Concrete is the most used construction material in the world. It underlies each aspect of modern infrastructure, from their tall ceilings and long-span roof systems and bridges, to roadways, concrete makes up the base layer of expansion and productivity.
However, not all concrete is the same. The strength of concrete will determine how much load the structure can resist when it is utilized and lifted, while also remaining intact for an extended period without falling apart. There are numerous aspects that need to be considered in terms of the strength of the concrete.
Table of Contents
Most Common Factors That Affect the Strength of Concrete
This guide to concrete expansion will explore more than a dozen aspects that are critical to the ultimate strength of concrete and therefore what builders and engineers need to consider so the structures they are creating are durable for the intended purpose of their work, and beyond!
1. Water to Cement Ratio (W/C Ratio)
Of the factors that affect the strength of concrete, this is probably the most important factor. The water-to-cement ratio simply relates to the amount of water in relation to the amount of cement, making a concrete mix.
The lower the water-to-cement ratio (W/C), the stronger the concrete material will be, while still leaving enough water available to evaporate before solidification, and generating voids within the structure. A W/C ratio typically falls between 0.45 to 0.60.
More water leads to more pores and capillary action, leading to weaker concrete structural characteristics. Less water means the mixture will not behave as well, which can impact the amount of compaction with the material and again lead to weaker properties.
2. Compaction of Concrete
Proper compacting of concrete is important to provide the most difficult air bubbles and voids with as little disturbance as possible. Improperly compacted concrete could result in a greatly reduced capacity, as much as 30-40 percent in some cases.
Proper compaction assures that the paste completely surrounds the aggregate and that all voids are filled, which equals a dense, strong mass.
Procedures for compacting concrete include mechanical vibration or hand tamping; this process is critical when pouring concrete for columns and slabs.
3. Quality and Type of Cement
Cement is the binding agent in concrete; the cement grade (e.g. 33, 43, 53,) determines how soon and how much strength will be achieved by concrete. Higher grade cements usually provide early strength and are best suited to high-strength concrete applications.
However, more cement also leads to a potential increase in shrinkage, which could lead to cracking, so there is a limit on how much cement can be used.
The recommended cement content is normally 300-450 kg/m³ and is dependent upon the exposure and structural application.
4. Aggregate Quality and Proportion
Aggregates are about 60–80 percent of the concrete volume (depending on the mix design) and therefore affect strength importantly. Both aggregate type and amount can affect the concrete final strength:
Shape: Angular aggregates bond better than rounded aggregates.
Size: Smaller aggregates have more particle surface area to bond with, but they may use more cement.
Cleanliness: Aggregates must be free from dust, silt, or organic impurities. Poor quality or contaminated aggregate means weak, brittle concrete.
5. Water Quality
Clean potable water is generally deemed necessary to use to mix concrete. Water with impurities like acids, oils, alkalis, or salts can affect hydration.
They may slow down or speed up setting time, and may even corrode the steel reinforcement. Seawater or wastewater should never be used unless specially designed mixes for marine use are used.
6. Curing Process
Curing is the process that maintains appropriate moisture, temperature, and time following the placement of concrete to allow hydration of the cement to occur. This stage is vital to gain strength and develop durability. As a general guideline,
70% of the strength will be gained in the first 7 days, and full strength at 28 days.
Popular methods of curing include ponding, sprinkling, wet coverings, and curing compounds. A poor curing process will result in surface cracks and dusting, and weak concrete.
7. Shape and Texture of Aggregates
Aggregates can be either angular, rounded, flaky, or elongated in shape. Angular aggregates bond more strongly to cement paste because they provide a larger surface area due to the aggregate’s roughness.
However, flaky and elongated aggregates can reduce the workability and increase the water demand of the concrete, which is detrimental to the concrete mixture. Cubical-shaped aggregates having good texture are best suited for high-performance concrete.
8. Maximum Aggregate Size
In general, a larger aggregate size equals a lower concrete strength. Williams and Eikmans (Art. No. 2371) attribute this notion to each cubic meter of concrete having fewer total cement-aggregate bond connections in a larger aggregate size.
In addition to having fewer aggregate-cement bond connections, larger aggregate size also increases the occurrence of segregation, internal bleeding, and uneven distribution of loads.
Aggregate sizes used for general structural bed, including aggregate size categories, 10 mm, 14 mm, and 20 mm, depending on application. Aggregate sizes utilized for high-strength applications will generally go smaller.
For example, when preparing the silica aggregate concrete for the UBC85 experiment, the aggregates used were chosen because they had a specific particle size.
