360R-06 Design of Slabs-on-Ground
Table 2.1—General comparison of slab types
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| Table 2.1—General comparison of slab types
Slab type Advantages Disadvantages Unreinforced concrete • Simple to construct • Generally costs less to install than slabs designed by other methods • Requires relatively closely spaced sawcut contraction joints • More opportunity for slab curl and joint deterioration • Large number of joints to maintain • Positive load transfer may be required at joints • Flatness and levelness may decrease over time Reinforced with deformed bars or welded wire reinforcement sheets for crack width control • Sawcut contraction joint spacing may be further apart than an unreinforced slab • Reinforcement is used to limit crack width • May reduce the long-term loss of flatness and levelness if reinforcement is continuous through the joints • May have higher cost than an unreinforced slab • Reinforcement can actually increase the number of random cracks, particularly at wider joint spacings • More opportunity for slab curl and joint deterioration • Positive load transfer may be required at joints Continuously reinforced • Sawcut contract joints can be eliminated where sufficient reinforcement is used • Eliminates sawcut contraction joint maintenance. Curling is reduced to heavy reinforcement and elimination of joints • Changes to flatness and levelness are minimized • Requires relatively heavy (at least 0.5%) continuous reinforcement placed near the top of the slab • Typically produces numerous, closely spaced, fine cracks (approximately 3 to 6 ft [0.9 to 1.8 m]) throughout slab Shrinkage-compensating concrete • Allows construction joint spacings of 40 to 150 ft (12 to 46 m). Sawcut contraction joints are normally not required • Reduces joint maintenance due to increased spacing of the joints reducing the total amount of joints and negligible curl at the joints • Increases surface durability and abrasion resistance (ACI 223, Section 2.5.7—Durability) • Requires reinforcement to develop shrinkage compensation • Window of finishability is reduced • Allowance should be made for concrete to expand before drying shrinkage begins. Joints should be detailed for expansion • Contractor should have experience with this type of concrete Post-tensioned • Construction or sawcut contraction joint spacings 100 to 500 ft (30 to 150 m) • Most shrinkage cracks can be avoided • Eliminates sawcut contraction joints and their maintenance • Negligible slab curl if tendons are draped near joint ends • Improved long-term flatness and levelness • Decreased slab thickness or increased load capacity • Resilience against overloading • Advantages in poor soil conditions • More demanding installation • Contractor should have experience with post-tensioning or employ a consultant with experience with post- tensioning • Inspection essential to ensure proper placement and stressing of tendons • Uneconomical for small areas • Need to detail floor penetrations and perimeter for movement • Impact of cutting tendons should be evaluated for post-construction penetrations Steel fiber-reinforced concrete • Increased resistance to impact and fatigue loadings when compared to slabs reinforced with bars or mesh • Simple to construct • Concrete containing steel fibers may require adjustments to standard concrete mixing, placement, and finishing procedures • Some fibers will be exposed on the surface of the slab • Floors subjected to wet conditions may not be suitable candidates for steel fiber because fibers close to the surface and in water-permeable cracks will rust Polymeric fiber-reinforced concrete • Helps reduce plastic shrinkage cracking • Simple to construct • High-volume macropolymeric fibers provide increased resistance to impact and fatigue loadings, similar to steel fiber • Polymeric fibers do not corrode • Micropolymeric fibers do not help in controlling drying shrinkage cracks • Joint spacing for micropolymeric fiber-reinforced slabs are the same as unreinforced slabs Structural slabs reinforced for building code requirements • Slabs can carry structural loads such as mezzanines • Sawcut contraction joints can be reduced or eliminated where sufficient reinforcement is used • Slab design should comply with ACI 318 • Slab may have numerous fine or hairline cracks if reinforcement stresses are sufficiently low |