Post-tensioned concrete represents a sophisticated evolution in reinforced concrete technology that enables longer spans, thinner slabs, and superior crack control compared to conventionally reinforced concrete. By placing high-strength steel tendons under tension after the concrete has hardened, post-tensioning actively compresses the concrete, counteracting the tensile stresses that cause cracking in ordinary reinforced concrete. This comprehensive guide examines the engineering principles, design methodologies, construction techniques, and long-term performance considerations for post-tensioned concrete slabs in buildings, parking structures, and bridge applications.
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The Principles of Post-Tensioning
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The fundamental concept of post-tensioning is elegantly simple: concrete is strong in compression but weak in tension—its tensile strength is only about 10-15% of its compressive strength. In conventionally reinforced concrete, steel reinforcement passively resists tensile stresses only after the concrete has cracked, with crack widths controlled by bond between the steel and concrete. Post-tensioning takes a different approach: high-strength steel tendons (typically 7-wire strands with an ultimate tensile strength of 1,860 MPa) are tensioned after the concrete has cured, creating a compressive stress in the concrete that offsets the tensile stresses from applied loads. The result is a concrete element that remains fully in compression under service loads, eliminating tensile cracking entirely in the uncracked state.
The post-tensioning force is applied using hydraulic jacks that pull the steel tendons to a specified percentage of their ultimate strength, typically 70-80%. The force is then locked off at the anchorages—permanent steel castings embedded in the concrete at the slab edges or at intermediate stressing points. The tendons are housed within plastic ducts or sheathing that prevents bond between the steel and concrete, allowing the tendon to move freely during tensioning and to move in response to long-term stress changes. After tensioning is complete, the duct is grouted (in bonded systems) to protect the tendon from corrosion and to create a mechanical bond between the tendon and the surrounding concrete.
The effective precompression in a post-tensioned slab typically ranges from 0.7-1.7 MPa (100-250 psi), which is sufficient to offset tensile stresses from gravity loads and restraint forces. The precompression is distributed through the slab cross-section, creating a stress state that neutralizes or dramatically reduces tensile stresses under service conditions. For a typical 200 mm thick slab with tendons at 1-meter spacing, each tendon stressed to 150 kN produces an average precompression of approximately 0.75 MPa—enough to keep the slab uncracked under its own weight and typical live loads. The benefits of this precompressed state include reduced slab thickness (typically 15-25% thinner than reinforced concrete slabs for the same spans), longer spans between columns (12-18 meters compared to 8-12 meters for reinforced concrete), and improved deflection control.
Bonded vs. Unbonded Post-Tensioning Systems
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Two principal post-tensioning systems are used in slab construction: bonded (grouted) and unbonded. Bonded post-tensioning, the original system developed by Eugène Freyssinet in the 1930s, consists of tendons housed in metal or plastic ducts that are grouted with cementitious grout after tensioning. The grout provides corrosion protection, creates a mechanical bond between the tendon and the surrounding concrete, and allows the tendon stress to be redistributed along its length if a localized failure occurs. Bonded systems are preferred for bridge construction and for structures where the tendons may be subject to severe exposure conditions or where redundancy against localized tendon failure is required.
Unbonded post-tensioning systems, developed in the 1950s for building construction, use individually greased and sheathed strands that are not grouted after tensioning. Each tendon is fully encapsulated in a plastic sheath filled with corrosion-inhibiting grease that protects the strand and allows free movement relative to the surrounding concrete. The anchorage at each end of the tendon transfers the post-tensioning force to the concrete. Unbonded systems are widely used in building slabs, parking structures, and residential foundations because they are simpler to install, require less specialized equipment for grouting, and allow easier replacement of individual tendons if damage occurs. The unbonded nature of the system means that the tendon stress is uniform along its entire length, simplifying analysis and enabling efficient use of the tendon capacity.
