Question:
Can I use a higher ratio between release strength and 28- or 56-day strength when using HPC in prestressed concrete girders?
Answer:
There are two approaches that can be used to answer this question. The first approach is from the perspective of what has been specified and achieved to date. The second approach is to look at what can be achieved with today’s technology.
In the first approach, consider data from the FHWA showcase bridge projects. Specified strengths at release ranged from 5500 to 10,800 psi (38 to 74 MPa). Specified design strengths ranged from 8000 to 14,700 psi (55 to 101 MPa). The ratio of specified strengths at release to specified design strengths ranged from 0.60 to 0.81. The ratio of measured strengths at release to measured strengths at 28 or 56 days ranged from 0.61 to 0.83. Based on these data, the ratios between release strength and 28- or 56- day strengths are similar to those specified and achieved with conventional strength concretes in prestressed concrete girders. On the other hand, Jerry Weigel reports on Page 1 of this newsletter that Washington State has found that release strength controls the design and that release and design strengths of 7500 and 8500 psi (52 and 59 MPa), respectively, provide optimum economy. This is equivalent to a ratio of 0.88.
In the second approach, consider methods that can be used to influence and control the rate of strength gain. Concrete that is heat cured exhibits a more rapid strength gain at early ages and a slower strength gain at later ages compared to the same concrete that is not heat cured. The heat may be provided by the internal heat of hydration or by an external source such as steam or radiant heat. The amount of internal heat and strength gain is also influenced by the properties of the cementitious materials. A finer cement will produce more heat of hydration than a coarser cement. The use of silica fume will produce higher earlier strengths whereas the use of fly ash will result in more strength gain at later ages.
Consequently, a higher ratio of release strength to 28- or 56-day strength is technically possible to achieve by selection of the cementitious materials and an appropriate curing temperature. However, depending on the application, it may be desirable to limit the maximum concrete temperature to 160°F (71°C) to reduce the likelihood of delayed ettringite formation. In addition, the selection of materials may be limited by those locally available. Therefore, consult your local producers to see what they can produce economically to meet the release strength in an acceptable time without greatly exceeding the specified design strength at a later age.