In its basic form Ready Mix Concrete is the mixture of sand, stone, cement and water. The cement (a dry fine powder) is combined with water to form a paste which is than mixed with fine and coarse aggregates (sand and stone respectively) to form a solid material when cured. The sand acts as a filler between the larger pieces of stone in Ready Mix Concrete. There can be other cementisous ingredients added to concrete such as fly ash, ground granulated blast furnace slag and silica fume in addition to a range of admixtures that can be used to enhance and change the properties of ready mix concrete.
Many people confuse the terms concrete and cement. Cement is the dry powdery material used to make concrete. The cement and concrete used in today's construction is similar to that used by the ancient Romans. Cement from the Latin term caementum, meaning rough-cut stone, is made by heating crushed limestone and clay from shale in a kiln. This process forms a huge mass called a clinker, which is then ground into a powder becoming cement. The cement is then mixed with water, sand and stone or gravel to make concrete. The cement and water form a paste that binds the other materials together as the concrete cures or hardens a process in the etymology of the term itself. Concrete is the combination of the Latin prefix com-, together and crescere, to grow. Cement must be kept in a dry moisture free storage facility and has a shelf life of several years. The cement comes from all over the world including the United States. Greco Bros. uses cement to produce Ready Mix Concrete the comes mainly from the United States and Greece, which is shipped to us via an intermediate cement terminal by dry bulk tractor trailers.
In the New York Metropolitan Area, the concrete sand used is a natural sand which is screened to New York State specifications by sand mines located in Eastern Long Island. Long Island sand is commonly known as some of the best sand in the world for Ready Mix Concrete. The most common stone used in the New York Metropolitan Area is crushed bluestone, which is commonly mined upstate New York or is imported from places such as Nova Scotia in Canada. Ready Mix Concrete is not limited to using the above mentioned aggregates. Manufactured sand and specialty coarse aggregates are available. Greco Bros. regularly stocks pea gravel and light weight aggregates for special applications or exposed aggregate designs. The gradation or sizes of the aggregated used is important for well consolidated strong concrete mix. Fine aggregates (sand) consist of particles with top size of under 3/8 of an inch.
Coarse aggregates (stone) are particles equal to or greater than 1/5 of an inch, but generally range between 3/8 and _ of and inch. A typical Ready Mix Concrete mix is comprised by volume of 10 to 15% cement, 14 to 18% water, 4 to 8% entrained air, 24 to 28% fine aggregate, and 31 to 51% coarse aggregate.
The paste which is made of water and cement is the foundation or basis of the strength of the concrete. The paste is quantified by the water cement ratio. The lower the water cement ratio the stronger the concrete mix. Simply put, water is both a friend and enemy to Ready Mix Concrete. It is needed to form the paste that binds to the sand and stone, but too much will break down the concrete mix and weaken its strength. Just enough water should be used to make the concrete workable.
There are various types of concrete mixes that can be designed based on specific job applications. Concrete mix designs can produce strengths in excess of 12,000 psi (pounds per square inch) for high rise applications to mixes with as little as 50 psi, which can be used as back fill and easily dug up and removed by the contractor later.
The most commonly used mixes in the New York Metropolitan Area range from 3000 psi to 4000psi for residential home and 3000 psi to 6000 psi for buildings and high rises. Our dispatching and quality control personnel can help you select the right mix for your project and if needed Greco Bros. has close relationships with several engineering and testing labs that can provide certified mix designs.
Ready Mix Concrete Mixes are often referred to in terms of how many 94lb bags of cement are in each cubic yard of concrete. The most common residential mixes are as follows.
3000 psi or 5 3/4 bag mix
3500 psi or 6 1/2 bag mix
4000 psi or 7 bag mix
The psi of a concrete mix is termed in a 28 day strength test. In other words a 3000 psi mix is tested by a lab facility after 28 days and must break at or exceed 3000 psi. All of the mixes provided by Greco Bros. in 28 days commonly break at the psi plus and additional 25%. A typical 3000 psi mix will break at 3750 psi and a 4000 psi mix will break at 5000 psi. This is a testimony to the quality of our Ready Mix Concrete.
It must be taken into account that mixes must be followed exactly by the producer and contractor or homeowner for these results to be achieved. If a 6 bag mix has had too much water added it probably will not have a break test result of 3000psi. Accordingly, a 7 bags mix left unprotected in temperatures under 35 degrees will also have a difficult time achieving 4000psi.
Special conditions and situations occur on almost every project. You may need a specific strength in less than 28 days, have a pour in extremely hot or cold weather or require a mix with an extraordinarily high slump (consistency) that is where Greco Bros. experience and expertise will aid you in providing the right mix for your project. Once again our dispatcher and quality control personnel are available to assist you in your mix selection.