1- Water – Cement Ratio (W/C Ratio)

The ratio of the weight of water to the weight of cement is called Water/Cement ratio. It is the most important factor for gaining the strength of concrete. The lower w/c ratio leads the higher strength of concrete. Generally, the water/cement ratio of 0.45 to 0.60 is used. Too much water leads to segregation and voids in concrete. Water/Cement ratio is inversely proportional to the strength of concrete. As shown in the chart below when the w/c ratio is increased the strength of concrete gets decreased and when w/c ratio is decreased then the strength of concrete increases.

2- Compaction of Concrete

Compaction of concrete increases the density of the concrete because it is the process in which air voids are removed from freshly placed concrete which makes the concrete compact and dense. The presence of air voids in concrete greatly reduces its strength .Approximately 5 % of air voids can reduce the strength by 30 to 40 %. As we can see in the above chart, even at the same water/cement ratio strength is different with different compaction accuracies. In the fully compacted concrete, strength is higher than the insufficiently compacted concrete.

3- Ratio of Cement to the Mixing Water 
For concrete that has to be compacted by a vibrator, a lower water cement ratio may be used. The maximum strength is derived at w/c = 0.4. When the water cement ratio is less than 0.4, there is improper consistency and workability of the cement and honeycombed structure.
4- Ratio of Cement to Aggregates
With an increase in the cement to aggregate ratio, the ultimate strength will increase to some extent. Aggregates are the key factors that will affect the impact strength of the concrete under aggressive load condition. Good quality aggregates will absorb less water content that is meant for hydration of cement.

5- Grading of Aggregate

Grading of aggregates determines the particle size distribution of aggregates. It’s the most important factor for concrete mix.  There are three types of graded aggregate Gap Graded Aggregate, Poorly graded aggregate and Well-graded aggregate. Well-graded aggregate contains all size of particles of aggregate. So that, they have a less amount of voids. The use of well-graded aggregates gives higher strength to the concrete.

6- Curing of Concrete

The higher the curing temperature, the greater is the rate of hardening of concrete. Ten hours curing at a temperature of about 90 degree Celsius will make attain 70% of its 28 days of strength.

Hence please remember, use as less water as possible during concrete mixing and use as more as possible after concreting.

7- The Rate of Loading

The strength of concrete increase with the increase in the rate of loading because at the high rates of loading, there is less time for creep. Creep produces permanent deformation in the structure at constant loading. So that, the failure occurs at limiting values of strain rather than the stress. In rapid loading, the load resistance is better than the slow loading.

8- Effect of Size of Maximum Size of Aggregates on the strength

The use of larger size aggregates leads to higher strength i.e larger the size, lower will be the surface area and the water requirement is lower. Hence a lower water cement ratio can be used which will result in higher strength. When large size aggregates is used due to the internal bleeding, the transition zone becomes weaker and will lead to lower strength. In the case of lean mixes, the larger aggregates will give higher strength.
9- Maturity Concept of Concrete

The strength development of concrete depends on both time and the temperature. The strength of the concrete is a function of the summation of the product of time and temperature. This summation is called as the maturity of concrete. Maturity = Σ ( Time x Temperature ). The maturity is measured in degree centigrade hours or degree centigrade days. The maturity of the concrete is helpful in estimating the strength of the concrete at any other maturity as a percentage of the strength of concrete at full maturity.

10- Weather Condition

Weather condition also affects the strength of concrete due to different reasons. In cold climate, exterior concrete is subjected to repeated freezing and thawing action due to the sudden change in weather. It produces deterioration in concrete. With the change in moisture content, materials expand and contract. It produced cracks in concrete.

11- Temperature

With the certain degree of temperature increase, the rate of hydration process increases in it which, it gains strength rapidly. Sudden temperature changes create a thermal gradient, which causes cracking and spalling of concrete. So that, the final strength of concrete is lower at the very high temperature.

12- Age of Concrete

With increase in age of concrete, the degree of hydration would be more. Hydration process is the chemical reaction of water and cement. Hydration produces the gel which plays a significant role in the bonding of particles of the concrete ingredients. Therefore, the strength of concrete increases with its age. Normally, concrete strength gets doubled after 11 years provided there are no adverse factors.

13- Gel-Space Ratio
The gel-space ratio can be defined as the ratio of the volume of the hydrated cement paste to the sum of the volumes of the hydrated cement and the capillary pores. The strength of the concrete can be more correctly related to this ratio. According to Power’s equation, the strength of the concrete can be expressed as : S = 240x3
Here, x = gel space ratio, the intrinsic strength of the gel in MPa is given by 240.
The gel space ratio can be calculated at any age and for any fraction of hydration of the cement. The relationship between the strength and the gel-space ratio is independent of the age.
Relationship between gel-space ratio and the strength

14- Gain of Strength with the Age

The strength at 28 days = f28 ; The strength at 7 days =f7  ;k2 = Constant Value vary from 3 to 6; k1= Constant value varies from 0.3 to 0.8 The rate of gain of strength will depend on the age. In the actual case, the strength of the concrete will develop beyond 28 days. The increase in the strength beyond 28days is taken in into consideration in the design of structures. The strength of the concrete at a lower age and 28 days depends upon the cement composition, fineness of the cement and the curing temperature. Concrete with lower water cement ratio gains strength, more rapidly than the mix with high w/c. The relationship between 7 days strength and the 28 days strength is: 𝛔 (28) = 1.4𝛔 (7) + 150 Here, The strength at 28 days = 𝛔 (28); The strength at 7 days =𝛔 (7), Another Formula is,  f28 = k2 (f7)k1

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