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Strong but slim concrete floors

By Marko Medjeral

October, 8, 2020

1. Slabs and decks from past to present

Last century experienced concrete boom, global urbanization, and eventually, high-rise construction. Large, tall, infrastructural, and industrial constructions especially need to be cost-effective, practical, and presentable. If they were subject to beauty ideals, slabs would try to “lose weight”. Steel must be factory-formed, but concrete adjusts to any mold, so slabs have been commonly over times poured on site. Alternatives have meanwhile emerged. (CIB5047, Ibrahim, 2007, slabs briefly,, concrete)

Figure 1. L: modern slab formwork systems are toilsome & costly (note: column capitals!), R: failure during slab casting

Floors and roofs support itself, human activity, and natural loads (dead, live, service or ambient load). Indoor or structural floors adjust shape to usage, where roofs usually are light and tilted for drainage (light materials or cellular/modular decks of light materials or joists).

Slabs have small depth/span ratio and rather parallel surfaces. Slab floors are usually massive, but slim, and when casted on site, mesh can be rigid and essentially (as name suggests) of massive nature. Various textbooks or building codes define load bearing in 1-2 directions, depending if long side to short >2, bearing is single sided. (wikipedia,, slabs)

Figure 2 Slab as part of structure, basic slab bay load paths and yield lines

Decks appear in ships or construction and can be defined as (wikipedia ,, woodmagazine) “…a floor-like surface… hull, superstructure, or deckhouse… cambered… strengthening the structure”. They are often outdoors, roof, terrace, platform, part of building, indoor construction systems, even include even cantilevers to an extent. Although decks are formerly wooden, composite steel or concrete (girder-joist), partly poured, is getting common (Figure 5), with rising prefabrication degree.


2. Advanced flooring challenge

Advancing manufacturing methods, including casting, can be made much more reasonable, by planning and investment. Combination of materials (wood, steel, concrete etc.) can make strong yet light (bubble-deck references1, 2) or flat (chapter 3: p. 5-6).

Figure 3. Bubble decks (left 1,2) can easier to construct than light and effective ribbed floor requiring complex mold (right 3), but still require planning of the construction

As cast in-situ slabs formwork, even in modular forms, can require space, cause trouble and risks, including cost, many composite and precast alternatives can be only option in most special cases. Slabs might need to be slimmer for economic, practical, (integration, installations, Figure 4) or even aesthetic reasons.

Figure 4. Various composite deck systems with varying degree of flexibility for integrated design – optimization requires good project planning and solution market knowledge, quickly constructed flexible solution is hard to achieve (source with details)

Even slim slabs can be precast (chapter 3: p. 5-6), or due fast frame construction, earn time and space for undisrupted slab construction (chapter 3: p.6-7). Different girder and joist selection can make quick composite slabs, even with planks or sheets. Prefabricated design can bring speed and minimize defects. (reference: current state-of-art in mechanization for precast concrete, 2, 3, 4, 5)

Figure 5. Left 1,2: Steel composite decking, typical still needs substantial propping supports during erection
Right,3: Composite steel beam with hollow core unit decking, requiring only minimal propping

Floors can carry its own weight and load, but can have limitations or local weaknesses (Figure 6). Such limitations are often shear related, especial concrete, which is brittle to tensional strains. Examples:

  • large shear at support or local point load (typically an issue for column-like slim support)
  • ground slabs can be casted in large pieces (bays), but need control over shrinkage openings
  • Spans with high deflection or lack of proper space for casting time supports

Figure 6. Slim floor failure and yielding mechanisms under shear and bend

Slab casted on ground is sensitive to subgrade conditions (advantages in small house) so it need rather level, firm, low moisture and chemical exposure bases. For large and important ground slabs, neglecting proper design can be costly. Deeper underground or water containing structures need to be water-tight also, which will need extra careful sealing between concrete casted at different times. Precast concrete blocks will have additional challenge since joints have 2 interfaces.

