Concrete Technology

Question 1
It is given that an aggregate mix has 260 grams of coarse aggregates and 240 grams of fine aggregates. The specific gravities of the coarse and fine aggregates are 2.6 and 2.4, respectively. The bulk specific gravity of the mix is 2.3.
The percentage air voids in the mix is ____________. (round off to the nearest integer)
A
2
B
4
C
8
D
16
GATE CE 2022 SET-2   RCC Structures
Question 1 Explanation: 
Given that,
Coarse aggregate = 260 gms
Fine aggregate = 240 gms
G_{CA}=2.6
G_{FA}=2.4
Bulk specific gravity G_m=2.3
Percentage air voids in the mix = ?
G_t (Theoretical specific gravity)

\begin{aligned} &=\frac{\Sigma W}{\Sigma \frac{W}{G}}\\ &=\frac{260+240}{\frac{260}{2.6}+\frac{240}{2.4}}\\ &=2.5 \end{aligned}
\begin{aligned} % \text{ air voids} (V_V)&=\frac{G_t-G_m}{G_t} \times 100\\ &=\frac{2.5-2.3}{2.3}\times 100\\ V_V&=8% \end{aligned}
Question 2
Match all the possible combinations between Column X (Cement compounds) and Column Y (Cement properties):
\begin{array}{|c|l|}\hline \text{Column X}&\text{Column Y} \\ \hline (i) C_3S & \text{(P) Early age strength} \\ \hline (ii) C_2S & \text{(Q) Later age strength}\\ \hline (iii) C_3A& \text{(R) Flash setting}\\ \hline & \text{(S) Highest heat of hydration}\\ \hline & \text{(T) Lowest heat of hydration}\\ \hline \end{array}
Which one of the following combinations is correct?
A
(i) - (P), (ii) - (Q) and (T), (iii) - (R) and (S)
B
(i) - (Q) and (T), (ii) - (P) and (S), (iii) - (R)
C
(i) - (P), (ii) - (Q) and (R), (iii) - (T)
D
(i) - (T), (ii) - (S), (iii) - (P) and (Q)
GATE CE 2022 SET-2   RCC Structures
Question 2 Explanation: 
C_3S- Responsible for early age strength
C_2S - Responsible for later age strength and lowest heat of hydration
C_3A- Flash setting and highest heat of hydration
Question 3
Which of the following equations is correct for the Pozzolanic reaction?
A
Ca(OH)_2 + Reactive Superplasticiser + H_2O \rightarrow C-S-H
B
Ca(OH)_2 + Reactive Silicon dioxide + H_2O \rightarrow C-S-H
C
Ca(OH)_2 + Reactive Sulphates + H_2O \rightarrow C-S-H
D
Ca(OH)_2 + Reactive Sulphur + H_2O \rightarrow C-S-H
GATE CE 2022 SET-1   RCC Structures
Question 3 Explanation: 
Pozzolanic materials have no cementing properties itself but have the property of combining with lime to produce stable compound.
Pozzolana is considered as siliceous and aluminous materials and when added in cement it have SiO_2 and Al_2O_3 form.
So, pozzolanic reaction :
H_2O + Reactive slilica-di-oxide + H_2O \rightarrow C-S-H gel or tobermonite gel
There are 3 questions to complete.

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