Questions & Answers of Concrete Structures

Two rectangular under-reinforced concrete beam sections X and Y are similar in all aspects except that the longitudinal compression reinforcement in section Y is 10% more. Which one of the following is the correct statement?

An RCC short column (with lateral ties) of rectangular cross section of 250 mm × 300 mm is reinforced with four numbers of 16 mm diameter longitudinal bars. The grades of steel and concrete are Fe415 and M20, respectively. Neglect eccentricity effect. Considering limit state of collapse in compression (IS 456 : 2000), the axial load carrying capacity of the column (in kN, up to one decimal place), is ______

An RCC beam of rectangular cross section has factored shear of 200 kN at its critical section. Its width b is 250 mm and effective depth d is 350 mm. Assume design shear strength $\style{font-family:'Times New Roman'}{\tau_c}$ of concrete as 0.62 N/mm2 and maximum allowable shear stress $\style{font-family:'Times New Roman'}{\tau_{c,max}}$ in concrete as 2.8 N/mm2. If two legged 10 mm diameter vertical stirrups of Fe250 grade steel are used, then the required spacing (in cm, up to one decimal place) as per limit state method will be ______

As per IS 456 : 2000, the minimum percentage of tension reinforcement (up to two decimal places) required in reinforced-concrete beams of rectangular cross-section (considering effective depth in the calculation of area) using Fe500 grade steel is ______

A reinforced-concrete slab with effective depth of 80 mm is simply supported at two opposite ends on 230 mm thick masonry walls. The centre-to-centre distance between the walls is 3.3 m. As per IS 456 : 2000, the effective span of the slab (in m, up to two decimal places) is ______

A singly-reinforced rectangular concrete beam of width 300 mm and effective depth 400 mm is to be designed using M25 grade concrete and Fe500 grade reinforcing steel. For the beam to be under-reinforced, the maximum number of 16 mm diameter reinforcing bars that can be provided is

A 6 m long simply-supported beam is prestressed as shown in the figure.

The beam carries a uniformly distributed load of 6 kN/m over its entire span. If the effective flexural rigidity $ EI=2\times10^4 $ kNm2 and the effective prestressing force is 200 kN, the net increase in length of the prestressing cable (in mm, up to two decimal places) is ______

According to IS 456 - 2000, which one of the following statement about the depth of netural axis $\style{font-family:'Times New Roman'}{{\mathrm\chi}_{\mathrm u,\mathrm{bal}}}$ for a balanced reinforced concrete section is correct?

A pre-tensioned rectangular concrete beam 150 mm wide and 300 mm depth is prestressed with three straight tendons, each having a cross-sectional area of 50 mm2, to an inital stress of 1200 N/mm2. The tendons are located at 100 mm from the soffit of the beam. If the modular ratio is 6, the loss of prestressing force (in kN, up to one decimal place) due to the elastic deformation of concrete only is _________

Let the characteristic strength be defined as that value, below which not more than 50% of the results are expected to fall. Assuming a standerd deviation of 4 MPa, the target mean strength (in MPa) to be considered in the mix design of a M25 concret would be

Group I gives a list of the methods and test apparatus for evaluating some of the properties of Ordinary Portland Cement (OPC) and concrete. Group II gives the list of these properties.

Group I Group II
P. Le Chatelier test 1. Soundness of OPC
Q. Vee-Bee test 2. Consistency and setting time of OPC
R. Blaine air permeability test 3. Consistency or workability of concrete
S. The vicat apparatus 4. Fineness of OPC

The correct match of the items in Group I with the items in Group II is

A simply supported rectangular concrete beam of span 8 m has to be prestressed with a force of 1600 kN. The tendon is of parabolic profile having zero eccentricity at the supports. The beam has to carry an external uniformly distributed load of intensity 30 kN/m. Neglecting the self-weight of the beam, the maximum dip (in meters, up to two decimal places) of the tendon at the mid-span to balance the external load should be _________________

In shear design of an RC beam, other than the allowable shear strength of concrete $ \style{font-family:'Times New Roman'}{\left(\tau_c\right)} $ , there is also an additional check suggested in IS 456-2000 with respect to the maximum permissible shear stress $ \style{font-family:'Times New Roman'}{\left(\tau_{c\;\max}\right)} $ . The check for  $ \style{font-family:'Times New Roman'}{\tau_{c\;\max}} $ is required to take care of

