Concrete MCQ Quiz - Objective Question with Answer for Concrete - Download Free PDF

Explanation:

Split-Tensile Strength

• Measures the indirect tensile strength of concrete using a cylinder placed horizontally and compressed diametrically.

• It is the lowest among all because concrete is inherently weak in tension.

• Typical value: 8–12% of cube compressive strength.

 Modulus of Rupture

• Also known as flexural strength, it represents the tensile strength of concrete under bending.

• Higher than split tensile strength but still much lower than compressive strengths.

• Generally 10–15% of cube compressive strength.

Cylinder Strength

• Measures compressive strength using a cylindrical specimen (150 mm × 300 mm).

• Lower than cube strength due to less confinement and aspect ratio effects.

• Typically 80% of cube strength.

 Cube Strength

• Standard compressive strength measured using 150 mm cube specimens.

• Highest among all due to uniform load distribution and shape effect.

• Used as the design strength in most Indian codes (e.g., M20, M25 grades).

Additional Information1. Split-Tensile Strength (Indirect Tensile Test)

• Test Method:
  A cylindrical concrete specimen is placed horizontally and loaded diametrically in compression until failure.
  The compressive force induces tensile stress perpendicular to the loading direction, causing the specimen to split.

• Specimen Size:
  Standard cylinder of 150 mm diameter × 300 mm height.

 Modulus of Rupture (Flexural Strength Test)

• Test Method:
  A beam specimen is placed on two supports and loaded either at one-third points (two-point loading) or at the center (single-point loading) until failure.

• Specimen Size:
  Standard size is 100 mm × 100 mm × 500 mm or 150 mm × 150 mm × 700 mm.

• Code Reference:
  IS 516:1959 (Section on Flexural Strength).

Explanation:

Water-Reducing Admixtures

• Purpose and Function:
  Water reducers decrease the required mixing water for a given workability, thus reducing the water-cement ratio and improving concrete strength and durability.

• Mechanism:
  These admixtures disperse the cement particles in the mix, reducing friction and allowing for better flow with less water.

• Common Compounds Used:
  Examples include lignosulphonates, hydroxylated carboxylic acids, and polycarboxylates (in modern applications).

• Applications:
  Used in residential and commercial concreting, precast elements, and for improving concrete finishes where water reduction is needed without sacrificing workability.

​Air-Entraining Admixtures

• Purpose and Function:
  These admixtures intentionally create microscopic air bubbles in concrete, which improve its resistance to freeze-thaw cycles, scaling, and deicing chemicals.

• Mechanism:
  The entrained air forms tiny voids that act as buffers when water in concrete freezes, preventing internal cracking.

• Common Compounds Used:
  Includes neutralized vinsol resin, wood resins, animal fats, and synthetic detergents.

• Applications:
  Essential for concrete used in cold climates, highways, airport pavements, and bridge decks, where freeze-thaw durability is critical.

Additional InformationSuperplasticizers (High-Range Water Reducers)

• Purpose and Function:
  Superplasticizers significantly increase workability and flowability without increasing water, or reduce water content drastically (up to 30%).

• Mechanism:
  They use advanced polymers (like sulphonated melamine, sulphonated naphthalene, and polycarboxylate ethers) to disperse cement particles efficiently.

• Common Compounds Used:
  Examples include sulphonated melamine formaldehyde (SMF), sulphonated naphthalene formaldehyde (SNF), and polycarboxylate ether (PCE).

• Applications:
  Used in self-compacting concrete (SCC), high-performance concrete (HPC), precast units, and mass concrete pours where flow and strength are both needed.

Accelerating Admixtures

• Purpose and Function:
  Accelerators are added to shorten the setting time and speed up early strength development, especially useful in cold weather.

• Mechanism:
  They promote faster hydration of C3S (tricalcium silicate) phase of cement, allowing earlier formwork removal or load application.

• Common Compounds Used:
  Includes calcium chloride (most common), triethanolamine, sodium thiocyanate, and calcium nitrate.

• Applications:
  Ideal for repair works, precast production, cold weather concreting, and early load-bearing structural elements.
  Note: Use of calcium chloride is restricted in reinforced concrete due to corrosion risk to steel.

Explanation:

• Larger Cube Size – Lower Strength:
  As the size of the concrete cube increases, the measured compressive strength decreases.
  This is due to a higher probability of internal flaws, stress concentrations, and heterogeneity in larger specimens.

• Size Effect:
  Known as the size effect in concrete strength, it results from non-uniform stress distribution and micro-cracking that become more prominent in larger specimens.

 Additional Information

• Standard Cube Sizes:
  The typical size for compressive strength testing is 150 mm cubes. For smaller elements, 100 mm cubes may be used.

• Why Smaller Cubes Show Higher Strength:
  Smaller specimens are more homogeneous and less likely to contain critical flaws, leading to higher strength readings.

• Code Considerations:
  IS 516 specifies correction factors when comparing strengths from cubes of different sizes to maintain consistency.

