Design of Exposed Ground Supported and Suspended Slabs. My Work Experience

Transcription

1 Design of Exposed Ground Supported and Suspended Slabs Design & Details That Impact Serviceability Rick Smith President Structural Services, Inc Richardson, Texas My Work Experience January 1987 My First World of Concrete, Houston, TX Astrohall / Astrodome December 1987 Graduated Oklahoma State University with BS in Construction Management Technology January 1988 Boremco Specialty Chemicals Regional Sales Manager for Might 150 Superplasticizer May Ribtec Carbon Steel Sales Manager Novocon / SI Concrete Systems Regional Sales Manager, Director of Technical Services June 2000 April 2014 Structural Services Inc.

2 Owner Expectations Well finished, blemish free sheen Flat (F F 35 to 50); Level (F L 25 to 35) Crack free (0 to 3% of panels cracked) Excluding Pour Backs! No relative movement at joints Cushion Tire - < differential movement under cross traffic Pallet Riders, Reach Trucks - < Low maintenance No problems which compromise serviceability for the intended uses Occupant / Tenant Expectations Generally, less than the foregoing, except: Serviceability for the intended uses.

3 Serviceability A serviceable slab is typically defined by reference to only the exposed surface: a flat, level, dense finished surface with stable joints and limited cracking..capable of carrying the applied loading by bearing on the ground. Major Concerns High (and low!) shrinkage mix (< 0.05% <) Cracking Joint Stability Curling Dominant Joints Load transfer Surface Delamination Aggregate Shadowing and Mortar Flaking Flatness and Levelness Moisture related problems

4 Risks vs. Costs High shrinkage mixes should not be accepted Low shrinkage mixes must be finishable; avoid surface flaking Joint Stability - Excessive shrinkage and/or dominant joints may lead to loss of load transfer and rapid joint deterioration. Consider light reinforcing (0.10%) continuous thru contraction joints or dowels in all joints Moisture related problems are avoided or at least attenuated by the presence of a vapor retarding sheet under the slab

5 BREAK THE NEWS! Engineering / Construction Realities Wide bay placements will always have some nonuniform set and color Floor flatness and levelness will always decay in time due to curling Crack widths Slab-on-Ground < (30 MILS) Suspended / Structural Slabs ± (15 MILS ± 10 MILS) Most Owners do not have a maintenance budget Construction schedules do not permit the slab to achieve specified MVER s or Relative Humidity

6 Design Philosophy Slabs-on-ground are not structural elements. SOG are designed as plain, unreinforced concrete elements using structural engineering principals. Reinforcement does not impact slab thickness SOG are designed for a safety factor against cracking at the extreme fiber in tension. First mode of failure should be flexure. Structural slabs Limit stress in reinforcement to approximately 20-ksi. Check area of steel for serviceability Cracking Cracking in slabs on ground is primarily caused by tension due to restraint to movement at structural and non-structural penetrations, stresses due to curling and subgrade restraint. Cracking due to superimposed loads is quite unusual.

7 Key Reference Documents ACI 360R-10 Design of Slabs on Grade ACI 302.1R-04 Concrete Floor and Slab Construction ACI 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials ACI 223R-10 Shrinkage Compensating Concrete Post-Tensioning Manual ACI Specifications for Structural Concrete ACI 117 Tolerances Context ACI 302 and 360 are guidelines which both include the statement; Reference to this document shall not be made in contract documents. ACI 301 is a prescriptive specification incorporated into the building code and referenced in the contract documents

8 PRESCRIPTIVE REQUIREMENTS ACI Specification for Structural Concrete; Section 11, Industrial Floor Slabs Must be designated such Minimum thickness 6 Minimum 4 aggregate base Minimum f c = 3,500 psi Maximum 6 slump Maximum air content 3% Capable of hard trowel finish Do not use silicate based surface densifier for curing

9 So what are the considerations for the design and construction of a slab on ground? Loads (analysis considers each independently) Distributed (SF 1.4 to 2.) Rack storage (SF (1.4?) 1.7 to 2.) Lift truck; wheeled traffic (SF 1.7 to 2.+) Speculative construction?