A reinforced concrete (RC) beam with width of 250 mm and effective depth of 400 mm is reinforced with Fe415 steel. As per the provisions of IS 456-2000, the minimum and maximum amount of tensile reinforcement (expressed in mm2) for the section are, respectively

For M25 concrete with creep coefficient of 1.5, the long-term static modulus of elasticity (expressed in MPa) as per the provisions of IS:456-2000 is ________

As per IS 456-2000 for the design of reinforced concrete beam, the maximum allowable shear stress $ \style{font-family:'Times New Roman'}{\left(\tau_{c\;\max}\right)} $ depends on the

Workability of concrete can be measured using slump, compaction factor and Vebe time. Consider the following statements for workability of concrete:

(i) As the slump increases, the Vebe time increases
(ii) As the slump increases, the compaction factor increases

Which of the following is TRUE ?

Consider the following statements for air-entrained concrete:

(i) Air-entrainment reduces the water demand for a given level of workability
(ii) Use of air-entrained concrete is required in environments where cyclic freezing and thawing is expected

Which of the following is TRUE ?

Consider the singly reinforced beam shown in the figure below:

At cross-section XX, which of the following statements is TRUE at the limit state ?

The development length of a deformed reinforcement bar can be expressed as $ (I/k)\;(\phi\sigma_s/\tau_{bd}). $. From the IS:456-2000, the value of k can be calculated as _________.

The composition of an air-entrained concrete is given below:

Water       : 184 kg/m3
Ordinary Portland Cement(OPC)       : 368 kg/m3
Sand       : 606 kg/m3
Coarse aggregate       : 1155 kg/m3

Assume the specific gravity of OPC, sand and coarse aggregate to be 3.14, 2.67 and 2.74, respectively. The air content is ______ litres/m3.

Consider the singly reinforced beam section given below(left figure). The stress block parameters for the cross- section from IS: 456-2000 are also given below (right figure). The moment of resistance for the given section by the limit state method is ______ kN-m.

 

A column of size 450 mm × 600 mm has unsupported length of 3.0 m and is braced against side sway in both directions. According to IS 456: 2000, the minimum eccentricities (in mm) with respect to major and minor principal axes are :

A simply supported reinforced concrete beam of length 10 m sags while undergoing shrinkage. Assuming a uniform curvature of 0.004 m-1 along the span, the maximum deflection (in m) of the beam at mid-span is _________.

According to the concept of Limit State Design as per IS 456: 2000, the probability of failure of a structure is _________.

In a pre-stressed concrete beam section shown in the figure, the net loss is 10% and the final pre-stressing force applied at X is 750 kN. The initial fiber stresses (in N/mm2) at the top and bottom of the beam were:

Match the information given in Group – I with those in Group - II.

Group – I Group – II
P Factor to decrease ultimate strength to design strength                      1 Upper bound on ultimate load
Q Factor to increase working load to ultimate load for design 2 Lower bound on ultimate load
R Statical method of ultimate load analysis 3 Material partial safety factor
S Kinematical mechanism method of ultimate load analysis 4 Load factor
   

While designing, for a steel column of Fe250 grade, a base plate resting on a concrete pedestal of M20 grade, the bearing strength of concrete (in N/mm2) in limit state method of design as per IS:456-2000 is ________________

The first moment of area about the axis of bending for a beam cross-section is

The flexural tensile strength of M25 grade of concrete, in N/mm2, as per IS:456-2000 is __________

The modulus of elasticity, E=5000fck where fck is the characteristic compressive strength of concrete, specified in IS:456-2000 is based on

The creep strains are

A rectangular concrete beam 250 mm wide and 600 mm deep is pre-stressed by means of 16 high tensile wires, each of 7 mm diameter, located at 200 mm from the bottom face of the beam at a given section. If the effective pre-stress in the wires is 700 MPa, what is the maximum sagging bending moment (in kNm) (correct to 1-decimal place) due to live load that this section of the beam can withstand without causing tensile stress at the bottom face of the beam ? Neglect the effect of dead load of beam. ________________

As per IS 456:2000, in the Limit State Design of a flexural member, the strain in reinforcing bars under tension at ultimate state should not be less than

Which one of the following is categorised as a long-term loss of prestress in a prestressed concrete member?

