Question

You are laying a concrete border around a circular pond ...The pond is 7 meters in diameter and is surrounded by a path 2 metres wide . The concrete needs to be 50 mm deep .. You are using premixed bagged concrete . 1 cubic metre requires 106 x 20 kg bags . How many 20kg bags of premixed concrete do you need

Answer 1

300 bags of 20 kg of premixed are required.

Explanation:

Given:

diameter of pond 'd'= 7 m (given)

radius of inner border of pond 'r'= d/2= 7/2 m

The radius of outer circular border 'R'= r+2= 7/2+2= 11/2m

Area of concrete border around the circular pond= area of outer circle- area of inner circle

So,
`\pi* R^2-\pi* r^2= \pi* (R^2-r^2)`

=> 22/7 x ((11/2)²-(7/2)²)

=> 22/7 x (121/4-49/4)

=>
`(22)/(7) * (72)/(4)`

=> 56.57 m²

volume of concrete needed= area x height= 56.57 x 0.05= 2.8285 cubic meter

Since 1 cube meter requires 106 x 20 kg bags,

2.8285 cubic meters require bags= 2.8285x106x20 kg

20 kg bags of premixed concrete required= 2.8285x106x20/20

=> 2.8285x106

=> 299.821 or 300 approximately