• Practical Implication:
  Structural elements often show lower in-situ strength than lab-tested small cubes, and design safety factors account for this discrepancy.

Explanation:

• Pulse Velocity = 4.0 km/sec:
  As per IS 13311 (Part 1): 1992, when the ultrasonic pulse velocity lies between 3.5 km/sec and 4.5 km/sec, the concrete is classified as of Good quality.

• Cross Probing Method:
  In this method, the transmitter and receiver are placed on opposite faces of the concrete, providing reliable internal condition assessment.

 Additional InformationUltrasonic Pulse Velocity (UPV) Method

• UPV is a non-destructive technique used to evaluate concrete's homogeneity, quality, and presence of internal flaws.

• IS Code Reference:
  The classification as per IS 13311 (Part 1):1992 is:

  • > 4.5 km/sec → Excellent

  • 3.5 – 4.5 km/sec → Good

  • 3.0 – 3.5 km/sec → Doubtful

  • < 3.0 km/sec → Poor

• Applications:
  UPV is used in assessing existing structures, checking uniformity, and detecting cracks, voids, and honeycombing.

• Advantages:

  • Non-destructive, quick, and simple to perform

  • Helps in structural health monitoring without damaging the concrete

  • Can be used for quality control during construction or investigation of deterioration in old structures

Explanation:

Concrete cast underwater should not fall freely through the water. Otherwise, it may be leached and become segregated. Concrete shall be deposited, continuously until it is brought to the required height. While depositing, the top surface shall be kept as nearly level as possible and the formation of seams avoided. The methods to be used for depositing concrete under water shall be one of the following:

a) Tremie:

The concrete is placed through vertical pipes the lower end of which is always inserted sufficiently deep into the concrete which has been placed previously but has not been set. The concrete emerging from the pipe pushes the material that has already been placed to the side and upwards and thus does not come into direct contact with water. When concrete is to be deposited underwater by means of tremie, the top section of the tremie shall be a hopper large enough to hold one entire batch of the mix or the entire contents of the transporting bucket, if any. The tremie pipe shall be not less than 200 mm in diameter and shall be large enough to allow a free flow of concrete and strong enough to withstand the external pressure of the water in which it is suspended, even if a partial vacuum develops inside the pipe. Preferably, a flanged steel pipe of adequate strength for the job should be used. A separate lifting device shall be provided for each tremie pipe with its hopper at the upper end. Unless the lower end of the pipe is equipped with an approved automatic check valve, the upper end of the pipe shall be plugged with a wadding of the gunny sacking or other approved material before delivering the concrete to the tremie pipe through the hopper, so that when the concrete is forced down from the hopper to the pipe, it will force the plug (and along with it any water in the pipe) down the pipe and out of the bottom end, thus establishing a continuous stream of concrete. It will be necessary to raise slowly the tremie in order to cause a uniform flow of the concrete, but the tremie shall not be emptied so that water enters the pipe. At all times after the placing of concrete is started and until all the concrete is placed, the lower end of the tremie pipe shall be below the top surface of the plastic concrete. This will cause the concrete to build up from below instead of flowing out over the surface, and thus avoid the formation of laitance layers. If the charge in the tremie is lost while depositing, the tremie shall be raised above the concrete surface, and unless sealed by a check valve, it shall be re-plugged at the top end, as at the beginning, before refilling for depositing concrete. It is a convenient and easy method for under water concreting.

b) Direct placement with pumps:

As in the case of the tremie method, the vertical end piece of the pipeline is always inserted sufficiently deep into the previously cast concrete and should not move to the side during pumping.

c) Drop bottom bucket:

The top of the bucket shall be covered with a canvas flap. The bottom doors shall open freely downward and outward when tripped. The bucket shall be filled completely and lowered slowly to avoid backwash. The bottom doors shall not be opened until the bucket rosts on the surface upon which the concrete is to be deposited and when discharged, shall be withdrawn slowly until well above the concrete.

d) Bags:

Bags of at least 0.028 m3 capacity of jute or other coarse cloth shall be filled about two-thirds full of concrete, the spare end turned under so that bag is square-ended and securely tied. They shall be placed carefully in header and stretcher courses so that the whole mass is. interlocked. Bags used for this purpose shall be free from deleterious materials. 

e) Grouting:

A series of round cages made from 50 mm mesh of 6 mm steel and extending over the full height to be concreted shall be prepared and laid vertically over the area to be concreted so that the distance between centers of the cages and also to the faces of the concrete shall not exceed one meter. The stone aggregate of not less than 50 mm nor more than 200 mm in size shall be deposited outside the steel cages over the full area and height to be concreted with due care to prevent displacement of the cages. A stable 1 : 2 cement-sand grout with a water-cement ratio of not less than 0.6 and not more than 0.8 shall be prepared in a mechanical mixer and sent down under pressure (about 0.2 N/mm2) through 38 to 50 mm diameter pipes terminating into steel cages, about 50 mm above the bottom of the concrete. As the grouting proceeds, the pipe shall be raised gradually up to a height of not more than 6 000 mm above its starting level after which it may be withdrawn and placed into the next cage for further grouting by the same procedure. After grouting the whole area for a height of about 600 mm, the same operation shall be repeated, if necessary, for the next layer of 600 mm and so on. The amount of grout to be sent down shall be sufficient to fill all the voids which may be either ascertained or assumed as 55 percent of the volume to be concreted.

