After 100 Years, Is It Time To Use Lightweight Concrete On Railroad Bridges? Author

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1 After 100 Years, Is It Time To Use Lightweight Concrete On Railroad Bridges? Author Reid W. Castrodale, PhD, PE, President Castrodale Engineering Consultants, PC 419 Winfield Blvd SE, Concord, North Carolina, Number of Words 7570 ABSTRACT Structural lightweight concrete has been used in bridges and other structures for nearly 100 years, including the central portion of the main span of the longest concrete box girder bridges in the world. However, some owners, design engineers and contractors are reluctant to use this material because they lack an understanding of the material. A recent resurgence of interest in using lightweight concrete for highway bridges has generated new information about the properties of modern lightweight concrete that should interest the railroad community, especially when span lengths of standard concrete box beams are limited by on-track crane capacities. This paper will discuss a range of structural and durability properties of lightweight concrete made with lightweight aggregates manufactured in the US that have potential benefits for bridge structures. The discussion will include findings regarding lightweight concrete used for prestressed girders with design compressive strengths up to 10 ksi and unit weights around 120 pcf. Long-term performance of lightweight concrete related to creep and shrinkage and also durability will be explored. Use of lightweight concrete to reduce structure mass for bridges in seismic regions will be explored, including the ductility of lightweight concrete substructure elements. The potential advantages of lightweight concrete for mass concrete elements, because of its lower coefficient of thermal expansion and lower modulus of elasticity, will be discussed. An understanding of the properties of modern structural lightweight concrete will enable bridge designers to use the advantages of this material to achieve greater economy, increased durability and longer spans. INTRODUCTION The rotary kiln process for expanding structural lightweight aggregate was patented in 1918 and commercial production began in The initial use of lightweight aggregate in the US was for building several ships at the end of World War I. One of these ships, the USS Selma, is still visible in Galveston Bay. The first known use of lightweight aggregate in bridge construction was in 1922 (1). Shortly after that, lightweight concrete with an air-dry unit weight of 95 pcf was used to construct the upper deck of the suspension spans on the San Francisco-Oakland Bay Bridge in 1936 (2). The use of all lightweight concrete was credited with saving $3,000,000 out of the original $40,000,000 total construction cost for the two bay crossing bridges. That deck, which has always had a conventional concrete wearing surface on it, is still in service today. This brief introduction to the development and early uses of manufactured structural lightweight aggregate demonstrates that the material has been used in the US for nearly 100 years. However, it is not as widely accepted as would be expected for a material that has been available for that long. In the period from 1960 to 1970, shipments of lightweight aggregate in the US doubled. However, by the mid- 1970s, shipments were about half of the peak that occurred in This drop was caused by the oil 660 AREMA 2016

2 crisis and the introduction of emission controls, both of which caused the price of the aggregate to increase. With the increased costs and controls, many plants closed and the aggressive promotion and research that had been conducted by the industry since the early 1950s stopped. Many engineers forgot about lightweight concrete. Its use continued mainly in elevated floor slabs for buildings and for concrete masonry blocks, with only occasional use for highway bridge decks. A notable recent use of lightweight concrete was the completion in 1998 of the Stolma Bridge in Norway. The bridge has the longest span for a segmental concrete box girder bridge in the world, with the central 604 ft of the 988 ft main span constructed with lightweight concrete (3). Recently there has been a renewed interest in using lightweight concrete for highway bridge construction to increase efficiency in design, reduce structure mass for seismic locations, and allow for hauling and erecting longer girders or other heavy precast elements that are being used more and more for accelerated bridge construction (ABC). As a result, there have been a number of recent research efforts that have focused on learning more about the behavior of modern lightweight concrete (4,5). Owners, design engineers and contractors are often reluctant to use lightweight concrete for bridges because of their lack of experience with or knowledge about the material, even though information has been available for many years on the appropriate use of lightweight concrete (1). The recent test results and production experience for modern lightweight concrete that are presented in this paper reveal that the material, which in most cases has properties equal or superior to conventional concrete, offers significant opportunities for improved efficiency, economy and durability for bridges and other structures, even those exposed to extreme environmental conditions. LIGHTWEIGHT AGGREGATE Structural lightweight aggregate is produced in the US using shale, clay and slate as a raw material. The materials are expanded at high temperatures in a rotary kiln to produce a vitrified, porous aggregate (Fig. 1). The vitrified material has a hardness similar to quartz. Properties of lightweight aggregate from different manufacturing plants vary somewhat, but aggregate from all sources can be used for structural concrete. FIGURE 1 The bulk density of coarse lightweight aggregate ranges from about 45 to 55 pcf, and from about 60 to 70 pcf for fine aggregate. The aggregate particle density is greater for smaller sizes. The largest lightweight aggregate grading used in the US is ¾ in. Water absorption of lightweight aggregate (LWA) is significantly greater than the absorption of normal weight aggregate (NWA), with the absorption ranging from 6 to more than 25% by mass, depending on the source. Most of the pores in lightweight aggregate particles are not inter-connected, which results in a relatively low absorption for such a porous material. To obtain more consistent workability and hardened concrete properties, lightweight aggregate is generally prewetted prior to batching to satisfy its absorption. Currently, 15 plants are manufacturing structural lightweight aggregate in the US. They are distributed across the country. Many are located along rail lines or on rivers to facilitate delivery. All but two are members of the Expanded Shale Clay and Slate Institute (ESCSI), which has a website ( from which information on the producers and various uses of lightweight aggregate can be obtained. Internal Curing Lightweight Aggregate Particle The absorbed water in prewetted lightweight aggregate provides an internal curing effect because it is released within the concrete as cementitious materials hydrate and react. This effect has significant beneficial effects on concrete properties and can improve the tolerance of concrete to conditions that may cause early cracking and reduce its durability. While all types of lightweight concrete mixtures in which the aggregate has been prewetted prior to batching will provide internal curing, significant effort has been AREMA

