Editor's Note: Ed Turner is dealing with some family health issues at the moment and will return shortly.
At the January, 2011 LGMI Conference in Atlanta, GA where the focus was on the identification of materials alternatives that would make paper machine systems lighter and greener, Dr. Reid W. Castrodale, VP of Technology for Carolina Stalite Company in Salisbury, NC presented information that leads to the conclusion that all concrete is NOT created equal.
Carolina Stalite Company is a producer of lightweight aggregates for use in structural and decorative concrete mixes. The aggregates are largely kiln-burned shale, clay, and slate that expand in a controlled and predictable way during the high temperature process. This process was patented in 1918 and has been widely used since. One of the first uses was for concrete ships constructed to support shipping during World War I (one still floats near Galveston, TX and others are in use near a Wood Yard on the Powell River in British Columbia). Both of the traffic decks on the San Francisco-Oakland Bay Bridge were manufactured using concrete containing lightweight aggregates or Light Weight Concrete (LWC)
OK LWC is different, but how different? By optimal use of lightweight aggregates concrete weights can be reduced from the “normal” 140-150lb/cu ft to as low as 95lb/cu ft—approximately a 35% reduction in weight while maintaining the functionality characteristics of the ‘normal’ mix. Densities can be controlled within this range by adjusting the mix according to the needs of the end use. Obviously this reduction can be of tremendous value in reducing transportation costs, handling and erection costs, and the cost of the structure needed to support the structure itself. The new Wembly, Australia stadium was constructed of pre-cast products using LWC to reduce these cost as well as eliminate the need for special cranes to lift the products and also to speed construction time to meet an especially tight schedule. Usually, these same goals apply in paper mill projects as well.
Dr. Castrodale reported that concrete using lightweight aggregates, due to lower modulus of elasticity, are less subject to microcracking and shrinkage cracking. He further reported that LWC exhibits up to a 30% reduction in thermal expansion. LWC has higher thermal insulating values than “normal” concrete mixes, up to a 59% reduction in Thermal Diffusivity. This characteristic leads to significant improvement in resistance to structural damage from fire. LWC’s higher resistance to heat transfer allows for a 20-25% reduction in structural thickness to achieve a 2hr fire rating. (In this case if fire rating is the controlling design parameter, a structure of LWC will be up to 51% lighter than a “normal” mix structure.) His data also indicated a significantly lower porosity which leads to a slower rate of moisture and external environment corrosives intrusion in the structure which may eventually corrode the reinforcing metals in the structure. On one structure exposed to frequent road salting, LWC materials exhibited a 50% lower chloride intrusion concentration at 1.0-1.5” depth than “normal” mixes. This testing showed chlorides had not penetrated beyond 2” depth in the LWC structures after 23 years of service. (Note; in most structures reinforcing metals are in excess of 2” from the surface of the concrete.)
In summary, LWC products should be considered by project managers who will need concrete and mixes that will provide
- Up to 35% reduction in the weight of the concrete structures involve with equal functional characteristic and the accompanying advantages
o Reduced size of structural elements
o Reduced size and/or number of foundation elements
o Reduced structural mass as related to seismic designs
o Reduced reinforcement and prestressing needs
- Up to 59% reduction in thermal diffusivity
- Near total elimination of early age cracking
- Significant reduction in penetration of atmospheric corrosives
- Demonstrated improved durability
- Reduced handling, transportation, and erection costs
After hearing Dr. Castrodale’s presentation, it is obvious that all concrete mixes are not created equal and that lightweight aggregates and lightweight concrete products can be a major contributor to LGMI goals.
