Air Content in Concrete

ACI 301 – Specifications for Structural Concrete currently limits the air content in concrete slabs that are to receive a hard trowel finish to 3%.  There is no magic to this value other than it has historically worked well to limit the probability of surface imperfections, such as delimitation, that can result when there is too much air in the mix.  The National Ready Mix Concrete Association (NRMCA) has proposed increasing this limit from 3% to 4%.  Presumably, the motivation to increase the limit is to reduce the amount of concrete trucks that are rejected at the project site because the measured air content exceeds 3%.  Most Contractors are not in favor of this change because it will increase their risk of having surface imperfections.  This proposed change has been voted down, for now.

Floor Flatness and Levelness

Floor flatness (FF) and floor levelness (FL) values for floor slabs are measured in accordance with ASTM E 1155 – Standard Test Method for Determining Floor Flatness and Floor Levelness Numbers.  This standard excludes measuring these values within 2-ft of any slab boundary, construction joint, isolation joint, block-out, penetration or other discontinuity.  As a result, the slab profile within this boundary is often very unflat and/or unlevel.  This results in issues with rack post leveling, lift truck traffic, robot traffic, and other sensitive equipment commonly being used on floors.  ACI 302 has put together a task group develop a method to control the slab profile in this area.  One proposed method is to use the “Q” value, which is a 24” curvature measurement.  This data is already collected using this ASTM test so it is just a matter of extracting it.  The task group will determine appropriate limits for the Q value and also determine how often this value should be measured along the slab boundary.  There are finishing and placing contractors on the committee who were concerned with this and with the cost it would add to construct the slab.  They said it will require more effort to accurately set the formwork at construction joints on a project and often times this formwork is set by the general contractor.

Finishing Suspended Slabs on Steel Deck

The Steel Deck Institute (SDI) currently recommends designing elevated slabs and framing for a construction live load of 20 PSF.  It is common these days for contractors to use finishing equipment that weighs in excess of 2000 LBS, which equates to about 50 PSF.  Aside from exceeding the design load of the structure, the deflection of the structure under this load makes finishing the floor and achieving the specified flatness tolerance difficult to achieve.  Another potential issue is the slab can delaminate from the deck as a result of the excessive deflection of the deck and repetitive fatigue loading of the equipment traveling across the slab. Even if the slab does not delaminate, the equipment will likely cause significant cracking in the slab. These are not usually an issue on floors that are designed for high live loads (e.g. 100 psf).  It is usually an issue for lower design live loads (e.g. 40 psf) that are further reduced (to as little as 16 psf) as permitted by the building code.  This is an example of yesterday’s standard not keeping up with today’s technology.  Contractors should hire an engineer to evaluate the floor structure for the proposed equipment. This could be the engineer of record or a third party engineer.  The floor may need to be temporarily shored or it may be cheaper to increase the design construction live load.

Limited Joint Slabs-on-Ground

The current edition of ACI 360 – Design of Slabs-on-Ground was published in 2010.  Since then the design of slabs with limited joints (sawcut contraction and construction) is becoming increasing popular.  It is estimated that 10% of industrial floor slabs being constructed today have joint spacings extended to the column lines or beyond.  The Committee hopes to publish the next version of this document in 2021 and is working to include provisions and recommendations for designing these types of floors.  There are several methods currently employed to achieve limited joint floors including chemical admixtures, increased mild reinforcing, high dosage fiber reinforcing or a combination of these.  If you would like to learn more about this cutting edge technology or to find out if it is appropriate for your project, please contact us.

Floor Flatness and Levelness

Slab flatness and levelness used to be measured with a 10-ft straightedge and a measuring tape.  It was common to specify that slabs achieve a flatness and levelness of 1/8” in 10-ft.  This method is still around but it is much more common to specify floor flatness (FF) and floor levelness (FL) values that are measured in accordance with ASTM E 1155 – Standard Test Method for Determining Floor Flatness and Floor Levelness Numbers.  We are frequently asked by clients what these numbers mean.  At right is a chart from ACI 117 – Tolerances for Concrete Construction that loosely correlates these numbers to the old straightedge method.

PES is looking forward to ACI’s Spring 2018 – Concrete Elevated, slated for March 25-29 at the Grand America & Little America in Salt Lake City, UT.  And if you’ll be there too, be sure to shoot Chad an email so he knows to look for you there.




Author: Chad Boyea, PE

Chad initially pursued a degree in architecture but then decided structural engineering was a better fit. As a Project Manager, Chad works on a host of project types ranging from hotels, manufacturing and distribution facilities, office buildings, retail developments, educational facilities, to healthcare facilities and historic preservation projects. He’s well versed working with structural systems involving concrete, masonry and steel design. Chad was named an Associate of the firm in January 2017.

Chad can be reached at cboyea@pesengineers.com