Prestressed Concrete Structures Multiple Choice Questions on “Crack Widths in Members”.
1. The width of cracks that developed in prestressed members is governed by how many factors?
Clarification: ACI committee and Nawy have indicated that the width of cracks that develops in prestressed members is governed by: the average strain at the level at which cracks are considered, the minimum cover to the tension steel, the overall depth of the member in the neutral axis depth.
2. The formula recommended by the British code BS: 8110 – 1985 for the estimation of surface crack width Wcr is?
a) Wcr = 3acrεm / 1+2(acr – Cmin/h-x)
b) Wcr = 3acrεm
c) Wcr = 3acrεm / 1+2(acr + Cmin/h-x)
d) Wcr = 5acrεm
Clarification: Wcr = 3acrεm / 1+2(acr – lmin/h-x) is recommended by the British code BS: 8110 – 1985 for the estimation of surface crack width based on the work of beedy, acr = distance from the point considered to the surface of the nearest longitudinal area, εm = average strain at the level where cracking is being considered, Cmin = minimum cover to the tension steel, h = overall depth of member, x = neutral axis.
3. The reason for smaller crack widths in slabs under service loads is?
a) H – x is large
b) H + x is small
Clarification: In structural members, εm is a maximum at the tension face if h-x is sufficiently small for the crack width at the tension face not to exceed the permissible limit of 0.3mm, it will not exceed the limit anywhere, this is the reason for smaller crack widths in slabs under service loads, provided the thickness does not exceed about 200mm.
4. The stabilized mean crack spacing acs can be expressed as ____________
a) C (A1/Ʃ0)
b) C (A1/Ʃ8)
c) C (A1/Ʃ5)
d) C (A1/Ʃ7)
Clarification: Nawy based on extensive research work, have developed empirical relations to predict the maximum width of cracks and their mean spacing in pre tensioned and post tensioned beams and according to the investigations the stabilized mean crack spacing, acs can be expressed as C (A1/Ʃ0)
At = effective concrete area in tension, Ʃ0 = sum of the circumferences of the reinforcing elements, C = empirical constant.
5. The width of cracks is influenced by?
a) Hollow stress
b) Linear stress
c) Net stress
d) Shear stress
Clarification: The widths of cracks are influenced by the net stress in the tendons, their surface characteristics and the effective area in tension.
Width of crack of Case 1: directly over a bar, the distance acr is equal to the concrete cover Cmin equation then reduces: Wcr = 3acrεm and Case 2: when distance acr is large: Wcr = 1.5(h-x) εmin.
6. The British and Indian standard codes on prestressed concrete maximum limiting crack for members exposed to aggressive environment is?
Clarification: The British and Indian standard codes on prestressed concrete prescribe maximum limiting crack widths of 0.1mm for members exposed to aggressive environment and 0.2mm for all other members.
7. A nano polymer range which meets wider expectations as aesthetics is known as ____________
Clarification: A nano polymer range which meets wider expectations as aesthetics, economics, durability and performance of manufactured concrete products and the application of nano technology in the production of nano polymers has revolutionized the concrete industry.
8. The construction joints should be located at points of ____________
a) Maximum shear
b) Minimum shear
c) Total shear
d) Average shear
Clarification: Construction joints should be planned in advance and preferably they should be located at points of minimum shear and they should be nearly perpendicular to the principal lines of stress and construction joints are generally either vertical or horizontal.
9. When the work has resumed the surfaces of the concrete previously placed should be cleaned of ____________
Clarification: When the work is resumed the surface of the concrete previously placed should be thoroughly cleaned of dirt, scum, laitance, loosely projecting aggregates and other soft material using stiff wire brushes and the surface should then be thoroughly soaked with clean water for two to three hours before further concreting using a thin layer of cement slurry.
10. In roadway slabs, construction joints should be formed?
Clarification: In roadway slabs, construction joints shall be formed vertical and in true alignment and shear layers in construction joints should be constructed as shown in working site plans and in the case of box girders webs, theses shear keys are normally shown on the plans to the full width.