Some floor designs are better suited to specific applications than others. Understanding an owner's or tenant’s requirements is critical to recommending a particular type of design.
Factors that determine the best solution are numerous and varied, and they interact to lead to a complex decision tree or matrix. These factors include, but are not limited to, facility use and conditions, construction conditions, load, floor tolerance, abrasion requirements, impact resistance, design, and last but not least, budget.
Let's walk through these factors:
- Facility usage. Spec warehouse, distribution center, freezer cold storage, food processing, manufacturing, etc.
- Conditions during the facilities’ use. Will it be a controlled environment with temperature and moisture control, or will it be unconditioned, ambient temperature and humidity? And will there be a vapor barrier under the slab to prevent water vapor emissions?
- Conditions during construction. Will the floor be placed in a controlled environment, under the roof, with downspouts and walls in place, or in the open, like in tilt-up construction?
- Loading:
- Rack, type, clear height, base plate size, and spacing;
- Automated Storage and Retrieval Systems (ASRS);
- Material handling equipment;
- Lift truck types and loads;
- Wheel types and sizes — Wheel type significantly affects joint spalling, which should play a vital role in deciding floor design type and the options for design upgrades like load transfer devices, joint fillers, and armored joints.
- Bulk materials or stacked goods;
- Loads and packaging or stacking devices — Packaging also affects the decision to use vapor barriers or not because dampness and mold can form under stacked materials if vapor is allowed to pass from the subbase through the concrete.
- Floor tolerance requirements:
- Random traffic, defined traffic, robotics;
- FF and FL numbers or F min numbers (Defined Traffic, VNA) — ACI guides suggest floor tolerances should be measured within 48 hours of construction. Still, subsequent floor curling can significantly reduce tolerances, so tolerances measured at the end of construction and throughout the facility's life need to be considered.
- Abrasion resistance requirement. What will operate on the floor?
- Impact resistance. What loads could be dropped? Will there be press pits or other equipment that imparts regular impact?
- Design life. How many years of service with little maintenance is required?
- Budget. What an owner wants and is prepared to pay for is always a consideration.
@Stephen - stock.adobe.com
New Design Alternatives
There are seven basic design choices for slab-on-ground construction:
- Unreinforced concrete slab.
- Slabs designed with load-transfer devices in joints. Commonly known as “strategically reinforced”. This design uses plate dowels in joints for positive load transfer but is otherwise unreinforced in the mid panel between joints. This design is often recommended to prevent joint spalling under lift truck traffic. Still, it also increases the bearing capacity of the slab by sharing the load into adjacent placements so the design thickness can be less than that of unreinforced for same stress.
- Slabs lightly reinforced to “enhanced aggregate interlock.” This design uses reinforcement to restrain the opening of joints so that the enhanced aggregate interlock provides load transfer to minimize joint spalling and reduce thickness or stress.
- Slabs where contraction joints are extended to column lines. These designs utilize fiber reinforcing and additives that can reduce shrinkage, curling, or both to allow for wider joint spacing and a significant reduction in saw cutting, dowels, or traditional reinforcement and joint filling. This design often includes some of the proprietary flooring systems that have become available over the past decade.
- Slabs reinforced to limit crack widths due to shrinkage and temperature restraint and applied loads. These slabs consist of non-prestressed steel bar, wire reinforcement, or fiber reinforcement, all with closely spaced joints; and continuously reinforced with traditional rebar or high dosages of steel fiber and free-of-saw cut contraction joints.
- Slabs reinforced to prevent cracking due to shrinkage and temperature restraint and applied loads. These slabs consist of shrinkage-compensating concrete; and post-tensioned.
- Structural slabs designed following ACI 318. These slabs are either:
- Plain (unreinforced) concrete; or
- Reinforced concrete.
Traditionally Jointed, Unreinforced Design
This design uses traditional concrete mixtures of portland cement, fine aggregates, coarse aggregates, and water. It is historically a simple, low-cost design. Still, it has some intrinsic limitations, namely increased shrinkage, curling, the need for short joint spacing, and significantly increased risk of joint spalling under lift truck traffic. Contraction joints should be per the most conservative recommendations of ACI 360.R-10, Figure 6.6 (approximately 12 ft. by 12 ft. depending on the shrinkage rates of the concrete mix and column spacing).