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12 Placed and Designed to Stay Flat and Level

13 Transitions / Handoffs

14 Get It On the Drawings!

15 Owner initiated criteria Anticipated loads, traffic Surface finish expected Flatness and levelness requirements Will decay impact operations? Tolerance for cracks (ACI 3%?) Joint stability and maintenance, (joint filler serviced after one year?) General maintenance, good housekeeping

16 Surface Finish and Tolerance Issues: can we get Div. 9 problems out of Div. 3? Slab moisture emissivity or RH may not comply with finish flooring requirements. Concrete and finish flooring surface flatness requirements use different tolerance systems. Finish flooring may require 1/8 in 10, i.e. superflat. Local market, site specific, actual or imputed criteria Geotechnical considerations Expansive / Collapsing Soils Characteristics of coarse and fine aggregates Concrete mix sources Flatwork subcontractor capabilities Available equipment; laser screeds, pan floats, early entry dry cutting saws Real estate market reality check

17 What else? Time of year that construction will occur Temperature and humidity variations. Cold weather protection Tiltwall project Crane on slab to erect steel Precast panels Getting water out of building CO / CO 2 Heaving

18 Open Air Warehouse Slab Sweating

19 Joint Filler Extrusion ANATOMY OF A FLOOR SLAB

20 Concrete Slab on Ground Contraction Joint aka - Control Joint Construction Joint aka - Formed joint Isolation Joint aka - Expansion joint Concrete Vapor Retarder Subgrade Subbase Concrete Slab on Ground Contraction Joint aka - Control Joint Random Crack Construction Joint aka - Formed joint Isolation Joint aka - Expansion joint Concrete Vapor Retarder Subgrade Subbase

21 Concrete Slab on Ground Contraction Joint aka - Control Joint Construction Joint aka - Formed joint Isolation Joint aka - Expansion joint Concrete Vapor Retarder Subgrade Subbase Concrete Slab on Ground Random Crack Dowel Basket Contraction Joint aka - Control Joint Smooth Dowel Plate Dowel Construction Joint aka - Formed joint Isolation Joint aka - Expansion joint Concrete

22 Concrete Slab on Ground Random Crack Deformed Reinforcement Fiber Reinforcement Smooth Dowel Plate Dowel Concrete Concrete Slab on Ground Deformed Reinforcement Smooth Deformed Dowel Reinforcement Plate Dowel Concrete

23 Concrete Slab on Ground Joint Filler Semi Rigid Semi Soft Specified Flatness (F F ) Levelness (F L ) Spectral Gloss Curing Compound Bond Breaker Sealer Chemical Densifier Mineral or Metallic Shake Concrete Reinforcement for Slabs can be Confusing ACI 318 ACI 318

24 ACI 360 The continuation of a small percentage of deformed reinforcement (0.1% of the slab cross sectional area) through sawcut contraction joints in combination with joint spacings (Fig. 6.6), has been used successfully by some designers to provide loadtransfer capability without using dowels. Recommended Joint Spacing

25 How Much Reinforcement ACI 360 ACI 360 As a general rule, the continuation of larger percentages of deformed reinforcing bars should not be used across sawcut contraction joints or construction joints because they restrain joints from opening as the slab shrinks during drying, and this increases the probability of out-of-joint random cracking To eliminate sawcut contraction joints, a continuous amount of reinforcement with a minimum steel ratio of 0.5% of the slab cross-sectional area in the direction where the contraction joints are eliminated is recommended. More Steel is Not Better

26 THERE IS NO PERFECT DESIGN 52

27 Unreinforced Strategically Reinforced

28 55 Nominally Reinforced

29 Dominant Joints Nosing Repairs

30 Installing Slip Dowels 59 Basket Runners & Wire Mesh 60

31 Hybrid Reinforced with Baskets All Too Common! #4 16,18, 24, etc

32 Reinforced for Crack Width Control 63

33 65 66

34 Cracking Expectations

35 Reentrant Corner Bars Doors

36 Design Schools for Serviceability

37 Grids, Ghosting, Cracks Composite Decks ANSI & SDI Reinforcement: Temperature and shrinkage reinforcement, consisting of welded wire fabric or reinforcing bars, shall have a minimum area of 0.075%, but shall not be less than the area provided by 6x6- W1.4 x W1.4 welded wire fabric. ANSI & SDI Neither welded wire fabric or fibers will prevent cracking; however, they have been shown to do a good job of crack control. The welded wire fabric must be placed near the top of the slab [3/4 to 1 inch cover) at supports and draped toward the center of the deck span. If a welded wire fabric is used with a steel area given by the above formula, it will not be sufficient as the total negative reinforcement.

38 Composite Decks Reinforced for Serviceability / Polishing Composite Decks Reinforced for Serviceability / Polishing

39 Fingers? Suspended Slabs

40 Reentrant Corners

41 Columns Thank You! Any Questions? Rick Smith Structural Services, Inc. Richardson, TX