Difference in Cement cost between Local Fundi mix ratio vs. specified ratio.
In Kenya, majority of the local construction developers who have no full architectural and structural engineering services rely on the local fundis and foremen to guide them on the mix ratios for cement, sand and ballast.
The local fundi mix ratio is usually 2 bags of cement for 4 wheelbarrows of sand to 8 wheelbarrows of ballast.
This represents a ratio of 1 to 4 to 8.
The specified mix ratio of for exposed Reinforced concrete is 1 to 2 to 4.
This represents 2 bags of cement to 1 wheelbarrow of sand to 4 wheelbarrows of ballast.
2 bags of cement make up 1 wheel barrow.
Example. 170m2 residential house.
Calculations when the house construction has used cement ratios of 2 bags cement to 4 wheelbarrows of sand to 8 wheelbarrows of ballast will be as below.
| FOUNDATION | |||
| BLINDING | |||
| CEMENT BAGS | 14.98 | ||
| SAND | 3,630.77 | ||
| BALLAST | 5,990.77 | ||
| STRIP FOOTING | |||
| CEMENT | 88.85 | ||
| SAND | 21,538.46 | ||
| BALLAST | 35,538.46 | ||
| FOUNDATION WALLING |
2000 stones |
||
| CEMENT |
29.17 |
||
| SAND |
12,376.00 |
||
| RING BEAM | |||
| CEMENT | 25.38 | ||
| SAND | 6,153.85 | ||
| BALLAST | 10,153.85 | ||
| SUPERSTRUCTURE WALLING | 3800 stones | ||
| CEMENT | 41.71 | ||
| SAND | 17,696.00 | ||
| BALLAST | 10,267.57 | ||
| ROOF | 2000ft | ||
| FLOOR SCREED | |||
| CEMENT | 23.76 | ||
| SAND | 5,760.00 | ||
| WALL PLASTER | |||
| CEMENT | 41.25 | ||
| SAND | 17,500.00 | ||
| WALL KEYING | |||
| CEMENT | 6.50 | ||
| SAND | 2,756.25 | ||
| TOTAL | |||
| COST | KES | ||
| CEMENT | 271.60 | 190,119.00 | 190,119.00 |
| SAND | 87,411.33 | 87.41 | 87,411.33 |
| BALLAST | 61,950.65 | 61.95 | 99,121.03 |
| TOTAL | 376,651.36 | ||
271 Bags of cement costing KES 190,000 will be used in total.
Calculations for the same house using specified ratios of 1 to 2 to 4 will be as below
| FOUNDATION | |||
| BLINDING | |||
| CEMENT BAGS | 27.81 | ||
| SAND | 3,371.43 | ||
| BALLAST | 5,562.86 | ||
| STRIP FOOTING | |||
| CEMENT | 165.00 | ||
| SAND | 20,000.00 | ||
| BALLAST | 33,000.00 | ||
| FOUNDATION WALLING |
2000 stones |
||
| CEMENT |
38.90 |
||
| SAND |
11,786.67 |
||
| RING BEAM | |||
| CEMENT | 47.14 | ||
| SAND | 5,714.29 | ||
| BALLAST | 9,428.57 | ||
| SUPERSTRUCTURE WALLING | 3800 stones | ||
| CEMENT | 41.71 | ||
| SAND | 17,696.00 | ||
| BALLAST | 10,267.57 | ||
| ROOF | 2000ft | ||
| FLOOR SCREED | |||
| CEMENT | 23.76 | ||
| SAND | 5,760.00 | ||
| WALL PLASTER | |||
| CEMENT | 41.25 | ||
| SAND | 17,500.00 | ||
| WALL KEYING | |||
| CEMENT | 6.50 | ||
| SAND | 2,756.25 | ||
| TOTAL | |||
| COST | KES | ||
| CEMENT | 392.07 | 274,450.41 | 274,450.41 |
| SAND | 84,584.63 | 84.58 | 84,584.63 |
| BALLAST | 58,259.00 | 58.26 | 93,214.40 |
| TOTAL | 452,249.44 | ||
392 bags of cement will be uses, costing KES 274,000.
this represents a 30% increase in cost of cement.
This represents a 16% increase in overall cost of concrete.
For most local constructions whereby developers engage fundis without architectural and engineering consultations, this is the amount of % savings they usually aim to get.
This method of achieving savings can be compared to an own goal since once structural defects check in, they are unrepairable and can result in the loss of the complete building, usually costing millions of shillings.
Developers should engage their architects and engineers in other methods that can result in the same % savings without compromising structural integrity.
Francis Gichui Kamau, Architect.
0721410684