3 directed toward developing the concept of replacing a portion of the fine aggregate in a conventional concrete mixture with prewetted fine lightweight aggregate to provide internal curing in concrete for which a reduction in density is not required. Weiss, et al. (6) have published a good introduction to this concept of internal curing of conventional concrete mixtures by using prewetted lightweight fines. Other Applications of Lightweight Aggregate While lightweight aggregate is typically used to make lightweight concrete, it is also useful for other applications that may be of value to the rail industry. These include using lightweight aggregate for a geotechnical fill (lower in-place weight and higher internal angle of friction) and storm-water filtration. Lightweight aggregate can be used to reduce settlements for embankments on soft soils or adjacent to other structures or to reduce design pressures on existing or new retaining structures. Lightweight aggregate was recently used in a water treatment facility for a Class I railroad locomotive shop. LIGHTWEIGHT CONCRETE Lightweight concrete (LWC) is made by using lightweight aggregate in a concrete mixture instead of conventional (normal weight) aggregate. When used to make concrete, lightweight aggregate is simply a lighter type of rock, so the same admixtures are used, and batching, placing and finishing can be accomplished using the same equipment. Lightweight concrete has been successfully pumped long distances or to high elevations when the lightweight aggregate has been prewetted before batching. With proper attention to mix design, lightweight concrete can have a high flow or be self-consolidating. The most common type of lightweight concrete in the US replaces all of the normal weight coarse aggregate with lightweight coarse aggregate to produce what is called sand lightweight concrete. A concrete mix in which all aggregate (both coarse and fine) is lightweight has the lowest density and is called all lightweight concrete. Lightweight and normal weight aggregates can be blended to achieve specified density concrete which can have any density between all lightweight concrete and normal weight concrete. Cost Lightweight concrete costs more than normal weight concrete because of the high temperature processing of the aggregate and increased shipping costs from a limited number of sources. It is difficult to give general cost estimates for lightweight concrete compared to normal weight concrete because of the many factors involved. However, sand lightweight concrete generally costs $20 to $50 / cu yd more than normal weight concrete; the cost of all lightweight concrete is usually somewhat less than twice the increase in cost for sand lightweight concrete. The higher cost includes the cost of the lightweight aggregate, but also increased quality control effort and increased risk since another factor (density) must be satisfied for acceptance of concrete. Lightweight aggregate should also be kept wet, although the effort required for this may not be much different than what is already done for other types of aggregate. The additional cost for lightweight concrete can be easily offset by savings in shipping and erection costs in many situations. It has also been shown that the quality of lightweight concrete is typically more consistent because of the additional quality control effort that is necessary. Material Properties Structural properties of lightweight concrete have been obtained from both research studies and production of lightweight concrete for structures. Properties measured in early tests sometimes showed a reduction in tensile and shear strengths and an increase in creep and shrinkage, so these relationships became accepted as typical and are the basis for current design provisions for lightweight concrete. However, recent test results for several properties of lightweight concrete are much closer to, or even exceed, results for normal weight concrete, as presented in the following (this behavior had also been observed in some earlier tests). Tests also show that lightweight concrete has essentially the same or even improved durability compared to normal weight concrete with the same quality and compressive strength. The modulus of elasticity and the coefficient of thermal expansion for lightweight concrete are reduced compared to normal weight concrete, but these properties can be beneficial in some situations. 662 AREMA 2016

