Rebar Couplers

During placing the steel in RC structure if the required length of a bar is not sufficiently available to make a design length then lapping is done. Lapping means overlapping of two bars side by side to achieve required design length. This is less than an optimum solution as it wastes extra steel and creates comparatively a weak joint. A mechanical coupler is a better solution for joining rebars in concrete.  Rebar couplers are designed to produce full strength of mechanical joints between reinforcing bars. It can connect rebars of same or different diameters laterally, vertically, slantingly. It is broadly applied in the Tunnel, Tower, Bridge, Subway, Airport, Nuclear Power station, High Rise Building, Reservoirs, Marine Structures etc

Raw Material

chemical compositionCSiMnCrNiCu
%0.42 -0.500.17-0.370.50-0.800.250.300.25
Mechanical propertyYield StrengthTensile Strength
Mpa355600

As  its made of carbon steel naturally plain couplers are prone to corrosion under concrete application, to resolve this issue now-a-days epoxy coated couplers are used in place of plain couplers to protect against corrosion

 

Rebar coupler advantages:

1. Good performance of anticorrosion extends service lifetime effectively

2. Raw material saving, High energy saving, high work efficiency

3. High dimension precision, reliable quality and stable performance

4. Improved fatigue strength of the connection

5. Solves bar conjunction problem

As per Decision Database, The worldwide market for Rebar Coupler is expected to grow at a CAGR of roughly 5.3% over the next five years, will reach 1230 million US$ in 2023, from 900 million US$ in 2017.

Epoxy coated rebar couplers

Epoxy coated rebar couplers are more durable because of added corrosion protection and have enhanced fatigue life due to shot peening effect it gets during shot blasting of it while epoxy coating process.

Quality of Reinforcement Steel Bars used for Construction of RCC Structure.

When a lot of steel received at site, First check the Manufacturer test certificate for its actual properties.
With each lot of steel, manufacturer should send a test certificate of same lot for test done at their laboratory.
Check for grade of steel mention in certificate and is as per the order or not.

Steel bars may have rusting on it, do check closely to know either it is acceptable or not.
Steel received should be free from any contamination like, mud, dust, oil and any other foreign material etc.
Bars should not have splits and any other deformation on it.

Causes of Rusting:
Primary steel which is made from pure iron ore are likely to get rusted quicker compare to secondary steel.
Bars may get rusted due to contact with water or air and atmospheric condition.

A brownish bars showing little rusting due to weathering are good for use.
Small amount of rust is good for bonding of steel and concrete.
If excessive scaling observed on the surface of bar, it should not be accepted.

Do Check for brand of steel, diameter and grade of steel embossed on steel bars.

Cut the samples of 1 meter in length, min 4 nos of bars from different bundles.
Measure the length of cut bars by measuring it on at-least 4 sides and average out the length of bar.
Weight the bar on weight scale and record it in register.
Calculate the actual average weight per meter of bar for at-least 3 samples.
Compare the result of it with theoretical weight given in IS 1786

Check the variation in weight is within limit or not as per IS specification.

After finding the results for nominal mass as satisfactory proceed further to do bend test.

Bend test should be carried out as specified in IS 1599 and using mandrels of size specified in IS 1786

Rebar sample should be bent at 180 degree as per procedure stated in IS 1599.


after this further process it to bend till 180 degree.

At site we can bend it on bar bending machine using appropriate size mandrel.
sample tested bar.

After bending the bar check the surface of bar opposite to bend side (which got tension, elongated due to bending) for cracks and rupture visible to a person with normal or corrected vision.

If there is no sign of rupture and cracks, rebar meets the requirement of bend test.

Further to this a rebend test also can be done at site, if required. (IS specifies for doing but if it passes bend test, in general it will pass the rebend test too. You cand do it at side provided you have required arrangement at site for this test)

For Rebend Test
First bend the bar to including angle of 135 degree.
Keep it in boiling water at 100 degree for 30 minutes.
Then cool it down for some time.
After cooling bent it back to including angle of 157.5 degree

The rebar should not show any rupture or cracks to a person with normal or corrected vision.

Mandrel to use for Rebend test as specified in IS 1786

Below is pic showing bend rebend test (Closely look at the direction of bending and re-bending, in order to do correct test)

After getting satisfactory results, you can approve the steel for further usage in actual construction.

Keep practice of getting steel tested from third party laboratory at 200 metric tone or at each lot received which ever is acceptable for your management.

I personally did a bend and rebend test for 25mm and 32 mm bar by witnessing it in third party laboratories.
When steel failed at my site and got passed in multiple third party test.

