What is welded Wire Mesh (Welded Wire Mesh Fabric)? It is an established fact that by mechanization or industrialization any and every productive activity invariably benefits in all respects of quality, efficiency of time and energy and elegance of human effort. The application of technology to any process helps achieve accurate control on all the required parameters. Reinforced Concrete Construction which is the backbone to any infrastructural project depends for its performance on its prime elements namely Concrete and Reinforcement. Just as mechanization of concrete production namely Mix design, Auto batching plants , Ready Mix technology and automated casting techniques have raised the standards and strengths of concrete to remarkable levels, the same is essential for reinforcement. It is high time we stopped doing the handicraft work of tying up individual bars. Usage of Welded Wire Fabric (WWF) is the easy and correct solution for achieving the requirements of quality, reliability, speed and efficiency. Welded Wire Fabric (WWF) is a prefabricated reinforcement consisting of a series of parallel longitudinal wires with accurate spacing welded to cross wires at the required spacing. The welding of the wires is achieved by electric resistance welding with solid-state electronic control and all the spacing are controlled by an automatic mechanism of high reliability. There is no foreign metal added at the joint and the intersecting wires are actually fused into a homogeneous section thereby ensuring permanency of spacing and alignment in either direction. The wires used in the fabric are cold drawn from controlled quality mild steel wire rods with carbon content generally less than 0.15%. The cold drawing through a series of tungsten carbide dies results in a high tensile strength and increased yield strength material of accurate dimensions. Further, each section of the wire gets inherently tested by the process itself for its characteristic physical properties
Transcript
What is welded Wire Mesh (Welded Wire Mesh Fabric)?
It is an established fact that by mechanization or industrialization any and every productive activity invariably benefits in all respects of quality, efficiency of time and energy and elegance of human effort. The application of technology to any process helps achieve accurate control on all the required parameters. Reinforced Concrete Construction which is the backbone to any infrastructural project depends for its performance on its prime elements namely Concrete and Reinforcement. Just as mechanization of concrete production namely Mix design, Auto batching plants , Ready Mix technology and automated casting techniques have raised the standards and strengths of concrete to remarkable levels, the same is essential for reinforcement. It is high time we stopped doing the handicraft work of tying up individual bars. Usage of Welded Wire Fabric (WWF) is the easy and correct solution for achieving the requirements of quality, reliability, speed and efficiency. Welded Wire Fabric (WWF) is a prefabricated reinforcement consisting of a series of parallel longitudinal wires with accurate spacing welded to cross wires at the required spacing. The welding of the wires is achieved by electric resistance welding with solid-state electronic control and all the spacing are controlled by an automatic mechanism of high reliability. There is no foreign metal added at the joint and the intersecting wires are actually fused into a homogeneous section thereby ensuring permanency of spacing and alignment in either direction.
The wires used in the fabric are cold drawn from controlled quality mild steel wire rods with carbon content generally less than 0.15%. The cold drawing through a series of tungsten carbide dies results in a high tensile strength and increased yield strength material of accurate dimensions. Further, each section of the wire gets inherently tested by the process itself for its characteristic physical properties
thereby offering a systematic reliability of material. The cold drawing operation unlike the cold twisting used in HYSD bars also doesn’t sacrifice the ductility of the material in any major way. The wires conform to IS: 432-Pt II/1982 which specifies an ultimate tensile strength of 570 N/mm2 and a characteristic strength of 480 N/mm2. Wires used for manufacture of fabric are generally manufactured in the range of 2 mm to 12mm diameter.
WWF is manufactured conforming to IS: 1566-1982 with long and cross wire spacing varying from 25 mm to 400 mm. Each of the rigidly welded intersection is capable of withstanding shear stresses up to 210 N/mm2 (IS: 4948/1974) on the reference area of the longitudinal wire. The fabric can be manufactured in widths up to 3000mm with lengths limited by transportation considerations. When supplied in ready to lay flat sheet form the standard length is 5500mm, otherwise the fabric can be supplied in roll form in standard lengths of 15m, 30m or 45m.
