Add more cement or less sand to make the concrete stronger. The higher the grade, the closer the sand-to-cement ratio is to an even one-to-one. This idea also operates in the reverse direction. A concrete contractor will be able to tell you what ratio of cement to sand is required for your specific situation.
As long as there's cement in the mix, you can strengthen it by adding more coarse aggregate (gravel, stone, etc.) or fine aggregate (sand). For example, if the original recipe called for 200 pounds of cement and 100 pounds of sand, then a contractor would expect you to use more cement than sand when making your batch again.
The finer the grain of the aggregate, the longer it will take the concrete to dry and the more resistant it will be to cracking from heat and other factors that could damage brittle materials such as glass, ceramics and stones.
Coarse aggregate is used to create a strong, durable surface on which to build with fine aggregate. The size of the pieces depends on how much wear-and-tear you want the finished product to withstand. Coarse aggregate can be as small as 2 inches if you plan to polish the surface after it has cured, or as large as 4 inches if you want the surface to look like new instead.
Numerous ways exist for increasing the strength of concrete:
The ratio of water used to cement utilized is the single most critical predictor of strength. Essentially, the smaller this ratio, the higher the ultimate concrete strength. Concrete that is mixed with a high volume of air (i.e., low density) will always be weaker than similar concrete that is mixed with a low volume of air.
Concrete that is mixed at a high temperature has greater initial strength and better durability when exposed to heat. However, if the mixture is cooled before placement, this advantage is lost. High-strength concrete requires more water per ton than normal-strength concrete to produce the same fluidity after mixing. This means that less aggregate is required to achieve the desired workability because more cement covers more area. The end result is a concrete slab that is very strong but also very dense.
Strength increases as concrete ages. This is due to the development of internal tensile stresses caused by the drying of the concrete. These stresses become more prominent as the age of the concrete increases. Concrete that has been placed in deep forms or trenches may require additional time before it can be used its full potential strength. Deeply embedded reinforcing bars can't transfer any load until after they have completely healed. This healing process usually takes a year or more.
To make bagged concrete stronger, use extra Portland cement. Hydrated lime can also be used. Sand extracted from volcanic lava with a high silica concentration should be used to build the strongest concrete. The sand should not be very fine or it will cause other problems for the concrete.
Concrete that is mixed and placed in forms or molds before it sets has many advantages over concrete that is poured into place. Concrete that is placed in forms or molds tends to have better uniformity of quality and consistency from batch to batch. It is possible to mix and place concretes with different drying times by using formwork that allows the required amount of water to be removed at the right time.
The most common method of making concrete stronger is to add more cement paste. Cement pastes are made up of water, limestone powder, and clay or shale. If you were to analyze one ton of standard portland cement, about 33% of its weight would be oxygen (O), 22% silicon (Si), 18% calcium (Ca), 10% iron (Fe), 4% magnesium (Mg), 2% aluminum (Al), and less than 1% other elements. Limestone is the main ingredient in cement paste and it is responsible for its ability to set into an hardened mass. When limestone dissolves in water, it releases carbon dioxide and hydrogen ions.