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CONEX Use of Shrinkage Compensating Concrete (SCC) In Pre-Stressed Concrete PRE-STRESSED CONCRETE: There are characteristics of shrinkage compensating concrete (SCC) that are similar to the objectives and methodology behind pre-stressed concrete. Pre-stressed concrete is defined as a concrete member with a pre-determined compressive force, or moment, built into the member so that the internal stresses, designed as a result of the members intended use, will be equal to or less then the pre-stressing stresses built into the member. Post-tensioning and pre-tensioning are methods of achieving pre-stressed concrete. The objectives and methodology of using SCC to enhance the properties of concrete is very similar to the objectives and methodology of using pre-stressed to accommodate structural loadings. Shrinkage cracking control, combined with the other inherent advantages of SCC, make SCC a better material for pre-stressed concrete members. SHRINKAGE COMPENSATING COMPONENT: ACI recognizes two methods of achieving SCC, ettringite crystal development or calcium hydroxide platelet crystal development. The inherent characteristics of the calcium hydroxide platelet system, developed by the use of CONEX, is the better system of the two due to its inherent likeness of chemical action during hydration of the cement in use. The advantages of the platelet SCC method makes it well suited for use in pre-stressed concrete in general and in post-tensioned concrete in particular. DESIGN CRITERIA: The primary design objective of pre-stressed concrete is to place a compressive force in the concrete member that would prevent the concrete from going into tension and failing under design load conditions. Failure usually being defined as tension or stress cracking occurring in the concrete member. The primary objective for using pre-stressed concrete is economic. This is due to the fact that a pre-stressed member, of the same physical dimensions as a conventional reinforced concrete member, will have a greater load carrying capability. Conversely, for a given design loading, a pre-stressed member will be smaller in dimension and weight then a conventional concrete reinforced member. SCC - HOW IT WORKS: Shrinkage compensating concrete (SCC) has a case history of placements free of shrinkage cracks due to the “pre-stressing” action (restrained expansion or RE) created within the concrete during hydration and curing. During this period several phenomena are taking place simultaneously within the concrete. The most important being expansion of the concrete matrix due to the chemical reaction of CONEX creating development of calcium hydroxide crystals, and bonding of the concrete to the reinforcing. While this is occurring, the RE created causes the concrete to go into compression. The calcium hydroxide system of expansion, that is the formation of the platelets that create the expansion, is approximately at the same rate as the curing of the concrete. As long as compressive stresses within the concrete are greater then the tensile stresses the concrete will remain in compression, and tension cracks will not appear. This characteristic is taken advantage of by using SCC in the construction of cast in place slabs on grade (warehouse floors, pavements, secondary containment structures for hazardous materials) and structural members (bridge decks, primary containment structures, buildings, etc.). The benefit of SCC in slabs on grade is the ability to place larger sections (i.e. 20,000 ft2 / 2,000 m2) without contraction joints and with a reduction, and often elimination, of edge curling. The advantage of using SCC in bridge deck and containment structures is the increase in imperviousness of the concrete as well as the lack of shrinkage cracking. SCC AS APPLIED TO PRE-STRESSED CONCRETE: Currently the use of SCC is expanding into the pre-cast industry and also into pre-stressed concrete applications. While there are examples of cast in place pre-stressed applications of SCC, it is still a long way off from being in general use. This is perhaps due to the lack of published data detailing the use of SCC in post-tensioned applications. Tests should be done to establish the required expansion for desired results. These tests would establish the amount of compressive stress in the member resulting from different dosages of CONEX and related degrees of post-tensioning. This would allow other member characteristics to be adjusted accordingly. An obvious goal of using CONEX is to increase the quality of an existing concrete product and/or reduce the production cost of that concrete product. The addition of CONEX will increase the internal compressive stresses in the post-tensioned concrete member(s) if the proper restraint is provided. How this will impact the member design and/or production methods will need to be developed, but it presents a different look at the potential of shrinkage compensation in pre-stressed concrete. CONCLUSION: In general, CONEX SCC is similar to and compatible with pre-stressed concrete, and acts interdependently with the cement in use, developing the following advantages: 1. A pre-stressed member with a higher degree of internal compression to assure greater crack control. 2. More impermeable concrete. 3. Better edge finishing. 4. Possibly a way to reduce production costs through less breakage and discard. Written by: Lawrence J. Valentine, PE Technical Field Representative IAI | What is CONEX | Chemistry | Concrete Characteristics | Facts about SCC | CONEX & Floor Slabs | Pre-Stressed | Links | | Return Home | CONEX | Projects | What's New | Our Services | Contact Us | |
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