A concrete slab is constructed utilizing various elements and principles of construction. This includes, for a concrete slab-on-grade, sub-grade preparation, sub-base and base preparation, overall depth of buildup, and the thickness of the concrete element. For a suspended concrete slab construction, this includes formwork, shoring, embeds, layout and other engineering principles. Concrete reinforcement is another imperative part of concrete slab construction. So, if you are asking how to build a concrete slab, you’ll find some valuable insight for both simple construction and more complex concrete slab construction in this article.
A concrete slab, whether structural or architectural, has to incorporate the right elements for construction to be sound structurally and architecturally fit for daily usage.
TABLE OF CONTENTS
Concrete Slab Build-up
Base & Sub-base Conditions
Design of a Concrete Slab
Formwork & Reinforcement for a Concrete Slab
Concrete Mix-Design Selection
Concrete Placing and Finishing
Concrete Slab Sealing and Curing
Conclusion
Concrete Slab Build-up
Firstly, the sub-grade preparation is paramount to ensuring that the subsequent base courses of support for the concrete remain intact.
For instance, a build-up for a concrete slab might encompass the sub-grade, a layer of crushed gravel, another layer of crushed gravel, and then the concrete. Other elements like radon barrier (poly) or high density rigid insulation might even be utilized to complete the entire build up. Regardless, all of this rests on the sub-grade conditions of the concrete slab.
So, with that information, the sub-grade conditions should be prepared to a condition which permits the subsequent layers of crushed as to not deform. Deleterious soils like poor clays could be removed. Alternatively, piling and further sub-surface structural measures could be taken to repair sub-grade conditions.
In any case, this layer of existing native soils should either be in good condition, and compacted, or removed and replaced with better soils and compacted.
DIY Tip: Try to ensure your grade, prior to putting down any crushed gravel, is a solid, flat ground. Ideally, the earth is hard compacted, and excavated to permit atleast 6″ of crushed gravel to go down for your slab (depending on structural requirements).
Photo Above: Accommodation for penetrating items like plumbing and electrical conduit also have to be factored in. Make sure to get these elements installed prior to your slab pour!
Base & Sub-base Conditions
On top of this layer, can be a layer crushed gravels or a geotextile grid. These provide a layer a stability for which to building a concrete slab is required. When placing crushed gravels, the compaction of this layer of crushed gravel is perfected through lifts. That means not all the crushed gravel gets placed at the same time, rather, smaller portions of crushed gravel are placed, then more gravel is placed on top etc.
The crushed gravel is used because it compacts well. Compaction of crushed gravel base courses and the subgrade help to ensure that the concrete slab does not have unnecessary sub surface settlement. This is where parts of the ground sink, over time, which can lead to unnecessary stressors on the concrete and structural damages to other elements.
Other considerations include the concrete slab itself.
DIY Tip: Ensure your crushed gravel gets tamped properly. This can be done with a specialized attachment from an excavator, using a motorized tamper, or even with a hand tamper if you must. The term ‘tamping’ indicates compacting crushed gravel(with moisture incorporated) to achieve a desired hardness for the layer of gravel. Use a bit of water if you must to achieve desired results.
Design of a Concrete Slab
Typically for bigger builds a concrete slab of approximately 6” or 150mm is used. This kind of depth of concrete is sufficient for most larger structures and residential structures.
A concrete basement slab construction for instance may only require 4″ or 100mm of thickness.
A concrete structural slab on grade can be poured as one section or as multiple sections in what are known as pour breaks. Pour breaks can also delineate certain additional requirements for steel reinforcement to accommodate for the interruption in monolithic concrete placement. Experienced concrete contractors know where pour breaks are and where they should be placed, accommodating steel reinforcement where it is required to make up for any loss of continuity.
Different steel reinforcement, depending on the size of the concrete element, provides the structural support required to add tensional resistance, in addition to compressive resistance for concrete slabs. This can be either top or bottom layers for a concrete slab, or just one layer.
It really depends on the function of the concrete slab. Thicker concrete slabs, such as approximately 8″ or 200mm would be more common for suspended concrete structural slabs.
Photo Above: Atleast two layers of rebar (reinforcement) can be seen in this photo. This would be regarded to as the top and bottom mat or layer of rebar in a concrete slab. Below the rebar, is shown a plastic barrier sheet. Construction of a concrete slab in progress!
Formwork & Reinforcement for Concrete Slab Construction
Overall building a concrete slab is not that complicated. You have to consider the sub-grade, sub-base, and base conditions for the element. Remember that a concrete slab is a layering of various construction elements which together makeup the element. Other concrete slabs, such as a suspended slab, are built totally differently.
