How much reinforcement in slabs on grade

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QUESTION
There seems to be differing opinions on the min. amount of reinforcement and on how it should be placed.

It will be nice to learn more from experienced friends:

  1. I understand that the reason behind such reinforcement is temperature and shrinkage, but definitely not the same amount as is specified in ACI318 for structural slabs. How much? How placed.

  2. A friend argues that since vapor barriers are placed beneath there will be no shrinkage problems. I can not find logic in this argument since as I understand shrinkage and temperature effects are considered for the sake of limiting cracks and vapour barrier may cut down shrinkage but if no further cover is made above slab, wont temperature changes affect this unreinforced construction.

REPLIES

JAE
There is a wide range of opinions, formulae, and values in the engineering literature for reinforcing of slabs-on-grade. ACI 318 values for minimum steel (0.0018 x gross area) are not specifically intended for concrete on grade.

PCA provides the traditional subgrade drag formula which is:

As = FLw / (2 x fs)

where As is the amount of reinforcing in sq. in. per lin. foot slab width

F = coefficient of friction between base and slab (typically 1.5)
L = slab length between free ends, feet, in the direction of the steel
w = weight of the concrete slab, psf (usually 12.5 psf per inch of thickness)
fs = allowable steel stress, psi 24,000 psi for 60 grade steel

There are other formulae as well, all of which give differing amounts of steel reinforcing. I’ve seen many engineers use 0.0014 x gross area of concrete. Some use 0.0018.

Generally, the reinforcing is detailed in the upper portion of the slab, or at least near the center of the slab depth. There have been articles published which argue that the reinforcing should be in the lower portion of the slab due to the nature of the drag forces on the bottom surface and the theoretical tensile stresses that develop.

We have begun to use more fiber reinforcing in the slabs-on-grades due to the high cost of setting bars and the difficulty contractors seem to have in properly chairing welded wire fabric. I have never, ever, seen WWF end up where you want it to and we have ceased specifying it in slabs. The fiber reinforcing, to date, has given us very good quality slabs.

Your friend, I believe, is mistaken in thinking that a vapor barrier under the slab does anything to reduce shrinkage. Conversely, it may actually hurt the slab by promoting slab curl. The slab will begin drying and shrinking in the upper surface of the slab, while the lower layers of the slab cannot cure out as fast due to the vapor barrier retaining moisture in the concrete. The top layers shrink while the bottom layers do not…this causes the slab to curl up, especially at the corners. When any load is applied to the slab, the slab cracks.

Hope this helps.

A set of notes I provided for some clients:

Design of Slab-on-Grade/Ground (SOG) Construction

General

SOG construction consists of placing a concrete slab on the existing native soil. The existing native soil may consist of a layer of engineered fill to bring the slab to proper elevation. The existing native soil, in many instances, is considered the sub-base.

On top of the sub-base, the base course is compacted. This provides additional bearing support and a generally flat surface.

On the flat base course, a concrete slab is constructed. The thickness of the concrete slab depends on the type of loading and the quality of the native soil on which the construction is founded. To prevent moisture from ‘wicking’ up through the concrete from the native soil, often a vapour retarder is installed between the base course and the concrete slab.

You can never guarantee cracking will not occur, but you can, however, minimise it with care. For proper construction, it is necessary to specify the proper base and sub-base, concrete mix design, provide control joints, and provide a manner of curing.

Concrete is a brittle material and to minimise random cracking, if the minimum dimension of the slab is greater than 5m, it is also necessary to provide proper control joints. The use of control joints should always be considered as part of the SOG construction.

For high quality or special purpose SOG construction, review of the Work should be considered as part of the project.

In many locales, there are specialty designers and contractors that work with SOG construction.

SOG construction is one of the most trouble prone and litiguous elements of concrete work; care and diligence is essential.

There are several good publications for SOG construction. These should be reviewed prior to commencing a significant SOG project.

Design

Most SOG construction is subject to minimal, static loading and these are generally infrequent. If frequent and moving loads are encountered, then the SOG should be constructed as a pavement. Pavement design includes consideration of both flexural stresses encountered and repetitive loading. If high loads and/or patterned loads are involved, then special consideration should be made to accommodate them.

