# WHAT IS TOTAL SURFACE INDEX OF AGGREGATES AND HOW TO CALCULATE IT?

|**Total Surface Index of Aggregates**

In my previous post I discussed about surface index of aggregate for individual aggregate grading. But in actual practice we use combination of different aggregates having different sizes. So now the question that comes to our mind is whether we can represent surface index of mixed aggregates or not. The answer is yes and we express it by **Total Surface Index**.

**How to Calculate Total Surface Index**

**Step-1**

Sieve the combined aggregates as per the sieves mentioned below and record the amount of material retained by corresponding sieves.

Sieves required for sieving = 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron & 150 micron.

**Step-2**

Find out the corresponding surface index values for different aggregate grading (refer the table below)

Sieve Size Within Which Particles Lie |
Surface Index For Particles Within Sieve Size Indicated |

80 mm to 40 mm | -2.5 |

40 mm to 20 mm | -2 |

20 mm to 10 mm | -1 |

10 mm to 4.75 mm | 1 |

4.75 mm to 2.36 mm | 4 |

2.36 mm to 1.18 mm | 7 |

1.18 mm to 600 micron | 9 |

600 micron to 300 micron | 9 |

300 micron to 150 micron | 7 |

< 150 micron | 2 |

**Step-3**

Multiply the percentage of material retained on its sieve by the corresponding surface index. Then add all the multiplied values and to their sum add a constant 330 and divide the result by 1000.

**Example Calculation for Total Surface Index**

Sieve size within which particle lie | % of particles within sieve sizes | Surface index for particles within sieve size | Calculating Total Surface Index |

20 mm – 10 mm | 55 | -1 | -55 |

10 mm – 4.75 mm | 15 | 1 | 15 |

4.75 mm – 2.36 mm | 7 | 4 | 28 |

2.36 mm – 1.18 mm | 7 | 7 | 49 |

1.18 mm – 600 micron | 7 | 9 | 63 |

600 micron – 300 micron | 7 | 9 | 63 |

Total = | 164 | ||

Adding constant 330 | 494 |

So Total Surface Index = 494/1000 = 0.494