The selection between bonded and unbonded systems depends on structural requirements, exposure conditions, construction preferences, and code requirements. For parking structures and other exposed applications where tendon corrosion is a concern, bonded systems with fully grouted tendons provide superior corrosion protection because the grout maintains an alkaline environment around the strand even if the outer sheathing is damaged. However, bonded tendons cannot be inspected, monitored, or replaced after grouting, while unbonded tendons can be individually inspected at the anchorages and replaced if necessary. The Post-Tensioning Institute (PTI) provides comprehensive specifications for both systems, including material requirements, installation tolerances, and quality control procedures.
Design Methodology and Structural Analysis
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The design of post-tensioned slabs follows the load-balancing method, an elegant approach developed by T.Y. Lin in the 1950s. The concept treats the post-tensioning force as an equivalent upward load that counteracts a portion of the applied gravity loads. A tendon draped in a parabolic profile between supports creates an upward vertical component along its length; the upward force from the tendon equals the downward force of the loads it is designed to balance. The load-balancing method simplifies the design process by separating the post-tensioning effects from the gravity load effects: the slab is first designed to be in equilibrium under the balanced loads (typically dead load plus 20-50% of live load), then checked for the unbalanced loads that produce bending moments and shear forces in the slab.
The structural analysis of post-tensioned slabs accounts for the secondary effects of the post-tensioning forces on the indeterminate structure. When a post-tensioned continuous slab is stressed, the tendons induce reactions at the supports that create secondary moments—these are the moments required to maintain compatibility of deformations at the supports. The secondary moments must be combined with the primary moments from the post-tensioning to determine the total moment distribution in the slab. For typical building slabs with balanced spans and uniform loading, the secondary moments are relatively small and can be accounted for through simplified analysis procedures. For irregular geometries, long cantilevers, or significant span variations, a more rigorous analysis using frame models or finite element methods is required to accurately determine the moment distribution.
Serviceability design considerations are particularly important for post-tensioned slabs. Deflection control is inherently improved by the precompression, which reduces the curvature of the slab under load. However, the long-term effects of creep and shrinkage in the precompressed concrete can cause gradual deflection increases that must be accounted for in design. The ACI 318 code requires that post-tensioned slab deflections be calculated considering the effects of creep, shrinkage, and relaxation of the prestressing steel. For long-span slabs, camber (an intentional upward curvature cast into the slab) is often provided to offset the anticipated long-term deflection, ensuring that the slab remains level under sustained service loads. The creep deflection of post-tensioned slabs is typically 2-4 times the immediate elastic deflection, with higher ratios for slabs with lower reinforcement ratios and higher sustained loads.
Construction Sequence and Methods
The construction of post-tensioned slabs follows a precise sequence that integrates the tendon installation with the conventional formwork, reinforcement, and concrete placement operations. The process begins with formwork installation, including slab edge forms, column capitals or drop panels, and openings. The formwork must be designed to support the weight of the wet concrete plus construction live loads, with deflection limits that ensure the slab is cast to its specified elevation and profile. For post-tensioned slabs, the formwork must also accommodate the slab’s camber profile, with the form surface shaped to match the specified camber geometry.
Tendon installation involves placing the sheathed strands or ducts in the slab forms, positioning them at the specified profiles and horizontal alignments. The tendons are typically placed at spacing of 600-1,200 mm in each direction, with concentrated bands over columns for two-way slab systems. The tendon profile is established by supporting the tendons on chairs or bolsters at the low points (midspan) and holding them down at the high points (over supports). For unbonded systems, the plastic sheathed strands are simply laid in position and tied to the reinforcement. For bonded systems, the metal or plastic ducts are positioned and the strands are inserted after the ducts are in place. Anchorages are positioned at the slab edges, aligned precisely with the tendon direction, and securely fastened to the formwork to prevent movement during concrete placement.