4 While much information is available, the length of this paper is limited. Additional information can be obtained from the references cited. Data presented in this paper are for specific concrete mixes. The comparisons between normal weight and lightweight concrete that are shown may not be representative of all mixtures because concrete properties are strongly dependent on aggregate type, cement source, and other factors. However, the reported properties of lightweight concrete demonstrate the potential behavior of lightweight concrete. Density Normal weight concrete (NWC) typically has a density of around 145 pcf; sand lightweight concrete generally has a density in the range of 110 to 125 pcf; and all lightweight concrete can have a density as low as 90 pcf. For comparisons in this paper, densities of 145 pcf, 115 pcf and 100 pcf will be used for normal weight concrete, sand lightweight concrete and all lightweight concrete, respectively. Using these values, the reduction in density from normal weight concrete (including an additional 5 pcf for all concrete types to account for reinforcement) is 30 pcf or 20% for sand lightweight concrete and 45 pcf or 30% for all lightweight concrete. Because lightweight aggregate has a higher absorption, lightweight concrete typically loses mass with time as unbound water migrates out of the concrete. The mass loss is greater when higher absorption aggregates are used. Because this mass loss is more significant than for normal weight aggregates, the term equilibrium density has been defined as the density achieved after moisture loss has occurred with time in ASTM C567 (7). The equilibrium density is generally reached after about 90 days of drying. Designers often specify the equilibrium density because it is a good estimate of the concrete dead load. However, the fresh or plastic density of the concrete must also be known or specified to use for material acceptance at the time of placement, as well as for formwork and shoring design. For high strength lightweight concrete using low absorption aggregate, the reduction in density with drying is expected to be minor and could be neglected. In this situation, designers can specify only the fresh density. Figure 2 presents a frequency plot of fresh density measurements taken during the production of lightweight concrete prestressed girders (8). The target fresh density was 123 pcf and the maximum permitted fresh density was 128 pcf. The variation in density can be caused by a number of factors such as air content or the quantity of absorbed moisture in the lightweight aggregate. FIGURE 2 Frequency Plot of Density Measurements During Lightweight Concrete Prestressed Girder Production (8) AREMA

5 Compressive Strength Typical design compressive strengths are easily achieved with lightweight concrete. Several projects in the US have used lightweight concrete in pretensioned bridge girders with design compressive strengths in the range of 8.5 to 10 ksi (8,9,10). The procedure used to design a lightweight concrete mix for a specified compressive strength is the same as for a normal weight concrete mix. Experience with field production of lightweight concrete has shown strength gain with time is comparable to normal weight concrete. Lightweight concrete mixes generally require a greater quantity of cementitious materials to achieve the same compressive strength. Tensile Strength It has long been assumed that the tensile strength of lightweight concrete is less than the tensile strength of normal weight concrete with the same compressive strength. As a result, design properties of lightweight concrete related to tensile strength have been reduced in design specifications for quantities such as shear and development length. However, recent tests indicate that tensile strengths for normal and high strength lightweight concrete may exceed the tensile strength assumed in the specifications for normal weight concrete with the same compressive strength. Recent test data on lightweight concrete bridge deck mixes reported by Byard and Schindler (11) illustrate this point. Researchers used three sources of lightweight aggregate for their test mixes. Test data in Table 1 show that the splitting tensile strength of lightweight concrete bridge deck mixes for the three lightweight aggregate sources and types exceeded the splitting tensile strength for the normal weight concrete control mix. Assuming a design compressive strength of 4,000 psi, the tensile strengths of lightweight concrete shown in Table 1 also exceed the expected splitting tensile strength for normal weight concrete of 6.7 f c = 424 psi. TABLE 1 Splitting Tensile and Compressive Strengths of Deck Concrete Mixtures (11) Splitting Tensile Strength (psi) Compressive Strength (psi) Aggregate Type NWC Sand LWC All LWC NWC Sand LWC All LWC NWA Slate LWA Clay LWA Shale LWA AREMA and AASHTO bridge design specifications (12,13) give designers the option to compute the design modification factor for lightweight concrete based on a specified splitting tensile strength. If the splitting tensile strength specified is equal to the expected tensile strength for normal weight concrete, a lightweight concrete member can be designed with a reduction factor equal to one for shear and other quantities related to concrete tensile strength so no reduction would be made and the design would be the same as if the concrete were normal weight concrete. Based on the data in Table 1, this approach could be taken by specifying a minimum splitting tensile strength of 424 psi for the lightweight concrete. 664 AREMA 2016

6 The splitting tensile strength of high strength lightweight concrete has also been shown to exceed the expected tensile strength for normal weight concrete with the same strength. Table 2 shows data for three projects for which the same high strength lightweight concrete mix was used (8). For all three projects, the specified compressive strength was 9,000 psi; therefore, the predicted tensile strength of normal weight concrete based on the specified compressive strength was = 638 psi, as shown at the bottom of the table. The average measured splitting tensile strength for all three projects was greater than 638 psi, and the minimum measured splitting tensile strength fell below this limit for only one of the projects. For additional information, the average compressive strengths for each project are also shown in the table, as well as the predicted splitting tensile strength computed using the average compressive strength. TABLE 2 Splitting Tensile Strength of Deck Concrete Mixtures (psi) (8) Project AWS Skagit SR162 Count Average Minimum Maximum Range Standard Deviation Average 28-day f' c 11,578 10,832 11,845 Predicted f ct from Average f' c Specified 28-day f c 9,000 9,000 9,000 Predicted f ct from Specified f' c AWS = Airport Way South (Prestressed Double Tees) Skagit = Skagit River Bridge (Prestressed Decked Bulb-Tee Girders) SR162 = SR162 Puyallup River Bridge (WF74G Prestressed Girders) Modulus of Elasticity The porous nature of lightweight aggregate reduces its stiffness, which in turn reduces the modulus of elasticity of lightweight concrete. The effect of concrete density on modulus of elasticity has been included in the equation used in US design codes since the 1960s. A new equation for the modulus of elasticity has recently been adopted in the AASHTO LRFD Specifications (13) that still includes the density of concrete, but the effect of changing density is greater because the exponent on the density has been changed from 1.5 to 2. The new modulus of elasticity equation is expected to give an improved estimate for lightweight concrete and for high strength concrete (14). Using the new equation, sand lightweight concrete with a density of 115 pcf is estimated to have a modulus of elasticity that is (115/145) 2 = 63% of the modulus for normal weight concrete with the same compressive strength and a density of 145 pcf; for all lightweight concrete with a density of 100 pcf, the modulus is estimated to be (100/145) 2 = 48% of the modulus for normal weight concrete. AREMA

