Problem No$.4(20\backslash \%)$

$($a$)(4\backslash \%)$ Briefly explain the following in the design of concrete mix:

$($i$)$ To achieve a certain slump, less water is needed for larger maximum aggregate size

$($ii$)$ The fraction of fine aggregates $($as $\backslash \%$ of total aggregates$)$ increases with slump

$($iii$)$ The cement content should not exceed $\backslash (550\backslash$mathrm$\{$~kg$\}/\backslash$mathrm$\{$m$\}^\{3\}\backslash )$

$($iv$)$ A maximum water$/$cement ratio is sometimes specified

$($b$)\backslash ((2\backslash \%)\backslash )$ A concrete with $70$ MPa mean strength at $28$ days is to be designed with crushed aggregates and R$52\mathrm{.}5$ cement. What is the w$/$c ratio for your trial mix?

$($c$)\backslash ((2\backslash \%)\backslash )$ After the mix design process, the required cement content is found to be $\backslash (600\backslash$mathrm$\{$~kg$\}/\backslash$mathrm$\{$m$\}^\{3\}\backslash )$ which is higher than the maximum allowable value. What would you do$?$

$($d$)\backslash ((4\backslash \%)\backslash )$ Briefly explain the following:

$($i$)$ The Young's modulus of concrete should be determined form cylinders rather than cubes

$($ii$)$ The ductility of steel obtained in the tensile test depends on the gauge length over which strain is measured

$($e$)(3\backslash \%)$ Besides microscopy, describe THREE experimental observations that reflect the presence of a porous$/$weak interphase between the aggregate and bulk cement paste in concrete.

$($f$)(2\backslash \%)$ Name TWO things you will do to make concrete with low chloride diffusivity, with brief explanatons.

$($g$)(3\backslash \%)$ When carbon dioxide penetrates into a reinforced concrete member, briefly describe the processes that will eventually lead to cracking of the concrete cover to the rebar.