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.

History of Rebar

Year Event
1848Jean-Louis Lambot
Used iron bars and wire mesh to reinforce several concrete rowboats…
1854William B. Wilkinson
Used iron bars and wire rope to reinforce the concrete floors of a two-story cottage.
1850-1880 François Coignet
A pioneer of reinforced concrete, Coignet was the first to use iron reinforced concrete in construction on a widespread scale. France
1889Gyozo Mihailich
Mihailich is credited with designing the first arch bridge to use reinforced concrete. Hungary
1911ASTM A15 published with grades 33 and 50
till 1967Plain MS bar of grade FE-250 in India
1968ASTM A615 published (replaced ASTM A15, ASTM A408, ASTM A431, ASTM A432, and portions of ASTM A305) with grades 40, 60, and 75
1973Epoxy-coated rebar first used in a U.S. bridge
1979ASTM A767 published for zinc-coated (galvanized) rebars
1981ASTM A775 for epoxy-coated rebars and ASTM D3963 for handling of epoxy coated bars published
1983Stainless steel rebar first used in U.S. bridges.
upto 1990CTD bars of grade Fe-405-415 used in India
1980-85TMT bars introduced in India
1985IS-1786 first published in India
1993IS-13620 for epoxy coated bars published in India
1996ASTM A955/A955M published for stainless steel rebars
2008ASTM A1055 published for zinc and epoxy dual coated rebars
2017IS-16651 for stainless rebar published in India

Rebar fabrication process

The fabrication of reinforcing steels into shapes suitable for fixing into concrete formwork is normally performed in advanced countries by specialist reinforcement fabrication centres. In these advanced countries very little reinforcement is cut and bent on-site. In India, the reinforcement bar fabrication is predominantly done at site by employing huge labour force and semi-automatic machines. Although the cutting and bending of steel reinforcement appears relatively straightforward, the steel reinforcement fabrication service centres are well equipped to do so in a consistently accurate manner. The service centres works under defined set of quality management criteria to ensure the accuracy of cutting and bending operations in order to achieve proper fit at site, and to maintain the required technical parameters defined by the clients.

Rebar Processing
The basic fabrication processes consist of cutting and bending the reinforcement steel. The actual process employed depends on the material being processed, whether bar or coil. At the Rebar processing centre coated or bare reinforcement are fabricated by cutting on shear lines, and thereafter bending on power bending machines. The processing ranges of these machines are from 16 mm diameter to 50 mm diameter. The reinforcement bar, in coil form, is obtained from the mills in Coil form in the size range of 6mm to 12mm and is bent to stirrups and most complicated shapes through the Automatic Link Bending machine to promise the delivery of dimensionally accurate stirrups and complex shapes just in time (JIT delivery).

Benefits to Construction Industry:

Image courtesy : unionrebar

• Accurate planning of the site programme
• An answer to severe skill and labour shortage
• Weather-independent construction
• Reduced construction time, thereby avoiding time and cost overrun
• Efficient use of limited space at congested sites
• Improved safety
• High accuracy on specified tolerances thereby avoiding rework at site
• Minimum wastage
• Dependency avoidance on power availability and power saving at site
• No investment in machinery for fabricating the reinforcement
• On time delivery every time

Benefits to Coated bars:

Image courtesy : ISPC

* No touch up at the site
* No damage to coatings
* Reduced transportation cost for the contractor
* Full range of rebar processing service-improved image
* Value-added service + saving generated due to a reduction of patch up compound

Myths about TMT bar

1. TMT has higher bond
Process of making TMT as such has no role in bond strength between rebar and concrete. Factors responsible for higher or lower bond are Rib geometry and concrete properties.
Rib geometry
-transverse rib height
-transverse rib length
-transverse rib angle of inclination
-transverse rib spacing
Concrete properties
-strength
-vibration/compactness
-use of chemical ingredient
Bond strength of any type of rebar in concrete can be tested with pull out test

2. TMT is corrosion resistant
in fact reverse is the truth. TMT bars are highly prone to corrosion due to its dual microstructure ie. Martensite and Perlite, Which provides galvanic coupling for corrosion. TMT bars at site shows corroded layer on outer surface.

3. TMT bar once in concrete never corrode
This is equally false belief that TMT bars gets corroded in outer atmosphere only and not inside concrete. As concrete is permeable to corrodents like moisture, oxygen, chloride etc Rate of corrosion can be different based on severity of atmosphere

4. TMT never fails in bend/strength
Every material has a defined range of properties which is prescribed in various nation/international standards beyond which it fails.