• ·ADVANTAGES OF WELDED WIRE FABRIC
• HIGHER CHARACTERISTIC DESIGN STRENGTH:
Though the structural behaviour of the fabric as reinforcement is similar to that for HYSD bars or Plain Mild steel bars, significant savings result due to the higher characteristic strength of WWF wires. The area of steel necessary for a required design moment is as per IS: 456-2000:
As = Mdes x (Load Safety Factor i.e.: 1.5 )
((Material Safety Factor i.e.: 0.87) x (Characteristic Strength i.e:480) X Lever Arm (i.e.: 0.808 x Deff for Fe480)
Simply from better characteristic strength point of view, usage of WWF with Fe480 grade results in savings in steel area or steel weight required to the tune of 13.55 % vis-a-vis HYSD bars of Fe 415 grade and to the tune of 47.92 % vis-a-vis Plain Mild Steel bars of Fe 250 grade.
• BETTER BONDING BEHAVIOR:
The bonding behaviour of WWF is significantly enhanced and different from that of HYSD or Plain Mild steel bars. As against the peripheral surface area which is responsible for bonding to concrete in the case of individual bars, the rigid mechanical interconnections by means of welds to cross wires are primarily responsible for stress transfer from concrete to steel and vice-versa in the case of WWF. Each of the rigid welds capable of resisting up to 210 N/mm2 ensure quick and complete stress transfer within 2 welded joints from the critical section. This behaviour of positive mechanical anchorage is acknowledged in specification of much lower lap splice lengths for WWF. A lap splice or a development length consisting of 1 cross-wire spacing comprising 2 welded intersections plus additional 100mm subject to a minimum of 150 mm total length is sufficient to develop a full strength lap. This aspect can result in savings of steel vis-a-vis HYSD bars by making easy the option to use a combination of fabrics/ steel areas provided to achieve curtailment of reinforcement with easy and short splices.
• BETTER AND ECONOMIC CRACK RESISTANCE WITH THINNER
WIRES AND CLOSER SPACINGS:
The behaviour of strong mechanical anchorage of the welds at each the intersections are further responsible in imparting an immense deal of homogeneity to the R.C.C section as a whole. The two dimensional uniform stress distribution of the fabric with the concrete achieves better plate behaviour in the slab. Further, WWF usage affords the possibility of using thinner wires at closer spacing. This serves most effectively in countering the non-load phenomena or strain induced stresses due to Shrinkage and Temperature changes. The close spacing of thinner wires and the two-way behaviour of WWF minimizes the crack widths and preserves structural integrity of the slab. This is particularly true for large span and large area structural and ground slabs. Further in cases where a designer is constrained to provide more than minimum reinforcement from the maximum bar spacing criteria, WWF affords enormous savings by providing reliable fabric with thinner wires at closer spacing.
For instance consider very common cases in residential slabs where load stresses are low but where minimum thicknesses of 75 to 125mm are used
*from serviceability or other reliability criteria,
The above aspect can be exploited to achieve savings in various cases of even designed steel area zones by providing minimum steel of suitable thinner WWF over all the zones and then adding extra layers of thicker designed steel WWF in the stressed zones of a slab.
• SAVINGS OF LABOUR, TIME AND BINDING WIRE:
The most obvious and clinching advantage in the use of WWF is the immediate and positive savings in labour and time. It is complete freedom from all the mundane fitter’s jobs. There is no cutting of bars, no marking and spacing them out, and above all no laborious tying of binding wires. There is saving of skilled fitters manpower and saving of helpers to cut and tie.
The fabric is available ready to lay on the shuttering. It is also ready for casting as the need for supervisors/ engineers to check the bar sizes & spacing is eliminated. The enormous savings in man-days and the associated cost vary from project to project depending upon the scale of the job and the repeatability of design. In repair jobs of critical nature where the structure is in service, the boon of time
saving with WWF cannot be understated. In return to all savings, the builder is always doubly benefited because he is assured of the job being done much more reliably and with much better quality. The designer too has lesser nightmares since he is assured that no bars have been missed or altered. Apart from savings in labour and time, there is direct savings in the consumption of binding wire. This consumption saved works out to about 2 to 5% of the reinforcement used in terms of cost of steel saved. Besides, the added benefit is avoidance of those dangling ends of binding wire which are the starting fuses for the virus of corrosion into the reinforcement.
