$10$

$8.(15$ Marks$)$ Design a concrete mix for a structural concrete beam consistent with the following:

Environment

Exposed to freezing and thawing but no chlorides

$f_c^{\text{'}}=27\mathrm{.}6$MPa

Minimum dimension of beam $=150mm$

Minimum space between reinforcement $=40mm$

$,$ Minimum cover over reinforcement $=40mm$

$,$ Standard deviation of strengths $=2\mathrm{.}1$MPa $($more than $30$ samples$)$

Air entraining admixtures allowed

Available Materials

Air entrainer:

Manufacturer specification $6\mathrm{.}3m\frac{l}{1}\%$ air$/100$ kg cement

Type GU cement

$,SG=3\mathrm{.}15$

Coarse aggregate:

$19$ mm NMAS, rounded gravel

$,$ SG $=2\mathrm{.}55$

$,$ DRUW $=1761k\frac{g}{m^3}$

Absorption $=3\mathrm{.}6\%$

Total Moisture $=2\mathrm{.}5\%$

Fine aggregate"

Natural sand

$,SG=2\mathrm{.}66$

$,FM=2\mathrm{.}47$

Absorption $=0\mathrm{.}5\%$

$,$ Total Moisture $=2\mathrm{.}0\%$