5. Any water-cooled bar are TMT
Water cooling hot bars are critical part of TMT bar processing but the process is called Quenching and is carried out under certain range of temperature to get desired microstructure which is responsible for higher strength of TMT bar. Water quenching if not done properly creates two process issues
Improper or peripheral quenching- this creates bar with variable mechanical properties in the bar, which is an safety hazard.
Tempering-this process imparts ductility and stress relieving effects to bar which harmonizes properties of rebar

6. TMT bars are earth quake resistant
In case of earthquake combined effects of concrete and TMT bar plays important role, TMT bar alone can’t resist forces of earthquake. Hence judicious selection of properties and integration of both is required

7. Scrap heated and rolled into bars are also TMT
Any scrap without known material history does not provide physical and mechanical properties defined in rebar standard, it may provide such wide variation that structural design requirement becomes first casualty and hence a safety concern.

8. TMT is the only rebar used in RCC

Now a days lot of innovations are happening in rebar field. Variety of Bars depending on site and environmental concerns are deployed. Some of the widely used type are Stainless steel, FRP, basalt, galvanized bars, epoxy coated bars

How to reduce surface preparation cost by Ervin Test ?

Testing in industrial life is routine and mundane thing but some tests you remember esp when its impact results in out proportion saving to bottom-line. Surface preparation with abrasive blasting is carried out in many activities like coating. Use of right abrasive results in right quality & best economy. I would like to share one such pleasant experience about Ervin test.

It is very small equipments consists of a wheel with two blades, a target and a re-circulating device-just like a miniature version of shot blasting machine.

Ervin test machine enables us evaluation of quality of metal abrasives esp durability [life] and transmitted energy [impact energy] of abrasives under controlled conditions, which are two key measures of the value and quality of metal abrasive. Durability of abrasives are determined by number of passes against the target that are required to reduce the abrasive to unusable size ie 100% replacement by test quantity.

We had calibrated the system using S-550 calibration standard shots and then tested abrasives available at our end, results were as follows:

AbrasivesDurability
[No.s of passes to replace 100%]
calibration standard- SS 5502946 [2900+-50 as per standard]
A-grit GP 252893
A-shots SS 3303043
B-grit GP 251091
B-shots SS3301483

Supplier A & B were both good name in industry B-shots gives half life compared to that of A-shots and B-grit gives one third life compared to that of A- grits. Results at that time were shocking, but resulted in very hefty saving in surface preparation cost.

This facility may be utilized for evaluation of shot and grits and for new vendor development.

It can also be used with a frequency inverter to establish optimum shot velocity of abrasive mix.

Thus Ervin test with simulation of real shot blasting process, helps to demonstrate media properties such as hardness, size, density and durability and how their interaction, determines the efficiency of energy transfer and the economy of abrasive cleaning process.

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

What is basalt rebar?

Rebars are backbone of construction industry. Rebars are made of steel and are in use ever since its innovated. Need is the mother of innovation. Problems like rebar corrosion led to various new type of rebar like epoxy coated bars, galvanized bars, stainless steel and frp rebars. Places where constructions are exposed to high levels of radio frequency radiations alongwith corrosion FRP and new entrant –rebar made of basalt are highly suitable.

Basalt rebars are made of volcanic rock called basalt- which is present abundantly in earh’s layer below crust

Basalt filaments are made by melting crushed volcanic basalt rock of specific mineral mixture at 1700 C, hot material is drawn through platinum  bushings and cooled to form filaments. This filaments are stronger then steel or FRP. Basalt rebar are made by pultrusion process using 80% basalt filaments and 20% epoxy as binder.

No chemicals are added to basalt during its manufacturing unlike FRP – so its environment friendly

Physical properties of basalt filaments like tensile strength, modulus of elasticity, temperature tolerance and resistance to acid & alkali damage are better than steel & glass fibers..

Basalt rebars have following advantages:

  1.    It does not corrode
  2.    Its stronger than steel and FRP rebars (tensile strength)
  3.    Higher chemical stability
  4.    Thermal expansion coefficient is similar to concrete.
  5.     Lighter than steel
  6.     Non conductor to heat & electricity.

                                                                                                                        Image courtesy: technobasalt

 

Basalt rebar clearly is ready to be used as a substitute for both steel and fiberglass rebar.  It is clear that steel in a concrete construction is a rust-spalling failure waiting to happen. Eventually moisture will get to steel wherever it is and no matter how well it is protected. It will then rust, swell, and cause the concrete to fail. With basalt rebar this issue is avoided forever.

