The strength of concrete typically increases with age. This is due to a process called hydration, which is the chemical reaction between water and cement particles in the concrete mixture. As the concrete hydrates, it goes through various stages of development, resulting in the formation of a hardened and durable material.

Here is a detailed explanation of how the strength of concrete changes with age:

1. Initial Setting and Hardening:
- During the initial stage, the concrete mixture undergoes the process of setting, where it transforms from a plastic state to a solid state.
- The concrete starts to harden and gains some initial strength within the first 24 hours after pouring.
- However, the strength at this point is relatively low and not suitable for structural purposes.

2. Early Age Strength Gain:
- As the hydration process continues, the cementitious materials in the concrete react with water molecules, forming chemical compounds that contribute to the development of strength.
- The early age strength gain is usually rapid, and significant improvements in strength can be observed within the first few days or weeks.
- This is mainly due to the formation of calcium silicate hydrate (C-S-H) gel, which is the primary binder in concrete.
- The C-S-H gel fills the voids between the cement particles, creating a dense matrix that enhances the concrete's strength.

3. Mid-Range Age Strength Gain:
- After the initial rapid strength gain, the rate of strength development slows down. This stage is often referred to as the mid-range age.
- The hydration process continues, and the concrete gradually gains strength over time.
- The formation of additional C-S-H gel and other hydration products contributes to the ongoing strength improvement.
- The rate of strength gain during this stage is relatively slower compared to the early age.

4. Long-Term Strength Gain:
- The strength development of concrete continues for an extended period, often spanning several years.
- Although the rate of strength gain decreases significantly, the concrete still continues to get stronger with time.
- This long-term strength gain is mainly attributed to the ongoing hydration reactions and the gradual densification of the concrete matrix.

In summary, the strength of concrete increases with age due to the continuous hydration process. The formation of C-S-H gel and other hydration products contributes to the development of a dense and strong concrete matrix. However, it is important to note that the rate of strength gain decreases over time, and the concrete may reach a point of saturation where the strength improvement becomes negligible.