9. Grading of Aggregate
Grading of aggregate means the distributive degree of aggregate sizes contained in a mixture. Considerable variability in grading exists between different aggregate particles, mixtures, and sources.
Well-graded aggregates without voids will have a lower demand for cement per unit volume and will also provide better overall concrete strength.
Well Graded: All particle grades are represented- greatest overall strength potential.
Poorly Graded: Limited particle range- leaves many voids; biggest weak points.
Gap Graded: Certain ranges missing; leads to honeycombing.
Having a well-graded mixture of phosphorus aggregates yields better overall workability and density of concrete mixtures.
10. Weather and Environmental Conditions
The environmental conditions that exist above and below concrete surfaces can greatly affect the strength of the concrete. In cold northern areas, repeated freezing and thawing cycles can cause cracking. In warm southern areas, evaporation rates can be so fast that plastic shrinkage cracking can occur.
When placing concrete, the project should be properly scheduled and managed to consider local weather conditions. It may be effective to use windbreaks, sunshades, curing blankets, and other weather mitigation techniques as necessary.
11. Temperature of Mixing and Curing
Temperature influences the rate of hydration. Warm temperatures can accelerate hydration, which can increase the potential for thermal cracking or a decrease in long-term strength. Cold temperatures inhibit hydration and strength gain.
When mixing and curing concrete, it is ideal to maintain a temperature range of 10–30°C to achieve full (or close to full) strengths.
12. Loading Rate
How fast a load is applied to concrete (in testing, those values are not always accurate in real structures) impacts the strength of the material perceived.
With quick loading rates, strength can be higher due to lower time for creep (slow deformation of a material in response to stresses), slow applied loading rates increase creep, causing lower measured strength.
Designs should consider input design models that include expected loading conditions, such as sudden impact loads or long-term static loads.
13. Age of Concrete
Concrete can gain strength over time. Many consider several years after placing concrete to be a ‘young’ aged concrete, due to continuous hydration. 28 days is the common measurement period for strength, so concrete can increase in strength for years if properly maintained.
This should be considered on long-term projects, where durability and future utilization, and performance are being considered.
14. Workmanship and Construction Practices
Concrete’s strength is not only dependent upon the materials, but from the methods used on-site. Inadequate workmanship leads to substandard mixing, placement, compaction, and curing.
Clearly, skilled workers, quality equipment, and methods of supervision will determine the success of concrete strength.
15. Use of Admixtures
Chemical and mineral admixtures, such as superplasticizers, retarders, accelerators, and fly ash, can change the properties of concrete.
Some admixtures will improve workability without additional water, which means that strength will be maintained. Various admixtures allow increased or decreased set-time and may be useful in extreme environments.
Improper dosages or poor quality admixtures can cause a reduction in strength, and therefore, concrete admixture testing and adequate quality control are required.
16. Formwork and Moulding Conditions
Although the repels the consideration of forms, it is to be discussed. The condition/ type/ and time of removal (formwork) can all influence how the concrete sets or gains early strength.
Well-supported and non-absorbent forms will ensure that shape and moisture are maintained during the initial curing stage, while removal of forms or moulds too early creates the potential for deformation or cracking.
Form release agents should also be selected carefully, as their residue can affect bonding and finishing.
17. Hydration Process and Supplementary Materials
The chemical reaction that takes place between cement and water is called hydration and is part of how strength develops.
Admixtures such as silica fume, GGBS (Ground Granulated Blast Furnace Slag), and fly ash can influence the hydration process by improving microstructure and decreasing permeability. These supplementary cementitious materials also lessen environmental impact and improve long-term strength.
Conclusion
Although the number of factors that impact concrete strength can be daunting, understanding their influence is critical to constructing strong, durable, and resilient concrete structures.
Whether you are a builder, civil engineer, or homeowner involved in any level of construction, knowledge about the factors that affect concrete performance will help drive informed decisions from raw material selection to curing completed products.
We can plan and decide to minimize factors for getting concrete done, and/or impact factors in a way that maximizes their benefits, and creates cost savings, or less maintenance or improves service life in our structures.
Quality concrete is the foundation to quality infrastructure. Numerous factors can influence concrete performance, however, if you prioritize long-term performance, it takes more than a bag of cement and some water. It means taking a balanced approach to science, materials, and process.
Sandip Agrawal, Polymer Engineer and MD of Sakshi Chem Sciences Pvt. Ltd., leads innovation in construction chemicals, shuttering oils, and industrial lubricants. With expertise in polymer science and eco-friendly solutions, he drives R&D and sustainable advancements, ensuring high-performance products for India and global markets.