What is Welded Wire Reinforcement (WWR)?

Definition

Welded wire reinforcement (WWR) is a grid of steel wires, welded at the crossings, that is cast in concrete to hold cracks closed. ASTM A1064 is the specification for the wire and the welded product. It covers carbon-steel wire produced from hot-rolled rod. The wire is cold-worked and is plain or deformed.

Names: WWR, WWF, wire mesh

ASTM Note 1 to A1064 says the product was historically called welded wire fabric, WWF, fabric and mesh. The wire reinforcement industry then adopted the term welded wire reinforcement. ASTM replaced the term “fabric” with “reinforcement” in A1064 and in the related specifications. The FAA documents on airfield pavement still say “fabric”. In French the product is the treillis soudé. The French standard NF A 35-080-2 covers the treillis soudé for reinforced concrete and follows the European standard EN 10080. A March 2022 edition replaced the 2013 edition.

Wire size and style designation

Each wire has a size number. The prefix W means smooth wire and D means deformed wire. The number is the nominal cross-sectional area of the wire in hundredths of a square inch. For example, W4 has an area of 0.040 in² and a diameter of 0.226 in. W16 has an area of 0.160 in² and a diameter of 0.451 in. Fractional sizes between the listed sizes are also available (TxDOT Item 440, 2.6).

A mesh style gives the wire spacing first and then the wire sizes. The style 6 × 12 - W16 × W8 means:

• longitudinal wires 6 in (150 mm) apart,
• transverse wires 12 in (305 mm) apart,
• smooth W16 wire in the longitudinal direction,
• smooth W8 wire in the transverse direction.

ASTM A1064 gives common wire sizes in its tables. It also says that actual wire sizes are not restricted to the sizes in the tables.

Coated mesh

ASTM A884 is the specification for epoxy-coated steel wire and welded wire reinforcement. P-501 lists it as an option for reinforcement. TxDOT requires Class A or Class B coating to A884 for epoxy-coated wire and WWR (Table 3). The related page on epoxy-coated rebar covers the coating system for bars.

What it does in an airfield slab

The FAA uses steel in a concrete panel to control cracks. It does not use the steel to stop cracks. The steel keeps the cracks that form tightly closed. The interlock of the cracked faces then holds the panel together, and less debris enters the cracks.

The FAA does not ask for this steel in every panel. AC 150/5320-6G requires it in a panel when the length-to-width ratio is above 1.25 or the shape is irregular. It also advises steel around embedded structures. Reinforced and continuously reinforced pavement is used to a limited extent at airports. On a federally funded project, structurally reinforced pavement needs approval from FAA AAS-100, and the FAARFIELD thickness design does not apply to it. Crack control steel does not change the panel thickness. The thickness is the same as for plain concrete.

Highway engineers know a similar system as jointed reinforced concrete pavement (JRCP). It uses contraction joints and a minimal amount of steel or wire mesh at mid-slab to hold cracks together. Its joints can be farther apart than in plain pavement, up to about 15 m (50 ft). Pavement Interactive reports that JRCP has fallen out of favor against plain jointed pavement and continuously reinforced pavement.

Mesh or bar mat

The FAA allows two forms of crack control steel. AC 150/5320-6G says the steel may be bar mats or welded wire fabric. P-501 lists ASTM A1064 and A884 for welded wire, and ASTM A184 or A704 for bar mats. A drafting note in P-501 says welded wire fabric shall be furnished in flat sheets only.

FAA layout of crack control steel

AC 150/5320-6G, paragraph 3.16.12.2.3, gives these values:

• Longitudinal wires are 4 to 12 inches (100 to 305 mm) apart.
• Transverse wires are 4 to 24 inches (100 to 610 mm) apart.
• The ends of longitudinal wires lap 12 inches (305 mm) or 30 wire diameters, whichever is larger.
• The ends of transverse wires lap 6 inches (150 mm) or 20 wire diameters, whichever is larger.
• The steel stays 2 to 6 inches (50 to 150 mm) from all edges and joints.
• In a panel thinner than 9 inches it lies about in the middle. In a thicker panel it lies in the upper third.

Texas DOT gives a general lap rule for WWR. A splice has an overlap of at least 2 cross wires plus 2 in on each sheet or roll. TxDOT also asks for mats of WWR to be fastened securely at the ends and edges (Item 440, 3.4 and 3.5). This is a state highway rule and not an FAA rule.

Specification and placement

Item P-501 in AC 150/5370-10H covers cement concrete pavement. The Engineer specifies whether the pavement has reinforcement. The reinforcement must be free of mud, oil and organic matter when the concrete is placed. P-501 accepts steel with rust or mill scale. A hand wire-brushed test specimen must still meet the dimensions, weight and tensile properties of the ASTM specification.

P-501 sets two ways to place the mesh (501-4.9):

• Two layers. The contractor strikes off the bottom layer so that the sheet lies full length on the concrete in its final position. The reinforcement goes directly on that layer, and then the contractor places the top layer. The contractor removes and replaces any part of the bottom layer that stays open for more than 30 minutes without cover. The same applies if initial set occurs.
• One layer. The contractor places the reinforcement before the pour. The contractor may also push it into the plastic concrete by mechanical or vibratory means after spreading.

ACI 302.1R-15 advises against pulling the mesh up from the ground after the pour, and against walking it into the concrete. ACI 301-16 limits the support spacing to 12 inches for wire smaller than W4.0 or D4.0. The FAA position rule shows why. Mesh that lies on the base is not in the middle or the upper third of the panel.

Inspection

The mesh lies inside the slab. A survey from the air sees the surface of the slab, not the mesh. P-501 names non-destructive rebar location devices, such as a pachometer, to check that dowel bars and tie bars are in place. The paragraph does not name the devices for welded wire. AC 150/5320-6G adds that ground penetrating radar (GPR) can locate rebar in concrete (E.1).