Philosophy of High Strength Cement/Concrete

Philosophy of High Strength Cement/Concrete

The world is passing through difficult and troubled times, and we live in a rapidly changing world. The construction industry is facing many challenges – global warming, climate change forces and the capability to achieve sustainable development and economic progress without damaging our environme...

The world is passing through difficult and troubled times, and we live in a rapidly changing world. The construction industry is facing many challenges – global warming, climate change forces and the capability to achieve sustainable development and economic progress without damaging our environment. The concrete industry in particular faces further challenges. There is extensive evidence to show that concrete materials and concrete structures all over the world are deteriorating at a rapid rate, and that we are unable to ensure their long-term durable service life performance. To confound this situation, we are also faced with an urgent need to regenerate our infrastructure systems if we are to eradicate poverty and provide a decent ‘quality of life’ for all the peoples of the world. Durability vs strength This paper shows that the current emphasis on high strength and very high strength, and the design philosophy of durability through strength for concrete materials and concrete structures is fundamentally flawed. It is this misleading concept and vision that is primarily responsible for the lack of durable performance of concrete in real life environments. To change this scenario, this paper advocates that concrete materials must be manufactured for durability and not for strength. It is shown that this concept of Strength through durability can be achieved through careful design of the cement matrix and its microstructure. If concrete is to be an eco-friendly, and sustainable driving force and construction material for social change, the need is to produce durable concrete with strengths of 30 to 60 to 80 MPa rather than very high strength concrete without an assured durable performance. Engineers are, by nature, fascinated and indeed obsessed, by high strength and very high strength concrete. Part of this fascination arises from the widely-held misconception that high strength concrete is, per se, highly durable. Making it last longer This intuitive association of strength with durability is again partly due to the current Ultimate Strength Design approach which creates an implicit belief and illusion that if concrete is proportioned to give high compressive strength, and then, if prescriptive code specifications in terms of cement content, water/cementitious materials (w/cm) ratios, types of cement, steel cover thickness and types and amounts of mineral and chemical admixtures are adhered to, then somehow the durable service life of the concrete structure will be automatically and adequately assured. The impetus for higher and higher concrete strengths also came from demands for exceptional increases in the height of high-rise buildings and for long span bridges. Indeed, the higher concrete strengths also brought in visible economics in terms of use of materials, increased usable space and shorter times of construction. As a result the latter part of the last century saw the development and use of concrete with compressive strengths from about 40 to 100/120 MPa and beyond. Holistic Durability Design Philosophy - an integrated material and structural design strategy - of strength through durability rather than of durability through strength where materials are manufactured for durability rather than for strength, and structures are designed for ductility and structural integrity. Holistic design envisages a global approach to all aspects of concrete and construction technology from material selection, design, construction, and maintenance to service life, integrating material characteristics with in situ performance. Use of cement replacement materials, design for material stability, design for structural integrity and design for sustainability are the key elements of this holistic durability design to achieve durable service life performance. About the author: Dr S B Hegde, Professor, Pennsylvania State University, United States of America.

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