The effect of a pattern point load may influence stresses in the concrete slab at point load locations adjacent to the load under consideration. This additional loading should be considered in the slab design. The design can include for point loading, line loading and wheel loading.

SOG construction can be designed based on a “drag formula” which tries to accommodate the shrinkage of the concrete and restraint of the granular base material.

SOG construction can also be designed by limiting the flexural tensile stresses generated by loading to a portion of the concrete modulus of rupture for flexure. The flexural stresses can be determined by elastic solutions or FEM studies.

The design may include for granular base materials as well as the sub-base. These can contribute to the modulus of subgrade reaction which is one of the ‘key’ soil parameters for SOG design. A multiple layered approach can be used; this is normally reserved for high quality, or special purpose SOG construction. Examples of this type of construction could be large cargo terminal buildings, airport runways, high speed highways, etc.

When designing for flexural tensile stresses, it is necessary to consider the probability of loading as well as fatigue issues.

Base and Sub-base

The sub-base can be proof rolled to check for uniformity of bearing materials. Soft areas can be excavated and or scarified and re-compacted with engineered fill. The more uniform the base and sub-base, the better the SOG construction.

The base should be a uniformly graded quality granular material that readily compacts. It provides a bearing surface for the concrete SOG over. There is research by both the American Concrete Institute (ACI) and the Portland Cement Association (PCA) that the effect of compacted granular base is minimal. To a lesser extent, it helps transfer loading from the concrete slab to the sub-base below. In additon, it can provide a smooth hard surface on which to construct the SOG.

The base aggregate under the slab should be carefully specified. A uniform degree of compaction with no soft spots and a relatively smooth surface is needed. A non uniform surface provides projections that can restrain a slab and promote cracking.

Vapour Retarder

To minimise moisture entering the SOG from the sub-base and granular base material below, it is common practice to place a polyethylene vapour barrier between the slab and base. To minimise curling of the slab, it is also practice, in some locales, to place 50mm or so of sand material between the polyethylene and the slab. This permits water from the concrete mix to seep into the sand layer and provides a more uniform moisture content throughout the concrete slab.

Concrete

Concrete testing can be predicated on using the compressive strength of concrete or, more correctly, the flexural tensile strenght based on ‘beam tests’.

The mix design must be carefully selected. A low slump concrete should be used to minimise shrinkage. a 75mm maximum slump is often specified. Note the use of the word ‘maximum’ and not just a 75mm slump. In some locales, a 100mm slump satisfies a 75mm specified slump. A superplasticiser can be utilised to achieve workable mix when a low slump is specified.

Consideration of a large proportion by weight of flyash should be made. This has the effect of reducing shrinkage, but causes a slower strength gain. Large amounts of flyash may have an effect on the finishing of the concrete surface.

If the slab is exposed to freeze-thaw conditions, the concrete strength, water:cement ratio, and air content should be carefully considered.

If the slab is exposed to deicing chemicals, there are additional considerations for the concrete design, such as strength, water:cement ratio, curing and sealing should be carefully considered.

If the slab is exposed to sulphates, there are additional considerations for the concrete design, such as sulphate resistane cement, strength, water:cement ratio, curing and sealing should be carefully considered.

If the slab is exposed to sulphates and chlorides, there are additional considerations for the concrete design, such as strength, water:cement ratio, curing and sealing should be carefully considered. Sulphate resistant cement is contraindicated for chloride resistance and a fly ash mix using 25% or 30% by weight of flyash.

The temperature differential between the top of the slab and the bottom of the slab should be minimised. This can be challenging when slabs are cast in sub-zero weather. Procedures for hot and cold weather concreting are applicable for SOG construction due to the general thinness of the elements involved.

Slab Thickness

The slab thickness is determined by the design loading and the quality of the base and sub-base materials below. Soil properties can be determined by a qualified geotechnical consultant.

You should try to use a 125mm slab thickness minimum. Many codes require the concrete thickness to be three times the maximum aggregate size. This permits the use of 40mm aggregate to minimise shrinkage.

If the slab is used for supporting loads in the same fashion as a strip or spread footing, then some codes require a minimum thickness of 200mm.

