Question: If the concrete in my precast, prestressed concrete bridge beams has achieved the specified compressive strength before the specified age, is there any reason not to transport and erect them?
Answer: For prestressed concrete bridge beams, the Engineer specifies a minimum concrete strength at time of prestressing force transfer. This is generally assumed to be a percentage of the 28-day strength specified by the owner. The Engineer may also specify a minimum concrete strength or concrete age at time of shipping or erection. For most precast, prestressed concrete beams, the specified strength at time of transfer controls the concrete mix proportions. Concrete strengths at 28 days or later are often in excess of the specified 28-day strength. It is, therefore, not unusual for the specified strength to be achieved before the specified age especially when the specifications require the beams to be at least 90 days old before placing the concrete deck and establishing continuity for live load. If continuity is established before 90 days, different design provisions are applicable.
Unless the project specifications specifically prohibit shipping before a certain age, the following should be considered in making a decision about shipment.
Camber. Camber in prestressed concrete beams begins when the prestressing force is transferred and continues at a decreasing rate until additional load is applied. A beam shipped and erected at an earlier age has less camber than the same beam shipped at a later age. Consequently, if the deck is cast at an earlier age, the depth of haunch above the beams has to be greater to maintain the specified deck thickness and achieve the riding surface profile. This effect is more critical with longer span beams.
Cracking. For design purposes, the flexural tensile strength of concrete is assumed to be proportional to the square root of the concrete compressive strength. In reality, the gain of tensile strength depends on many factors. If the tensile strength gain is relatively slower than the compressive strength gain, the beams are more susceptible to cracking if shipped earlier. Owners may wish to specify that the modulus of rupture of the concrete be measured in accordance with AASHTO T 97 (ASTM C78) with a requirement that the modulus of rupture exceeds either 550 psi (3.8 MPa) or 7.5
psi (0.62 MPa) for normal weight concrete, where fc‘ is the specified concrete compressive strength at 28 days. For sand-lightweight concrete and all-lightweight concrete, the modulus of rupture should exceed 6.4 and 5.6 psi (0.53 and 0.47 MPa), respectively.
Curing. Some owners have specific curing practices for certain concrete mix designs to ensure adequate hydration of the cementitious materials. The resistance of concrete to chloride penetration does not develop at the same rate as concrete strength. High performance concrete mixes with supplementary cementitious materials may, therefore, require extended wet curing to provide a low permeability concrete prior to installation. An example would be piles intended to be placed in seawater.
Summary. Clearly, the more conservative approach is to wait until the specified age before shipping. Exceptions can, however, be made but need to be addressed on an individual basis as there may be factors other than compressive strength to consider.
The answer to this question was provided by William N. Nickas, Managing Director, Transportation Systems, Precast/Prestressed Concrete Institute.