Concrete has changed the face of the entire construction industry. It is undeniably one of the strongest and durable materials man ever created.
But the fact is, this too has certain weaknesses. The most telltale signs are of course the cracks that may appear on the face of a building.
Their cracks can be due to many reasons. They can be either minor ones due to normal wear and tear, or these can result from deep-set faults too.
There are many factors that abet cracks in a building. Harsh weather, faulty cement or excess water are some of the factors that lead to cracks. Design deficiencies too can result in cracks.
Let’s delve into 3 main factors of concrete failure.
3 Factors of Concrete Failure
When you discuss construction cracks or failure of concrete, you can club the various reasons under three primary heads. It can be due to:
1) Design Issue
Many times, a building may develop cracks due to core design problems. But they may not be only due to the difference in planning.
Sometimes, it can be also a result of
1. Wrong Admixture
We all know that cement needs adequate water content for proper placement of cement.
Often the presence of additional water in the wet stage may result in cracks after drying.
This is why different types of water reducers are introduced. Different types of water reducers, superplasticizers and retarders are used to reduce the water amount.
They alter or modify the concrete’s properties. But if these are not mixed in the right proportion, it can result in placement issues and grouting problems. Ultimately that goes on to impact the strength of the building.
2. Using Wrong Grade of Concrete
The concrete grade is very important in determining the final strength of the building. But what do you understand by that?
Well, it essentially refers to the compressive strength of the concrete you are using.
An inaccurate ratio can even lead to damages to your core structure. So the safety element in the concrete design is crucial.
Once the concrete is hardened, there is precious little left to be done. So, before the concrete is mixed, you have to get the measurements rights.
3. Size of Aggregates
The term Aggregate refers to the long list of coarse particles or medium-sized particles that are used for constructing any structure. Most times this also includes sand and slag.
The aggregates are important base material for laying the foundation as well. You have different aggregate sizes for the different construction.
They essentially add to the strength of the building. But inaccurate sizes of these can also lead to a potential concrete failure.
Most master masons have a pre-determined list of different size of aggregate for different kinds of constructions.
The ones that are used for laying roads will be very different from buildings.
4. Shape of Aggregates
The aggregates in concrete add to its strength, texture and durability. That is why the size of aggregate is also very important.
If the aggregate size is big, it reduces the quantity of cement and water required.
So, overall, depending on the kind of construction you need, this size has to be monitored.
Based on their sizes, they can be divided into 2 categories:
- Coarse aggregates: Any aggregate particle that is bigger than 4.75 mm is classified as coarse.
- Fine aggregates: Sand or crushed stone below 9.55 mm comprises of fine aggregates.
5. Grading of Aggregates
Good grading of aggregates is crucial for sound construction.
By that, we imply that a given sample must contain the different types of aggregates in a pre-decided ratio.
This is what will ensure that there is minimum void in the composition. When the amount of void increases, the aggregate grading is termed poor.
With aggregates making up as much as 85% of the concrete mass, this grading is crucial.
A higher-grade aggregate will inevitably require less of the cement paste to fill up the void. That translates into stringer structures with a lesser chance of shrinkage. It also reduces the overall cost of the concrete mix.
6. Surface Texture of Aggregates
The surface texture of the aggregate particle is often as important as its shape. The resistance, workability and durability of the concrete are all dependent on this key factor.
Quite understandably, as these aggregates provide strength, it is best to opt for rough aggregates. The rough texture also helps in better bonding of the concrete and makes the mix fairly more sustainable.
They also provide a larger surface to the cement to bond. So all in all in contributes more effectively to the overall sustainability of the structure.
2) Environment Issue
Weather affects the construction quality to a large extent. Different weather conditions have a different impact on the construction quality. This is where it is important to account for such damages.
Moreover, the construction basics need to factor in these possibilities and how to reduce it.
1. Corrosion of Steel Reinforcement
Steel reinforcements are an important element of construction. But at the same time safeguarding these from weather-related wear and tear is important.
Corrosion has in fact resulted in the widespread destruction of different types of structures.
Normally good quality concrete stops corrosion of the inner steel reinforcement. This is because they keep the inner alkaline environment intact.
However, with the passage of time, the carbonation process results in gradual erosion of the alkaline cover. This is when corrosion sets in.
But adequate cement content coupled with proper curing after compacting slows down this process significantly.
