The strength of concrete or, in other words, the Compressive Strength of concrete is the most important factor taken into consideration in construction planning besides the strength of the reinforcement. The factors that affect the strength of concrete must be known to develop the required strength.
There are key factors as discussed in the article Concrete Compressive Strength and Concrete Testing that affect the strength of concrete.
They are as follows.
- Quality of materials such as cement, coarse aggregate, fine aggregate and water
- Water-cement ratio
- Air trapping
- Aggregate suggestion (coarse: fine)
- The ratio of aggregate to cement
- Healing time
- Use of additives
- Concrete Compaction
- Time after concreting
The individual factors that affect the strength of concrete are explained below.
1. Quality of materials
Four main materials are used to make concrete.
- cement
- Coarse-grained aggregate
- Fine chips
- and water
The effect of individual materials can be discussed separately.
cement
Cement is the material that creates the bond between the aggregates after reacting with the addition of water. The hydration process can be represented by the following equation.
Using quality cement improves bonding and strength. The strength of the cement depends on the date of manufacture.
Likewise, there are several aspects that must be taken into consideration when evaluating the quality of cement.
- Packing date
- Color
- To scrub
- Introduction by hand
- Swim test
- Smell test
- Presence of lumps
- Form Test
- Strength test
Additionally, there are other cement tests to be carried out to determine the quality of the cement.
- fineness
- Compressive strength
- Heat of hydration
- Initial and final setup time
- solidity
- Normal consistency
In general, cement is considered one of the most influential factors in the strength of concrete.
Aggregates
Aggregates are the materials that bind to the cement paste after reacting with water. Therefore, the quality of the aggregates influences the strength of the concrete.
Generally, 80% of the concrete volume is made up of aggregates.
There are two types of aggregates: coarse aggregates and fine aggregates.
To maintain the quality of the aggregates, the following factors are taken into consideration.
- Particle size distribution/granulation
- shape and texture
- Moisture content
- specific weight
- Reactivity
- solidity
- Apparently density
The above properties affect the workability, finish, bleeding and segregation of fresh concrete, and affect the strength, shrinkage, density and durability of hardened concrete.
2. Water-cement ratio
The water-cement ratio is one of the most important factors that affect the strength of concrete. Depending on the water-cement ratio, the compressive strength is defined in the mix compositions.
If we need a specific quality of concrete, we first select a suitable water-cement ratio to proceed with the mix design.
As shown in the figure above, increasing the water-cement ratio reduces the compressive strength of concrete. The water-cement ratio can be increased by increasing the water content or decreasing the cement content.
Currently the use of Additives Reducing the water content to maintain the expected workability has had a significant impact on this relationship.
The use of new additives brought even more benefits to the industry, but also risks. All this will be discussed in the final part of this article.
3. Air trapping
In general, the inclusion of air in concrete reduces its strength.
A 1% increase in air void volume reduces resistance by 5%.
However, the air voids trapped inside improve the concrete's resistance to damage from freezing and thawing cycles.
Furthermore, it improves the workability of concrete.
The following figure also shows the variation in the compressive strength of concrete with the water-cement ratio due to air inclusions.
4. Full participation
Aggregate proportions have a great influence on strength.
There are generally fine and coarse aggregates. Sand and quarry dust are used as fine aggregates.
For all these additives, the properties must be checked according to standards and the grain size of the materials must also be within the acceptable range.
Any change to the source material requires verification.
As particle size has a great influence on compressive strength, it must be checked regularly.
The strength of concrete depends on the mixing proportions of cement, water, coarse and fine aggregates. Different strengths can be achieved with different mixing ratios.
When creating mixing plans, the mixing ratio of the aggregates is considered one of the most important factors.
5. Ratio of aggregates to cement
Aggregates make up the largest proportion of concrete volume: fine aggregates and coarse aggregates.
After the cement reacts with water, it bonds with the aggregates, creating concrete.
Cement content and aggregate volume ratio are related to strength, as shown in the following figure from a technical document.
6. Healing period
The duration of concrete curing directly influences the development of concrete strength. The article published on this website Factors Affecting Concrete Curing Time provides detailed information about concrete curing time.
The following figure, taken from the website, illustrates the influence of curing time on the development of resistance.
As shown in the figure and due to the importance of curing concrete to achieve its strength, curing should not be avoided.
In addition to increasing strength, hardening concrete also improves its durability, preventing the formation of cracks during hardening, etc.
7. Use of additives
Nowadays, additives are almost exclusively used in all concreting work. This brings with it other advantages.
Current developments in this industry allow additives with multiple functions, such as retardant and water-reducing effects.
As we know, Flowing Agents are often used as additives because of their advantages. These additives are categorized into chemical additives.
Superplasticizers reduce water requirements by 15 to 20% without compromising workability, resulting in dense, high-strength concrete. This increase in resistance is achieved with the same cement content, reducing the amount of water.
Additionally, we can reduce the cement content by adding a superplasticizer to maintain the required strength. This allows us to save some money.
However, the dosage of additives must correspond to the manufacturer's instructions.
Overdose can also lead to a reduction in compressive strength.
8. Compaction of concrete
It is quite clear that compaction has a direct impact on strength.
Poorly compacted concrete has lower resistance, as shown in the following figure.
Poorly compacted concrete has more voids, reducing the adhesion between the aggregates.
Therefore, it is very important to sufficiently compact the concrete.
9. Time after concreting
The age of concrete is an indicator of the development of concrete strength.
Over time, the strength of concrete increases, but not proportionally.
Time is the main factor that reflects the strength of concrete. As concrete ages, its strength increases. However, after a certain period of time the increase in resistance is minimal.
