Construction Interview Questions & Answers

Ties or Rings are commonly used in columns or beams

• To avoid buckling of columns or beams
• To bind or hold the reinforcement bars properly in position
• And to resist against the shear failure.

In columns, the reinforcement ratio is totally depending on the stresses developing in the structures. The maximum and minimum reinforcement ratio are

Longitudinal Reinforcement – For cross-sectional area of longitudinal reinforcement

            Minimum Reinforcement = 0.8%

            Maximum reinforcement = 6%

Average 4% is provided in the field works but the amount of steel is different for different RCC structures as per the stress developed in it.

Due to the following reasons: –

Concrete is strong in compression but weak in tension. Thus, we use steel as reinforcement to make the concrete strong in compression as well as in tension. If we only use steel, the structure becomes expansive. If we only use concrete, the structure becomes weak. Therefore, for safe and economical structure we have to use steel with concrete.

Another main reason for the use of steel is that the coefficient of thermal expansion of steel as well as of concrete is almost same, so the bond between them didn’t break on expansion and contraction. If we use any other material, due to the difference in thermal coefficient they lose out their bond at the time of expansion and contraction.

Steel is ductile material and concrete is brittle in nature, thus stopping any sudden failure.

Yes, Steel is a metal and when we use it with concrete then it is known as reinforcement. It is widely used with concrete because coefficient of thermal expansion if steel and concrete is approximately same otherwise, we can go for other reinforcing material like aluminum, brass, bamboo etc. Currently rigorous research is being conducted to replace steel by some other material like bamboo, because it is economic and environment friendly.

Water-cement ratio is inversely proportional to strength of concrete so on the increase of water- cement ratio, the strength of concrete decreases. Water to cement ratio is defined as the mass of water to that of cement in the concrete mix. Generally, w/c ratio taken as 0.3 to 0.6. If the w/c ratio is on the higher side, some water will still be left after hydration process. This water will gradually evaporate, leaving some voids in the concrete block and the presence of voids results in greatly reduced strength.

Rusting of steel is not always bad if it is very mild or in limit. If there is only very little rusting the bond strength or bond performance depends on the type of bas reinforcement under consideration.

For Plain round bars – Here the bond got its strength from adhesion and interface friction between concrete and steel. The rust on plain round bars improves the bond performance by the formation of rough surfaces which increase the friction between steel and concrete.

For Deformed bars – Here the bond got its strength by mechanical lock formed due to ribs on steel. The rust on deformed bars weakens the bond performance by filling the gap between the ribs and evening out the original deformed shape.

Concrete spalling is break away of concrete surface so that the steel inside it, is exposed. It typically begins when the steel reinforcing embedded within the concrete member rusts. Contrary of popular belief, concrete is porous. So, when steel exposed to air and water, a chemical reaction takes place where iron oxide (rust) is produced. The production of iron oxide includes a volumetric expansion of the bar by up to 6 times the original volume. These expansive forces can cause the concrete to delamite or to crack, spall and break off.

NOTE

Spall – A spall is defined as flakes of material that are broken off of a larger solid body.

Digits 33, 43 and 53 represents 28 days compressive strength in N/mm2 of standard cube of face area 50 sq.cm made up of cement mortar 1:3.

Two major types of volume change of interest to civil engineers are creep and shrinkage. Creep is the increase in strain due to sustained constant load. In most structures above ground, creep is taking place under drying conditions. Drying out of water from the interior of concrete leads to shrinkage.

Major Constituents of concrete are following: –

1. Cement – is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces and fills the voids between aggregates (coarse as well as fine aggregate)
2. Fine Aggregate – fine aggregates fills voids present in coarse aggregates and reduces shrinkage & cracking,
3. Coarse Aggregate – Coarse aggregates increases the crushing strength of concrete, makes concrete solid hard mass, reduces cost of concrete by occupying major volume in concrete.
4. Water – Water is needed for the hydration of cement but not all is used up for this purpose. Part of this added water is to provide workability during mixing and for placing

Stages of concreting are:

1. Batching,
2. Mixing,
3. Transportation,
4. Placing,
5. Compaction,
6. Finishing and
7. Curing,

As we know, seawater contains a number of dissolved salts that affect our structure physically and chemically. But the main durability concern for structures nearby seawater is the corrosion of the reinforcement resulting from chloride ingress. Mainly due to splash and tidal zones. To be durable under seawater exposure conditions, concrete must have an adequate cover and low permeation properties with the appropriate choice of cementitious materials. Seawater should never be used as mixing water for the production of reinforced or prestressed concrete structures.

Hydraulic lime provides a faster initial set and higher compressive strength in a very short time and eminently hydraulic lime will set in more extreme conditions like underwater constructions.

Low heat cement should be use for mass concreting to prevent development of cracks. In this cement the rate of development of strength is low but 28 days strength is same as ordinary Portland cement.

The rounded aggregate has minimum voids and minimum ratio of surface area to volume, thus requiring minimum cement paste to make good concrete but due to absence of interlocking, these aggregates are not suitable for high strength concrete.

Size of aggregate is governed by following

1. Size of Structural member – Aggregate should go to each corner of member and cover reinforcement completely.
2. Distance between two main bars – Aggregate should be small enough so that it can pass through the distance between two main bars. Due to this reason, it is kept 5 mm less than the distance between two main bars.
3. Minimum Cover – Size of aggregate should be 5 mm less to minimum nominal cover. If aggregate size is more than the nominal cover provided for member, then there is possibility of exposure of reinforcement to environment.

In general, 20 mm nominal size coarse aggregate is used for most if the work but in case of massive concreting, like dam’s construction, 40 mm and even higher size can be used. For extremely thin slab, 10 mm nominal size aggregate is used for a better finish.

Concrete mix design is the calculation of proportion of constituent elements (like cement, coarse aggregate, fine aggregate, water and sometime admixture also) to achieve desired degree of workability of fresh concrete and desired strength, durability, surface finish of hardened concrete.

The characteristic strength of concrete is the strength of material below which not more than 5% of the test results are expected to fall. It is denoted by “fck”.

Air-Entrainment improves durability, workability and plasticity but it have an adverse effect on the strength of concrete. The decrease in strength is usually proportional to the amount of entrained air. For each percent increase in air content, the compressive strength reduces approximately by 1.4 MPa.

Water used for mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalies, salts, sugar, organic materials etc. Potable water is preferable. The pH value of water shall not be less than 6. Sea water is also not preferable because of presence of harmful salts in sea water. Water found satisfactory for mixing is also suitable for curing of concrete.