Why Does Concrete Crack Over Time?
Some things in life are inevitable. For instance the sunrise and sunset. It’s just gonna happen.
Cracks in concrete are similar. Inevitably, concrete will eventually crack.
But why does concrete crack in the first place?
To begin, it helps to understand what concrete is made of. Concrete consists of a mixture of Portland cement, water, and aggregate (sand and stone). When this mixture is poured into forms to create the foundation of a house, a chemical reaction continues for days and weeks as this liquid turns into a solid.
What can go wrong?
How Do Foundation Cracks Happen?
1. Adding too much water to the mix
- the more water,the easier concrete is to move around and shape(that’s why it’s tempting to do it)
- the problem is,that the more water there is in the mix, the more the concrete will shrink as it dries
- this shrinkage forces the concrete to pull apart, resulting in cracks
2. ConcreteDries Too Quickly
- to reach maximum strength, concrete needs to stay hydrated during the chemical process (which, as we said, takes days and weeks)
- unless the concrete is cured properly by spraying it with water, covering with material that stays wet, or applying a waterproof paper-it will dry out too soon and become vulnerable
3. Improper settlement
- When concrete is poured into a form, it is not as dense as it needs to be
- 10 to 30 percent of the mixture contains air pockets
- If left alone, the concrete will set up in this porous,honey-combed condition,resulting in weak areas
- To combat this, a vibrator should be used to agitate the particles in the mix so that the entrapped air escapes
- This enables the concrete to be as compact and strong as possible
Do External Factors Cause Concrete To Crack?
We discussed three factors that can cause the actual concrete to crack
- A) Too much water in the mix
- B) Concrete drying too quickly
- C) Improper settlement
But what about external factors? Can they cause concrete to crack?
Yes, they can.
It’s like with us as individuals.
We each have certain internal characteristics, like our genetics. But we are also affected by external factors, such as our environment and experiences in life.
Let’s consider a few external factors:
1) Thermal Contraction
- Concrete is affected by temperature(it expands when it is warm and contracts when it is cold)
- This would be fine if concrete foundations were not restrained by other parts of the structure, by internal reinforcement, or by the ground
- But they are, so this conflict between contraction and restriction can cause concrete to crack over time
- This is especially a factor in climates with a wide range in seasonal temperatures, like the Northeast
2) Subgrade Settlement
- The subgrade is what is underneath the concrete slab of a foundation
- In a perfect world, it would consist of solid rock-but it’s usually soil
- This soil supports the weight of the house
- If the sub-grade doesn’t stay consistently dense and level, problems will develop
- Poor soil conditions or changes in soil moisture content can cause the foundation to sag or shift
- This uneven stress creates cracks eventually(think of how your back feels after a night spent lying on an uneven surface, such as an old mattress or on the ground underneath your tent while camping)
3. Applied Stress
Foundation walls do not enjoy stress any more than you or I.
If the stress applied to us, is constant or extreme, we can crack or break down over time. Our concrete walls can do the same due to these causes:
- a) Building load
- How much weight does the foundation support?
- Has it increased over time, perhaps because of an addition?
- The heavier the load, the more stress the foundation feels-which can result in cracking
- b) Earth load
- The soil leaning against the foundation walls causes pressure and stress
- Some soil types exert greater pressure than others (for example, clay and silt soils exert greater pressure than sandy soil)
- Additional soil, such as when a raised patio is put in along the back wall of a house, greatly increases the lateral pressure on that wall
- Heavy vehicles parked near the foundation walls add to the pressure exerted by the soil
- a pool that is installed too close to the foundation will increase the earth load as well
- c) Hydrostatic pressure
- Water can build up in the soil on the outside of the foundation walls, especially when a house is situated on a hillside or it has been especially rainy or snowy
- This constant pressure can be too much for the walls to support and they can crack
- Also, due to the porous nature of concrete, the water can eventually work its way through the walls into the interior of the basement (efflorescence-white mineral deposits on interior walls-is evidence of water pushing in)
Poor concrete…. It’s being attacked from within and from the outside. No wonder it eventually cracks.
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