Reinforced Cement Concrete

Reinforced Concrete

 

Picture 1 : Concrete Reinforce Diagram

Background

 
Concrete is a mixture consisting of sand, gravel, stone, or other aggregates are blended into one with a paste made ​​of cement and water to form a rock-like mass. Sometimes, one or more additives are added to produce concrete with particular characteristics, such as the working facilities (workability), durabilitas, and hardening time. As the substance of other rock-like substance, the concrete has a high compressive strength and tensile strength is very low. Reinforced concrete is a combination of concrete and steel where the steel reinforcement work provides a strong pull that does not belong to the concrete.

 
In a reinforced concrete building structures, particularly in the column happens axial bending moment and styles that work together - the same. Moment - this moment caused by the load or the gravity eksentris can cause lateral loads such as wind and earthquake, or it could also be caused by an unbalanced load floor. Therefore, each component of the structure of such cross-beams and columns should be planned for each style strong internal occurs, both the bending moment, axial style, sliding style or torque that arises as a response of the structure to external influences.

A. Defenition reinforced concrete structures 

Reinforced concrete material is a material made of concrete and steel reinforcement. Combination of both material produces building materials have good properties of each building material. Concrete has a good character, that have a high capacity press. However, concrete has a bad character, that is weak if burdened pull. While the steel reinforcement has a high capacity to pull, but have hit a low capacity for a slim shape (to be susceptible to buckling of the compressive load). However, by placing a section of concrete rebars in tension will pull eliminate the lack of concrete on the pull. Similarly, when the section of concrete rebars placed suffering press, concrete around the rebars together will prevent reinforcement rebars sengkan suffer buckling. Such an explanation of why the combination of the two building materials producing new building materials which have properties better than the properties of each of these materials has previously combined. Here we will show something relating to building materials concrete and steel reinforcement. 

Concrete is a building material made of cement (Portland cement or other hydraulic cement), sand or fine aggregate, gravel or coarse aggregate, water and with or without additives. Press strength concrete used for planning purposes is determined based on the strength of hit concrete at 28 days. Even now we can produce concrete with a strength of over 100 MPa press, the general press strength concrete used in planning ranges from 20-40 MPa. As described previously, the concrete has high strength press but has a low tensile strength, only ranges from 8% to 15% of the strength of push. To overcome the shortcomings of this concrete material then found a new building material by adding steel reinforcement for reinforced concrete sections, especially the experience to pull. Steel rebars used for planning must use steel reinforcement threads / fin (DEFORMED bar). Whereas plain rebars (plain bar) can only be used for spiral reinforcement and tendons, except for particular cases.

  

B. Strengths and Weaknesses In A Material Reinforced Concrete Structures 
1. Excess:

 
Reinforced concrete may be the most important construction materials. Reinforced concrete is used in various forms for almost all structures, large and small - buildings, bridges, paving roads, dams, dindingpenahan land, tunnels, bridges that crosses the valley (viaduct), drainaseserta irrigation facilities, tanks, and so on. The huge success of concrete as a construction material that is universally pretty easy to understand if seen from the many advantages it has. The advantages include:

 
a) The concrete has a compressive strength that is relatively high compared to most other materials.

 
b) Reinforced concrete has a high resistance to fire and water, even the best material for building structures in contact with water. In the event of a fire with an average intensity, rod-rod structure with concrete cover thickness yangmemadai as a protective reinforcement only padapermukaannya any damage without collapse.

 
c) reinforced concrete structure is very sturdy.

 
d) Reinforced concrete does not require high maintenance costs.

 
e) Compared with other materials, concrete has aged very long service life. In normal conditions, reinforced concrete structures can be used up at any time without losing its ability to withstand the load. This can be explained from the fact that the concrete strength is not diminished with the passage of time more and more growing even in a matter of years, due to the lengthy process of compacting cement paste.

 
f) Concrete is usually the only material that is economical to tread foundations, basement walls, pedestal pole bridges, and buildings like that.

 
g) One characteristic of concrete is its ability to be molded into the shape that is very diverse, ranging from plates, beams, and columns that is simple to roof dome and a large shell.

 
h) In most areas, the concrete made ​​from local ingredients are cheap (sand, gravel, and water) and requires only a relatively small amount of cement and steel reinforcement, which may have to be brought in another from area.

 
i) labor skills needed to build a reinforced concrete construction is lower when compared with other materials such as steel structure.