• THE ONLY FEASIBLE AND ESSENTIAL ALTERNATIVE FOR
GROUND SLABS :
Concrete Slabs on ground including roads and pavements are often ignored and neglected from the provision of reinforcement. This however is most unfortunate since ground slabs are more often than not subject to many times greater loads than they were supposed to bear and further the base and sub-soil conditions are mostly quite unreliable. In such a scenario, the presence of at least some reinforcement makes a world of difference. WWF usage provides the only practical and easy solution for reinforcing slabs on ground. A plain concrete slab under conditions of sub-soil erosion or movements or due to temperature changes coupled with heavy traffic loading will develop cracks which collapse the integrity of the surface. The tendency to use extra high strength concrete or extra thickness of concrete to minimize cracking does not solve any problems since the strains induced by drying shrinkage or temperature contraction do not appreciably change with thickness. cost of a WWF reinforced slab is also more or less similar to that of a slightly thicker unreinforced slab. Usage of WWF serves to control cracking and crack width in both directions. It ensures that even if a crack develops the cracked faces are held together and the aggregate interlock is maintained. The amount of reinforcement to be provided in ground slabs is generally designed by the Subgrade-Drag Procedure where
A typical 150mm thick ground slab with joints spaced 6m apart by the above design would need As = 2 x 6 x 0.375 x 9600 / (2 x 230 ). i.e.: 94 mm/2 or 0.063% steel. WWF of 100 x 100 x 3.1 x3.1mm of 1.23 kg/m2 would be sufficient for this slab.
Other design procedures such as the Confirmed Capacity Procedure, Temperature Procedure, Equivalent Strength Procedure and Crack Restraint Procedure covering the unconventional topic of ground slab reinforcement design from all angles have been exhaustively covered in the paper ‘Innovative Ways to Reinforce Slabs on Ground’ by Robert B Anderson.
• FLEXIBILITY OF HANDLING AND PLACING:
The usage of thinner wires lends the fabric as extremely flexible in handling. Coupled with the availability in long lengths in roll form, WWF provide the ideal and convenient solution for all kinds of repair work by Re-plastering or Guniting. The same aspect makes WWF indispensable in thin elements such as precast partitions, shelves, fins, and ferrocement or ferrocrete products such as ferrocrete water tanks etc. WWF is the only solution for the thin and though spine of thin and efficient structural elements as folded plate roofs, folded plate precast roof girders or hyper shells.
· WELDED WIRE REINFORCEMENT: NOMENCLATURE:
Welded wire is produced from a series of longitudinal and transverse high strength steel wires, resistance welded at all intersections. The wires are produced from controlled-quality hot-rolled rods which are cold-drawn through a series of dies reducing the rod to the specified wire diameter. This wire is then fed into a rigid grid of reinforcement. The manufacturing process can be varied to accommodate various style changes and dimensions. However, consideration should be given to the complexity of the change. The manufacturing variables are listed in the general order of time involved, starting with the most time consuming:
1. Longitudinal wire spacing
2. Longitudinal wire size
3. Width
4. Side and end overhangs
5. Transverse wire size
6. Transverse wire spacing
7. Length
The more difficult machine changes require greater quantities per item, in order to offset the additional production time required. Generally, it is more economical to order a few basic sheet sizes and styles than to specify many variations in the sheet. Quantity requirements for each change usually vary between producers.
The cross-sectional steel area is the basic element used in specifying the required wire size. The nomenclature used to indicate wire size is a letter followed by a number. The letter “W” identifies a plain wire and the letter “D” a deformed wire. The number which follows is the cross-sectional area of the wire given in hundredths of a square inch. For example: W16 denotes a plain wire with cross-sectional area of 0.16 sq. in.; D7.5 indicates a deformed wire with a cross-sectional area of 0.075 sq. in.
