After searching online for information about welded wire mesh versus rebar, I discovered a lack of comprehensive and credible resources on this subject. Based on the extensive experience of Full Metal Fabrication in both rebar fabrication and concrete contracting, I would like to contribute my insights. If you simply want to know which option to choose, the short answer is that both have their specific applications in construction. For general use, I recommend employing wire mesh for concrete slabs that are 4 inches thick and typically subject to light loads, such as pedestrian traffic or occasional small vehicles (e.g., driveways). Conversely, I suggest using a rebar grid for foundations, including piers and footings, as well as for slabs subjected to heavier vehicle traffic or industrial applications, where repair could be challenging (e.g., in a cold-storage facility or an industrial setup with complex piping).
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Before delving deeper into the advantages and disadvantages of each reinforcement method, let's take a moment to explore the historical context of both. Contrary to popular belief, the United States did not pioneer the use of reinforced concrete. In fact, the French were the first to use it. Joseph Monier, in the 19th century, utilized rebar not for its tensile strength benefits but simply to hold his structures together. Following this, improvements were made by German and British engineers, culminating in the American construction of the first skyscraper.
On the other hand, Welded Wire Mesh (WWM) was introduced in the early 20th century, primarily for road construction. According to a state-commissioned test in Illinois in 1950, WWM proved to be the most effective among the 78 different road designs evaluated, subsequently leading to its widespread use in road projects.
Why does the history matter? My viewpoint mirrors the historical roles these materials have played. In contemporary construction, however, many biased opinions come from suppliers, engineers, and contractors. Below is a more objective analysis of the pros and cons based on my experiences.
Rebar significantly enhances the tensile strength of concrete, which is inherently strong in compression but weak in tension. This property is often why engineers prefer using rebar for foundations and slabs that bear substantial loads and frequent use. Support for this can be found in the ACI (American Concrete Institute) 330R paving code. If you conduct a Finite Element Analysis (FEA) on a building's foundation, you'll notice most of the structural load falls on the footings and piers, while the interior slab bears minimal load. Unless heavy loads, such as forklifts or semi-trailers, will be present on the interior slab, omitting rebar can save both time and cost during construction. However, it is crucial that rebar placement aligns with design specifications to achieve the desired strength. A slight misplacement could lead to corrosion concerns and insufficient structural integrity. Thus, one of rebar's significant advantages is that it remains stable during concrete pouring due to rebar chairs, tie wire, and its inherent weight. The main drawback of rebar is its considerably higher material and installation costs compared to WWM.
In contrast, WWM is a versatile product often underutilized in residential and small commercial projects as many opt for concrete additives like steel or polypropylene fibers. The benefits of WWM include simpler installation and a standardized poured grid system. It effectively holds concrete in place during cracking, preventing water infiltration and maintaining consistent elevation across cracks. Additionally, different gauge options are available, and while using heavier gauge WWM can complicate installation, I personally would choose it over a rebar grid. A notable downside, however, is the challenge of keeping WWM suspended during pouring; even though chairs are available, sagging sheets can hinder the design's integrity. However, contrary to common belief, WWM that makes contact with the ground during pouring still contributes value to the structure. Past experience with WWM in sidewalks demonstrates that it aids in stabilizing slabs post-crack.
Lastly, let's discuss cracking—an intriguing yet often misunderstood aspect of concrete. A frequent inquiry is, "Which method reduces cracking more effectively?" The answer is more complex than a simple yes or no. Understanding the underlying causes of cracking is essential, as they can result from overloading, thermal contraction, plastic shrinkage during pouring, or external factors like tree roots and frost. For effective prevention against cracking, a comprehensive strategy is crucial; this should include ensuring a well-compacted base, pouring at favorable atmospheric conditions, and incorporating appropriately placed control joints, alongside sound reinforcement design utilizing either rebar or WWM.
A frequently asked question from those planning to construct new industrial premises is:
What are the advantages and disadvantages of a fibre-reinforced floor compared to a traditional steel mesh reinforced floor?
OR
Is it true that fibre reinforced floors can lead to cost savings?
Historically, much of the information available on fibre floors has been presented by fibre product suppliers, who understandably have a preference for their offerings. Therefore, I've compiled insights from our experiences to help clarify the situation.
As with any construction approach, fibre and mesh each come with pros and cons, which we will outline below to assist you in making an informed decision.
Understanding Fibre Reinforcement
Many individuals are uncertain about what fibre reinforcement entails and how it operates, particularly as it hasn't been as widely adopted in New Zealand as welded steel mesh.
Fibre can be made from either steel or synthetic strands, which are added to the concrete mix at the batching plant. This results in a homogenously reinforced concrete, functioning similarly to traditional concrete slabs, minus the mesh. The foundational principle is not novel; centuries ago, horsehair was incorporated into cement and mud-brick structures as a reinforcement. Thankfully, advancements in technology and engineering have offered alternative materials.