A concrete beam prestressed with a parabolic tendon is shown in the sketch. The eccentricity of the tendon is measured from the centroid of the cross-section. The applied prestressing force at service is 1620 kN. The uniformly distributed load of 45 kN/m includes the self-weight.

The stress (in N/mm2) in the bottom fibre at mid-span is

The cross-section at mid-span of a beam at the edge of a slab is shown in the sketch. A portion of the slab is considered as the effective flange width for the beam. The grades of concrete and reinforcing steel are M25 and Fe415, respectively. The total area of reinforcing bars (As) is 4000 mm2. At the ultimate limit state, xu denotes the depth of the neutral axis from the top fibre. Treat the section as under-reinforced and flanged (xu > 100 mm).

The value of xu (in mm) computed as per the Limit State Method of IS 456:2000 is

The cross-section at mid-span of a beam at the edge of a slab is shown in the sketch. A portion of the slab is considered as the effective flange width for the beam. The grades of concrete and reinforcing steel are M25 and Fe415, respectively. The total area of reinforcing bars (As) is 4000 mm2. At the ultimate limit state, xu denotes the depth of the neutral axis from the top fibre. Treat the section as under-reinforced and flanged (xu > 100 mm).

The ultimate moment capacity (in kNm) of the section, as per the Limit State Method of IS 456:2000 is

A 16 mm thick plate measuring 650 mm x 420 mm is used as a base plate for an ISHB 300 column subjected to a factored axial compressive load of 2000 kN. As per IS 456-2000,The minimum grade of concrete that should be used below the base plate for safely carrying the load is

Consider a reinforcing bar embedded in concrete. In a marine environment this bar under goes uniform corrosion, which leads to the deposition of corrosion products on its surface and increase in the apparent volume of the bar. This subjects the surrounding concrete to expansive pressure. As a result, corrosion induced cracks appear at the surface of  concrete. Which of the following statements is TRUE?

The cross-section of a thermo-mechanically treated (TMT) reinforcing bar has

consider a bar of diameter "D"embedded in a large concrete block as shown in the adjoining figure, with a pull out of force P being applied. Let σb and σst be the bond strength (between the bar and concrete) and the tensile strength of the bar, respectively. If the block is held in position and it is assumed that the material of the block dies not fail, which of the following options represents the maximum value of P?

Consider two RCC beams, P and Q, each having the section 400 mm × 750mm (effective depth,d=750mm) made with concrete having a τcmax=2.1 N/mm2 . For the reinforcement provided and the grade of concrete used, it may be assumed that the τc =0.75 N/mm2 .The design shear in beam P is 400 kN and in beam Q is 750 kN.Considering the provisions of IS 456-2000, which of the following statements is TRUE?

As per India standard code of practice for prestressed concrete (IS:1343-1980) the minimum grades of concrete to be used for post-tensioned and pre-tensioned structural elements are respectively

A doubly reinforced rectangular concrete beam has a width of 300 mm and an effective depth of 500mm. the beam is reinforced with 2200 mm2 of steel in tension and 628 mm2 of steel in compression. The effective cover for compression steel is 50mm. Assume that both tension and compression steel yield. The grades of concrete and steel used are M20 and Fe250 respectively. The stress lock parameters (rounded off to first two decimal places) for concrete shall be as per IS 456:200.

The depth of neutral axis is

A doubly reinforced rectangular concrete beam has a width of 300 mm and an effective depth of 500mm. the beam is reinforced with 2200 mm2 of steel in tension and 628 mm2 of steel in compression. The effective cover for compression steel is 50mm. Assume that both tension and compression steel yield. The grades of concrete and steel used are M20 and Fe250 respectively. The stress lock parameters (rounded off to first two decimal places) for concrete shall be as per IS 456:200.