7 Figure 3 is a plot of modulus of elasticity test data versus compressive strength collected as part of NCHRP Project (15). The wide scatter in data is typical of modulus of elasticity results for concrete. Points are color coded to identify the density of the concrete. Lines are plotted on the figure to represent values for the new AASHTO equation for modulus of elasticity for three densities of concrete corresponding to normal weight, sand lightweight, and all lightweight. The equation appears to be a reasonable lower bound for the data. FIGURE 3 Measured Concrete Modulus of Elasticity with Plot of New AASHTO Equation (15) 666 AREMA 2016

8 Creep and Shrinkage As with tensile strength, it has generally been assumed that the creep and shrinkage of lightweight concrete is greater than for normal weight concrete with the same quality and compressive strength. Early data indicated this relationship. However, recent tests of high strength lightweight concrete used for prestressed concrete girders (8,9,10) show that creep and shrinkage of higher strength lightweight concrete are in the range of normal weight concrete, as illustrated in Figures 4 and 5 for concrete mixtures with similar design compressive strengths. It is expected that the internal curing effect of the lightweight aggregate greatly reduces the early age (autogenous) shrinkage that is usually large in high strength mixes. FIGURE 4 Creep Coefficient and Specific Creep for Lightweight and Normal Weight Concrete (8) FIGURE 5 Shrinkage of Lightweight and Normal Weight Concrete (8) AREMA

9 The same studies mentioned above also show that prestress losses for lightweight concrete girders are within the expected range for normal weight concrete. This finding allows current code expressions for estimating creep, shrinkage and prestress losses to be used for lightweight concrete without modification. Thermal Properties The coefficient of thermal expansion for structural lightweight concrete is less than for normal weight concrete with the same strength. This is recognized by the AASHTO LRFD Specifications (13) which provides values for both normal weight and lightweight concrete: 6 x 10-6 / deg F and 5 x 10-6 / deg F, respectively. The reduced thermal expansion is beneficial for long-span structures and may also have potential for controlling thermal effects in mass concrete placements. Tests results for different types of typical deck concrete mixtures using lightweight aggregate from three sources (11) indicate that the coefficients of thermal expansion for sand and all lightweight concrete mixtures were about 80% and 65%, respectively, of the coefficient for the normal weight concrete mixture, as illustrated in Table 2. In addition to a reduced coefficient of thermal expansion, lightweight concrete responds to changes in the ambient temperature more slowly than does normal weight concrete. The combination of these factors would lead to a significant reduction in the thermal expansion and contraction of the deck in long bridges. TABLE 3 Coefficient of Thermal Expansion ( / F) (11) Aggregate Type NWC Sand LWC All LWC NWA 6.2 Slate LWA Clay LWA Shale LWA The combined effects of the reduced coefficient of thermal expansion and the reduced modulus of elasticity for lightweight concrete are expected to provide benefits for mass concrete placements by allowing a greater tolerance to temperature differentials that occur within mass concrete. Research sponsored by ESCSI is currently underway to study this potential application of lightweight concrete. Ductility for Seismic Applications Lightweight concrete is an obvious option for bridges designed for seismic loadings because its reduced weight decreases the seismic demand on a structure. However, material tests and field experience show that lightweight concrete is typically more brittle (less ductile) than normal weight concrete with the same compressive strength. Therefore, lightweight concrete was expected to have limited use for structural elements where ductility is required during seismic or other extreme loading events. Tests of lightweight concrete bridge piers subjected to seismic loading by Kowalsky, Priestley and Seible (16) have demonstrated, contrary to expectations, that lightweight concrete, when properly detailed, will perform as well as normal weight concrete A later series of tests reported by Hendrix and Kowalsky (17) concluded that the strength of the lightweight concrete shear-resisting mechanism appears to be lower than the normal-strength mechanism when subjected to reversed cyclic loads so strength reduction factors were proposed for use in design. However, the researchers recognized that, while the proposed concrete strength reductions were significant, the reduced mass of a lightweight concrete structure would reduce the seismic lateral forces and the reduction in shear demand will more than compensate for the reduced strength of the concrete shear-resisting mechanism. 668 AREMA 2016