The lack of spalling leads to one more advantage for basalt rebar. Construction codes call for concrete cover- spacing steel reinforcement at least 3 inches from the surface of the concrete. This is not necessary when using basalt rebar. A slab or panel can be made as thick or thin as is needed for structural integrity. If one inch of concrete is sufficient, a panel can be one inch thick with no risk of failure from spalling.

Comparison of physical properties- Steel, FRP and Basalt rebars.

ItemSteelGFRPBasalt rebar
Tensile strength(MPa)350-650500-900700-1300
Yield strength(MPa)280-420--
Compressive strength(MPa)350-650400-550450-550
Tensile elastic modulus(GPa)20035-4555-75
Coefficient of Thermal
expansion(*10-6 /C)-transverse
11.721-2321-22

courtesey: pulwell composites co.Ltd

 

How to tell the difference between paint and fusion bonded epoxy coating

Hundreds of paint formulations are being sold in market each claiming the best benefits. Each paint has its own chemistry and set of properties so it is essential to match it with requirement of application to get the maximum advantage esp when value addition difference is too wide and the opportunity cost difference is too high eg. In functional coatings like rebars.

When pigments are fine tuned, it is difficult to differentiate between regularly painted rebar and fusion bonded epoxy coated rebar as visually both look alike. However with careful observations and some tests to validate the same it can be easily identified & differentiated between normally painted rebar and fusion bonded epoxy coated rebars.

ParameterPaint Fbec
Image
Raw materialContains VOC+solvents-pollution
Liquid
Lot of wastage due to overspray
Environment friendly
Powder
100% used
ProcessSand blasting
-health hazard
-poor Ra-poor adhesion
Primer coat+top coat
Brush applied, air cured, poor adhesion
Site application-unreliable quality
Abrasive blasting
Single uniform coat
Fusion bonded-strong adhesion
Plant applied-controlled quality
Properties VisualDull matt-like finish
Bulging
Air pockets
Sagging, dripping
Solid smooth finish
Almost uniform thickness
DamageEasily damaged
Corrosion spreads underneath film
Chips & scratches easily
Hard & tough film
Prevents spread of corrosion around damaged area
Scratch resistant
Sharp edgesDifficult to cover, less thicknessMore thick cover
Contour ribsFilled & coveredLess thickness
DurabilityFew years80-100 years
Tests
Cathodic Disbondment>4 mm<3mm
Applied Voltage ResistenceH2 Evolution & Rusting within a weeklasts more than 30 days
GlossLowHigh
Dielectric resistancepoorstrong
Film ThicknessLow and non-uniformUniform
Hardness< 16 Knoop>16Knoop
Salt spray-scratched panelvery less>1000Hrs
Boil Adhesion TestFailPass

Above table summarizes the salient differences between paint and FBEC which will be helpful to site engineers in achieving desired design life of RCC structures by protecting their rebars from undue corrosion.

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

 

 

Dowel bars in concrete slab

Concrete has to face lot of hostile environmental forces. Concrete can expand or contract because of temperature, stresses due to various kind of load etc to maintain functional capacity in such way so that it does not get cracked, allow transfer of load without faulting at joints, in practice this is achieved by small steel rods called Dowel bars.

corrosion risk -dowel bars
                           Dowel bar

Dowel bars are place at transverse joints of concrete pavements to provide a connection between two slabs. Dowel bars are used in new construction like roads and rehabilitation projects such as slab replacement.

  1. Dowel bars are the device through which wheel loads are transferred from one slab of pavement to the next.
  2. Dowel Bars will significantly reduce the magnitude of the stresses and deflections at the slab joints.
  3. Dowel Bars prolong the useful life of concrete pavements.

 

Dowel bars  will extend service life of the project by improving load transfer and prevent faults at joints. Dowel bar of  30, 32, 36, 38 & 40 mm  Dia and 510 mm long are generally used.

 

Higher Corrosion Riskcorroded dowel bar in slab

Dowel bars are at higher risk of corrosion due to following reasons:

  1. Joints allow direct passage to aggressive agents like oxygen, moisture, chloride etc to the surface of dowel bar. It is further aggravated due to direct exposure to high and low temperature.
  2. Bond between dowel bar and concrete also allow easy diffusion to aggressive ions due to cyclic horizontal movements between the slabs.

 

Corrosion prevention

Dowel bars should be protected from corrosion although joints are sealed to keep water penetration to a minimum; water will seep over time and may corrode unprotected bars.

Typically dowel bars are protected from corrosion by application of epoxy coating or stainless steel clad bars. Epoxy coating’s smooth and almost friction-less surface provides easy horizontal movement in concrete so that slabs move independently which prevents slabs from excess stresses.