When i did witness test, i shocked to see many laboratories don’t have the equipment’s to test it, as their current set up don’t allow higher diameter bars to get bend and rebend test.
Either machine reach its maximum capacity or their is chance of accident if we do it as per specification (That same laboratories gave me report stating rebar failure, this is real condition at least in my area of NABL accredited labs)

So my suggestion to all, when you get doubt on quality of steel, you personally witness the test process to understand either steel passes the test or not.

To do value addition to company and your self, you can implement following practice during unload of steel at site;
Count the number of bars received.
Record the average length of bar by doing random measurements.
Calculate the actual weight of steel received as per theoretical weight and compare it with actual total weight.
Make comparative statement showing the variation in steel weight as per brand of steel.
Show it to your management which brands are supplying overweight steel.

Overweight steel though it is as per tolerance provided in IS 1786 will cause a loss of money to your company, by understanding the which steel brand manufacture to optimum level and produce less overweight steel you can reduce the indirect loss to your company.

If i receive 25mm steel with 3% over weight, technically there is nothing wrong. Steel meets the requirement of IS 1786.
Site will suffer for 3% wastage without actually wasting steel and waste count will always add this unaccounted 3% loss as wasted by site engineer unless it is recorded.
Saving those 3% may cause gain of crores of rupees which are going directly to pocket of supplier and causing indirect costs to project due to which site engineers suffers a lot to get promotion. It may help you to get up in your career.
This is just a one example of steel, if you do correct work you can save a lot to company and get return benefit for doing such jobs.

This is something comes from experience and not told in any books (Now a days i wont read book, if any author had catched it then i am not aware about it, to write something like this author should have practical experience of project )

Hope this will help you in doing better testing of steel bars and satisfaction of doing it right

courtesy : https://civil4m.com/threads/how-to-check-quality-of-reinforcement-steel-bars-used-for-construction-of-rcc-structure.715/

Use of corrosion inhibitors in concrete

Corrosion inhibitors are chemicals that can slow down or prevent corrosion of reinforcing steel in concrete. Corrosion inhibitors were first investigated in the 1960’s.

Some early inhibitors included sodium nitrite and the sodium and potassium salts of chromate and benzoate. Studies found that the sodium and potassium salts reduced the strength of the concrete and gave mixed results on corrosion inhibition. However, other inhibitors have shown promise as methods for protecting reinforced concrete from corrosion damage (Virmani and Clemena 1998).

A common inhibitor used today, calcium nitrite, was developed to be used in concrete as a noncorrosive set accelerator (Berke and Rosenberg 1989)

Corrosion inhibitors are typically divided into three categories: anodic inhibitors, cathodic inhibitors, and organic inhibitors. Anodic inhibitors, made up of chromates, nitrites, molybdates, alkali phosphates, silicates, and carbonates, act by minimizing the anodic part of the corrosion reaction.

These inhibitors form an insoluble protective film on anodic surfaces to passivate the steel. Some anodic inhibitors, such as nitrites, can cause accelerated corrosion and pitting if they are not used in large enough quantities. Cathodic inhibitors, consisting of zinc, salts of antimony, magnesium, manganese, and nickel, form an insoluble film on the cathodic surfaces of the steel. They are usually less effective than anodic inhibitors, but are also safer. Organic inhibitors, including amines, esters, and sulfonates, block both the anodic and cathodic reaction on the entire surface of the metal (Virmani and Clemena 1998).

the inhibitors not only participate in reducing the rate of corrosion but it also takes part in the properties like compressive strength of the structure.

The effectiveness of inhibitors depends mainly on the concentration of inhibitor, more the concentration more is the inhibitor effect on corrosion efficiency decreases with increase in time.. The application requires transport of inhibitor to the reinforcement where it has to reach sufficiently on the surface of reinforcement to protect the steel against corrosion or to reduce the rate of ongoing corrosion.

Following considerations should be taken care while evaluating use of inhibitors in concrete for corrosion protection of rebar:

* Long term stability & performance of inhibitors
* Inhibitors effect on corrosion propagation after corrosion initiation
* Inhibitors effect on concrete physical properties over the service life
* Inhibitors should remain chemically intact & physically present (not leaching or evaporating)
* Insufficient dosage of inhibitors may impact on corrosion progression
* Amount of cracking concrete resistivity decrease

Rebar Calculator

Rebarcalculator enables calculations/conversions of rebar from  weight(MT),  Surface are(M2),   No. of bar(12 meter length each) and total Length(meter) in any combinations.

To Calculate from Weight(MT) and convert it into   Surface are(M2)  for example Dia 10 MM & Quantity 5 MT select the unit  MT from green panel  then select the unit to be converted as output ie. M2   from Blue panel.  Press calculate to get the required conversion.