10 Durability Long-term durability is always a concern for railway bridges. While the durability of concrete depends on a wide range of factors, the two primary factors are permeability and cracking. Compared to normal weight concrete with the same quality and compressive strength, lightweight concrete has been shown to have reduced cracking and a more complete reaction of cementitious materials that reduces permeability. More information on the durability of lightweight concrete can be found in Castrodale and Harmon (18). Properties of lightweight concrete that contribute to enhanced durability include: Internal curing effect from prewetted lightweight aggregate that improves hydration of cement and allows more complete reaction of supplementary cementitious materials for more effective use of cementitious materials and reduced permeability Improved bond between lightweight aggregate particles and the paste that is both mechanical and chemical, which is due to slight pozzolanic activity of lightweight aggregate Improved quality of the interfacial transition zone (ITZ) [which is the interface between the aggregate and paste] that significantly reduces its porosity that contributes to increased permeability in conventional concrete Reduced modulus of elasticity that contributes to reduced cracking potential Elastic compatibility between lightweight aggregate and paste that provides a more homogeneous stiffness of the concrete composite, reducing micro-cracking around aggregate particles that increases permeability in conventional concrete Reduced coefficient of thermal expansion An example of concrete permeability test results for a sand lightweight concrete highway bridge deck follow (19): the specifications required a maximum permeability of 2,500 coulombs using an accelerated version of the rapid chloride permeability test. The average permeability measured was 989 coulombs, which was based on 17 samples over the 6-month construction duration. The maximum and minimum test results were 1,467 and 593, respectively. Lightweight concrete has very good freeze-thaw resistance when properly proportioned, batched and placed. This seems counter intuitive since lightweight aggregate is more porous. However, the pores are not all connected, so water cannot penetrate deeply into the aggregate. Other features of enhanced durability mentioned above also contribute to the freeze-thaw durability of lightweight concrete. It has performed well on bridge decks in northern climates. The improved cracking resistance of lightweight concrete bridge deck mixtures is illustrated in Fig. 6 for concrete placement scenarios in a) Fall (73 deg F) and b) Summer (95 deg F) (11). The plots show the stress that developed in restrained concrete specimens when subjected to the heat of hydration and daily temperature variations that are typical of an 8 in. thick bridge deck. Lines for each type of concrete end when the restrained concrete cracks. These data show that cracking was significantly delayed for lightweight concrete mixtures made with an expanded slate lightweight aggregate, when compared to normal weight concrete. Both the sand lightweight concrete (SLW) and all lightweight concrete (ALW) did not crack within 4 days after placement, so the researchers forced them to crack by reducing the concrete temperature. The mix designated IC is a concrete mixture in which a portion of the conventional fine aggregate was replaced with prewetted lightweight aggregate fines to provide internal curing. The research report also includes test results using two other sources of lightweight aggregate (shale and clay) with very similar results. AREMA

11 FIGURE 6 Restrained Stress Development for Lightweight Concrete: a) Fall and b) Summer Placement Scenarios (11) APPLICATIONS OF LIGHTWEIGHT CONCRETE FOR RAILWAY STRUCTURES The reduced density of lightweight concrete provides benefits for several types of rail structures. Prestressed Concrete Slab Spans, Box Beams and Girders Lightweight concrete has been used for highway bridge decks since the 1920s and for prestressed concrete girders since at least the 1960s (1). Prestressed concrete box beams and girders are commonly used on railway structures. For highway structures, lightweight concrete can be used for bridge girders and decks to extend span ranges, but for railway structures, the live load is much larger so the reduced dead load does not make a significant difference in most designs. Therefore, the main justification for using lightweight concrete is usually for improvements in handling and erection. If sand lightweight concrete is used, the weight of an element is reduced about 20%, or the span length could be increased approximately 20% for the same weight. Lighter elements may also reduce shipping and erection costs. Precast Concrete Elements Precast concrete elements are widely used to reduce construction time for railway structures. These elements, such as bent caps, abutments, backwalls, and grade beams, are usually large and heavy. A pile cap for a typical trestle may be 24 ft x 4.5 ft x 5 ft. Neglecting any voids, such a cap would weigh 81,000 lbs using normal weight concrete. With sand lightweight concrete, the cap would weigh 20% less, or 64,800 lbs. Using all lightweight concrete, the cap would weigh 30% less, or 56,700 lbs. While lightweight concrete costs more, the reduced weight can provide savings in shipping and erection. 670 AREMA 2016

