Tuesday, 21 June 2011

The accelerated curing of concrete

The accelerated curing of concrete with saturated steam at atmospheric pressure or with radiant heat sources is widely practiced in many countries. These heating processes are commonly used Ln the production of plant-manufactured precast and prestressed concrete products to achieve early-age-strength development. Numerous research studies of accelerated curing have been published in the period from 1950 to 1980.The American Concrete Institute (ACI) Committee 517 published a report entitled "Low Pressure Steam Curing" in the August, 1963 AC1 Journal.This committee also prepared a "Recommended Practice for Atmospheric Pressure Steam Curing of Concrete" which was adopted as a standard in July, 1970. This committee also published a state of the art on accelerated curing of concrete in the November-December, 1980 AC1 Journal. These AC1 publications reference hundreds of different studies on accelerated curing of concrete materials.

 

The use of radiant heat as an alternate method of accelerating the curing of concrete became of widespread interest in the 1960's. Numerous methods of using radiant heat have been applied to the curing of precast, prestressed concrete. These radiant heat systems were developed to help reduce the energy losses associated with exhausting live steam to the atmosphere. The Prestressed Concrete Institute (PCI), along with the AASHTO Subcommittee on Prestressed Concrete prepared in 1973 a proposed change to the Steam Curing Specification which was contained in the AASHTO "Standard Specification for Highway Bridges", Division II, Construction; Section 4, Concrete Structures; Article 2.4.33, Prestressed Concrete; Sub-Article E, Steam Curing. The revised Article 2.4.33 was retitled "Accelerated Curing with Low Pressure Steam or Radiant Heat" and it contained a section "Curing with Low Pressure Steam", as well as a section "Curing with Radiant Heat".

 

This newly prepared AASHTO specification introduced for the first time the use of the ASTM C403-70 method to determine time of setting of concrete mixtures. This test technique had not been used previously in AASHTO, ACI, PC1 or other similar highway department specifications, although most specifications did require a waiting period or delay period prior to applying significant heat to the concrete.

 

This AASHTO specification also removed the requirement that the accelerated-cured concrete should not be exposed to temperatures below freezing for six days after accelerated curing, or that six days of additional water curing be provided after the accelerated curing. This proposed specification was adopted by AASHTO and was included in the 1974 Interim Specification, Bridges, as Interim No. 18. It was subsequently included in the 1977 AASHTO "Standard Specifications for Highway Bridges", Twelfth Edition, in Section 2.4.33, Section E. While the new AASHTU specification was a significant improvement over the earlier AASHTO, AC1 or PC1 specifications, the development of this specification occurred simultaneously with the beginning of the energy crisis which began during the period from 1973 to 1975. Thus, this new and current AASHTO specification does not incorporate the energy-saving concepts that the precast, prestressed concrete industry has developed in the period from 1975 to 1980 when numerous in-plant experiments dealing with energy conservation were made across the United States and Canada.

 

These in-plant tests and experiments have resulted in new concepts of using lower water/cement ratio concrete, high-range water reducers, efficient thermal barriers and short-cycle curing periods that result in significant energy conservation. The short-cycle curing periods were generally not researched in the last thirty year period primarily because energy was so inexpensive, and constituted a relatively insignificant factor in the cost of plant-produced precast concrete. The cost of accelerated curing has increased dramatically since 1975 and must now be accounted for in the product cost as an item of significance. In addition, if this energy becomes extremely limited or totally unavailable, the precast, prestressed concrete industry would then need to develop a curing technology and specifications which would allow routine daily production using energy-efficient curing techniques while still providing the highest quality and durability in the finished concrete product.

0 comments:

Post a Comment

Note: only a member of this blog may post a comment.

Related Posts Plugin for WordPress, Blogger...