The welded wire reinforcement style identifies the spacing and size of the transverse and longitudinal wires and takes the format: 6 x 12—W16 x W8, where the longitudinal wire spacing is 6 in. with wire size W16 and the transverse wire spacing is 12 in. with wire size W8. The complete designation also includes the dimensions of the fabric sheet such as: 90″, (+1″ +3″) x 20—0″ where the width (given in inches) is equal to 90 in., with side overhangs of 1 in. on one side and 3 in. on the other for an overall width of 94 in., and the length is equal to 20 ft.—O in. The standard end overhang, equal to one half the transverse wire spacing, is assumed unless otherwise specified. It is important to note that the length is the tip-to-tip dimension of the longitudinal wire (20 ft.—0 in. in above example) and that the tip-to-
tip dimension of the transverse wires is called the overall width, equal to the width plus both sides overhangs (94 in. in above example).
· WELDED WIRE REINFORCEMENT: DESIGN CODES AND
SPECIFICATIONS:
The use of welded wire as a structural concrete reinforcing material is governed by codes such as the ACI 318 Building
Code and by specifications such as ASTM A-82, A-185, A-496, and A-497. These references provide the necessary criteria
for designing with the unique structural grid of reinforcement provided by welded wire. The following is a summary of
ACI code specifications which pertain to the use of bent welded wire:
Welded wire reinforcement, both plain and deformed, is defined as deformed reinforcement (Ref. ACI 318, Section 2.1).
Current ASTM Standards for welded wire allow up to 80,000 psi yield strength and refer to local building codes for stress/strain tests when structural welded wire reinforcement is specified. If 60,000 psi, fy or lower is specified, the ASTM Standards state that fy shall be the stress corresponding to a strain of 0.50%. The ACI building code states that when yield strength, fy exceeds 60,000 psi, fy shall be the stress corresponding to a strain of 0.35%. (Ref. ACI 318, Sections 3.5.3.4, 3.5.3.5 and 3.5.3.6)
BENDS AND HOOKS: (Ref. ACI 318, Section 7.2.3)
Inside diameter of bends in welded wire used for stirrups and ties shall not be less than four wire diameters for deformed wire larger than D6 and two wire diameters for all other wires, both plain and deformed. Bends with inside diameters of less than eight wire diameters shall not be less than four wire diameters from nearest welded intersection.
LATERAL REINFORCEMENT
Equivalent areas of welded wire may be used to furnish the lateral reinforcement requirements specified in ACI 318, Section 7.11.
Design yield strength of shear reinforcement shall not exceed 60,000 psi, except that the design yield strength of welded deformed wire shall not exceed 80,000 psi. (Ref. ACI 318, Section 11.5.2).
Design yield strength of non-prestressed torsion reinforcement shall not exceed 60,000 psi. (Ref. ACI 318, Section 11.6.3.4)
Design yield strength of shear-friction reinforcement shall not exceed 60,000 psi. (Ref. ACI 318, Section 11.7.6)
Anchorage of web reinforcement for each leg of a simple U-shaped stirrup formed from welded wire must meet one of the following: (Ref. ACI 318, Section 12.13.2.3).
(1) Welded wire may be used as shear reinforcement when the wires are located perpendicular to the axis of the member. (Ref. ACI 318, Section 11.5.1.1,b)
(2) One longitudinal wire located not more than d/4 from the compression face and a second wire closer to the compression face and spaced not less than 2 in. from the first wire. The second wire shall be permitted to be located on the stirrup leg beyond a bend, or on a bend with an inside diameter of bend not less than8 wire diameters.
· WELDED WIRE REINFORCEMENT: PRODUCTION:
The fabrication of welded wire reinforcement into various structural shapes is readily accomplished with 3 basic pieces of equipment, a wire mesh welding line like HK-2400 & HK-300, a wire mesh bending machine like HBM – 360 and a wire mesh cutting machine like HKM-360.
The bending machine provides the flexibility of adjusting to various wire spacing, angles of bend and bending radii. This equipment is
manufactured in sizes ranging in length from 8 to 40 feet. Capacities range from the small wire sizes used primarily in precast operations to heavy W45 structural wires, 0.757 in. diameter. The sheets of welded wire are bent on the machine by an arm which rotates through an angle of 0° to 180°, shaping the wires around the mandrels. This arm can be preset to stop at any angle and the mandrels can be varied to meet the design requirement for bend radius and wire spacing.