Its main advantages are lower initial cost and ease of construction. Still, its disadvantages are significant, including reduced F-number tolerances due to uncontrolled curling, future joint maintenance, damage to material handling equipment, a higher incidence of OSHA claims for lift truck drivers, and the increased need for saw cutting.
Traditionally Jointed, “Strategically Reinforced” Design
This design also uses traditional concrete mixtures of portland cement, fine aggregates, coarse aggregates, and water. No traditional reinforcement is required in the mid panel, but plate dowels are used in both sawn contraction joints and formed construction joints.
It is historically a simple and effective design that performs well under lift truck traffic. Still, it does not reduce shrinkage, curling, or reduce joints significantly. With confidence in the load transfer provided by dowels designers and an appropriate mix design designers can stretch joint spacing slightly but should still stay within the range provided by ACI 360.R-10, Figure 6.6 (approximately 15 ft. by 15 ft.). The recent availability of dowel design software that optimizes the use of dowels for specific loads and applications has further reduced the cost of this design option.
Its main advantages over unreinforced slabs are thinner slabs and performance under lift truck traffic. It advantages over other designs are ease of construction, and large placements for a quicker schedule. Its main disadvantages still relate to the number of joints required, and the reduction in F number tolerances due to the curling of traditional portland cement mixes.
Traditionally Jointed, “Lightly Reinforced for Enhanced Aggregate Interlock”
This design uses traditional concrete mixtures of Portland cement, fine aggregates, coarse aggregates, and water with traditional steel rebar reinforcement (typically, < 0.1 percent steel by cross-section) run continuously through saw-cut contraction joints with plate dowels in construction joints.
Its advantages and disadvantages are the same as those of a “strategically reinforced” design but also includes an increased propensity for dominant joint openings, which can cause some construction and contraction joints to open wider than normal. So, special details like more conservative doweling recommendations to accommodate wider joints, hybrid joints, or armored construction joints should be considered.
Features & Risks of Using Traditionally Jointed Designs
From an environmental perspective, the “strategically reinforced” and “lightly reinforced for enhanced aggregate interlock” designs allow for thinner sections. If the typical thickness for a 50-ft. clear building with 6 storage levels per PCA method in a non-seismic zone is 8 in. then the same PCA design would require a minimum of a 9 in. section for the same stress if positive load transfer is not adopted. The reduction in section provides a reduced need for materials and freight, but no other reduction in cement usage is affected.
Dowel manufacturers have software to optimize the dowel design and reduce the usage of steel in “strategically reinforced designs,” and if designed properly, glass fiber reinforced polymer (GFRP) rebar can be substituted to improve sustainability in “lightly reinforced for enhanced aggregate interlock” designs.
Type 1L cement can be used to reduce the carbon footprint further. Still, it can introduce negative effects, including increased curling, reduced abrasion resistance, and retarded set, which can impact finishing and saw-cut quality. The increased exposure to silica dust due to the amount of saw cutting required is also a consideration from an EPD point of view.
From a performance, serviceability, and life expectancy point of view, designers should be cautioned to avoid value engineering alternates that remove dowels or the 0.1 percent rebar from designs that provide load transfer. From a cost perspective, the reduced section compensates for the additional design cost of providing positive load transfer.
Most traditionally jointed designs come with a standard one-year warranty.
Slabs Where Contraction Joints are Extended to Column Lines
This design uses either high dosages of macro-synthetic fiber (usually 7.5 lb. per cubic yard of concrete), lighter dosages of macro-synthetic fibers with shrinkage-reducing admixtures, or steel fibers at various dosages.
This design type is gaining market share quickly due to the reduced need for sawcutting, load transfer devices, and joint filling maintenance. Joints are located at column lines (typically 40 to 65 ft.), combining sawcut contraction joints and formed construction joints.
Its main advantages lie in the reduced number of joints (1/3 or 1/4 of traditionally jointed designs), significantly reducing joint filling maintenance, and the amount of silica dust generated during saw cutting operations. Some proprietary systems employ additives that increase concrete strengths with a reduced Portland cement requirement, making them more sustainable. Many of the proprietary systems reduce curling, which allows for a further reduction in concrete sections. Some additives can increase air content in the mix, so additional de-foaming additives may also be required to reduce the risk of delamination.
More comprehensive construction and contraction joints may require more conservative doweling with well-maintained joint filling or armored joints in highly trafficked aisles. Manufacturers should be consulted for dowel design recommendations for the wider joint openings. Extended warranties are available depending on the system employed.