The moment of resistance of the section is

The modulus of rupture of concrete in terms of its characteristic cube compressive strength (fck) in MPa according to IS 456:2000 is

For limit state of collapse, the partial safety factors recommended by IS 456:2000 for estimating the design strength of concrete and reinforcing steel are respectively

A rectangular concrete beam of width 120 mm and depth 200 mm is prestressed by pretensioning to a force of 150 kN at an eccentricity of 20 mm. The cross sectional area of the prestressing steel is 187.5 mm2. Take modulus of elasticity of steel and concrete as 2.1×105 MPa and 3.0×104 MPa respectively. The percentage loss of stress in the prestressing steel due to elastic deformation of concrete is

Column I gives a list of test methods for evaluating properties of concrete and Column II gives the list of properties

Column I Column II
P. Resonant frequency test 1. Tensile strength
Q. Rebound hammer test 2. Dynamic modulus of elasticity
R. Split cylinder test 3. Workability
S. Compacting factor test 4. Compressive strength

A reinforced concrete structure has to be constructed along a sea coast. The minimum grade of concrete to be used as per IS: 456-2000 is

In the design of a reinforced concrete beam the requirement for bond is not getting satisfied. The economical option to satisfy the requirement for bond is by

Un-factored maximum bending moments at a section of a reinforced concrete beam resulting from a frame analysis are 50, 80, 120 and 180 kNm under dead, live, wind and earthquake loads respectively. The design moment (kNm) as per IS: 456-2000 for the limit state of collapse (flexure) is

A reinforced concrete column contains longitudinal steel equal to 1 percent of net cross-sectional area of the column. Assume modular ration as 10. the loads carried (using the elastic theory) by the longitudinal steel and the net area of concrete, are Ps and Pc respectively. The ration Ps/Pc expressed as percent is

A pre-tensioned concrete member of section 200mm × 250mm contains tendons of area 500 mm2 at the centre of gravity of the section. The prestress in tendons is 1000N/mm2. Assuming modular ratio as 10, the stress (N/mm2) in concrete is

A reinforced concrete beam of rectangular cross section of breadth 230 mm and effective depth 400 mm is subjected to a maximum factored shear force of 120 kN. The grade of concrete, mains steel and stirrup steel are M20, F415 and Feb 250 respectively. For the area of main steel provided, the design shear strength τc as per IS : 456-2000 is 0.48N/mm2. The beam is designed for collapse limit state

The spacing (mm) of 2-legged 8 mm stirrups to be provided is

A reinforced concrete beam of rectangular cross section of breadth 230 mm and effective depth 400 mm is subjected to a maximum factored shear force of 120 kN. The grade of concrete, mains steel and stirrup steel are M20, F415 and Feb 250 respectively. For the area of main steel provided, the design shear strength τc as per IS : 456-2000 is 0.48N/mm2. The beam is designed for collapse limit state

In addition, the beam is subjected to a torque whose factored value is 10.90 kNm. The stirrups have to be provided to carry a shear (kN) equal to

Consider the following statements:

I. The compressive strength of concrete decreases with increase in water-cement ratio of the concrete mix.
II. Water is added to the concrete mix for hydration of cement and workability.
III. Creep and shrinkage of concrete are independent of the water-cement ratio in the concrete mix.

The TRUE statements are

The percentage loss of prestress due to anchorage slip of 3 mm in a concrete beam of length 30 m which is post-tensioned by a tendon with an initial stress of 1200 N/mm2 and modulus of elasticity equal to 2.1 x 105 N/mm2 is

A concrete beam of rectangular cross-section of size 120 mm (width) and 200 mm (depth) is prestressed by a straight tendon to an effective force of 150 kN at an eccentricity of 20 mm (below the centroidal axis in the depth direction). The stresses at the top and bottom fibres of the section are

Consider the following statements:

I. Modulus of elasticity of concrete increases with increase in compressive strength of concrete.
II. Brittleness of concrete increases with decrease in compressive strength of concrete.
III. Shear strength of concrete increases with increase in compressive strength of concrete.

The TRUE statements are

A singly reinforced rectangular concrete beam has a width of 150 mm and an effective depth of 330 mm. The characteristics compressive strength of concrete is 20 MPa and the characteristics tensile strength of steel is 415 MPa. Adopt the stress block for concrete as given in IS 456-2000 and take limiting value of depth of neutral axis as 0.48 times the effective depth of the beam.

The limiting value of the moment of resistance of the beam in kN.m is

A singly reinforced rectangular concrete beam has a width of 150 mm and an effective depth of 330 mm. The characteristics compressive strength of concrete is 20 MPa and the characteristics tensile strength of steel is 415 MPa. Adopt the stress block for concrete as given in IS 456-2000 and take limiting value of depth of neutral axis as 0.48 times the effective depth of the beam.

The limiting area of tension steel in mm2 is