Concrete is one of the most widely used building materials in the world. It is durable, versatile, and relatively inexpensive. One of the key properties of concrete is that it continues to get stronger over time through a process called curing. This raises the question: does concrete continue to get harder forever or is there a limit? Let’s take a deeper look at how concrete gains strength over time.
How Concrete Gains Strength
When concrete is first mixed, it starts out as a wet, plastic mixture that can be poured into molds and shapes. At this point, it has very little strength. Here are the key steps that give concrete its initial strength gain:
- Hydration reactions – When water is added to cement powder, it triggers chemical reactions that cause the cement to harden and bind the aggregate together.
- Gel formation – Hydration produces calcium-silicate-hydrate (CSH) gel which coats each particle and binds them into a solid mass.
- Crystal formation – Calcium hydroxide crystals also develop during hydration, providing rigidity.
These processes cause concrete to start gaining strength rapidly in the first 7 days. After 28 days, a concrete mix will typically achieve most of its designed compressive strength, usually around 60-75%. However, hydration continues at a slower rate over many years.
Does Concrete Continue to Get Stronger?
Yes, concrete does continue to gain strength over time, but the rate of gain declines significantly. Here are some key points:
- The hydration reactions that produce CSH gel and calcium hydroxide progressively slow down over time. The reactions do continue for years but at a decreasing rate.
- The concrete continues to gain strength over time, but only at a fraction of the initial rate. Most standards estimate the strength gain after 28 days to be about 5-20% per decade.
- Testing shows that even after 10 years, concrete can gain anywhere from 10-40% additional strength compared to its 28-day strength.
While concrete gets harder with age, the additional strength gain declines over time. There is likely a practical upper limit, but concrete can continue to gain measurable strength over a period of decades.
Factors Affecting Long-Term Strength Gain
There are several factors that influence the extent to which concrete gains strength over the long-term:
- Water-cement ratio – Concrete with lower water content will have higher long-term strength gains.
- Curing conditions – Proper curing with moisture increases long-term strength.
- Porosity – Denser concrete with lower porosity has higher strength development.
- Cement content – Higher cement content leads to more hydration and higher long-term strength.
- Temperature – Colder weather slows hydration reactions and reduces strength gain.
Optimizing these factors allows concrete to continue gaining strength over many years. On the other hand, improper curing or high water content can severely limit long-term strength development.
Does Concrete Get Brittle with Age?
There is a common perception that as concrete gets harder, it also becomes more brittle. However, research shows that concrete’s durability and ductility can continue to improve over time:
- The CSH gel produced by ongoing hydration results in densification of the internal structure. This reduces voids and pathways for aggressive chemicals to enter.
- Calcium hydroxide crystals also fill in capillary pathways, further reducing permeability.
- Reduced permeability results in improved resistance to freeze/thaw damage, scaling, and chemical attack.
- Ongoing hydration of cement particles glues the aggregates more tightly together. This can improve tensile strength.
With proper curing, the increases in strength and density confer improvements in ductility and durability that can counteract brittleness. However, inadequate curing can leave concrete more susceptible to cracking and breakdown over time.
Does Concrete Ever Stop Getting Harder?
Concrete never truly stops gaining strength and hardness, but the practical rate of gain does decrease progressively over time. Here are some considerations:
- Hydration slows significantly but does not stop. CSH and calcium hydroxide continue forming at decreasing rates.
- Strength gain after 10+ years becomes minimal and difficult to accurately measure. It may reach a practical plateau.
- However, very slow hydration and pozzolanic reactions are theorized to persist for over 100 years in some cases.
- Well-cured concrete likely continues to gain at least some strength over a century or more, albeit at diminishing rates.
While concrete gets harder with age, the process gradually slows down. For engineering purposes, strength gain after a few decades becomes negligible. However, chemical reactions may cause concrete to continue gaining hardness over its service life.
Does Age Affect Concrete Strength Testing?
The age of concrete does impact strength testing results. Here are some considerations:
- Testing concrete cylinders or cores at 28 days will give lower results than testing years later. The structure may be stronger than the 28-day tests indicate.
- For accurate assessment of in-place strength, cores extracted from older structures need to be tested. Compressive strength can be 10-40% higher than 28-day cylinder tests.
- Nondestructive testing based on hardness (e.g. Schmidt hammer) produces higher results on older concrete. The apparent surface strength increases with age.
- Building codes define standard 28-day strength values. Increase factors account for longer-term strength gain in design.
When evaluating the strength of existing concrete structures, the age of the concrete must be considered. Testing methods and building codes account for the fact that concrete in older structures likely has higher strength than indicated from 28-day cylinder testing.
Does Concrete Get Stronger Underwater?
An interesting question is whether concrete can gain strength when cured underwater, such as in bridges or marine applications. Here is what the research shows:
- Underwater curing provides water for hydration but leaches away calcium hydroxide, a key strength-giving product of hydration.
- In the first 28 days, underwater curing results in 10-15% lower compressive strength compared to moist air curing.
- However, over the long-term, the continual water exposure allows hydration to continue at higher rates compared to drying out.
- Over several years, underwater concrete can surpass air-cured concrete in strength. Longer submersion leads to higher ultimate strength.
While underwater curing hampers early strength gain, it allows prolonged hydration which can ultimately produce very high long-term strengths in submerged concrete structures.
Conclusion
Concrete is unique among construction materials in that it continues to gain strength over decades. Ongoing hydration and pozzolanic reactions increase the density and hardness of concrete over time. However, the rate of strength gain drops off significantly after the first month. Proper curing conditions are critical to allow concrete to achieve its maximum long-term strength and durability.
While concrete never stops getting harder, the practical strength increases diminish over time. After a decade or more the additional strength gain is minimal. For engineering purposes, concrete can be assumed to reach a plateau after which age has negligible effect on strength. However, even slow hydration may continue for a century or more, leading concrete to become stronger the longer it remains in service.