This will be useful to site engineers, Managers, Project leaders in rolling, civil construction and coating industry to estimate cost and operational detail.

If you find this is useful, please comment on my blog and suggest of similar or any other areas in your business which can be improved with such software/applications.

You can download Rebarcalculator  free of cost from below. this application will easily run on Windows  but may  or may not  work   on Mac OS X, Linux, Solaris,  FreeBSD,  or  AIX

 

 

Do Corrosion Resistant Steel bars resist corrosion?

A number of manufacturers have come up with a new type of higher strength re-bars which are claimed to be corrosion resistant generally known as CRS.

These bars have dual micro structures i.e. the surface layer is tempered martensite while the inner shell is ductile ferrite-pearlite to increase corrosion resistance of these bars certain elements like copper , chromium,  and phosphorus are also added.  Copper plugs pores in the rust. Phosphorous in the form of P2O5  acts as an inhibitor and chromium helps in spinel oxide layer.These  protective layers is formed  on the surface when they come in contact with atmospheric oxygen and  moisture. This layer is formed over a period of time. In concrete oxygen is  not available. Hence, the protective coating may not form when such bars  are used as reinforcement in concrete. Obviously, there appears to be no advantage in using such bars in place of mild steel. Even, when exposed to atmosphere in marine environment the chlorides present in the air break the protective layer thereby reducing corrosion resistance. Therefore, in marine atmosphere these bars do not give good resistance to corrosion as in normal atmosphere. Secondly, their chemical composition is such that it may lead to pitting corrosion which is highly undesirable. In mild steel bars conforming to IS:1786, although is less corrosion resistance, but corrosion occurs all over  the surface without pitting.

Effects of alloying element

Phosphorus :          Higher phosphorus content contributes to the increase in strength and Corrosion resistance                                            properties but brings brittleness due to the formation of low euctoid phosphicles in the grain                                          boundary. Also lowers the impact Value at sub zero temperature level (transition temperature)                                      and tenacity to cracking during welding

Copper  :                Being a pearlite stabiliser, it increases the strength and corrosion resistance Property [only when                                   wet and dry cycle present]

Chromium :            Present as an impurity from the scrap and influences carbon  equivalent; Weldability and                                                increases corrosion resistance property.  High chromium bars not suitable for welding.

Result of Laboratory test on FBEC and CRS bars:

appvolt

 

   Result  of Applied voltage test:

  1. FBEC : after 30 days no sign of corrosion.
applied voltage resistance test

 

  CRS :   after 24 Hrs most of the bar gets dissolved in similar condition testing.

CRS

 

  1. What various standard recommends about CRS

 

aci

 

Best site practices for epoxy coated reinforcing steel

Unloading

unloading coated bar

Use spreader bar or strong back with

multiple pick-up points to minimize sags.

 For lifting, use nylon or padded slings; not

bare chains or cables.

 Storage

storage of coated bar

Unload as close as possible to the point of

Concrete placement to minimize rehandling.

Store coated and uncoated steel separately.

 Store bundles on suitable materials, such as

timber cribbing.

 Space timber cribbing to minimize sagging.

 Use nylon strapping or other non-abrasive

material.

covered bar

 If outdoor storage is to exceed 30 days, cover

coated bars with suitable opaque material and

minimize condensation

Shearing & Bending

bending

 Contact points on shearing and bending

equipment to be a suitable protective material.

 Inspect bars after bending and repair any

Cracks with patching material.

Cutting 

Power shears or chop saw (avoid flame cutting).

cutting

Repair cut ends.

Welding

 1.  Remove coating from the areas to be welded.

2.   Cleaned it with brush, remove any dust particles
3.   Weld properly prepared area
-reinforcing steel should be welded according to the American Welding Society,

AWS D1.4/D1.4M Structural Welding Code – Reinforcing Steel.
-While in situ welding of epoxy coated bars adequate ventilation to dissipate fumes must be provided
-Tack welding is not permitted.
4.   Clean all carbon deposits & slag from weld and HAZ
5.   Apply patch up  as usual practice.
– the damaged areas should be repaired using patch materials meeting ASTM  A775/A775M.

Patch up Material 

Use 2-part patching material, approved

by the coating manufacturer.

 Follow manufacturer instructions.

Repair of damaged epoxy coating, cut ends,

cracks and abrasions

step 1

Step 1.

Remove rust and contaminants from the

damaged area to be patched with a wire

brush

step 2

step 2       

Mix the patching material according to the

manufacturer’s instructions. Use patching

material prior to end of pot life.

step 3

Step 3.          

Apply the patching material to the repaired

area. Follow the patch material manufacturer’s

instructions.

step 4

 Step 4.

