The factors affecting the strength of concrete are mentioned below. These factors can be either dependent or independent of each other when comes to the concrete strength. Most of the factors are interrelated in certain means. The primary factor that has a higher influence on the strength of the concrete is the mix design factors. Each of them is mentioned and explained in detail one by one.
Factors Influencing Strength of Concrete
The factors affecting the strength of concrete are:
1. Ratio of the cement to the mixing water

2. Ratio of cement and aggregate

3. Grading of the aggregate

4. Shape, size, texture of the aggregate

5. Mode of compaction

6. Curing method and the curing temperature

7. Concrete Mix

8. Rate of Loading

9. Moisture conditions

10. Age of the cement

11. Gel  space ratio

12. Age of concrete

13. Maximum size of the aggregates

14. Water Cement Ratio

15. Porosity

1. 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.
2. 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.
3. Grading of the Aggregates
Among the three group of aggregates we have: Gap Graded, Poorly graded and Wellgraded aggregates, the use of wellgraded aggregates imparts higher strength to the concrete.
4. Type and the Size of Aggregate
A rounded spherical shaped aggregate when compacted contains fewer voids than an irregular and flaky aggregate of the same nominal size. Therefore, the rounded spherical aggregates give higher strength.
Larger the aggregate size, smaller will be the surface area for developing the gel bonds that will lead to lower strength of concrete. Larger aggregates will also bring a heterogeneous structure for the concrete thus making the non  uniform distribution of the loads when it is stressed.
5. Degree of Compaction
Inadequate compaction leading to air void content of 5% and 10% results in a loss of strength of 30% and 55% respectively. The concrete compacted by the vibrator displays higher strength even up to a water cement ratio of 0.3.
6. Curing Method and Curing Temperature
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.
7. Rate of Loading
The strength of concrete increase with the increase in the rate of loading. At low rates of loading, there is more time for creep to occur so that the increase of strength with the rate of loading provides. The evidence for the theory that the failure occurs at limiting values of strain rather than the stress.
8. Moisture Conditions of the Specimen
The strength of the concrete will depend on the moisture content at the time of loading. Dry cubes of concrete may have drying shrinkage and bond failure that will lead to smaller strength. The moisture content in concrete provides lubrication effect and will reduce the strength.
Also, the specimen should be tested immediately after taking it out of the curing tank in order to give uniformity to the results as compared to the testing of samples which may have dried at different degrees.
The strength of dry samples = (1.1 to 1.2) x strength of the saturated sample
9. Age of the Cement
With time and age, the strength of the cement concrete reduces since it will set up by absorbing moisture from the atmosphere.
10. Size of the Specimen relative to the maximum size of the aggregates
Generally, 150mm cubes are specified irrespective of the size of the aggregates. However, for aggregates that are less than 25mm in size, 100mm cube is allowed.
11. GelSpace Ratio
The gelspace 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 = 240x^{3}
^{}
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 gelspace ratio is independent of the age.
Fig.2.Relationship between gelspace ratio and the strength 
12. Gain of Strength with the Age
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)
f_{28} = k_{2} (f_{7})^{k1}
13. Maturity Concept 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.
14. 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.
15. Water  Cement Ratio (W/C Ratio)
In engineering practice, the strength of the concrete at a given age and cured in water at a specified temperature is mainly dependent on two main factors. They are the:
 Water  Cement Ratio
 Degree of Compaction
Abram's Watercement Ratio Law:" For any given condition of test the strength of the workable concrete mix is dependent only on the water cement ratio".
Lesser the water cement ratio for a workable concrete mix, the strength will be greater. For Abram's law, it follows that provided the concrete is fully compacted, the strength is not affected by the aggregate shape, type, texture or the grading, workability and the richness of the mix. The cement requires about 1/5 to 1/4 of its weight of water to become completely hydrated.
The strength of the concrete increases with the cement content and decreases with air and water content. In 1918, Abrams presented his classic law in the form
S = A/B^{X}
^{}
In 1897, Feret formulated a general rule defining the strength as,
Here, S = strength; c,e and a are the volume of the cement, water and air respectively; k = constant;