2. weakness
To be able to optimize the use of concrete, planners must be familiar with the advantages. Reinforced concrete weaknesses include:

 
a) Concrete has a very low tensile strength, thus requiring the use of tensile reinforcement.

 walls
b) Reinforced concrete requires a concrete formwork to hold firmly in place until the concrete hardens. In addition, temporary support or brace may be needed to keep the formwork remains in place, such as the roof, , and similar structures, to the parts of this concrete is strong enough to withstand its own weight. Formwork is very expensive. In the United States, the cost of formwork ranged from one-third to two-thirds of the total cost of a reinforced concrete structure, with a value of about 50%. It is clear that to reduce costs in the manufacture of a reinforced concrete structure, the main thing to do is to reduce the cost of formwork.

 
c) The low strength per unit weight of concrete resulted into heavy reinforced concrete. It will be very influential on the structure-long-span structures where the dead weight of concrete that will most greatly affect the bending moment.

 
d) The properties vary widely due to the variation of the concrete-mix proportions and stirring. In addition, concrete pouring and care can not be handled as expeditiously as is done in the production process of other materials such as steel and timber structures.

C. Properties of Reinforced Concrete

 
Depth knowledge of the properties of reinforced concrete is very important before starting design of reinforced concrete structures. Some properties of reinforced concrete, among others:

 
1. Compressive Strength

 
Compressive strength of concrete (f'c) is done by testing concrete cylinders with a diameter of 150 mm and height 300 mm. At the age of 28 days with a certain loading rate. Over a period of 28 days is usually placed concrete cylinder Mdalam a room with constant temperature and humidity 100%. Although there is a strong concrete that has a maximum of 28 days from 17 MPa to 70 MPa -140, mostly concrete has a strength in the range of 20 MPa to 48 MPa. For general applications, use concrete with a strength of 20 MPa and 25 MPa, while for the construction of prestressed concrete 35 MPa and 40 MPa. For some applications, such as for columns on the lower floors of a high-level building, concrete with strength up to 60 MPa have been used and can be provided by the manufacturers of ready-mixed concrete (ready-mix concrete).

 
The values ​​of the compressive strength of concrete as obtained from the test results is strongly influenced by the size and shape of the elements of the test and how the assignment. In many countries, the test specimen used was a cube containing 200 mm. concrete-concrete for the same test, testing of cylinders 150 mm x 300 mm produce a compressive strength of only about 80% of the values ​​obtained from testing of concrete cube test.

 
Concrete strength concrete can be switched from 20 MPa to 35 MPa concrete without the need for additional labor and cement in excessive amounts. The estimated increase in the cost of materials to gain additional strength as it is 15% to 20%. But to get the concrete strength above 35 or 40 MPa concrete mix design required a very careful and close attention to details such as mixing, placement, and maintenance. This requirement raised the cost relatife greater. Stress-strain curves in the figure show the results achieved behind from a cylinder compression test on a standard 28-day test of strength diverse. 

D. Static Modulus of Elasticity

 
Concrete does not have a definite modulus of elasticity. Its value varies
depending on the strength of concrete, concrete age, type of loading, and the characteristics and comparison of cement and aggregate. In addition, there are several definitions of the modulus of elasticity:
a) The initial modulus is the slope of the stress-strain diagram at the origin of the curve.
b) tangent modulus is the slope of one of the tangent (tangent) on the curve at a particular point along the curve, for example, at 50% of maximum strength of concrete.

 
c) The slope of a line drawn from the origin point of the curve to a point on the curve somewhere between 25% to 50% of the maximum compressive strength is called secant modulus.
d) Modulus of the other, called the pseudo modulus (apparent modulus) or long-term modulus, determined using the stress and strain obtained after the load is given for some time. 

ACI Regulation states that the formula to calculate the modulus of elasticity of concrete which has a concrete weight (wc) ranged from 1500 to 2500 kg / m 3.

where:
wc: weight of concrete (kg / m3)
fc ': the quality of concrete (MPa)
Ec: modulus of elasticity (MPa)
Dynamic Modulus of Elasticity

E. Modulus dynamic elasticity

 
Dynamic modulus of elasticity, which corresponds to regangan-
instantaneous strain is very small, usually obtained from sonic test. Its value is usually greater than 20% -40% and the static modulus of elasticity is approximately equal to the modulus of the initial value. Dynamic modulus of elasticity is usually used in the analysis of structures with seismic load or a collision. 

F. Comparison of Poisson

 
When a concrete receiving compressive loads, the cylinder is not only high but also experience reduced expansion (expansion) in the lateral direction. Comparison of lateral expansion to longitudinal approach is referred to as the Poisson ratio (Poisson's ratio). Its value varies from 0.11 to 0.21 and a high quality concrete for low strength concrete, with an average value of 0.16. It seems there is no direct relationship between the value of this ratio with values​​, such as water-cement ratio, length of treatment, aggregate size, and so on. In most of the design of reinforced concrete, the effect of Poisson's ratio is not too concerned. However, the influence of the comparison must be considered when we analyze and design the arc dam, tunnel, and the structures of other statically indeterminate.