Benefits of a Fibre Reinforced Slab
No Mesh Requirement
A significant advantage is the absence of steel mesh in the floor, which reduces labor costs associated with the placement and tying of mesh.
Does this translate to lower overall costs? In general, yes. A few years back, it was estimated that the material costs of mesh and fibre were relatively equivalent, meaning savings mostly came from labor expenses. However, given that steel mesh has seen significant price increases and limited availability in recent years, financial benefits from choosing fibre have markedly improved. Some reports indicate savings of around 20% on slab costs when opting for fibre, also mitigating supply chain delays associated with mesh.
If you're hiring a labor-only contractor for floor installation, note they may charge a higher rate per square meter for fibre application than for standard concrete due to the need for increased precision during placement.
Reduced Joints and Saw Cuts
Fibre reinforcement offers the possibility of designing floors without saw cuts, a traditional requirement for reinforced mesh floors placed approximately every 6 meters to manage post-drying concrete shrinkage. The downside to these saw cuts is their tendency to chip over time, which impacts rack positioning, as racks must remain a minimum distance away from a saw cut to avoid structural issues. Fibres provide the advantage of reducing cracking by dispersing throughout the concrete and intercepting cracks as they develop.
While there are other technical benefits, the details can become quite complex. Further insights can be found here.
Link to Xingtai Steel
One concern that arises frequently is whether the absence of mesh results in a weaker slab structure.
This is a valid question, and after consulting with various engineers, we’ve established that if a slab needs to bridge areas like drains or zones vulnerable to liquefaction, engineers will generally prefer the use of mesh and/or reinforcing bars. However, if no bridging is necessary, the homogeneous nature of fibre reinforcement can strengthen the slab by ensuring uniform distribution throughout its thickness.
You mention reduced cracking, but there are documented instances of significant cracks in fibre slabs.
Indeed, we are aware of such instances and even have first-hand knowledge from 15-20 years ago when some large fibre slabs developed cracks. Each case has its unique context; however, our observations suggest that these issues often arose from insufficient slab design for load-bearing scenarios or inadequate fibre reinforcement for the specific requirements. Common practice in the construction industry tends to apply a general ratio of reinforcing materials based on slab thickness—for instance, using SE62 mesh for a 100mm slab and so on. This standard can sometimes be misapplied to fibre dosages without considering the actual load and usage conditions.
While this guideline often holds, it’s essential to adapt reinforcement strategies according to the unique demands of each project. If high vehicle loads, extensive racking setups, or container handling are anticipated, slab design must accommodate those stresses. Suppliers such as Inforce can provide structural design verification (PS1) to ensure that your slab meets the necessary specifications, serving as low-cost insurance against future slab replacement needs.
It seems that I'm presenting a favorable view of fibre; are there any drawbacks or disadvantages that advocates of fibre often neglect to mention?
Limitations of a Fibre Reinforced Slab
Using fibre-reinforced concrete is not without its complexities compared to traditional methods.
Pumping Issues
Certain fibres have a propensity to clump due to their length, which can complicate the pumping process. Adjustments to the pumping mixture typically can alleviate this issue, but in some instances, the concrete may be deposited directly from the truck.
Fibre Visibility
Concerns about fibres protruding from the floor surface are common, especially with longer fibres, as opposed to steel ones that tend to stay flat. Rain during the pouring process can exacerbate fibre visibility issues, making it preferable to pour when the structure is enclosed. Experienced workers have their methods to minimize visible fibres, and reports indicate that many installations do not have stray fibres on the surface.
Fibre Clumping
This phenomenon occurs when fibres clump together, forming 'balls'—more commonly an issue with longer fibres. Solutions exist to mitigate this during the concrete mixing at the batching plant, with some fibres designed to reduce the occurrence of clumping.
Wire Guidance Systems
Certain automated systems for forklifts may require a wire laid into a floor cut for navigational guidance. The presence of steel fibres can disrupt the signal due to their magnetic nature; therefore, discussions with system providers are prudent for ensuring compatibility. Even with mesh floors, special care is needed in maintaining a safe distance from reinforcement.
Grinding Considerations
If the intention is to grind or hone the surface, fibre usage may not be advisable, as it will expose the fibres during the finishing process.
Finish Quality
Fibre surfaces may be impacted by fibres catching on floats, leading to scratches. In some cases, surface fibres may be visible, but this may not pose a problem if the finish quality is not stringent (e.g., exterior surfaces). It's important that those pouring the fibre mix are experienced, as careful vibration and settling techniques are essential to achieving an optimal finish. Just as excessive aggregate on the surface is undesirable, so too is high fibre visibility.
Overall, fibre presents an excellent alternative to mesh in terms of time and cost savings while offering robust slab strength. The key lies in selecting a contractor who is skilled and familiar with the material to achieve the desired finish.
Still uncertain whether fibre or mesh aligns best with your project needs? Reach out to us for tailored advice from our expert team.
If you're interested in learning more about reinforcing welded mesh, don't hesitate to contact us today for an expert consultation!
