Corrosion of steel in reinforced concrete
Today I want to continue talking about the durability of concrete, specifically about the corrosion of steel in reinforced concrete. How does it work?...
Today I want to continue talking about the durability of concrete, specifically about the corrosion of steel in reinforced concrete. How does it work? For what is this? How to avoid it? In this article, you will find everything you need to know about this aspect.
Electrochemical corrosion of the reinforcement
As I have already mentioned, this corrosion occurs through an electrochemical or galvanic mechanism caused by the presence of water and oxygen inside the concrete.
On the other hand, this process is influenced by different factors :
- The characteristics of concrete.
- The 'cover thickness ', ie the distance between the steel bars and the concrete surface.
- The temperature. The higher the temperature, the faster the oxidation process.
- The presence of chlorides, which accelerates the process.
Depassivation of the concrete reinforcement
Generally, the steel in concrete tends to passivate due to the alkalinity given by portlandite or CaOH2 from the hydration of the cement and its alkalis.
However, this passivation can be lost for a number of reasons:
1. CARBONATION
In carbonation, atmospheric CO2 penetrates the air-filled capillaries of the concrete and reacts with calcium hydroxide, giving rise to calcium carbonate. The pH of the concrete must be between 12'5 and 13'5 to properly protect and passivate the reinforcements inside. The presence of calcium carbonate lowers the pH of the medium that surrounds the steel bar to values close to 9, resulting in a more acidic medium than it should and that begins to cause a constant, uniform, and progressive corrosive effect of the steel.
The time required for the penetration front of the CO2 to reach the steel bars depends on the porosity of the concrete, the thickness of the coating of the steel bars, and the humidity of the air.
2. CHLORINE IONS
Chloride ions destroy the passivating layer of the steel in a specific way, giving rise to pitting corrosion that spreads in-depth, being able to section the bars and especially the prestressing wires, which are thinner and with a more delicate crystalline structure. This is why the maximum chloride content is limited to 0.4% in reinforced concrete and 0.2% in prestressed.
In a strict sense, the critical content of chloride s depends on the degree of carbonation of the concrete and the relative humidity of the environment. For corrosion to occur, there must be a significant drop in pH so that the steel remains depassivating, and this drop may be caused by the presence of acidic substances in the concrete.
Measures to avoid the corrosion of the armor
As measures to be taken against the corrosion of steel reinforcements, it is worth highlighting:
- Use good concrete with quality components, with a cement dosage above the minimum and with the maximum water/cement ratio indicated by the instructions.
- Give a suitable coating thickness to the steel bars depending on the environment in which the structure is located and respecting the minimums indicated in the instruction.
Today I bring an article a little more technical than the previous ones but that I think cannot be missing in a blog on this subject. It is concrete, that material that we civil engineers like so much for its excellent resistance properties and relatively low price.
However, as those of you who are involved in the construction world will already know, concrete has a great weakness. The tensile strength of concrete is much lower than its compressive strength, around 10, and limits, in many cases, the use of this material.
How can we alleviate this defect? Here I leave you some brushstrokes of the most common methods to solve this problem so characteristic of concrete.
This article is part of a series on concrete tensile strength. Here you have the other two articles in case you are interested in consulting them:
- Direct tensile tests of concrete
- Indirect tensile tests of concrete
Reinforced concrete
Assembling the concrete is undoubtedly the traditional method par excellence. It is about integrating the properties of "plain" concrete and reinforcing steel. Both materials must work as one, so it is necessary that they be intimately united and interact through the adhesion forces that develop on their contact surfaces.
That is why the surface of the steel must be rough (corrugated steel rods) and the concrete must be vibrated after being placed in the molds.
Steel, unlike concrete, has similar mechanical characteristics both in tension and compression. When faced with tensile stresses, concrete cracks easily, ceasing to resist. The steel prevents collapse from occurring, joining the two "pieces" and resisting those tensions that concrete is unable to resist.
Prestressed concrete
This concept is newer and more complex than that of reinforced concrete. Any beam is subjected to two main loads: its own weight and external loads. As a consequence, some fibers in the section are betrayed and others are compressed.
Due to the characteristics of concrete, the compressed fibers will work properly, on the other hand, the tensile fibers will do so relatively inefficiently (similarly to what happened in the case of reinforced concrete).
