Concrete Materials

Concrete has been known in the world of Civil Engineering as a construction material that has many advantages. Concrete can be produced by mixing coarse aggregate and fine aggregate with little added adhesive (cement paste).

Concrete Advantages: The price is relatively cheap (sand, gravel, cement, water), high compressive Strength, wear-resistant, fire resistant, weather resistant, not rust, almost does not require special care during usage

Concrete Weakness: tensile strength of concrete is very small when compared to compressive strength, Getas, Hard obtained concrete watertight

In order to obtain high compressive strength of concrete that there are several factors that need to be addressed:

The type and quality of semen
The type and shape of the surface of the aggregate
Concrete treatment conditions
The ambient temperature around the concrete
age of concrete
The resulting concrete is strongly influenced by the material - material mixture used. The materials used such as water, sand, gravel, cement own terms - the specific requirements for bonding generated by the material - the material can be good in order to produce concrete with good quality.

1. Water
Water is one of the mixed materials in the manufacture of concrete. Water is needed to help the survival of the reaction of cement (without water, cement will not be able to react / harden), as well as a lubricant between the grains - grains aggregate so that the concrete is easy to work.

Terms - the terms of the water used in the manufacture of concrete mix:

Should not contain sludge (may slow the initial bonding concrete), oils, acids, alkali (can lead to reduced bond between aggregates with cement paste), salt (can corrode reinforcement), organic matter
In general, we recommend the water used is potable water, tasteless, odorless, and does not muddy when blown air.

2. Cement
Portland Cement is a hydraulic cement produced by softening clinker consisting of silicate - calcium silicate that is hydraulically with gypsum as an additive to adjust the initial bonding cement. Fineness of cement affects the ease of processing the concrete, and can reduce bleeding  (rising amount of water to the concrete surface).

Cement hydration process takes place when cement reacts with water. At the beginning of hydration generated Ca (OH) 2 ,  etteringite , and C 3 S 2 H 3  which form the  coating  on the cement particles. This resulted in hydration reactions persist for 1-3 hours (cement paste is still plastic and  workable ). This period ended with the outbreak of the coating  and re-hydration reaction occurs and the  initial setting  is imminent that the start time of the dough occurs until it starts to happen a certain rigidity in which the dough begins not  workable . And the  final setting  is achieved when the dough reaches the full rigor. Bonding time ( setting time ) of cement is influenced by several things:

The content of C 3 A, the greater will cause the  setting time  is short
The content of the cast, the greater will cause the  setting time  is long
Cement increasingly fine led to  the setting time  is getting shorter
There are several types of Portland cement, namely:

Type I ( Normal Portland Cement )
For general use which does not require any special requirement. Cement type is not well used on the mass of the building because it can cause large temperature difference between the inside and outside of the building so that it can lead to cracking - cracking hardening.

Type II ( Modified Portland Cement )
In use the necessary resistance to sulphate. Both are used for buildings such as pears, pillars, retaining wall is thick, dams and docks)

Type III ( High Early Strength Cement Prtland )
In use requirements demanded high early strength after bonding occurs. Both are used to manufacture - the manufacture of concrete that must be used or should be removed  bekesting his. (Example: highways and airfields)

Type IV ( Low Heat Portland Cement )
Lower heat of hydration is required in its use. Both are used for the manufacture of such mass building dams.

Type V ( Sulfate Resisting Portland Cement )
In use demanding requirements of highly resistant to sulphate. The use of this type of cement for the building - the building located in concentrated sulfuric (example: dams and breakwaters)

3. Aggregate

The purpose of the aggregate in the concrete mix is ​​to save on the use of Portland cement (so the price of concrete admixture become cheaper), generating considerable strength and reduce shrinkage of concrete hardening.

Aggregate grading is one factor that is very concerned, because if the grain - grain aggregate of uniform size will be generated a large pore volume, otherwise the size of the grain - grain aggregate vary the pore volume will be small. This is because the small grains will fill the pores between the large grains. Mix concrete with little pore volume is desirable because then it becomes a little connective material usage.

a) Fine aggregate (sand)

Natural sand may be obtained from the soil / sand quarrying (Bullet sharp, hard, and free of the salt content), at the bottom of the river / river sand (Bullet smooth and round), and the waterfront / marine sand (Bullet smooth, rounded, and contain salt, so it is not good for use as an ingredient mixed concrete as it can cause corrosion of the reinforcement). Good sand is used as an ingredient in the manufacture of concrete mix is ​​sharp and hard grained (sand quarrying), not easily broken, and does not contain the mud.

b) coarse aggregate (gravel)

Terms - requirements that must be met by a coarse aggregate or gravel in the concrete mixture is grained hard (not crumbly) and not porous in order to produce concrete hard and properties of translucent water is small, does not contain clay more than 1%, did not contain the reactive alkali (can lead to the development of concrete).

The maximum grain size of the aggregate:

Should not exceed 3/4 times the distance between the steel reinforcement net or between reinforcing steel mold (formwork)
Must not be greater than 1/3 times thicker plate
Must not be greater than 1/5 times the smallest distance between the field side of the mold
Concrete made tractable should have properties (easily stirred, channeled, poured and compressed). This trait is influenced by several factors: the amount of water in the concrete mixture were increasingly accompanied by the addition of cement, the concrete will be more tractable, a good gravel gradation item, use gravel that is round, and clamminess between the material - the building blocks of concrete. Then, if after the concrete cast on the structural elements of a building, but after a strong test lab turned out she insisted on concrete does not qualify for that of the RKS, the things we can do that:

Analyzing the structure of the building with the actual quality of concrete (on the ground)
Later in the test  hammer test  on concrete has been cast in the
Then, when they are not qualified to do the test pembeban directly to the structural elements that have been cast in the
If they are not qualified to do the retrofitting of the structural elements of the
(Source: Lecture Materials Construction Materials Technology, Civil Engineering, University of Atma Jaya Yogyakarta)