Question:

Published on: 28 November, 2022

**Discuss the role of Gel-space ratio. How can it be determined? Calculate the gel-space ratio and theoretical strength of a sample of concrete made with 500gm of cement with 0.5 w/c ratio on full hydration and 40% partial hydration. Discuss the factors which influence creep in concrete?**

Answer:

Instead of relating the strength to w/c ratio, the strength can be more correctly related to the solids products of hydration of cement to the space available for formation of this product. Powers and Brownyard have established the relationship between the strength and gel/space ratio. This ratio is defined as the ratio of the volume of the hydrated cement paste to the sum of volume of the hydrated cement and of the capillary pores.

**Calculation of gel/space ratio for complete hydration:-**

Let, C = wt. of cement in gm.

Vc = Sp. Volume of cement = 0.319 ml / gm

Wo = volume of mixing water in ml.

So, gel-space ratio = \(\frac{volume\ of\ gel}{Space\ available}\) = \(\frac{0.657 C}{0.319 C + Wo}\)

**Calculation of gel/space ratio for partial hydration:-**

Let, α = fraction of cement that has hydrated.

So, gel-space ratio = \(\frac{volume\ of\ gel}{Space\ available}\) = \(\frac{0.657 C \alpha}{0.319 C \alpha+ Wo}\)

C = 500 gm; Wo = 250 x 0.5 = 250 gm

w/c ratio = 0.5

Gel/Space Ratio For Complete Hydration

= 0.657 C / ( 0.319 C + Wo ) = 0.8

Theoretical strength of concrete = 240 x 0.8^{3} = 123 MPa

Gel/Space Ratio For 40% Hydration

= 0.657 C α / (0.319 C α + Wo ) = 0.42

Theoretical strength of concrete = 240 x 0.42^{3} = 17.78 MPa.

Factors which influence creep in cement are:

- Influence of aggregate.
- Influence of mix proportions.
- Influence of age.

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**What are flaky and elongated aggregates? How can the shape of aggregate influence strength of concrete? Following is the result of a sieve analysis of 500 gm aggregate:**

IS sieve size |
10 mm |
4.75 mm |
2·36 mm |
1·18 mm |
600 micron |
300 micron |
150 micron |
lower than 150 micron |

Weight retained in gm |
0 |
10 |
50 |
50 |
95 |
175 |
85 |
35 |

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28 November, 2022

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