The Strength Of Concrete
A binder is a substance that coheres and adheres other constituents together to form a well integrated and unified mixture. Binders, whether in the form of a liquid or a powder-like substance, harden chemically or physically to bind aggregates, fibers, and other filler materials.
In concrete, the most used binding agent nowadays is cement, particularly Portland cement, which is an inorganic binder. Portland cement’s raw material consists mainly of lime, silica, alumina and iron oxide. Over 90% of the cement is composed of oxides which interact together to form more complex compounds, such as C3 S, C2 S, C3A, and C4AF:
- Tri-calcium silicate = 3CaO*SiO2 = C3S
- Di-calcium silicate = 2CaO*SiO2 = C2S
- Tri-calcium aluminates = 3CaO. Al2O3 = C3A
- Tetra-calcium alumina-ferrite = 4CaO.Al2O3 .Fe2O3 = C4AF
The main effects of these compounds on concrete properties mainly are:
- C3S: is responsible for the initial set and early strength of concrete.
- C2S: it affects the strength of concrete at later ages.
- C3A: it contributes to the strength development in the first few days and it is the first compound to hydrate. The main disadvantage of this compound is its high heat generation and its interaction with soils and water containing moderate to high sulfate concentrations.
Since the hydration process of C3A is very fast, it has a great impact on the hydration rate of clinker and the workability of fresh concrete which can be controlled by proper amount of gypsum mixed with the clinker, which leads to the final form of Portland cement.
C4AF: it contributes very little to the strength of concrete even though it hydrates very rapidly.
The main hydration equations are the following:
Tri-calcium silicate + Water à Calcium silicate hydrate + Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O à 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + heat
Di-calcium silicate + Water à Calcium silicate hydrate + Calcium hydroxide + heat 2 Ca2SiO4 + 5 H2O à 3 CaO.2SiO2.4H2O + Ca(OH)2 + heat
Calcium hydroxide + Silicic acid à calcium silicate hydrate
Ca(OH)2 + H4SiO4 à CaH2SiO4*2 H2O
The main contributor to the strength of concrete and the engineering properties of cement paste is the reaction product calcium silicate hydrate gel (C-S-H gel), which is the hydration product of C3S and C2S. Another hydration product of the same compounds is Ca(OH)2, which reacts with amorphous silica from pozzolanic material to produce additional calcium silicate hydrate gel, which is the main process of pozzolanic reaction.
B-Filler
Filler material concrete can be divided into two main categories, aggregates and inert fillers. Aggregates and inert fillers have different roles and influences on fresh and hard concrete’s mechanical and physical properties.
B-1-Aggregates
Aggregate is a granular material such as gravel, crushed stone, sand, lime rocks, synthetic or even crushed hydraulic-cement concrete, used with cement and water to produce mortar or concrete. Aggregates in a concrete mix should comply with the following qualities:
- They should not react with any other constituents in the concrete mix, they should be inert.
- They should be hard enough to withstand scratching and abrasion.
- They should be strong enough to withstand vibration, normal tension forces, and compression in normal mixture.
- They should be of an acceptable quality and free of contaminants that would affect the quality of the mixture.
It is economical for a concrete mix to contain as much aggregates as possible if not forgoing other mechanical and physical properties, which would also enhance the volume stability and durability compared to cement paste alone.
Aggregates can be classified as natural mineral aggregates, synthetic, and recycled.
Natural mineral aggregates are aggregates made from natural sources like rocks, which are mainly
- igneous,
- edimentary, and
- metamorphic.
Igneous rocks are formed after magma or lava cools and solidifies. These rocks may be crystallized or not. Some examples of these rocks are: granite, Syenite, Diorite, Gabbro, Peridotite, Pegmatite, volcanic glass, Obsidian, Pumice, Tuff, Scoria, Perlite, Pitchstone, Felsite, and Basalt.
These rocks are mainly made of minerals such as silica, silicates, and others. Minerals like quartz, feldspar, hornblende, mica, pyroxene, and olivine make up over 92% of the common igneous rocks thus being easy to distinguish.
This type of rocks is considered as an ideal aggregate especially granite and basalt because of their hardness, strength, and toughness.
Sedimentary rocks are formed when weathered fragments of other rocks are compressed and cemented together by pressure. Some examples of these rocks are: Conglomerates, Sandstone, Quartzite, Graywacke, Subgraywacke, Arkose, Claystone, Siltstone, Argilite, Shale, Carbonates, Limestone, Dolomite, Marl, Chalk, and Chert.
The main minerals found in sedimentary rocks are quartz, carbonates such as calcite and dolomite, sulfates such as gypsum and Anhydrite, iron sulfides such as Pyrite, and some silicates, Feldspar and Muscovite.
This type of rocks is cost effective since it is found near the surface, and it is widely used as an aggregate in concrete especially sandstone and limestone. However sedimentary rocks could vary from poor to excellent aggregates depending on their hardness, density, and absorption capacity.
Metamorphic rocks are pre-existing rocks like igneous or sedimentary rocks that have transformed their texture, crystal and mineral structure or composition due to high pressure and temperature under the earth’s crust (this process is called metamorphism). Some examples of these rocks are: Marble, Meta-Quartzite, Slate, Phyllite, Schist, Amphibolite, Hornfels, Gneiss, and Serpentinite.
The main minerals found in metamorphic rocks as a result of the metamorphism process are Sillimanite, Kyanite, Staurolite, Andalusite, and some Garnet. This type of rocks could be considered as poor to excellent aggregate especially marble, slate, and schist, according to hardness and density, however lamination is undesirable.
Aggregates are divided into two main types according to their size and dimensions, coarse and fine aggregates.
B-1-a-Coarse Aggregates
According to sieve analysis, particles that are retained on sieve No.4, or by other words particles that have dimensions greater than 4.75mm, and pass through a76mm screen, are considered as coarse aggregates.
