GATE 2019 CE (CE2) – Question 31

Construction Materials · Concrete: constituents, mix design, short-term and long-term properties · 1 mark · Numerical answer

Required characteristic concrete compressive strength is 25 MPa; observed standard deviation is 4 MPa. Trial mixes with w/c percentages 45, 50, 55 and 60 have average cube strengths 35, 25, 20 and 15 MPa respectively. Find the mix water–cement ratio in percent, rounded down to the lower integer.

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Correct answer: 46 to 46

Explanation

Target mean strength is $25+1.65(4)=31.6$ MPa. Interpolate between 35 MPa at w/c 45% and 25 MPa at 50%: $45+(35-31.6)(5/10)=46.7\%$. Round down as instructed: **46%**.