Continuously Reinforced with Traditional Rebar to Reduce Crack Widths
This design uses steel rebar reinforcement in high volumes to hold cracks tight. Owners and tenants must accept that this design type is “designed to crack” but that the reinforcement will hold cracks sufficiently tight for the floor to remain serviceable under lift truck traffic.
Primary reinforcement can be designed in one direction for long, narrow strip placements or two directions for mass placement, such as random traffic areas. ACI 360R-10 recommends a minimum of 0.5 percent steel by cross-section, but many designers specify a minimum of 0.6 percent. Joint spacings can be up to 300 ft., although even longer distances have been achieved.
This design is not widely used due to its increased cost and lack of sustainability due to the significantly increased requirement for steel reinforcement. It can, however, provide excellent results in some facilities with heavy lift truck traffic and high tolerance F-min “Superflat” floors. Designers need to consider the optimum positioning of steel to hold cracks tight at the slab's surface without reducing tolerances attributed to the effect the rebar has on the concrete placement and screeding process. Properly designed GFRP rebar can be substituted to improve sustainability.
Continuously Reinforced with High-dosage Steel Fibers
High dosages of steel fibers extend joint spacing, hold micro-cracks tight, and increase impact resistance in the concrete. Typical dosages of steel fiber to extend joint spacing to 160 ft. by 160 ft. are 60+ lb. of type 1 or type 2 steel fiber per yard of concrete.
The design's main advantages are the elimination of saw-cut contraction joints and the ability to extend joint spacing. Curling is not naturally reduced, so attention to mixture proportioning, the right dowel design for load transfer, and armored joints should be considered to avoid joint spalling of the remaining construction joints, which can open to 1 in. or more. Shake-on hardeners can minimize the number of steel fibers at the slab's surface but may also increase the propensity for delamination if not properly installed.
From an environmental point of view, this design can vary significantly depending on many factors, including the dosage of fiber required, the distance from the fiber factory, and whether proprietary additives are used to reduce the cement content required. Warranties vary depending on the design-build contractor.
Reinforced Shrinkage-compensated Concrete Slabs
Shrinkage-compensated concrete slabs blend cement types, including ordinary portland cement (Type 1 or Type 1L) and expansive cement (usually Type K cement) in the mix. The expansion of the Type K cement in the early stages of the concrete hydration stretches the reinforcement.
When properly designed, the reinforcement restrains a portion of the concrete's normal drying shrinkage, thus offsetting the contraction of the floor. This results in fewer joints and less curling. The requirement for reinforcing can vary depending on the expansion of the concrete. Traditional steel or GFRP rebar can be used, and joint spacings of 100 ft. by 100 ft. are regularly accomplished with no mid-panel cracking.
The system's main advantages are reduced jointing and the elimination of cracking. And, shrinkage-compensated concrete can also reduce curling, allowing slab thickness to be reduced. Qualified designers, consultants, and contractors with experience in designing and constructing shrinkage-compensated concrete floors should be used. Some of the specialty flooring contractors that construct with this design also provide extended warranties.
Post-tensioned Concrete to Prevent Cracking
Post-tensioned floors have been used with great success on many high-tolerance floors. Post-tension tendons are positioned in the concrete in either one direction for long, narrow aisles or two directions for randomly trafficked floors. The tension is applied with jacks at predetermined times during the hydration period.
The resulting tension allows for very long sections of concrete (300 ft.+ and just one aisle wide) without any joints or visible cracks. The post-tensioning of floors also increases their load-carrying capacity and can allow for thinner sections. Most of the joints can be placed in the rack flu spaces, and the tendons can be lowered at the ends of the long placements to pull any curling at the joints down. Qualified designers, consultants, and contractors with experience designing and placing post-tension floors are required.
Advancements in Slab Design Options & Analysis
Many factors contribute to a floors load carrying capacity and performance but few more than the amount of curling. The magnitude of curl affects the floors load carrying capacity, flatness and levelness tolerances, and its propensity for cracking and joint spalling.
The increased number of floor design types provides designers with options and recent advancements in slab design software allows for analysis that considers curling, joint spacing, concrete shrinkage, and joint stability as well as the more typical inputs of loading, concrete strength, concrete thickness and subgrade support. The availability of slab design and dowel design software allows designers to optimize by choosing designs that curl less or that mitigate its effects.