12 The author spoke with a precaster who proposed using lightweight concrete pier caps to reduce shipping costs for a Class I Railway trestle replacement project. The reduced weight will allow the shipper to haul the caps using a blanket permit instead of requiring an overweight permit for each cap. The contractor liked the idea because it will make cap installation easier. The precaster had already used lightweight concrete on an earlier project for the same railway. Seismic Applications In regions where seismic loading must be considered in design, lightweight concrete will reduce the structure mass, which reduces the seismic demand on foundations and substructures. This benefit has been used for highway bridges, including the Benicia-Martinez Bridge in California, which is a 7,434-ftlong segmental concrete box girder with main spans of 659 ft (20). The box girder superstructure is sand lightweight concrete except for the pier tables. In a conversation with the author, one of the lead designers indicated that the entire box girder, as well as the pier columns, would have been lightweight concrete if seismic research (16) had been completed in time. Other Uses of Lightweight Concrete in Superstructures Lightweight concrete may provide benefits for several other applications in railway bridge superstructures. For long span bridges with ballast decks, the deck could be constructed with lightweight concrete to reduce the dead load of the structure, improving the capacity of the structure or retaining the load rating when a replacement superstructure may weigh more than the existing structure and the substructure is being reused. Lightweight concrete could also be used for ballast curbs and walkways added to precast concrete beams or slabs. For decks on through girder bridges where the bottom flange is in the tension zone of the bridge, the lower stiffness of lightweight concrete should reduce cracking because the strains to which the deck slab are subjected during passage of a train cause less stress in the lightweight concrete compared to a normal weight concrete deck, reducing the cracking potential. Mass Concrete Applications With a lower coefficient of thermal expansion and lower modulus of elasticity, lightweight concrete has a potential advantage for mass concrete elements. For example, using sand lightweight concrete typically provides a modulus of elasticity of 65% of normal weight concrete and a coefficient of thermal expansion of 80% of normal weight concrete. For a given change in temperature, the tensile stress developed in concrete is proportional to the product of the coefficient of thermal expansion and the modulus of elasticity, so the stress in lightweight concrete would be 65% x 80% = 52% of the stress in normal weight concrete. Because of its insulating qualities, the initial peak temperature in lightweight concrete will be greater than for normal weight concrete, but the increase is more than offset by the reduced stress caused by the change in temperature. Lightweight concrete has not yet been used to reduce the cracking potential of mass concrete, but research is underway and results appear promising. High-Early Strength Concrete Applications When concrete is placed in situations where it must gain strength rapidly so a structure can return to service quickly, it often suffers from early deterioration. The high cement content required to achieve high strengths at early ages often results in high shrinkage that causes cracking. Using lightweight aggregate in such mixtures can provide three benefits: 1) reduced shrinkage from the internal curing effect 2) improved hydration of cementitious materials which cannot be cured using conventional methods since they must be reopened to traffic at a very early age 3) reduced modulus of elasticity of lightweight concrete which reduces the stress and possible cracking that may result from any shrinkage strains that may develop AREMA

13 CONCLUSIONS The information presented on lightweight aggregate, lightweight concrete, and the material properties of lightweight concrete demonstrate that lightweight concrete is a proven material that can be used in a number of ways for railway structures. Lightweight concrete can have an immediate benefit for precast concrete elements that may be limited in size because of weight or to reduce shipping costs and facilitate installation. Guidelines for its use are already provided in the AREMA Manual for Railway Engineering (12); however, its provisions related to lightweight concrete design should be updated and expanded. So, nearly 100 years after its introduction, it appears that now is the time to begin to use lightweight concrete for railway structures. REFERENCES 1 T.Y. LIN INTERNATIONAL, Criteria for Designing Lightweight Concrete Bridges, Report No. FHWA- RD , Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 1985, 153 pp. 2 WOODRUFF, G.B., Light Weight Concrete Pavement on the San Francisco-Oakland Bay Bridge, ACI Journal, Proceedings 34(1), 1938, pp MELBY, K., Long Span Bridges with Lightweight Concrete in Norway, Nordic Road & Transport Research, 15 (2), 2003, pp COUSINS, T., ROBERTS-WOLLMANN, C. and BROWN, M.C., High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks, Report 733. National Cooperative Highway Research Program, Transportation Research Board, Washington, DC, 2013, 81 pp. 5 LOPEZ, M., KAHN, L.F. and KURTIS, K.E., Creep and Shrinkage of High-Performance Lightweight Concrete, ACI Materials Journal, 101(5), 2004, pp WEISS, J., BENTZ, D., SCHINDLER, A. and LURA, P., Internal Curing, STRUCTURE 19(1), 2012, pp ASTM C567-05, Standard Test Method for Determining Density of Structural Lightweight Concrete, ASTM International, West Conshohocken, PA, 2005, 3 pp. 8 CHAPMAN, D.D., and CASTRODALE, R.W., Sand Lightweight Concrete for Prestressed Concrete Girders in Three Washington State Bridges, Paper 81, National Bridge Conference, Nashville, TN, Precast/Prestressed Concrete Institute (PCI), Chicago, IL, 2016, 22 pp. 9 LILES, P. and HOLLAND, R.B., High Strength Lightweight Concrete for Use in Precast, Prestressed Concrete Bridge Girders in Georgia, HPC Bridge Views, 61, 2010, pp HOLLAND, R.B. and KAHN, L.F., High Strength Lightweight Concrete Properties of the I-85 Ramp over State Route 34, HPC Bridge Views, 61, 2010, pp BYARD, B.E. and SCHINDLER, A.K., Cracking Tendency of Lightweight Concrete, Highway Research Center, Auburn, AL, 2010, 82 pp. 12 AREMA Manual for Railway Engineering. Lanham, MD: American Railway Engineering and Maintenance-of-Way Association (AREMA). 13 AASHTO AASHTO LRFD Bridge Design Specifications. 7 th Ed. Washington, DC: American Association of State Highway and Transportation Officials (AASHTO). 14 GREENE, G.G. and GRAYBEAL, B.A., Lightweight Concrete: Mechanical Properties, Report No. FHWA-HRT , Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 2013, 176 pp. 15 RIZKALLA, S., MIRMIRAN, A., ZIA, P., RUSSELL, H. and MAST, R., Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Flexure and Compression Provisions, 672 AREMA 2016

