Chad Boyea, P.E. was our man on the streets at the recent ACI Spring Conference held April 17-21 at the Hyatt Regency & Wisconsin Center in Milwaukee. While there Chad had the opportunity to swing by the Milwaukee Art Museum to see Santiago Calatrava’s Burke Brise Soleil as well enjoy some of the City’s great beer and brat culture.
He also sat in on a variety of programs ranging from how revised energy codes will impact the continued use of concrete and masonry, how sustainable practices, such as the use of Fly Ash, are changing the market and a host of other topics.
We were intrigued by the knowledge Chad brought back to our team and in the interest of sharing that information, we present….
Eight Things Learned at the ACI Spring 2016 Conference
8. High Strength Steel in the Slab: Volume of Steel versus Placement or Number – Most slabs-on-ground are designed as unreinforced concrete but reinforcing is installed in the top third of the slab to control crack widths. High-strength, pre-tied reinforcing mats are gaining popularity with contractors for use in slabs-on-ground and tilt-up wall panels. The advantages over conventional mild reinforcing include reduced field labor in placing and tying the reinforcing and a reduction in the area of steel due to the increased yield strength of the steel. The reduction in steel area may be acceptable if the reinforcing is used for strength only; however, it is not acceptable if the reinforcing is used for crack width control. Why? Because the modulus of elasticity of the steel, which is a ratio of the stress to strain (i.e. how much the steel stretches under a given load), is independent of the steel’s yield strength. That is, the modulus of elasticity equals 29000 ksi for steel with a yield strength of 80 ksi and 60 ksi. In simple terms, the higher strength steel can support more load by stretching further but this results in larger crack widths. The bottom line is that the pre-tied mats can be substituted but the specified area of steel must be maintained when the reinforcing is used for crack-width control.
7. 2016 marks the Portland Cement Association Centennial Anniversary
6. ACI Committee 302.1R – Guide for Concrete Floor and Slab Construction published a new version of the document last fall and is now beginning a new cycle. In this iteration, the committee is adding a chapter on aesthetic concrete. Exposed slabs, such as polished slabs, are becoming increasingly popular, particularly in school and retail projects. The first step in a successful aesthetic concrete project is coordination between the architect and the engineer. The engineer has to understand the desired finishes so the slab can be properly designed and detailed and the necessary adjustments made to the concrete specifications. The next important step is for the contractor to hold a pre-construction meeting with all parties involved in placing and finishing the concrete, especially the concrete finishing subcontractor and the polishing subcontractor. The finishing contractor must properly finish the slab to allow the polishing subcontractor to create the desired aesthetics. The most common issue is a slab that is not finished with a uniform surface density and not finished with the proper flatness. There is a common misconception that the polishing contractor will come in and cleanup any surface defects. This often requires deeper grinding than what the polishing contractor anticipated and requires cutting deeper into the aggregates which can change the desired appearance. Look for more discussion on this topic in the next version of the guide.
5. Tolerances for the Depth of Sawcut Contraction Joints in slabs-on-ground is on the Horizon – The structural engineer is responsible for specifying the depth of the sawcut at slab contraction joints. A commonly specified depth is the slab thickness divided by 4 or 5. The location of the reinforcement is then specified to be just below the depth of the saw-cut. There is currently no specified tolerance on the depth of the sawcut. Often times contractors end up cutting much deeper into the slab to ensure the contraction joint activates. This results in a couple of issues mainly, cutting through the slab reinforcement and the amount of joint filler required to fill the joints full depth. Committee 302 is adding a +/- tolerance limit to the depth of the joint to ensure the reinforcement is not cut and the contractor responsible for filling the joints can accurately bid a project.
4. The Milwaukee Museum of Art is closed on Mondays. Please make a note.
3. To tie the tilt-up panels to the slab-on-ground or not to tie the tilt-up panels to the slab-on-ground? – It is common practice in the Southeast to tie the tilt-up concrete wall panels to the slab-on-ground with reinforcing bars to resist out-of-plane lateral loads. In other parts of the country, tying the panels to the slab is not so common. It is well known that tying the panels into the slab will restrain the slab from undergoing its normal volume changes due to temperature and shrinkage. This often results in restraint cracks in the slab which can have a significant impact on the function of the facility. There are alternatives to tying the panels to the slab including designing the panels to span to the footings without any lateral support from the slab-on-ground, tying the panels to a waste slab below the slab-on-ground, or setting the panels on a cast-in-place concrete foundation wall that is poured up to the slab level. These alternatives often come at price, such as thicker tilt-up panels. The potential for restraint cracks in the slab should be discussed early on in the project and alternative options to tying the panels to the slab should be reviewed if shrinkage cracks will adversely affect the facilities usage.
2. The 2016 Version of ACI 301 – Specifications for Structural Concrete will be published within the next couple of weeks. This is a reference specification that the engineer or architect can cite in the project specifications to make applicable to any project. One major revision to the document worth mentioning is the requirement to chair welded-wire reinforcement in concrete slabs. ACI 301 was previously silent on this topic and the typical construction method is to lay the reinforcement on the subbase and lift it up into place with hooks after concrete placement. Often times, this results in the reinforcement ending up near the bottom of the slab instead of within the top third of the slab where it is most effective at controlling crack widths. ACI 301 now requires welded wire reinforcement smaller than size W4.0 (diameter < 0.226 inches) to be supported at the specified height with continuous chairs at 12 inches on-center. This change will surely affect the cost of slabs reinforced with welded wire reinforcement and will require changes in the concrete placement methods that contractors can use.
1. ACI Committee 360 – Design of Slabs-on-Ground is continuing to make chapter revisions to the guide document. One revision worth discussing is the inclusion of guidance for wide-panel slab design and construction. High dosages of macrosynthetic fiber reinforcement have been successfully used in industrial slabs to extend joint spacing’s to the column lines and beyond. The reduced amount of joints in the slab means less saw-cutting, less joint filler material, and fewer dowel baskets for the contractor, all of which can offset the costs of the fiber reinforcement. Another benefit with reduced joints in the slab is the reduction in long-term maintenance costs of refilling and repairing the joints. The success of this type of slab is dependent on a number of factors including a properly prepared subbase, use of double slip sheets below the slab to reduce subbase friction, a low shrinkage concrete mix, and proper placement and finishing techniques. I was able to observe a slab in a cold-storage facility in Wisconsin that utilized this type of design. The slab joints were extended to every other column line – approximately 90 feet! In a more conventionally designed slab without reinforcing or with a nominal amount of reinforcing for crack-width control, the joint spacing would have been approximately 15 feet in each direction. I observed negligible slab curling at the joints and noticed a single hairline crack in the entire floor.
Did you attend the conference? If so, what were your take aways from the event? Was your favorite session on Autogenous and Drying Shrinkage, Concrete’s Expansion Behavior or something else entirely? Let us know what you learned at the ACI Spring Conference in the comments section. And let Chad know if he should look for you at the Fall 2016 — Revolutionary Concrete conference in Philly slated for October 23-27.