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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? 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 :

  1. The characteristics of concrete.
  2. The 'cover thickness ', ie the distance between the steel bars and the concrete surface.
  3. The temperature. The higher the temperature, the faster the oxidation process.
  4. 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:


  1. Use good concrete with quality components, with a cement dosage above the minimum and with the maximum water/cement ratio indicated by the instructions.
  2. 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.

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, 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.


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 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.

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 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 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 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.


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 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.


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 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

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 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.


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 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.

What is the difference between sandcrete and reinforced concrete?

 


Sandcrete is a weak mortar-like building material with little compressive strength and zero tensile strength. I used it in Hollywood in a breakaway wall effect.

Concrete has more cement which is the glue that binds the rock aggregate and sand into concrete. It has lots more compressive strength but still not a lot of tensile strength.

So to make a strong column, vertical steel rods in compression are kept from blowing out with horizontal tension bands. In a horizontal beam or slab, the top chords of steel are in compression, but the bottom ones are being pulled in are in tension. The steel is ductile, which means it can flex with the concrete and it adds that critical tensile strength allowing us to create mighty bridges.



Soil excavation by machine and by hand


Machine excavation and material extraction

Once the stakeout is done, the machine proceeds with the excavation of as much earth as possible.

For this purpose, the total amount of earth to be excavated will be calculated by means of topographic plans and the projection of the new building. This volumetric computation is performed when there is a topographic survey of the terrain under analysis.


Contract and measurement method

The excavation of virgin earth material and the respective extraction and hauling off the ground consist of a single item within the unit price analysis.

The earth excavation contract with the contractor will then be based on the projected volume. This projected volume must be multiplied by a swelling factor according to the type of terrain to be excavated.

  • 10% swelling for loose terrain
  • 15% for compact sand
  • 20% to 25% for clays and clays with gravel
  • 30% to 40% for rocky material
  • 20% to 70% for demolition debris

Many times when the projected volume is not available, the earth extraction contract is made by machine on the number of dump trucks that leave the land. However, under this model, there is no amount calculated a priori that allows a budget for the work to be made for the excavation item.

Excavation by hand

Excavation by hand is done after all the earth excavated by machine has been removed. This type of excavation (by hand) is carried out for the footings and retaining walls sector.


Although a certain percentage of footing excavation can be done by machine, care must be taken that the machine does not exceed the footing offset depth, since if this happens, the accidentally excavated soil would have to be filled in and compacted, running the risk. that the compacted soil does not remain the same as the naturally consolidated soil.

Although a certain percentage of footing excavation can be done by machine, this excavation must be fully budgeted as if it were by hand. This is because the volume that the machine will help excavate the pit where the footing will go cannot be estimated and in the end, the profiling of the hole must always be done by hand. Therefore, the worst case is adopted, which is that everything is excavated by hand.

Types of excavation by hand

Soft ground excavation

As its name says, it is the excavation of organic soil, sand, or loosely compacted clay.

Hard ground excavation

It consists of the excavation of overconsolidated clay soils or gravelly and rocky soils. This type of excavation is more expensive than the previous one.

Excavation with exhaustion

In cases where the water table is high, and when excavating water must be pumped to allow the workers to work, this pumping must be included in the excavation costs.

Excavation at high depths

There is another type of excavation at high depths where more than one person is already needed to accomplish the job, as well as equipment such as ladders, shirts or rings to support the ground, and even winch, depending on the amount of material to be extracted.

Filling and compacting

Finally, for metric computations, the filling and compaction of the earth must also be measured, referring to the voids to be filled after the foundations have been emptied.


Construction site preparation

 


It consists of the adaptation of the construction environment to the minimum needs for habitability and work

In this sense, the installation of works is carried out by building temporary environments of different use according to the needs of the construction.

  • The most commons are:
  • Deposit
  • Porter or Caregiver Room
  • Bath
  • Environment or cover for tools and clothes of Workers
  • Perimeter fence

Finishing and improvisation of environments

Most of the time while excavating the land and building the first floor slabs, the environments mentioned above will be highly improvised.


Even at the time of demolishing the old environments prior to the excavation, materials are recovered that can be used to build the environments of the worksite.

When environments are built for the installation of tasks, it is normal to build them on the internal perimeter of the land, since this sector often does not harm the construction of the building.


Bath position

The bathroom is usually accommodated as close as possible to the old chambers of the land, corresponding to the old house or demolished property. This is done to prevent sanitary pipes running through construction and disrupting regular work.


Electrical installations

Electrical installations in the field are usually the elements relegated to the last place in importance, however, it is one of the main inputs in construction.

The cables derived from the electric power meter usually do not have the necessary insulation, or they tend to be routed on the floor and without the necessary support.

A recommended way to have this issue available is through a board like the one shown:



Cost

The Faenas installation could be one of the most difficult items to calculate since it varies according to the type of finish that you want to give it.

The environments can be in the exposed brick partition or in contrast, be huts armed with wood, paint, and all the necessary comforts of an office.
It all depends on the budget you have, the size of the work, and even the factor of how close the offices and logistics area is to the land.

One way of budgeting direct costs for the installation of works could consist of estimating two workers working full time for a week, and adding the amount of material to be used for that purpose.

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