14 Report 595. National Cooperative Highway Research Program, Transportation Research Board, Washington, DC, 2007, 28 pp. 16 KOWALSKY, M. J, PRIESTLEY, M. J. N. and SEIBLE, F., Dynamic Behavior of Lightweight Concrete Bridges, ACI Structural Journal, 97(4), 2000, pp HENDRIX, S. E. and KOWALSKY, M. J., Seismic Shear Behavior of Lightweight Aggregate Concrete Square Columns, ACI Structural Journal, 107(6), 2010, pp CASTRODALE, R.W. and HARMON, K.S., Durability of Lightweight Concrete Bridges, Paper 48. National Bridge Conference, Orlando, FL, Precast/Prestressed Concrete Institute (PCI), Chicago, IL, 2008, 35 pp. 19 CASTRODALE, R. W., and ROBINSON, G. M., Performance of Lightweight Concrete Bridge Decks Paper 75. Concrete Bridge Conference, St. Louis, IL, National Concrete Bridge Council, Chicago, IL, 2008, 15 pp. 20 MURUGESH, G. and CORMIER, K., When Lighter is Better, ASPIRE, 1(3), 2007, pp AREMA

15 After 100 Years, Is It Time To Use Lightweight Concrete On Railroad Bridges? Reid W. Castrodale, PhD, PE Castrodale Engineering Consultants Concord, NC 674 AREMA 2016

16 Introduction to LWA and LWC Lightweight Aggregate (LWA) Manufactured product Process developed in early 1900s not new! Just a lighter rock Lightweight Concrete (LWC) LWA is used to reduce density of concrete Material properties Uses of LWC for railway structures Many more details are given in the paper! Development of LWA In early 1900s, Stephen Hayde discovered how to manufacture lightweight aggregates from shale, clay and slate Some bricks bloated during burning Development of rotary kiln process began in 1908 in Kansas City, MO Patent for expanding aggregates using the rotary kiln process was granted in 1918 First use of LWC was for several ships in World War I USS Selma Galveston Bay Development of LWA LWC used on San Francisco-Oakland Bay Bridge built in 1936 Used all LWC (95 pcf air dry) for upper deck of suspension spans Lower deck was reconfigured for highway traffic in 1958 using LWC Both decks are still in service today - protected by wearing surfaces LWA is a manufactured product Raw material is shale, clay or slate Heated in kiln to about 2200 deg. F Gas bubbles form in softened material Gas bubbles remain after cooling Clinker is crushed and screened Properties of LWA Vitrified ceramic material Hardness equivalent to quartz Pores reduce density, but have limited connectivity Results in increased absorption But does not act like a sponge Properties of LWA Relative density for rotary kiln expanded LWA Range from 1.3 to 1.6 Relative density for normal weight aggregates (NWA) Range from 2.6 to 3.0 A particle of expanded slate LWA soaked in water with fluorescent yellow dye for 180 days, then split open. Absorption at time of test was 8% by mass. Twice the volume for same mass Half the mass for the same volume 1 lb. of each aggregate AREMA

17 Absorption of LWA Typical absorption (24 hours) Natural (Pumice) 30% Expanded Clay 20% - 40% Expanded Shale 14% - 30% Expanded Slate 6% - 10% LWA is generally pre-wetted before batching Critical for workability and pumping Necessary for internal curing Gradations of LWA After crushing and screening, LWA fractions are blended for Uniformity of specific gravity Optimal gradation LWA conforms to ASTM C330 gradations and other properties Coarse LWA sizes shown Fine LWA (sand) is also available MRE Chapter 8 Art /4" 1/2" 3/8 does not allow LW fines 5/16" Fines ESCS Manufacturing Plants in US Uses of LWA Other than LWC Geotechnical fill low density, high angle Storm & waste water filtration media 15 plants in the US See for locations of member companies NS DeButts Diesel Shop Using LWA to make LWC Replace some or all NWA with LWA LWA is just a lighter rock Same batch plants and mixing procedures Same admixtures Can use same mix design procedures Roll-o-meter for measuring air content Cost of LWC Increased cost of LWA High-temperature processing Shipping from the manufacturing plant LWA has higher absorption than NWA Prewet aggregate, especially for pumping Prewetting also provides internal curing Benefits can compensate for cost 676 AREMA 2016

