DESIGN AND CONSTRUCTION OF SLAB-ON-GROUND: APPLYING ACI 318

By Eamonn Ryan, derived from an ACI podcast

The design and construction of slabs-on-ground are critical aspects of modern concrete engineering, ensuring structural integrity, durability, and minimal maintenance over the lifespan of a building. This presentation by Scott Tarr, North S.Tarr Concrete Consulting, delves into the key considerations highlighted during a recent ACI presentation, focusing on the application of ACI 318 and ACI 360 standards to achieve robust slab performance.

This presentation will present the discussion items from the article as this was a series of questions submitted to ACI (American Concrete Institute) regarding the requirements of ACI 318 and they are often misunderstood.

At the heart of the discussion lies the necessity to comprehend and apply ACI codes effectively. Tarr emphasised the fundamental principles outlined in ACI 318. Notably, ACI 318 Section 14.8 specifies that while the code does not apply directly to slabs-on-ground unless they transmit vertical or lateral forces, these slabs still bear substantial loads and forces. Therefore, they require careful structural design to ensure load-carrying capacity.

Concrete’s innate propensity to crack necessitates meticulous planning to mitigate long-term maintenance issues, particularly in high-traffic areas like industrial facilities. Tarr highlighted ACI 360 guidelines, which offer criteria to manage cracking in slabs-on-ground effectively:

• ACI 360 allows cracking solely beneath saw-cut contraction joints, facilitating controlled cracking patterns that minimise maintenance concerns.
• Techniques such as using conventional steel reinforcing bars (rebars) at a minimum of 0.5% of the slab’s cross-sectional area or incorporating fibers help control crack widths, enhancing durability.
• Methods like shrinkage-compensating concrete or post-tensioned concrete aim to maintain concrete in compression, thereby avoiding tensile cracking altogether.

Regarding reinforcement, Tarr clarified that slabs-on-ground can be designed with no reinforcing steel (0% reinforcement). However, for enhanced performance and to meet specific project requirements, considerations of joint spacing, depth, and timing as per ACI 360 guidelines are crucial. These guidelines ensure that joint design aligns with concrete shrinkage potentials, minimising the risk of uncontrolled cracking between joints.

In contemporary practice, there’s a growing trend towards extended joint designs that exceed standard recommendations. This approach requires careful attention to:

• Minimising concrete shrinkage through mix design adjustments and environmental controls.
• Limiting constraints that exacerbate shrinkage-induced stresses within the concrete slab.
• Incorporating features like smooth dowels or enhanced aggregate interlock to stabilise joints and prevent excessive crack widening.

By integrating these principles into engineering practice, stakeholders can mitigate risks associated with cracking, delamination, and surface defects, thereby enhancing the overall quality and longevity of concrete slab constructions.

Best practices for performance and durability

The design and construction of slabs-on-ground are pivotal to ensuring the longevity and performance of concrete floors in various environmental conditions. Tarr elaborated on the critical role of reinforcement in managing cracks in slabs-on-ground. Unlike unreinforced slabs, which are prone to wide, uncontrolled cracking upon shrinkage, reinforced slabs distribute tensile stresses more effectively. However, excessive reinforcement (above 0.1%) can lead to multiple, tighter cracks as the slab continues to shrink. Thus, adhering to ACI 360 guidelines, which recommend a minimum reinforcement of 0.1% for enhanced aggregate interlock, strikes a balance between crack control and structural integrity.

The discussion also addressed the impact of total air content on slab performance, particularly concerning surface blisters and delaminations. Concrete with a total air content exceeding 3% poses a risk during finishing operations. As power equipment manipulates the concrete, entrained air bubbles can coalesce into larger voids near the surface, resulting in blisters. Moreover, these voids may flatten into lenses, causing surface delamination—a significant concern for aesthetics and durability.

When slabs-on-ground are exposed to environmental factors such as freezing and thawing (Exposure Category F per ACI 318), proper protection measures become crucial. Concrete must be air-entrained to withstand these conditions effectively. However, achieving a hard trowel finish complicates this requirement, as air-entrained concrete is unsuitable for such finishes. Alternative methods like screeding, bull floating, and broom texturing are recommended to maintain durability while achieving desired surface finishes.

Continued research and innovation will further refine these practices, enhancing the resilience and sustainability of concrete slab constructions in diverse environments. By prioritising quality assurance and adherence to ACI standards, stakeholders can confidently address the challenges of modern construction, ensuring robust and enduring concrete infrastructure for generations to come.

Maintaining a dry surface for slabs-on-ground is paramount, especially under conditions prone to freezing and thawing (Exposure Category F). Scott emphasises the challenge of keeping slabs dry during construction phases, particularly over winter periods when the building may not be fully enclosed. To combat this, a densified hard trowel finish is recommended. This finish delays the critical saturation of concrete, reducing the risk of surface defects such as scaling caused by freeze-thaw cycles. Prompt removal of water puddles and ponds with scrubbers and squeegees further minimises these risks, as evidenced by examples where scaling occurred only in ponded areas.