2. Sulfate Attack
This is another key culprit. This can be due to both external factors and internal ones as well.
The external sulfate attack is primarily sulfate penetration in concrete from external sources.
One of the common triggers is groundwater containing sulfate enters the concrete mix.
It can result in cracking, expansion and the cement coming apart from the aggregate.
The internal sulfate attack is a big risk if the source is incorporated in the original concrete mix. You have to understand this is the reason aggregates have to be cleaned thoroughly before mixing. Possible contamination or even additional gypsum in cement can result in this contamination. It weakens the internal structure severely.
3. Finish-Related Delamination
In very simple terms delamination of cement refers to the separation of the surface layer from the rest of the body.
Normally this surface layer is relatively thin. You can understand it quite easily from the hollow sound of tapping it.
This happens if the troweling time is not set in sync with the initial setting. The timing if the trowel is, in fact, the most important cause of this phenomenon.
This can even create small pockets of water accumulation if left unattended for long. That can further affect the structural strength.
4. Freeze-Thaw Deterioration
Concrete quality can deteriorate significantly from the freeze-thaw action. This is a risk when the concrete saturation point reaches 90% and above.
In this case, water fills up all the pores. When it solidifies into ice, it expands 9% in volume. But if there is no additional space accounted for this volume expansion, it results in damaging the property eventually.
When the same phenomenon keeps happening over an extended period, the concrete gets distressed severely.
Normally this is the reason concrete with high water content is considered less frost resistant.
5. Alkali-Silica Reaction (ASR)
In very simple terms, it is also referred to as concrete cancer. The concrete begins to swell as a result of the reaction between the cement and silica matter in the aggregate.
As a result of this, the strength of the concrete begins to weaken, its starts spalling and eventually this damages the building as a whole.
In case of serious distress, buildings may have to be demolished as well.
6. Overloading of Concrete Structure
This is one of the biggest safety breaches in construction. Every concrete structure is geared to handle a certain amount of overload, any additional load will damage the structure for sure. It can begin with cracks in concrete but eventually result in severe distress.
Buildings can also collapse as result of this perpetual overload. It can result in other additional defects in the overall structure as well.
7. Thermal Damage
As the name indicates, this refers to any possible damage to the structure or concrete due to excessive heat.
Almost till about 300-degree centigrade, there is normal thermal expansion. However, beyond that point, there is water loss due to high temperature. That results in the concrete shrinking. One of the most notable examples is the 1996 Channel fire.
The fire reduced the overall thickness of the concrete inside the tunnel connecting France and England. The high-temperature exposure also results in excessive spalling and severe water loss as a result.
Essentially if it turns pink, that is a danger sign. It means the concrete is damaged completely.
8. Bacterial Corrosion
Bacteria per se does not have any distinct impact on concrete. But the acid production as a result of bacterial reaction results in significant damage to the concrete.
Especially, there are many bacteria that produce harmful gases like hydrogen sulfide. The building corrodes as a result of continuous exposure to such damaging gases can result in corrosion.
Using limestone in the aggregate can help contain the impact to a large extent.
3) Workmanship Issue
But concrete failures can be a direct outcome of inefficient workmanship too.
1. Rehandling of Concrete after Initial Set
Unplanned rehandling of concrete is one of the most problems resulting in concrete failure. Different concrete mix proportions give different results.
The entire process of mixing, transporting and handling of concrete is a step by step process. You also have to maintain a strict order in doing it. It is always better to plan it in the proper way.
Avoid rehandling of concrete as much as possible. Once the initial set is done, any alteration can affect the construction quality significantly. This can also lead to possible cracks.
2. Bad Water-Cement Mixing Ratio
The water and cement ratio is very important in creating a durable and good quality concrete.
Adequate quality of water is required to improve the workability of the cement.
But again, at the same time, when this extra water evaporates it results in shrinkage, cracks and other faults.
The strength of the concrete is also directly impacted by the water content. While super-plasticizers reduce the amount of water used, mixing in the appropriate ratio remains very important.
Conclusion
Failure of concrete is essentially the fall out of following inappropriate construction practices.
Good quality concrete is an assimilation of several elements and maintaining the right ratio is crucial.
Most times the crack in concrete is a reflection of underlying fault and design deficiencies. The key to great construction is paying attention to details.
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