Allow the repaired area sufficient curing time,

as specified by the manufacturer’s instructionsbefore placing concrete.

Bar Placement

bar placing

Lift and set the steel into place without dragging.

Use plastic or bar supports coated with non-conductive material.

Use pvc coated tie wire.

Use power shears or chop saw to cut steel and not flame cutting.

Concrete Operations

concrete opration

Minimize traffic over the steel.

Avoid traffic and concrete hoses on placed steel.

Use plastic headed vibrators to consolidate concrete.

Corrosion rate measurement by Half-cell potential

The half-cell potential measurement is an electrochemical technique commonly used by engineers to assess the severity of corrosion in reinforced concrete structures.

Corrosion of steel reinforcement is a major factor in the deterioration of highway and bridge infrastructure. A survey of the condition of a reinforced concrete structure is the first step towards its rehabilitation. A rapid, cost-effective and non-destructive condition survey offers key information on the evaluation of corrosion, and aids in the quality assurance of concrete repair and rehabilitation and in the prediction of remaining service life.

 

                       Schematic showing basics of the half-cell potential measurement technique

 

Traditional way to assess the severity of steel corrosion is to measure the corrosion potential, since it is qualitatively associated with the steel corrosion rate. One can measure the potential difference between a standard portable half-cell, normally a copper/ copper sulphate (Cu/CuSO4) standard reference electrode placed on the surface of the concrete with the steel reinforcement underneath. We have developed use of standard calomel reference electrode[ at almost fraction of cost]  available in our lab as  illustrated above. The reference electrode is connected to the negative end of the voltmeter and the steel reinforcement to the positive.

    Half cell potential reading      Corrosion activity
>-80 mv Low risk of corrosion{10 % risk}
-80 mv to -230 mv Intermediate corrosion risk
<-230 mv High corrosion  risk {90 % risk}
<-380 mv Severe corrosion

      

         Criteria for corrosion of steel in concrete for calomel reference electrode

 

A more negative reading of potential is generally considered to indicate a higher probability of corrosion. However, this general “rule” may not always apply because of the complexity of today’s concrete and repair technologies. eg Not directly applicable to epoxy coated & galvanized bars

It must be stressed that the half-cell potential measurement only reveals the corrosion probability at a given location and time. Long-term monitoring of the half-cell potential reading is more meaningful.

Concrete Coatings-limitations

Various external hostile environmental substances, such as, water, carbon dioxide, oxygen, chlorides, sulphides and biological organisms are transported from the atmosphere into the concrete and attack steel and concrete in different mechanisms causing premature deterioration of reinforced concrete challenging its durability resulting in premature failure of the structures.

  1. Concrete is a porous material having high gas, vapour and liquid permeability leading to deterioration of reinforced concrete structures. chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion of rebar contributes to the deterioration of concrete.

2.  The protection of concrete should actually begin at the conceptual stage and meticulous strategies are adopted            for protecting the concrete from both internal and external environments. Various coating materials and                       application methods for concrete surface repair and strengthening have been developed. However, selection                 criterion for these materials has not been established yet at the current moment. Selecting procedures of                       concrete coating materials must focus on deteriorating mechanisms diagnosed carefully by the conditions of                 target structures. For instance, in case of salt damage, repair policy should consider corrosion environment and          deteriorating condition to determine symptomatic indications such as, 1) removal of permeated chloride ions, 2)           penetration block of chloride ions, moisture, and oxygen, 3)derusting of rebar, 4) corrosion-control method                (coating or potential control). However, it is still ambiguous to determine which is the best material and coating          system, because there is not enough, durability data to estimate.

concrete coatings
concrete coatings

 

3.  it should be noted that all resin materials are not totally resistant and impermeable to all aggressive agents                  and do not provide a total protection. Chemical/physical degradation of resins and debonding of coatings are              the major phenomena affecting the durability of surface protection. The mechanisms of destructive processes             in such heterogeneous materials as resin composites are complicated and not completely understood.                             Degradation of resins mainly involves swelling, dissolution and scission of molecular chain bonds. A wide                     variety of reactions is possible for resin degradation. The transport of gases and liquids aggressive to substrate             into or through the coating is the major problem of its delamination. There are many parameters that                             influence  the deterioration process of coatings, such as chemical agents, temperature, solar radiation,                           pressure, abrasion, cyclic temperature-moisture changes etc. All these parameters can occur simultaneously or             they can be complementary to one another.

  1. Diffusion rate of corrosive ingredients is higher than FBEC

5.  Concrete coatings suffers from poor durability of the coating and loss of corrosion protection in the areas                       where the coating is damaged

  1. Concrete requires continuous maintenance and therefore recurring cost.