Prestressed Concrete Principles
Prestressed concrete is a form of reinforced concrete in which compressive stresses are intentionally introduced into the member before it is subjected to service loads. The precompression counteracts the tensile stresses that develop under load, reducing or eliminating cracking and allowing longer spans than conventional reinforced concrete. The prestressing force is applied by tensioning high-strength steel strands or bars that are either bonded to the concrete after curing or remain unbonded within the member. Prestressed concrete offers the advantages of reduced member depth, longer spans, improved crack control, and reduced deflection compared to conventionally reinforced concrete. The construction of prestressed concrete requires specialized knowledge of prestressing systems, anchorage details, and stress control during fabrication and erection.
Pretensioning is a method of prestressing in which the steel strands are tensioned before the concrete is placed. The strands are stretched between fixed abutments in a long prestressing bed, and the concrete is placed around the tensioned strands. After the concrete has cured to adequate strength, the strands are released from the abutments, transferring the prestressing force to the concrete through the bond between the steel and the concrete. Pretensioning is used primarily for precast, prestressed members manufactured in permanent plants where the pretensioning bed can be reused for many production cycles. The transfer length required to develop the full prestressing force through bond is typically 50 to 100 strand diameters, depending on the strand surface condition and concrete strength.
Post-tensioning is a method in which the steel strands are tensioned after the concrete has hardened. The strands are placed in ducts or sleeves within the concrete before placement and are tensioned against the hardened concrete using hydraulic jacks that bear against bearing plates at the member ends. After tensioning, the strands are anchored by wedges that grip the strands and transfer the force to the bearing plate. The ducts are then grouted to bond the strands to the concrete and provide corrosion protection. Post-tensioning is used for cast-in-place concrete structures, bridges, and tanks, and is particularly advantageous for long-span floors in buildings where the reduced member depth provides significant savings in building height and material costs.
Loss of Prestress
The initial prestressing force applied to the strands decreases over time due to various time-dependent effects that must be accounted for in design. Immediate losses occur during the stressing operation and include elastic shortening of the concrete, friction losses in post-tensioning ducts, and anchorage seating losses. Elastic shortening occurs as the concrete compresses under the prestressing force, reducing the strain in the strands. In pretensioned members, the elastic shortening loss is calculated based on the concrete stress at the centroid of the strands and the modular ratio of steel to concrete. whitney stress block for reinforced concrete flexural design. minimum reinforcement requirements for beams in aci 318. interaction diagram for reinforced concrete column design. In post-tensioned members, the elastic shortening loss is reduced because the concrete compresses progressively as each strand is tensioned.
Time-dependent losses include creep of the concrete, shrinkage of the concrete, and relaxation of the prestressing steel. Creep is the continued deformation of the concrete under sustained compressive stress and reduces the strain in the strands, decreasing the prestressing force. Shrinkage of the concrete as it dries over time also reduces the strand strain. Relaxation of the prestressing steel is the reduction in steel stress under constant strain over time. The total time-dependent loss typically ranges from 15 to 25 percent of the initial prestress, depending on the concrete properties, the environmental conditions, and the level of prestress. The losses must be estimated in design to ensure that the effective prestress after all losses is adequate for the service load requirements.
The effective prestress after all losses must be sufficient to control cracking and deflections under service loads while maintaining adequate safety at ultimate load. The allowable stresses in the concrete at transfer and under service loads are limited by the ACI 318 code to prevent excessive cracking and to ensure long-term durability. The allowable concrete compressive stress at transfer is typically 0.60 times the concrete compressive strength at the time of transfer. The allowable tensile stress under service loads depends on the exposure condition and the level of crack control required. For members exposed to corrosive environments or where water tightness is required, no tension is permitted under full service loads.
Prestressed Concrete Slabs
Post-tensioned slabs are widely used in building construction to achieve longer spans and thinner slabs than conventionally reinforced concrete. The unbonded post-tensioning system uses greased and sheathed strands that are placed in the slab before concrete placement and tensioned after the concrete cures. The unbonded strands can be stressed individually and are replaceable in the event of damage. The banded-tendon distribution concentrates the prestressing tendons in bands over the column lines in one direction, with uniformly distributed tendons in the perpendicular direction. The concentration of tendons in the column band provides efficient resistance to the negative moments over the columns while the distributed tendons provide uniform compression across the slab.
The design of post-tensioned slabs follows the balanced load method, where the vertical component of the tendon force balances a portion of the applied load. The upward force from the draped tendon profile counteracts the downward gravity loads, reducing the net load that must be carried by flexure. The balanced load is typically 60 to 80 percent of the slab dead load, leaving the remaining dead load and the full live load to be resisted by the flexural strength of the slab. The deflection of post-tensioned slabs is reduced by the balanced load effect, allowing longer spans and thinner slabs. The minimum slab thickness for post-tensioned slabs is typically span divided by 45 for interior spans and span divided by 40 for exterior spans, compared to span divided by 33 for conventionally reinforced slabs.
Design Standards and Building Code Requirements
All construction work must comply with the applicable building codes and industry standards that establish minimum requirements for structural safety, fire protection, accessibility, and energy efficiency. The International Building Code provides the comprehensive framework for building design and construction in most jurisdictions. The code requirements for each building element depend on the occupancy type, the building height, the type of construction, and the seismic design category. The designer must review all applicable code provisions during the design phase to ensure that the design complies with every requirement. The permit review by the building department verifies that the design documents demonstrate compliance with the applicable codes before construction begins.
The material standards published by ASTM International, the American Concrete Institute, the American Institute of Steel Construction, and other organizations provide the specifications for material properties, testing methods, and quality control procedures. These standards ensure that the materials used in construction meet the minimum quality requirements for the application. The reference standards are incorporated into the building codes by reference, making them legally enforceable requirements. The contractor must verify that all materials meet the applicable standards through mill certifications, test reports, and product labeling. The quality control testing during construction verifies that the installed materials achieve the specified properties.
Construction Methods and Installation Procedures
The proper installation of construction materials and systems requires adherence to the manufacturer’s instructions and industry best practices. The installation procedures for each product are developed through testing and field experience to achieve the specified performance. The contractor must ensure that the installation crew is properly trained and qualified for the work. The quality of the installation is verified through inspections at each stage of the work. Any deviations from the specified procedures must be approved by the designer before proceeding. The documentation of the installation process provides the record of compliance for future reference.
The sequencing of construction activities affects the quality and efficiency of the work. The work must be planned so that each activity is performed in the correct order and with adequate time for preparation and curing. The protection of completed work from damage by subsequent activities is essential for maintaining quality. The coordination between different trades working in the same area requires careful scheduling and communication. The site conditions including weather, temperature, and humidity affect the installation procedures and must be considered in the planning. The contingency plans for adverse conditions ensure that the work can proceed safely and efficiently under varying conditions.