The most important factor that affects the strength of concrete is water-cement (W/C) ratio. Lower the W/C ratio, higher would be the strength of concrete. However, there is a cap on this ratio such as 0.5 (max) and near about 0.2 (min). For complete hydration of Portland cement W/C shall be 0.18 (minimum) whereas the extra water (beyond 0.18 ratio) provides workability to the mix. As the W/C ratio increases, strength of concrete would reduce and so the durability properties. The W/C ratio can be lowered by utilizing water reducing admixtures such as plasticizers (upto 20% reduction in water demand) and superplasticizers (20 to 30% reduction). .

Further to the previous answer, w/c ratio (more accurately water/binder ratio) defines the capillary porosity. So, assuming that your workability is sufficient to ensure compaction, then a lower w/b ratio will ensure a more durable concrete.

But, there are more factors to consider than w/b ratio alone, e.g. binder type, minimum cement content, cover depth, etc.. For example, and this is a very specific case, a lower w/b ratio may encourage the structural engineer to use a more slender structural element, thus reducing your cover depth. This may be detrimental to durability (hence the requirement for minimum cover, but at a risk of over-engineering a structure).

The easy way to understand the w/c ratio by believing that the more amount of water in a concrete mix, the more dilute the cement paste will be. This not only affects the compressive strength, but it also affects the tensile and flexural strengths, the porosity, the shrinkage and the colour.

The more the w/c ratio is increased (that is, the more water that is added for a fixed amount of cement), it means strength of concrete is reduced. This is mostly because adding more water creates a diluted paste that is weaker and more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is a large excess of water, that water bleeds out onto the surface. The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.

Using a low w/c ratio is the usual way to achieve a high strength and high-quality concrete, but it does not guarantee that the resulting concrete is always appropriate unless the aggregate gradation and proportion are balanced with the correct amount of cement paste. High-quality concrete can obtain from good mix design and a low w/c ratio are just one part of a good mix design.

Please see this article.Article Role of water/cement ratio on strength development of cement mortar

The water-cement ratio affects both the concrete strength and durability . According to Abram’s Law, the concrete strength at a given age and normal temperature decrease with increasing the water-cement (W/C) ratio assuming full compaction of concrete have been done. Permeability of concrete to water depends mainly on the W/C ratio, which determines the size, volume and continuity of the capillary voids.

The small increase of W/C ratio can increase the concrete permeability to a great extent. Again, permeability is the most important Characteristics determining the long-term durability of reinforced concrete exposed to seawater as it controls the diffusion of aggressive salt-ions into the concrete. ACI 318-83 requires that normal –weight concrete subjected to freezing and thawing in a moist condition should have a maximum W/C ratio of: 0.45 in case of curbs, gutter, guard rail or their sections and 0.50 for other elements.

I completely agree with that, the more the w/c ratio is increased (that is, the more water that is added for a fixed amount of cement), it means strength of concrete is reduced. This is mostly because adding more water creates a diluted paste that is weaker and more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is a large excess of water, that water bleeds out onto the surface.

Dear Dr. Khalid , Thank you so much for your interesting and valuable comments.

Yes, the use of low w/c ratio is the usual way to achieve a high strength and high-quality concrete, but it does not guarantee that the resulting concrete is always appropriate unless the aggregate gradation and proportion are balanced with the correct amount of cement paste. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself.

The following paper ( suggested by Dear Dr. Khalid) is very interesting and useful:

Role of water/cement ratio on strength development of cement mortar