When working with concrete, there are numerous factors that affect the overall strength of the finished material. To produce the desired strength for fresh concrete, all these factors must be taken into consideration, as they all play an integral role within the setting and formation of the product. Listed below are the top six factors affecting the strength of concrete.
Raw Material Quality
The initial factor that must be addressed when creating concrete is the raw materials that you wish to use to make said concrete. Concrete is formed of four parts; cement, water, coarse aggregate, and sand (or fine aggregate), and the quality of all these components will affect the strength of the concrete.
The quality of the cement used should be tested to make sure it is the appropriate type of cement for the desired role, though in terms of strength the most important aspects are the fineness of the cement, as the finer the cement is then the faster the mixture will hydrate, and consequently the quicker it will strengthen as it dries, and the manufacturing date as cement loses its strengthening capabilities over time (roughly within 3 months of the date it was produced).
Aggregate has several components that must be considered when planning to make concrete, such as its size, shape, and texture. There are also several types of aggregate, such as natural (sand and gravel), crushed rock (usually quarried or excavated rock), and recycled aggregate (made from construction waste or demolished concrete). For high-strength concrete, it is advised that you use either crushed rock or recycled aggregate, as the crushing process imbues the aggregate with strong compaction qualities, and using recycled materials means that you are using products that have already been through the process and bonded with the other raw materials so, in essence, you are using pre-existing concrete to make more concrete.
The sand that will allow you to make the strongest form of concrete is sharp sand, which is also known as grit sand, or, more helpfully as, concrete sand. Typically made of granite or limestone, this coarse sand is of a grittier texture to other types of sand and is filtered vigorously to make sure that no large stones or impurities are present within it.
The ratio in which you add the different components of the concrete together is an incredibly key factor in creating strong or weak concrete. By adding more or less of just one of the raw materials will create a completely different form of concrete, and while different forms of concrete are better utilised in different situations, to create the strongest form of concrete it is advised that you use a ratio of 1:2:3:0.5, 1 part cement, 2 parts sand, 3 parts aggregate, and 0.5 parts water. By adding less water, you are creating a dryer mixture, which means that the concrete will start to solidify and strengthen more quickly, and by adding more cement and less sand, you are providing the mixture with more of the bonding agent to help create a robust finished product.
Compacting concrete during the setting process has a tremendous impact on the cohesion of the aggregate and cement. The less compact the concrete is, the more voided space and air pockets you will have between the aggregate, thereby meaning that the strength of the concrete directly correlates with the compression of the wet concrete. If concrete is compacted poorly and is filled with 5 to 10% voided space then the strength of the finished product will decrease by roughly 30 to 40%, so it is imperative that you achieve the correct compressive strength of concrete for whichever project you are working on.
Heat plays a key role in the strength of concrete, though it is not a simple correlation between the temperature and the strength of concrete. Concrete that is dried at a higher temperature will of course gain strength quicker than that of concrete dried at a lower temperature, however, in the long-term the sheer strength of the concrete that was dried at a higher temperature will be lower, as the cement has less time to form well-structured bonds and will be more porous. To achieve the strongest bonds, you must find a middle ground of drying the concrete out at a temperature that does not allow for the cement to become too porous and dry prematurely, but also for the concrete to also dry at a reasonable pace so that the bonds form before too much air and voided space fill up the finished product. It is advised that you maintain a temperature of between 12°C and 22°C if you wish to achieve the strongest form of concrete after 28 days of setting.
Curing concrete is the process in which you allow the dried concrete to age, in a controlled environment, so as to achieve the desired properties (i.e. strength). Curing entails several of the aspects we have discussed already, mainly the temperature and compaction of concrete, but the curing process has a few more factors that must be taken into consideration. The humidity of the environment you cure your concrete in impacts the strength drastically as the cement needs continuous moisture, at high levels, around 85-90% humidity, to complete the hydration process properly. The curing time also impacts the properties of the finished product, as this reflects the amount of time the concrete has to complete its internal reactions and reach your desired state. Typically, curing processes can range, depending on the type and subsequent use of the concrete, but for the best results the rule of thumb is to let the concrete cure for up to 28 days.
While this is more of a long-term factor, there is a direct correlation between the strength and age of concrete, if not impacted by any adverse external factors. Several studies have shown that the strength of concrete can double or triple from its initial strength after 28 days to its strength after 10, 25, or 50 years. All the factors previously discussed in this article will impact this variable, and while the thought of how strong your concrete will be in over a decade’s time may not be at the forefront of your mind, the significance of the strength increase, if the conditions are favourable, should make this an important factor when planning to use concrete for certain projects.