To solve this, in the prestressing a state of initial compressive stresses is created in such a way that future tensions are reduced to values that can be assumed by the concrete or simply disappear.
In this way, concrete is subjected to two systems of forces:
- One caused by internal forces caused by prestressing
- Other caused by external loads and own weight
As a final note, it should be noted that the prestressing action is more efficient, it is placed eccentrically with respect to the centroid of the section. But be careful, since an excess of eccentricity could produce the opposite effect and that the concrete would crack on the opposite side only under the force of the prestressing.
Fiber-reinforced concrete
The last method is based on the incorporation of a certain proportion of fibers in the concrete to improve its properties. The fibers improve some characteristics such as toughness, control of the cracking process, and flexural strength.
Therefore, the brittleness of the concrete is reduced and there is a "stitching" effect of the cracks, reducing the opening of the cracks. The most commonly used fibers, in descending order, are steel, glass, and polypropylene.
And this is the most basic of the basics. Without a doubt, an essential in a blog like this. In any case, this is just a contact that I hope will serve the curious or those who are starting in this world.
Tension concrete, how to solve it?
Today I bring an article a little more technical than the previous ones but that I think cannot be missing in a blog on this subject. It is concrete,...
Download free Manufacturer specific BIM object files. Revit Families are components you use to build your model, such as walls, windows, stairs, doors, etc. Each Revit family can have multiple types, such as different
Part 01
Part 02
Revit Family Free
Download free Manufacturer specific BIM object files. Revit Families are components you use to build your model, such as walls, windows, stairs,...
What is AutoCAD and what is it for?
Well, before explaining the program, you should bear in mind that its acronym stands for Computer-Aided Design ("Computer-Aided Design" in its acronym in English).
This definition appeared in the sixties and early seventies of the twentieth century when the design of mechanical parts began to be carried out using computers, mainly in the automotive and automotive sectors.
At that time, computers were capable of presenting different views of drawing layout and the generation of plans with photo-based systems, making the work of engineers much easier.
With the development of IMB's Personal Computers, the predecessor of AutoCAD appeared, known as MicroCAD, which although it had more limited functionalities, features, and tools, represented a great leap in the work of CAD or Computer-Aided Design systems.
System requirements for AutoCAD 2022
OS: Microsoft Windows 10 (64-bit only) (version 1803 or higher)
CPU:Basic:2.5–2.9 GHz processor / Recommended:3+ GHz processor
RAM: 8 GB / Recommended: 16 GB
Display Resolution:
Conventional Displays:1920 x 1080 with True Color
Resolutions up to 3840 x 2160 supported on Windows 10, 64-bit systems (with capable display card)
Disk space: 6.0 GB
Download Link
Download x-Force Universal FlexNet patch for Autodesk 2022
AutoCAD 2022
What is AutoCAD and what is it for?Well, before explaining the program, you should bear in mind that its acronym stands for Computer-Aided Design...
A Cantilever slab is a structure that is fixedly supported at one side
Note: Careful while detailing your reinforcement
1- You know the dimension of your slab ( Thickness, Width, and length ). then Go for Load Analysis to get the maximum design load combined in KN/m or
2- Next go for structural Analysis to get your maximum forces (i.e maximum shear and moment )
3- Then calculate the minimum area of reinforcement required. Also, calculate the minimum area of reinforcement and compare the two.
4- Detailed your reinforcement arrangement. ( provide more at the area of the maximum bending moment )
5- Check for shear. if shear reinforcement needs to be provided or not.
6- check for deflection.
However, An easy way is to use design software such as Staad, ETABS….. and so on. using the software can give you everything within 15 minutes.
How can we design a cantilever slab?
A Cantilever slab is a structure that is fixedly supported at one sideNote: Careful while detailing your reinforcement1- You know the dimension of your...
Is there a way to connect a new reinforced concrete slab on an existing reinforced concrete beam, given that the beam doesn't have steel dowels to connect with?
Yes its possible, but I agree with Greville Wood’s answer and genuinely don’t think he is touting for work.
Depends on the loads involved and connecting to the existing set concrete needs specific techniques and materials. Plus you won’t know the reinforcement on the existing beam. It’s a recipe for an expensive mistake.
The safest way is to break back the concrete to establish what reinforcement exists, that way you can physically reconnect to the existing concrete providing it hasn’t degraded or spalled.
