mixes IS456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40.
In this designation the letter M refers to the blend and the number to the specified 28 day cell strength of blend in N/ mm2.
3. Design Mixes In these mixes the performance of the concrete is specified by the developer but the blend proportions are determined by the patron of concrete, except that the minimal cement content can be laid down ideal of Mix Design Following are the ideal of blend design
1. To achieve the designed/ wanted plasticity in the plastic stage.
2. To achieve the asked minimum strength in the toughened stage.
3. To achieve the asked continuity in the given terrain conditions.
4. To produce concrete as economically as possible.
Que3.2. What are the colorful principles of proportioning of blend design?
Answer Principles of Mix Design Following are the colorful principles of blend design
1. The terrain exposure condition for the structure.
2. The grade of concrete, their characteristic strength’s and standard diversions.
3. The type of cement.
4. The types and sizes of summations and their sources of force.
5. The nominal maximum sizes of summations.
6. Maximum and minimal cement content in kg/ m3.
7. Water cement rate.
8. The degree of plasticity of concrete grounded on placing conditions.
9. Air content inclusive of detrained air.
10. The outside/ minimum viscosity of concrete.
11. The outside/ minimum temperature of fresh concrete.
12. Type of water available for mixing and curing.
13. The source of water and the contaminations present in it.
Que3.3. bandy the Abram’s water/ cement rate law and its validity. How strength of concrete is estimated by Abram’s law.
Answer Abram’s Water/ Cement rate Law
1. According to Abram’s law the strength of completely compacted concrete is equally commensurable to the water- cement rate.
2. Then the water- to- cement rate is the relative weight of the water to the cement in the admixture. For utmost operations, water- to- cement should be between0.4 and0.5, lower for lower permeability and advanced strength.
Validity If not duly compacted, the concrete blend will contain large valids, which contribute to porosity. therefore, at low water/ cement rate where full contraction is hard to achieve, Abram’s law isn’t valid.
Que3.4. What are the different factors in the choice of blend proportions?
Answer Factors impacting Choice of Mix Design According to IS 4562000 and IS 13431980 following are the factor affecting the design of concrete blend
1. Grade of Concrete
i. The grade of concrete gives characteristic compressive strength of concrete.
ii. The grade M20 denotes characteristic compressive strength fck of 20 N/ mm2.
iii. Depending upon the degree of control available at point, the concrete blend is to be designed for a target mean compressive strength( fck) applying suitable standard divagation.
2. Type of Cement
i. The advanced the strength of cement used in concrete, lower will be the cement content.
ii. The use of 43 grade and 53 grade of cement, gives saving in cement consumption as much as 15 and 25 independently, as compared to 33 grade of cement.
3. Maximum Nominal Size of summations It’s designated by the sieve size advanced than larger size on which 15 or further of the total is retained.
ii. The maximum nominal size of total shouldn’t be further than one- fourth of minimal consistence of the member.
iii. For heavily corroborated concrete members as in the case of caricatures of main shafts, the nominal outside size of the total should generally be confined to sum lower than the minimum clear distance between the main bars or 5 mm less the minimal cover to the underpinning, whoever is lower.
4. Grading of Combined summations
i. The relative proportions of the fine and coarse total in a concrete blend is one of the important factors affecting the strength of concrete.
ii. For thick concrete, it’s essential that the fine and coarse total be well canted.
5. Maximum Water/ Cement rate The lower the water/ cement rate, the lesser is the compressive strength.
6. Plasticity Plasticity of fresh concrete determines the case with which a concrete admixture can be mixed, transported, placed, compacted and finished without dangerous isolation and bleeding.
7. continuity continuity bear low water/ cement rate.
ii. It’s generally achieved not by adding the cement content, but by lowering the water demands at given cement content.
iii. Water demand can be lowered by through control of the total grading and by using water reducing cocktails.
Que3.5. Write short note on quality control of concrete.
Answer
1. The strength of concrete varies from batch to batch over a period of time.
2. The sources of variability in the strength of concrete may be considered due to variation in the quality of the element accoutrements , variations in blend proportions due to batching process, variations in the quality of batching and mixing outfit available, the quality of supervision and workmanship.
3. These variations are ineluctable during product to varying degrees.
4. Controlling these variations is important in lowering the difference between the minimal strength and characteristic mean strength of the blend and hence reducing the cement content.
5. The factor controlling this difference is quality control.
6. The degree of control is eventually estimated by the variation in test results generally expressed in terms of the measure of variation.