Reinforcing

It is common to place reinforcing steel in the upper one-quarter of the slab thickness. This is somewhat contrary to flexural mechanics. Maximum flexural moments occur at the bottom fibre of the slab. In addition, tractions produced by the base material on the bottom of the slab, tend to increase the bottom fibre tension and put the top fibres of the slab in compression. The reason for placing the steel in the top is to minimise cracking on the top surface which is subject to ‘wear and tear’. Cracking of the top surface is not aesthetically pleasing because it is visible. Cracking of the underside is not so noticeable.

Reinforcing is often placed in a single layer near the top of the slab. It is common to place the reinforcing to provide a concrete cover equal to the depth of the sawcut.

Cracks on the top become accentuated with time due to objects moving over the top surface.

Although some jurisdictions permit reinforcing steel to be placed at five times the slab thickness, a spacing of three times the slab thickness should be considered.

The effect of reinforcing in small amounts is somewhat nebulous and its main function is to hold the concrete sections together to help develop the aggregate interlock at the fractured surface. For improperly timed, sawcut joints, is also helps distribute cracking a little better and minimises crack widths.

It is common to provide 0.2% of the concrete area as reinforcing steel area. This proportion can be increased to 0.5% or 0.6% to largely ‘eliminate’ visible cracking. The cracks still occur, but they are much more frequent and have a greatly reduced crack width.

Control Joints and Sawcutting

Unreinforced, or minimally reinforced, slabs usually have control joints located at 35x to 40x the slab thickness, but not greater than 5m or so. This is recommended by the ACI SOG committee.

If the structure is unheated, then the sawcuts should be at a closer spacing.

It should be noted that the time for sawcutting is critical. This is more important for a thin slab. A thin slab reacts to changes in temperature, and humidity.

The sawcuts should be made with an “early entry” saw, or ‘Soff-Cut’ saw, that permits sawcutting within two to four hours of the completion of the floor finishing. Sawcut timing is critical. Without an early entry saw, sawcutting should commence within 6 to 8 hours after finishing. CSA A23 stipulates that sawcutting should commence as soon as possible. Concrete should have sufficient strength to prevent the aggregate from ravelling behind the saw blade. If too much time passes, sawcutting is superfluous and the location of the microcracking has determined where the cracks will form.

The depth of sawcut should be a minimum of one-quarter of the slab thickness.

The sawcutting pattern should correspond with any interior columns.

For irregular shapes, the sawcutting pattern can be shown on the construction documents.

In addition to sawcutting, construction joints should be located at approximately every twenty metres.

Projections or re-entrant corners in the slab that will restrain movement should be detailed so they are isolated.

After the initial shrinkage has occurred, sawcuts should be filled with a caulk material that adheres to the concrete sawcut face and provides support for the concrete adjacent to the sawcut. This can be a polyurethane material that has a hardness to prevent the ingress of particles. For heavier loaded slabs, the caulk hardness should be increased.

Curing

Curing is best done by covering with a saturated curing blanket for a minimum of 4 days. Alternatively, a good curing agent, with a high solids polyvinyl chloride content, can be specified. Curing compounds using polyvinyl acetate products (PVA) should be avoided due to their poor ‘track record’.

Special care should be made for SOG’s cast in the open air to prevent undue evaporation by exposure to direct sunlight and any wind. Additional care should be taken for thin slabs.

2 Likes

@dik…excellent info

Thanks… compliment’s appreciated… Do you have anything to add? be picky… It can be posted here as a ???.

A week back I had a reply to some comments I had made about mortar joints and will include the material in the next re-write of my Historic Brickwork paper… posted on the ‘other site’; I’m not very active on there…

Dik

@Ron From the other side… Make sure your clients sign off on the type of slab loading, use, and flatness… I’ll incorporate that in the next edition… BTW, your book’s great.

Dik

@dik…thanks. Your info is excellent. Hope you don’t mind if I quote you from time to time!

Be honoured… thanks.

Dik

@Ron looked it over again and have to update it with a little more information on the design and loading and the importance of the client ‘signing off’ on the loading… I do that with my project notes all the time. I also should add a little more information about slab ‘flatness’ and some of the pitfalls. Just up to my eyeballs in stuff right now…

Dik