Figure 7. Left: Forlift liftin heavy opjects high cannot allow much tilt, Middle: level slab on ground finished and polished
Right: Warehouse during usage, note high logistics shelves (racks), with large point loads under legs

Figure 8. Damage to slab on ground can be expensive to repair. Reasons of damage can be uncontrolled cracking, local ground settlement, slab (reinforced concrete) or dowel (joint) failure (concrete-slab-repair/, PU-Joint Fillers, powerpile ground bearing)

Figure 9. L: Large water-containing precast concrete structure, R: Precast concrete wall vertical joint and sealing method using epoxy and rubber tape (more: Conseal, Sika)

3. Advanced floor solutions

Ground level

Although not typically highly active part of frame, slabs on grade can carry high loads and participate as foundation. For industrial floors and joint design, see TR34.4 (British concrete society, 2013 edition notes, Figure 7). Large bays and strict quality and dimensional requirement for cast slab can require special concrete (fibres, Silwfer) armored control joints with dowels 

Figure 10. Large bay construction armored joints with shear dowels

Opposed to ground slabs above, suspended precast ground floors can be quick and practical (Figure 11).

Figure 11. Fast suspended floor on top of varying ground, that can be made slim as well

In addition, many underground structures can be made using prefabricated ways

Flat diaphragm and slim floor

Indoor parking, industry, warehouse, or traffic on deck can impose heavy loads and need large spans. Slab or its joints might also be part of vibration damping or lateral load resisting system (diaphragm, see fib 74). These reasons challenge further modular and precast construction. For slim floors a common issue, shear or shear punching can become determining.

Figure 12. Diaphragm s with varying rigidity for structural resistance of a building (ETABS analysis, ASCE 7-05 definitions)

Shrinkage or cracking due flexure or shear stresses in concrete might occasionally need additional care.

  • Shear and punching resistant structures (Figure 13, left), solution: Shear rail, p. 2/ Enhanced
  • Cambered steel beams and concrete deck for flat and quick slab finish (video, Figure 13, right)

Figure 13. Slim floors reinforced, securing concrete cross-section shear

Fast and flat can avoid potential delays or optimize building space. Precast composite slim floor (Figure 14) is not only great for suspended floors, but also fast high deck assembly way, as well as optimum for moderate spanned and loaded offices, parking and similar constructions, when lateral resisting frame or cores are present.

Figure 14. Precast floor using hollow core unit deck with casted joints and steel composite beam filled with concrete on site

High-rise slabs

Tall buildings can utilize a variety of slabs, but due to need for lateral resistance, they need often high degree of in-situ pour (fib 73, 2014). Shear wall constructions and cores supporting slabs (often high-rise slide-form built) can utilize joint reinforcement or starter bar elements with shear key to speed-up casting significantly (starter alternative, reference1, 2 , examples: Figure 16, Figure 17).

Figure 15. Wall and slab connections or splicing are tricky to achieve (especially in smooth casting using climbing forms, that might require bending joint reinforcement at least once)

Figure 16. Rebate bars and fast coupling support fast tower casting with sliding form

Related high-rise with compact spaces plank casting can be economic option for reduced propping or insulated cantilever creation.

Figure 17. Quick precast plank floors supporting quick vertical frame casting

Figure 18. Achieving flat slabs in simple and fast ways

Figure 19. fib 73 bulletin – tall buildings slab comparison (borrowed, resized, flat slab highlighted as optimum according source)




CG Concrete Slab Repair

Climbing formworks



Design of steel fibre reinforced concrete slabs on grade, Prof. Johan Silfwerbrand, 2000


CEF, formwork failures

fib bulletin 43 Structural connections for precast concrete buildings

fib bulletin 73 Tall buildings, 2014; p.29 (tall building slab feasibility comparison)

fib bulletin 74 Planning and design handbook on precast building structures

High- rise buildings – Needs & impacts, CIB5047, 2007

Horizontally reinforced shear stud system:

Peikko deltabeam

Peri formworks

Punching:, including uncited material at least from: Uncited: ACI318 design, papers, books, Peikko 2013, p. 2, PSB shear rail TMA, possibly Max Frank old material

Slab system options listed by architect,

Soil stabilization, lift and foundation enhancement by injection of expansive agent,,

Titus floor repairs

Tufflon polyuretan filler solution


Flexible support, HCS


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