As per table 5 of IS 456 : 2000 

Minimum cement content, Maximum water-cement ratio, and Minimum grade of concrete for different exposures with normal weight of aggregates of 20 mm Nominal 

Explanation:

Non-tilting drum mixer 

• A reversing drum mixer (also commonly called a non-tilting mixer) is a type of concrete mixer that produces concrete in single batches.
• The entire drum rotates around its axis as materials are loaded through a charge chute at one end of the drum and exit through a discharge chute at the opposite end of the drum.

Tilting drum mixer 

• Tilting drum mixer means the drum will discharge concrete by tilting downwards.
• It is a rapid discharge process and is used for larger projects.
• Rapid means it delivers concrete by gravity that is tilting the drum downwards because of this the concrete mix obtained will not be subjected to segregation.
• Low workable concrete containing large-sized aggregates greater than 7.5cm is mixed efficiently with this tilting type mixer.

Mixing efficiency depends on some of the factors as follows:

• Shape of the drum
• Angle of the drum
• Size of blades
• Angle of blades

Explanation:

From the above table, It is observed that as the compaction factor increases slump increases.

Slump test is the most general test that is used for the measurement of workability of concrete. It helps in understanding the internal work done by concrete for self-compaction.

Concept:

Theoretical strength of concrete is = 240 x3

x → gel-space ratio = 0.59

Calculation:

Theoretical strength = 240 × (0.59)3

Explanation:

Schmidt’s Rebound Hammer

The rebound hammer is also called as Schmidt hammer.

The rebound hammer test is a non-destructive testing method of concrete that provides a convenient and rapid indication of the compressive strength of the concrete. 

The Schmidt hammer provides an inexpensive, simple and quick

Note:

The results are affected by factors such as smoothness of surface, size and shape of the specimen, moisture condition of the concrete, type of cement and coarse aggregate, and extent of carbonation of surface.

The rebound hammer is a surface hardness tester for which an empirical correlation has been established between compressive strength and rebound number.

But the thickness of concrete cannot be estimated by this method.

Other non-destructive methods of testing are:

1. Ultrasonic pulse velocity test

It is mainly used to measure the time of travel of ultrasonic pulse passing through the concrete and hence concrete quality.

2. Pull out test

This technique can thus measure quantitatively the in-situ strength of concrete when proper correlations have been made.

3. Penetration method

This test is developed to measure the chloride permeability of in-place concrete non-destructively.

4. Radioactive methods

It can be used to detect the location of reinforcement, measure density and to check whether honeycombing has occurred in structural concrete units.

Explanation:

Lime Concrete:

• Lime concrete is a composite mixture of hydraulic lime(or slaked lime) as binding material, sand as fine aggregate, and gravel as coarse aggregate in appropriate proportions.

Cement Concrete:  

• Cement concrete is a composite mixture of cement as binding material, sand as fine aggregate, and gravel as coarse aggregate in appropriate proportions

Difference between lime concrete and cement concrete:

• ​Lime hardens much more slowly than cement-containing mortars, making it much more workable.
• Lime is also less brittle and less prone to cracking, and any cracked areas can absorb carbon dioxide and mend over time.
• The cement hardens very quickly but may be too strong for some applications, e.g., working with old bricks.

Explanation

As per clause 5.4 of IS 456 : 2000

Potable water is considered satisfactory for mixing Concrete and the permissible limits for solids is shown in table below:

Explanation:

Super-plasticizers

They are admixtures that work on surfactant property, in which they disperse and deflocculate cement particles thus making concrete flowing, pourable, and easily placed.

Examples: Sulfonated Melamine formaldehyde resin, sulfonated naphthalene-formaldehyde resin, Mixtures of saccharates, and acid amides.

Note:

(Super-water reducers) - 15 to 30 % water reduction.

Purpose:

1. To increase the workability of concrete without any change in the composition of the mix.

2. To reduce the water content of mixing water, to reduce the water/cement ratio resulting in the increase of strength and durability of concrete.

3. To reduce cement and water contents in the concrete to reduce the cost of production of concrete. The reduction in cement and water contents reduces the creep, shrinkage, and heat of hydration.

4. To slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture and remove air bubbles.

Explanation:

(i) Aerated 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.

(ii) Aerated concrete can be manufactured by:

• By the formation of gas by chemical reaction within the mass during liquid or plastic state.
• By mixing the preformed stable foam with the slurry.
• By using finely powdered metal (usually aluminium powder) with the slurry and made to react with the calcium hydroxide liberated during the hydration process, to give out large quantity of hydrogen gas.

Explanation

As per clause 5.4 of IS 456 : 2000

Potable water is considered satisfactory for mixing Concrete and the permissible limits for solids is shown in table below: