Using Concrete in Buildings
The Egyptian used clay and hay for bonding materials and were said to have built the great pyramids using lime and gypsum. The Ancient Romans were the first who mixed small gravel and coarse sand mixed with hot lime and water to paved their road and build their aqueducts. They were known to have mixed horsehair with the mixture to lessen shrinkage.The first commercial grade cement that is still used today is the Portland cement invented by English Joseph Aspdin an English inventor. He made a strong cement by burning grounded chalk and crushed clay in a kiln for lime until the carbon dioxide evaporated. The cement was called Portland for the color resembled the limestone found in Portland Island near Wales. Today there are several types of Portland specification but the usual "all-purpose" cement used are the Type 1 and Type 1A. Now, there's the other type of cement used called Pozzolan cement - indicative of the ancient Roman's Pozzolona.
Cement is a key ingredient in producing concrete. In a nutshell concrete is a mixture of cement and aggregates like sand and gravel and water. There are various applications for concrete mixture. You can use it for house foundations, posts or columns, floor slabs, pavement and walkways, bridges, arches, and high-rise buildings. Because of its numerous applications there are different ratios or proportion of the three materials to produce the needed strength of concrete that you need.
World's Number One Building Material
Not only is concrete the preferred building material in the UK but also the world over. There is an over 3.8 billion cubic meter of cement produced worldwide every year. Concrete is used for building foundations, posts and columns, walls, flooring, pavement and roads, roof, bridges, arches and pipes. You can build a lot of structures using concrete as it is durable and easy to maintain. The cement in concrete is also used for mortar.
Concrete is superb as material for tilt-up construction. Tilt-up constructions are used to stack high-rise buildings at a very fast pace. Concrete is also used for decorative purpose such as stamped concrete where designs are "stamped" or impressing patterns and colour into concrete pavement or walkway
Different Strength of Concrete Mixes
C7.5P is a lean mix of concrete that has a ratio of 1:3:6. This used for general paving like garden path and walkways. The 1 is for concrete, the 3 for sand and the 6 for gravel.
C15P mix could be used for various concrete slabs that are low bearing floors preferably with steel reinforcement. The ratio for this is 1:2:6 where 1 is concrete(kg), 2 is sand (kg) and 6 is coarse aggregates (%). If you are planning to add
A medium strength mix is called a C20P mix where P is Portland cement. The ratio for this mix is 1:2:4 where 1 is cement, 2 is sand and 4 is coarse aggregate or gravel. The mix could work for floor slabs that would normally only carry light live load with no heavy dead load. Note that this mix is not advisable to use for house foundations unless specified by the architect or structural engineer. All types of mix would take 28 curing days to achieve its maximum compressive strength of 20 Newton's per square mm
C25P and C30P are of higher compressive strength and are normally used for big structures and high-rise buildings. The ratio for C25P is 1:1 ½:3 and for C30P it's 1:1:2. If you are going to use pre-mixed aggregates then the ratio for C15P becomes 1:6 where 1 is cement and 6 is pre-mixed aggregates. For C20P the ratio would be 1:4 and for C30P is 1:3.
Ready-mixed and hand-mixed concrete
As long as you know the right proportion of the concrete strength that you intend to mix, then you won't have any problem as long as you follow the ratio and water proportion. Mixing on site is preferred if the mixture needed is less than 1 cubic meter. Hand mixing is do-able but it is definitely hard work as cement causes burn on the skin, and heavy to mix. Renting a powered mixer would do your back and your hands more good. It would also assure you of an even-mixed concrete. If the job entails more than 1 cubic meter in one batch, then consider ordering ready-mixed concrete. Ready-mixed concrete is expensive. The mix has additives that make the concrete dry faster and easy to work with. If you are on a tight budget it would be better for you to mix the concrete on site but be sure to rent or buy a portable cement mixer.
In case you do use a concrete mixer, there is a right way to do your batch. First, add 75% of the water into the mixer and then 50% of the aggregates. Next add the whole bag of cement and then the rest of the remaining aggregate and water. Be sure to agitate the concrete mixer after each addition. To know of your mix has the right consistency, a slump test is done. It is done by pouring the mixed concrete into a cone shaped container with its base on the ground. After the cone has been filled up, the cone is taken away leaving the concrete to achieve its "slump". The allowable maximum slump for a C20 P is 50 mm.
Today the use of ready-mixed concrete for big projects is a necessity. The ready-mixed concrete are delivered in batches according to the compressive strength specified by the structural engineer. To add tensile strength to a building, reinforcement bars are added and spaced according to the architect or structural engineer's design and specifications too.
There are several additives that you can add to concrete mix to make it waterproof or denser or dry faster. There are ready-mixed aggregates to mix with cement so you don't have to deal with buying sand and gravel for your mix. However ready-mixed aggregates are more expensive than buying sand and gravel for your concrete mix.
"The "P" in C30P stands for perscribed mix, not Portland"
R.H