18 Relative Cost Comparison Concrete Technology Corp., Tacoma, WA LWC mix was used by design/build team for PS deck girders to replace span on I-5/Skagit River Bridge 123 lb/cf & f' c = 9 ksi LWA was shipped by rail from Charlotte, NC to Tacoma, WA LWC Material Properties Density Compressive Strength Tensile Strength Modulus of Elasticity Creep and Shrinkage Thermal Properties Ductility for Seismic Designs All of these topics are discussed in the paper LWC Material Properties Density Compressive Strength Tensile Strength Modulus of Elasticity Creep and Shrinkage Thermal Properties Ductility for Seismic Designs Spectrum of Densities for LWC All LWC and Sand LWC are defined in ACI 318 Only Sand LWC currently allowed in MRE All LWC Sand LWC NWC LW Fine NW Fine NW Fine LW Coarse LW Coarse NW Coarse Density ranges (in lb/cf) are approximate and reflect the variety in LWA types Spectrum of Densities for LWC New definition in AASHTO LRFD Bridge Specs No more All LWC or Sand LWC LWC NWC [min. 95] < LW Fine NW Fine NW Fine LW Coarse NW Coarse NW Coarse Densities shown are lb/cf Blend LWA & NWA to achieved desired density Design Compressive Strength of LWC Minimum compressive strength by ASTM C330 2,500 psi Most LWAs can achieve 5,000 psi Some LWAs may achieve 7,000 to 10,000 psi Consult concrete or LWA supplier for information on mix designs with desired strength and density AREMA

19 Compressive Strength of LWC Data from Concrete Tech LWC girders ASTM C39 Creep of LWC Data from Concrete Tech LWC girders ASTM C512 Compressive Strength (psi) LWC NWC Creep Coefficient NWC LWC Age (days) Age (days) Shrinkage of LWC Data from Concrete Tech LWC girders ASTM C157 Shrinkage (%) NWC LWC Durability of LWC A major concern for transportation structures LWC has major benefits Internal curing Improved bond between LWA & paste Improved quality of interfacial transition zone Elastic compatibility between LWA & paste Reduced modulus of elasticity Reduced coefficient of thermal expansion Age (days) These result in reduced permeability & cracking Cracking Tendency of LWC Results from Tests at Auburn Univ. Concrete Stress (psi) Concrete Stress (psi) Concrete Age (hr) Spring 73 F Summer 95 F Uses of LWC for Railway Structures PS Concrete Slabs Spans, Box Beams & Girders Precast Concrete Elements Seismic Applications Other Uses of LWC in Superstructures Mass Concrete High-Early Strength Concrete All of these topics are discussed in the paper Concrete Age (hr) 678 AREMA 2016

20 Uses of LWC for Railway Structures PS Concrete Slabs Spans, Box Beams & Girders Precast Concrete Elements Seismic Applications Other Uses of LWC in Superstructures Mass Concrete High-Early Strength Concrete Prestressed Concrete Beams & Slabs Reducing the dead load of highway bridge elements by using LWC improves span capabilities This benefit is not available for railway structures because of the much greater live loads However, LWC can still be used to reduce the weight of prestressed concrete beams and slabs Span lengths can be increased if length is limited by rail-mounted crane capacities Hauling and erecting costs can be reduced Precast Concrete Elements LWC can be used to reduce the weight of precast concrete elements Pile caps Abutments & Backwalls Reducing weight reduces hauling & erection costs Example: 24 x4.5 x5 pile cap weighs 65 kips with sand LWC (115 lb/cf) instead of 81 kips PC caps made in AL for project in SC could be trucked using blanket permit instead of special permits when LWC was used Savings more than paid for extra cost of LWA Mass Concrete Properties of sand LWC compared to NWC Reduced modulus of elasticity (E c ) About 70% of NWC Reduced coefficient of thermal expansion (CTE) About 85% of NWC Nearly the same tensile strength Considering only these parameters: for a given temperature change, stress in sand LWC would be 70% x 85% = 60% of the stress in NWC Cracking tendency is significantly reduced Simplified comparison shows potential for LWC High-Early Strength Concrete Used for repairs and closure joints These mixes usually have a very low w/cm Prone to high shrinkage and cracking Adding prewetted LWA to the mix provides internal curing by carrying curing water into the concrete Reduced shrinkage & E c reduced cracking Improved hydration reduced permeability Result is more durable concrete Internal Curing with Prewetted LWA Test pour for 10Mgal tank - Highlands Ranch, CO Internal Curing vs. No Internal Curing Concrete placed at 92 F air temp. & 20% RH No conventional curing of any type was applied With internal curing Without internal curing One day after placement AREMA

21 Conclusions Using LWC for railway structures can make sense Potential savings in time and money Immediate benefits for some precast elements MRE Chapter 8 should be reviewed for possible updates regarding LWC It Is Time To Use LWC On Railroad Bridges! Thank you Reid W. Castrodale, PhD, PE Castrodale Engineering Consultants Concord, NC reid.castrodale@castrodaleengineering.com 680 AREMA 2016