Concrete placement for post-tensioned slabs requires special attention to achieving complete consolidation around the tendons, anchorages, and reinforcement congested zones. The concrete must flow freely through the tendon spacing without displacing the tendons from their specified positions. The maximum aggregate size is typically limited to 20 mm to ensure adequate flow through the reinforcement and tendon grid. The concrete compressive strength at the time of tensioning (typically 20-28 MPa) must be verified by testing field-cured cylinders that are stored alongside the slab. Tensioning should not begin until the concrete has achieved the minimum specified strength, which typically requires 3-7 days of curing depending on concrete temperature and mixture proportions.
Stress Transfer and Anchorage
The anchorage zone is the most highly stressed region in any post-tensioned structure and requires careful design to prevent splitting, spalling, or bearing failures. The concentrated post-tensioning force at the anchorage must be distributed into the concrete over a relatively small area, creating triaxial compression and tension stresses that can exceed the concrete’s capacity if not properly reinforced. The anchorage zone design includes the local zone (the immediate area around the anchor, typically reinforced with closely spaced spirals or stirrups) and the general zone (the region over which the concentrated force spreads to become uniformly distributed across the slab cross-section).
Bursting stresses—tensile forces that develop behind the anchorage as the concentrated force spreads into the slab—are the primary concern in anchorage zone design. These stresses are resisted by bursting reinforcement (typically closed stirrups or spiral reinforcement) placed within the general zone. The amount of bursting reinforcement is calculated based on the tendon force, the anchorage geometry, and the concrete strength. Spalling stresses at the slab surface near the anchorage are controlled by surface reinforcement and by maintaining adequate edge distance between the anchorage and the slab edge. Bearing stress at the concrete-anchorage interface must not exceed the limits specified in ACI 318, with the concrete bearing capacity enhanced by the confining effect of the surrounding reinforcement and the triaxial stress state.
The long-term performance of post-tensioned slabs depends on the durability of the tendon corrosion protection system. For unbonded systems, the grase and plastic sheath provide the primary corrosion protection, with additional protection at the anchorages provided by caps filled with corrosion-inhibiting compound. The sheathing must be inspected for damage during installation and repaired before concrete placement. For bonded systems, the grout must completely fill the duct without voids, with grout injection proceeding from the lowest point to the highest point to ensure complete filling. Grout quality is verified through testing of compressive strength, expansion characteristics, and bleed water. The Post-Tensioning Institute’s specifications require that unbonded tendon sheathing have a minimum thickness of 0.8 mm for interior applications and 1.2 mm for exterior applications, with additional corrosion protection requirements for aggressive environments including parking structures and marine structures.
Applications and Advantages
Post-tensioned concrete slabs are used in a wide range of building types where long spans, reduced floor-to-floor heights, and superior crack control are desired. In parking structures, post-tensioning enables clear spans of 16-18 meters between columns, eliminating columns from parking aisles and improving traffic flow and parking efficiency. The crack control provided by the precompression reduces water penetration through the slab, protecting the reinforcement and the vehicles below from corrosion damage. In office buildings and hotels, the thinner slabs made possible by post-tensioning reduce the overall building height, potentially eliminating one or more stories within a fixed building height limit. In residential construction, post-tensioned slabs on grade provide crack-resistant floor systems that protect floor coverings and reduce moisture-related complaints.
The economic advantages of post-tensioned concrete include reduced material quantities (15-25% less concrete and 50-70% less conventional reinforcement compared to reinforced concrete), faster construction cycles (slabs can be tensioned and forms stripped within 5-10 days compared to 14-21 days for reinforced concrete), and reduced building height (each 100 mm of slab thickness saved translates to approximately one additional floor in a 30-story building). The total cost savings for post-tensioned building frames typically range from 10-25% compared to conventionally reinforced concrete frames, with the savings increasing for longer spans and taller buildings. When combined with the improved serviceability and reduced maintenance requirements, post-tensioned concrete represents one of the most cost-effective structural systems available for multi-story building construction.