With suspended concrete slabs, concrete is poured onto a series of forms, shoring, and falsework. These elements of temporary construction aid in supporting the concrete slab prior to it being cured fully. Final curing of a concrete slab is essential to ensuring schedule progress, in addition to manipulation and specialized utilization of formwork/shoring to allow for advanced subsequent concrete slab placements.
Photo Above: Concrete reinforcement chairs serve to elevate concrete reinforcement (rebar) off the ground to provide the necessary elevation and concrete reinforcement cover for structural soundness when placing concrete for a concrete slab. If rebar was left on the ground, inadequate cover could be a concern for both structural integrity for the slab construction, but also potential for corrosion/excessive rusting of the rebar over time.
Mix-Design Selection
Mix-design selection and concrete exposure design classification is another area where the selection of concrete strength rating is imperative to ensuring that both the building is structurally sound, and can be built at an appropriate schedule progression. Anyway you look at it, for serious structural requirements, mix design selection is a job for an engineer usually. For larger scale residential projects and multi-residential projects the mix design selection is something that an experienced concrete contractor will be able to handle.
Mix-design criteria should be established by an engineer for serious projects but if you are just putting a concrete slab in your backyard for a shed or garage then approximately 35 MPa should be plenty sufficient.
The MPa compressive strength for a concrete slab is generally going to be higher than structural concrete mix strength for concrete driveways, however, this is not always necessarily the case.
Concrete Placing & Finishing
Photo Above: Part of the finishing process for a concrete slab involves moving concrete to ensure level and even distribution throughout concrete formwork. Additional usage of finishing tools would be required to finish this freshly placed concrete.
Concrete Placement & Finishing
Concrete placement is the process of actually pouring concrete in the desired location, or placing it.
This process can include wheelbarrow, concrete buggy, shovels, concrete chutes, pumps, and rakes. All of these tools are intended to spread and place concrete in a manner that makes it appropriate to be finished.
Concrete finishing process is the stage after placing concrete whereby skilled workers utilize a variety of hand and mechanical tools to ensure a desired level of finish. Some examples of concrete finishes include stamped concrete, broom finish concrete, exposed aggregate concrete, and power-trowel smooth finish concrete.
Common concrete finishing tools: edger, groover, bull float, screed boards, hand float, steel trowel, power trowel (walk-behind or ride-on), broom, concrete vibrator etc.
Photos Above: The right concrete formwork makes all the difference! Concrete formwork has to be stable enough to support the lateral forces of an accumulation of placed fresh concrete!
Implementing formwork is essential to the overall success of any concrete slab pour! Utilizing fabricated and custom forms makes light work of any concrete slab build, whether on grade, or a suspended concrete slab.
Photos Above: A suspended structural concrete slab element with two layers of rebar. Formwork and falsework utilized provides support utilizing posts to a suspended concrete structure prior to the structure being completed. This remains in place until deemed worthy to be removed by a structural engineer or formwork engineer and suitable concrete strength has been achieved to pour subsequent floors/slabs of concrete.
Another important method to consider when finishing concrete slabs is the implementation of slab joints. These joints are intentionally placed to control the location of cracks that may develop due to shrinkage and temperature changes. By creating these predetermined lines of weakness, slab joints serve as controlled stress-relief points, allowing cracks to form in a predictable and manageable manner rather than randomly across the slab surface.
Concrete Slab Sealing and Curing
Concrete curing is the process by which a concrete slab reaches design strength. This time can vary, with most concrete slabs, in about 28 days of curing for final design strength to be warranted. Within about 7 days, a large amount of strength can be achieved.
Curing aside, with a concrete slab, you also have to consider the sealing and level of finish! A concrete sealer, either acrylic, or penetrating, or even a concrete hardener might be utilized to improve the aesthetic and durability of a concrete slab. There are various concrete sealing and curing compounds on the market, so get ready to do some research! For a start, the right sealing compound for a concrete slab provides additional aid during the curing process, by helping to trap moisture in the slab to ensure design strength is achieved.
Summary
In conclusion, building a concrete slab can be serious work!
Building a concrete slab involves careful planning and execution of several construction principles. For slab-on-grade applications, this includes proper sub-grade and base preparation, accurate buildup depth, and appropriate slab thickness.
Suspended slab construction, on the other hand, requires additional elements like formwork, shoring, embedded components, and precise layout based on structural design.
Reinforcement is essential across both types to ensure durability and structural integrity. Whether you’re tackling a straightforward project or a more complex build, this article has offered valuable insights into the key steps and considerations for successful concrete slab construction.
For all your Calgary concrete needs, turn to the professionals at Deluxe Concrete Services! Their team of trusted concrete professionals are standing by!
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