The existing concrete will have to be primed and the new concrete replaced, not always easy. The new concrete may have to be polymer modified. That why it is best to check with an engineer.
SNRN 4/07/2021 Admin Bandung Indonesia
Is there a way to connect a new reinforced concrete slab
Is there a way to connect a new reinforced concrete slab on an existing reinforced concrete beam, given that the beam doesn't have steel dowels...
The main principle is that concrete has strong resistance to compression failure therefore it is strong in compression however it is weak in tension resistance to bending that is why steel compensates this weakness hence its strong in tension Having more percentage than the specified will cause the concrete to act as steel structure which is basically is not steel aimed structure besides that the area occupied by more reinforcement will weaken the concrete to act in compression resistance and punching shear and brittle failure may occur especially in the perpendicular structure such as columns at the time of ultimate strength, in conclusion, the standards have undergone plenty of experiences by experts to bring forward simple references for a designer I hope you have gained insight hints.
Rebar or reinforcement is expensive. Using more than recommended will be uneconomical. Moreover, providing adequate spacing between the rebars will be difficult. IS 456:2000 recommends minimum and maximum spacing which has to be fulfilled.
One more disadvantage of providing excess rebar is that the placement of concrete will become difficult. The concrete should be placed uniformly in the framework which would be affected by the presence of excess rebar.
Then the concrete will fail first, which is a brittle material so you will not get any warnings of failure beforehand, which means a sudden failure without warning and it can therefore cause loss of lives… this is a very unwanted situation. The rebar should be below a certain limit so that when the failure will happen, the rebar will start to fail, which is a ductile material so it gives a warning before the collapse and can save lives.
What will happen if we use too much rebar in concrete?
The main principle is that concrete has strong resistance to compression failure therefore it is strong in compression however it is weak in tension resistance...
There is no difference between ‘T’ rebars and ‘H’ rebars, as they both refer to high tensile steel. The letter ‘T’ comes from older codes of practice and ‘H’ is the more modern or current standard. Always use H to avoid confusion with T = Top and B = Bottom. So for example:
5H10 T&B means five 10mm diameter rebars top and five 10mm rebars bottom of the slab or beam. Also you can place two lines above one another as so:
4H16 T
4H25 B
Which would be a common way of showing a beam had different bar sizes top and bottom.
Note that the grade of rebar should be specified in your calculations and drawings using the tensile strength and ductility grade, B500A, B500B and B500C.
What is the difference between T rebars and H rebars?
There is no difference between ‘T’ rebars and ‘H’ rebars, as they both refer to high tensile steel. The letter ‘T’ comes from older codes of practice...
Rather than advising you on which brand of cement to be used, I would advise you on the type of cement to be used because that plays an important role in the quality of construction
For plastering works, use blended cement like PPC or PSC. These cement have less heat of hydration. That means during the cement reaction with water, they produce less heat. Less heat means less cracks on the wall surface. Less cracks and less porosity will decrease any water seepage.
As far as brand is concerned, most of the brands provide cement of good quality these days. You can go with the brand that is locally well known and gives you the best rate. Big brands claim huge premium and it is not worth a value for individual house constructions.
As different stages of construction require different types of cement, Understanding the types of cement, its substitutes, and the availability of local brands should empower you to choose the right cement required for your work thus avoiding any concerns on the availability of cement.
As Cement has a limited shelf life and loses strength over time, cement should be best consumed within a month or at most within two months of its production. The rate at which cement loses its strength depends on the storage conditions.
Which is the best cement used for plastering?
Rather than advising you on which brand of cement to be used, I would advise you on the type of cement to be used because that plays an important role...
That depends on what country you are in. It is specified in different ways. Hopefully, that will come out in other answers, rather than just assuming that we are all in the same plot of land (as is usually the case on Quora). Note also that the way one country does it is not necessarily the best, regardless of how much they puff out their chest or point to their population size.
Grading of Concrete.
Grading of concrete can be defined as the designation of concrete based upon its strength and mix characteristics for its use. In this designation, concrete is graded as M10, M15, M20, M25, and so on, where M specifies the mix and the number corresponds to the strength of concrete after 28 days in N/mm^2. These grades also represent the ratio of aggregates to be used with respect to the cement. Some of these mix ratios are as follows
M10-(1:3:6)
M15-(1:2:4)
M20-(1:1.5:3)
M25-(1:1.2.2.4)
For gaining concrete strengths above M25 design mix has to be prepared as per the standards.
