ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297000
44 | ASPIRE , Spring 2013 F H WA S tructural lightweight concrete (LWC) has been used successfully in bridges and structures for a century. The benefits of using LWC are many and varied. The main benefit is in the reduction of dead load in: • allowing bridge deck widening with few or no changes to the existing structure, • improving seismic structural response, • designing longer-span bridges, • fabricating smaller structural elements and thinner sections, • lowering handling and transportation costs, and • reducing foundation and substructure costs. Lightweight aggregates for LWC may be from volcanic sources, byproducts from coal burning, or manufactured by expanding shales, clays, and slates (ESCS). ESCS lightweight aggregates are the most commonly used in modern LWC construction. ESCS aggregates are structurally strong, dimensionally stable, physically durable, light in weight, highly absorptive and retentive of water, environmentally friendly, and great at controlling cracks. Features of LWC LWC is concrete whose density has been r e d u c e d t h r o u g h t h e u s e o f l i g h t w e i g h t aggregates. For use in highway bridges and structures, LWC normally uses ESCS as coarse and/or fine aggregates within the concrete mixture proportions. LWC with compressive strengths up to and above 8 ksi are possible. In practice, a concrete with a density less than 135 lb/ft 3 is considered LWC. Concretes w i t h d e n s i t i e s b e t w e e n 1 2 0 a n d 1 3 5 l b / ft 3 commonly use a mixture of conventional and lightweight aggregates, while those below 120 lb/ft 3 use all lightweight coarse and fine aggregates. LWC with densities between 100 and 135 lb/ft 3 are appropriate for use in structural applications. The use of LWC in bridges does necessitate some special considerations. The AASHT O LRFD Bridge Design Specifications require that special reduction factors be applied to the resistance afforded by LWC. These provisions are in the process of being updated to accurately reflect the modern LWC mixture proportions commonly deployed in bridges. Use of LWC also requires special quality control processes, including the assurance of aggregate saturation at the initiation of concrete mixing. Research on Performance S t r o n g i n t e r e s t i n t h e u s e o f LW C f o r b r i d g e s h a s r e s u l t e d i n a s e r i e s o f r e s e a r c h p r o j e c t s t h a t h a v e a i m e d t o c h a r a c t e r i z e t h e s t r u c t u r a l p e r f o r m a n c e o f t h i s c l a s s o f c o n c r e t e . T h e N a t i o n a l C o o p e r a t i v e H i g h w a y R e s e a r c h P r o g r a m ( N C H R P ) P r o j e c t 1 8 - 1 5 t i t l e d " H i g h - P e r f o r m a n c e / H i g h - S t r e n g t h L i g h t w e i g h t C o n c r e t e fo r B r i d g e G i r d e r s a n d D e c k s ," l e d b y r e s e a r c h e r s a t Vi r g i n i a Te c h , wa s r e c e n t l y p u b l i s h e d a s N C H R P Re p o r t N o . 7 3 3 . T h i s e f f o r t f o c u s e d o n s t r u c t u r a l per formance of LWC with densities below 125 lb/ft 3 . The Federal Highway Administration (FHWA) research group also recognized the potential value of LWC and has recently completed a series of nearly 100 full-scale structural tests characterizing the shear, flexural, strand bond, and mild-steel bond performance of a variety of LWCs with densities of 125 to 135 lb/ft 3 . This density range is commonly referred to as specified density concrete and is commonly used in structural elements requiring both reduced weight and high performance. Curr ently, the FHWA r esear ch gr oup is w o r k i n g w i t h A A S H TO B r i d g e Te c h n i c a l Committee T-10 to compile r esult s fr om a l l r e l e v a n t s t u d i e s , d e v e l o p p r e d i c t i v e relationships, and draft proposals to update design recommendations pertaining to the variety of density ranges that occur within the LWC class of materials. The results of recent research are promising, indicating that some of the design restrictions placed on LWC decades ago may no longer reflect the performance that can be achieved with modern structural LWC. Lightweight Concrete in Highway Infrastructure by Ben Graybeal and M. Myint Lwin, Federal Highway Administration Shear test of lightweight concrete prestressed girder at Turner-Fairbank Highway Research Center. Photo: Gary Greene, PSI Inc. AspireBook_Spr13.indb 44 4/1/13 11:02 AM