1. What will be the tensile strength of a concrete specimen of grade M30?
a) 3.83 N/mm2
b) 38.33 N/mm2
c) 0.38 N/mm2
d) 0.038 N/mm2
Explanation: As per IS 456, the empirical formula \(f_{cr}=0.7\sqrt{f_{ck}}\) is used to compute the tensile strength of normal concrete. The concrete specimen of grade M30 would have a characteristic compressive strength of 30 N/mm2. Therefore, the tensile strength is computed as \(0.7f_{cr}=0.7\sqrt30=3.83 N/mm^2\).
2. Which of the below methods is the best method to compute the modulus of elasticity?
a) Initial tangent modulus
b) Secant modulus
c) Tangent modulus
d) Initial secant modulus
Explanation: The stress and strain relationship are plotted and various slopes are drawn in it. The slope of the tangent to the stress-strain curve gives the tangent modulus. The slow a tangent taken at the origin is called the initial tangent modulus. The secant modulus is taken as the ratio of stress to strain at different points as per the ASTM code.
3. Concrete should be placed and finished during the ______ stage.
a) Dormant
b) Cooling
c) Hydrating
d) Mixing
Explanation: During the dormant stage, concrete remains plastic and it does not have any heat production. The presence of ettringite slows the hydration process and it is also responsible for the attainment of early strength.
4. ______ admixture is used to help the pavement withstand the freezing and thawing cycle.
a) Retarders
b) Plasticisers
c) Accelerators
d) Air-entraining
Explanation: Air-entraining admixtures are used to entrain small bubbles into the concrete mix. These bubbles help in withstanding the damage due to freezing and thawing. Retarders are used to slow down the initial setting time, plasticisers help in reducing water content and accelerators help speed up the hydration process.
5. The relationship between the strength gain, time and temperature of the concrete mix is called ______
a) Setting
b) Maturity
c) Creep
d) Curing
Explanation: Maturity is the term used to relate the three parameters of concrete – strength, time and temperature. A graph is plotted between strength gain and time-temperature and it is called the maturity curve. Maturity is an index value.
6. Clay lumps up to 2% are allowed in fine aggregates used for the pavement concrete.
a) true
b) false
Explanation: As per IRC 15, the amount of clay lumps in the fine aggregate must not be more than 1%. The fine aggregates must be free from other organic and deleterious materials, but it is not possible to avoid them completely. Therefore, the 1% condition has been laid down.
7. Which of the below mineral admixtures can be used up to 50% by weight of cement?
a) Lime
b) GBFS
c) Silica fume
d) Fly ash
Explanation: GBFS is granulated blast furnace slag and asper the specifications in IRC 15, it can be added up to 50% by weight of Portland slag cement. Fly ash up to 20% can be added with OPC. Silica fume can be added up to 10% of OPC. Lime is not used as an admixture.
8. A concrete cube of 150 mm was subjected to a compression test. The load at failure was found to be 700 kN. What would be the tensile strength of concrete and can it be used for PQC?
a) 3.9, yes
b) 4.42, yes
c) 3.9, no
d) 4.42, no
Explanation: Tensile strength can be found out using empirical formula, \(f_{cr}=0.7\sqrt{f_{ck}}\). The compressive strength can be found as load per area of the specimen. So, the compressive strength can be found as
\(f_{ck}=\frac{700}{150×150}=0.0311 kN/mm^2≈ 31.11 N/mm^2\)
\(f_{cr}=0.7\sqrt{f_{ck}}=0.7×\sqrt{31.11}=3.9 N/mm^2\)
The compressive strength for pavement is obtained as 31.11 N/mm2 and flexural strength as 3.9 N/mm2. As per IRC, the minimum grade of concrete for rigid pavement is M40, having 40 N/mm2 compressive strength and 4.42 N/mm2 flexural (tensile) strength. So, the sample cannot be used for PQC.
9. Plastic shrinkage is also known as drying shrinkage
a) true
b) false
Explanation: Plastic and drying shrinkage are two different terms. The shrinkage that occurs in a concrete pavement during its early life i.e. when it is plastic, is termed as plastic shrinkage. The loss of water that occurs later during its drying period, is called the drying shrinkage.
10. Which of the below sieve designation and the percentage by weight passing the sieve pairs are matched incorrectly?
a) 31.5 mm – 100
b) 9.5 mm – 30 to 55
c) 600 micron – 8 to 30
d) 75 micron – 0 to 5
Explanation: The gradation for PQC as per IRC 15 consists of a table with details on the percentage that should pass through the corresponding sieve. The sieve designation 9.5 mm has the range of passing between 45 and 65.