As per IS 456 2000 Concrete Grades are denoted by M10 M15 M20, M25, etc where M refers to Mix and Number stands for the Compressive strength of Mix attained in 28 Days
As per the mix, the density varies, the higher the grade higher the density. As concrete is a versatile structural building material, made by mixing cement, aggregate, and water
The proportions of cement, fine and coarse aggregate are important if satisfactory concrete is to be produced.
here comes the importance of Grading refers to the determination of the particle-size distribution for aggregate (Which includes Fine and Coarse). Grading limits and maximum aggregate size are specified because these properties affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete.
The strength is derived mainly from the bonding of particle to particle by the hardened cement paste
SNRN 4/04/2021 Admin Bandung Indonesia
What is the grading of concrete?
That depends on what country you are in. It is specified in different ways. Hopefully, that will come out in other answers, rather than just assuming...
Concrete is one of the most used construction materials in the world today. It can be placed in forms to create almost any shape or placed in flat slabs thousands of feet long and inches or feet thick, it can be structural columns and beams supporting tall buildings or bridges, or it can be the hull of a boat that floats underneath the bridge and out to sea.
Concrete is a mixture of cement, usually Portland cement, coarse and fine aggregates (think of coarse as gravel and fine as sand, though other materials may be used). When cement is mixed with water and the appropriate aggregates, it becomes plastic (fluid), and it is placed in forms (molds) to create the shape the user is building. Hydration, a reaction between the dry components of the cement and water causes the concrete to set up, or harden into a stone-like material that is the foundation (literally) of the construction industry today.
Other answers concern themselves with the change in the water to cement ratio if rain occurs before the concrete has set. Having placed and finished concrete for many years, I haven’t seen that happen unless the rain comes while the concrete is being placed. As soon as the concrete has been screeded and bull-floated, the paste is pretty much established, in fact, the process of bull floating is partly to raise paste to the concrete surface. While hard, driving rain or long sustained rainfall may disturb the paste, it doesn’t penetrate through the top layer of the aggregate unless the surface isn't sloped enough for the water to run off.
Ideally, if you expect rain, you will cancel the concrete placement. If you are not able to, then plan on covering the concrete with plastic sheeting as soon as possible after placing and floating the concrete. Uncover the concrete only if it begins to set too hard for adequate finishing, or if the rain passes by. Failing to cover the concrete may allow the cement paste to wash away, leaving very little workable material to finish, but trying to finish the concrete while water is standing on the slab will certainly allow the water to work into the concrete, and this will weaken it.
Keep in mind that the hydration process that causes concrete to set produces heat, so covering the slab while the concrete is wet may cause the set to speed up, and if the concrete remains covered until the concrete begins to set quickly, it will be difficult to achieve an acceptable finish.
What happens if it rains after you pour concrete?
Concrete is one of the most used construction materials in the world today. It can be placed in forms to create almost any shape or placed in flat...
A home's form and exterior construction have a significant impact on its energy quality. The height, width, and depth of the structure are all included in the shape. These are also known as the footprint of a house. The walls, roof, windows, doors, and cladding make up the external frame, also known as the building envelope. A home's footprint and envelope may either make it more energy-efficient or cause it to use more energy. Easy or uncomplicated shapes are more energy-efficient to heat and cool than homes with complex or irregular shapes.
Along with the form and envelope of a house, the location and orientation of a home must also be considered. The home's relationship to the land, the sun, and the wind would have a significant impact on its overall energy performance. The east and west orientations should be carefully considered since these exposures would have a more substantial solar heat gain in hot climates.
To minimize solar heat gain, fewer windows should be installed in east- and west-facing walls. Furthermore, the home's narrowest elevations should face these orientations to minimize wall exposure to the summer sun while allowing the low angle rays of the winter sun to warm the southern exposure when required in cooler climates. Consider the cold prevailing winds and orientation, as well as landscaping and windbreaks, in cold climates.
SNRN 4/04/2021 Admin Bandung Indonesia
Does the structure of the building affect the temperature inside the house?
A home's form and exterior construction have a significant impact on its energy quality. The height, width, and depth of the structure are all included...
