Test: Concrete Technology- 1 - SSC JE MCQ

According to IS 456:2000 Art No. 7.1 The slump of concrete used for hand placed pavement construction is 25-75 mm and degree of work ability is low for this slump value.

IS 456 : 2000 states that the concrete in the member represented by a core test shall be considered acceptable, if the average equivalent cube strength of core is equal to at least 85% of the cube strength of the grade of concrete specified, but no individual core has a strength less than 75%.

The use of fly ash in portland cement concrete (PCC) has many benefits and improves concrete performance in both the fresh and hardened state. Generally, fly ash benefits fresh concrete by reducing the mixing water requirement and improving the paste flow behavior.

Fly ash benefits to fresh concrete are:

1. Improved workability: The spherical shaped particles of fly ash act as miniature ball bearings within the concrete mix, thus providing a lubricant effect. This same effect also improves concrete pumpability by reducing frictional losses during the pumping process and flat work finishability.

2. Decreased water demand: The replacement of cement by fly ash reduces the water demand for a given slump. When fly ash is used at about 20 percent of the total cementitious, water demand is reduced by approximately 10 percent. Higher fly ash contents will yield higher water reductions. The decreased water demand has little or no effect on drying shrinkage/cracking. Some fly ash is known to reduce drying shrinkage in certain situations.

3. Reduced heat of hydration: Replacing cement with the same amount of fly ash can reduce the heat of hydration of concrete. This reduction in the heat of hydration does not sacrifice long-term strength gain or durability. The reduced heat of hydration lessens heat rise problems in mass concrete placements.

Fly ash benefits to Hardened concrete are:

1. Increased ultimate strength: The additional binder produced by the fly ash reaction with available lime allows fly ash concrete to continue to gain strength over time. Mixtures designed to produce equivalent strength at early ages (less than 90 days) will ultimately exceed the strength of straight cement concrete mixes

2. Reduced permeability: The decrease in water content combined with the production of additional cementitious compounds reduces the pore interconnectivity of concrete, thus decreasing permeability. The reduced permeability results in improved long-term durability and resistance to various forms of deterioration

3. Improved durability: The decrease in free lime and the resulting increase in cementitious compounds, combined with the reduction in permeability enhance concrete durability.

Following are the methods of underwater concreting:

1. Tremie method

2. Pumping technique

3. Hydro valve method

4. Pneumatic valve method

5. Skip method

6. Tilting pallet barge method

7. Preplaced aggregate concrete

8. Toggle bags method

8. Bagged concrete method

9. Caissons

Note: Underwater concreting using tremie method is convenient for pouring large amount of high flowable concrete.

Aerated concrete or Cellular concrete is made by introducing air or gas into a slurry composed of Portland cement or lime and finely crushed siliceous filler so that when the mix sets and hardens, a uniformly cellular structure is formed. Though it is called aerated concrete it is really not a concrete in the correct sense of the word. As described above, it is a mixture of water, cement and finely crushed sand. Aerated concrete is also referred to as gas concrete, foam concrete, cellular concrete. In India we have at present a few factories manufacturing aerated concrete.

The only disadvantage of the mixer is sticking of concrete to bottom of drum. To overcome this a method called buttering of mixer is applied in which some amount of cement mortar is mixed in the mixer before mixing first batch of concrete.

Rate of application of load has a considerable influence on the strength test results. If the rate of application of load is slow, or there is some time lag, Then it will result into lower values of strength. The reason behind this is creep. Due to slower application of load, the specimen will undergo some amount of creep which in turn increases the strain. And this increased strain is responsible for failure of test sample, resulting lower strength values. That’s why with the increased rate of loading during testing of concrete specimens, the compressive strength of concrete increases.

Concrete is not a homogeneous material like steel which is strong in both tension as well as compression. It is a composite material and is obtained by mixing cementing materials, water and aggregate and sometimes admixtures). Concrete is not able to resist direct tension (in comparison of its ability to resist direct compression) because of its low tensile strength and brittle nature.

When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate. Flaky aggregates in concrete leads to decrease in strength due to its weak interlocking phenomenon.

Adding water to the cement starts a chemical reaction called hydration. As hydration proceeds over time, the cement and water are transformed into beneficial calcium silicate hydrate compounds. These compounds are the glue that hold the aggregates together, creating the hard, solid material we know as concrete. There are other compounds that form during the hydration process, but they are not responsible for strength. Also, Aggregate is a inert material and does not take part in chemical reaction.

Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain, thus setting time decreases. Calcium chloride is generally used in cold weather to hasten the setting time and produces an early finish of the concrete. Calcium chloride can effect the characteristics of concrete causing temperature rise, increased internal stresses, corrosion of unprotected reinforcement, a decrease in the resistance to freezing and thawing, an increase in the attack of sulphates, and an increase in the amount of drying shrinkage between 10 and 50 percent.

M-25

Split tensile strength of concrete is (10 - 15)% less than flexural tensile strength of concrete

Flexural tensile strength =

= 2.97 N/mm²

= 11.88%

So, the most approximate Answer is (7 to 11)%.

Height of slump cone is 300 mm, bottom diameter is 200 mm and top diameter is 100 mm.

By applying air entraining agent, the workability of concrete can be increased. The air film will cause lubrication between the coarse aggregates, thus improving the workability of concrete. At the same time segregation and bleeding can also be improved.

While Rebound Hammer, CAPO/Pullout, Windsor probe and ultrasonic pulse velocity tests give indirect evidence of concrete quality, a more direct assessment on strength can be made by core sampling and testing, which is a destructive test. Cores are usually cut by means of a rotary cutting tool with diamond bits.

See clause 6.2.5.1 of IS 456:2000, strain that develops due to constant sustain loading is called creep strain but in the initial age of concrete, creep strain of concrete is higher than later age. However, elastic strain remains constant throughout. So creep coefficient  decreases with time.

Modulus of rupture as obtained by performing flexure test on concrete gives the theoretical maximum tensile stress under bending on concrete specimen.

The soundness of cement is its ability to resist volume change. According to Le Chatelier Method for testing soundness of cement, high alumina cement should not have an expansion of more than 5 mm (IS:6452-1989). See table for other cements.

STANDARD SPECIFICATIONS

Rounded aggregates have less surface area than any other shape of aggregates and require less cement paste for lubrication.

Shrinkage of concrete depends upon the relative humidity and time. Shrinkage of concrete is inversely proportional to relative humidity and directly proportional to time.