The cutting equipment can be a simple hand tool capable of cutting one wire at a time or larger powered equipment which cuts the full width of a sheet in one operation. This powered equipment allows the use of more economically manufactured sheets of wire reinforcement. The bending and cutting equipment are comparatively low cost investments which require no special skills for efficient operation. Both machines, operating on electric power, can be conveniently moved from one project to another, lending themselves very readily to on-site construction, precast operations and use in fabricating shops.
· ADVANTAGES OF BENDING WELDED WIRE REINFORCEMENT: -
Bending welded wire fabric literally adds a third dimension to concrete reinforcement. It provides the structural engineer with new options in design. The welded wire can be bent to the desired shape
and placed where it is needed. Equally important, the contractor can be reasonably sure that it will remain intact as placed. Here are some of its many advantages:
EXCELLENT BONDING AND DEVELOPMENT CHARACTERISTICS
The welded cross wires of welded wire reinforcement provide unique anchorage for the reinforcement. ACI 318 code provides for the use of either hooked or straight “U” stirrups when designed from wire reinforcement. The straight “U – shaped stirrup can be designed from plain welded wire when at least two separate longitudinal wires are located in the anchorage zone. The use of the “U” shaped stirrups eliminates several bends allowing stirrup cages to be formed in less time. Effective designs using welded deformed wire for stirrups have been developed using both the development length of the deformed wire in addition to the weld shear strength, to meet anchorage requirements.
OPTIMUM USE OF LABOR/SIMPLIFIED SUPERVISION
Equipment is basic and easily operated by construction crews who require no special training. Sections of bent welded wire reinforcement, with the steel spacing already fixed, are quickly set
into place, therefore reducing supervision and simplifying inspection of the reinforcement.
BETTER CRACK CONTROL
The high efficiency of small wire sizes and closely spaced reinforcement serves to distribute and equalize the stresses that may result in cracking. Research’ has shown that closely spaced wires, 2 to 4in. apart, in welded wire represent the most favourable type of reinforcement for shear and torsion.
MINIMIZES WELDING PROBLEMS
Because welded wire reinforcement is made from low carbon, cold-drawn steel it has greater weld ability, therefore reducing special fabrication problems.
BENDING AND PLACEMENT TIME REDUCED
Fabrication and placement of individual rebar as stirrups takes up to five times longer than bent units of welded wire, depending on the stirrup spacing.2 Only when stirrup spaces were greater than 30 in. were the individual bars found more economical.
WELDED WIRE REINFORCEMENT Fabric : Applications:
CAST-IN-PLACE CONSTRUCTION
Recent high-rise construction projects have shown significant savings when using welded wire stirrup reinforcement: Midway through construction of a 32story of office building the stirrup reinforcement was converted from bars to welded wire. “once the stirrups were used, production shot up, steel placement costs dropped and the slab construction cycle was reduced from 9-10 days to 6 days . . . for a time/labour savings of 75%.” Contractors on similar high-rise construction projects have reported reduction in bending and placement of stirrups from 16 man-hours per ton for rebar stirrups to 8 man-hours per ton to place welded wire reinforcement stirrups.
OTHER CAST-IN-PLACE APPLICATIONS
PRECAST/PRESTRESSED CONSTRUCTION
The preshaping and assembly of welded wire reinforcement is a natural time saver in the production of precast/ prestressed products: The precaster of utility vaults, box culverts and other underground precast products achieved a significant savings in reinforcing case assembly time by using welded wire reinforcement. The assembly of the reinforcement for a typical manhole structure 6′x12′x6′ once required three hours to assemble from bars. With welded wire this same cage takes 40 minutes.
The use of shaped welded wire reinforcement results in similar savings of time and money in the production of prestressed box beams and single and double-tee beams.
Welded wire reinforcement, shaped to the contours of 3- tiered risers for a large stadium, helped the precaster of these prestressed components to achieve assembly line efficiency by reducing handling and placement time for the reinforcement. The wire reinforcing rigidity assured correct position in the riser forms and correct concrete cover.
