CONCRETE BRIDGE TECHNOLOGY Shasta Viaduct Arch Bridge: Improved by Pumping a Lightweight Concrete Solution

By Ric Maggenti, California Department of Transportation (retired), and Sonny Fereira, California Department of Transportation

The Shasta Viaduct Arch Bridge is two alignments are parallel. Given the connecting the two ribs were also located on southbound Interstate 5 steep hillside, each rib is footed on carbon-fiber wrapped to improve the (I-5), in Shasta County, Calif., about 15 thrust blocks at differing elevations arch struts’ performance. miles north of Redding and just north of and the chord lengths of each rib vary Lake Shasta. This new bridge replaces accordingly. Thus, the left rib spans 366 Lightweight Concrete the 78-year-old cast-in-place (CIP), ft while the right rib spans 400 ft. Mixture Proportions conventionally reinforced concrete The original NWC 28-day design

Sidehill Viaduct. The new arch bridge, The asymmetric ribs complicated the compressive strength f'c of 4000 psi was which was partially opened to traffic design analysis. After the initial design increased to 5500 psi for the LWC. The in July 2019, is part of an improved and completion of the arch ribs and increased strength eliminated any need horizontal alignment at that portion of columns, the structure was evaluated to adjust the shear, creep, modulus of southbound I-5. An arch support system using a nonlinear, second-order analysis elasticity, and modulus of rupture values was chosen to avoid conflict with the with time-dependent effects. Following used in the original design. In design Union Pacific Railroad tunnel that passes the evaluation, modifications to the codes, these empirically derived material beneath the bridge. initial design were made to ensure properties based on the compressive the structure’s design life. The most strength are assumed to be higher The Shasta Viaduct Arch Bridge is a significant modification was lowering for NWC than LWC. By increasing the six-span, post-tensioned, CIP, continuous the dead load on the arch ribs by using compressive strength of the LWC, the concrete box-girder superstructure, lightweight concrete (LWC) instead of values of these properties converge to with four of the spans supported by a normalweight concrete (NWC) in the those of the lower-strength NWC that two-ribbed arch. The arch support was superstructure box girder including were used in the original design. A target designed to be within the perimeter of deck, webs, bottom slab, intermediate equilibrium density for the LWC of 120 the superstructure. To accommodate and abutment diaphragms, and lb/ft3 was selected for the design; in the the horizontally curved alignment, the bent caps. All other bridge elements interest of time, an oven-dry density of ribs making up the arch are asymmetric, used NWC, including the bridge rail. no more than 112 lb/ft3 was used for with each rib having a lateral kink near Capacity of the ribs was increased the trial batches. With a density of 120 its apex. These kinks have different by providing a 1-ft-thick NWC collar, lb/ft3, there was no need for lightweight angles; therefore, four separate placed by shotcrete, around the lower sand; therefore, local sand was used in the alignments make up the ribs and no 17 ft of the 7-ft-square ribs. Struts mixture. The same LWC mixture was used

Plan for the Shasta Viaduct Arch Bridge showing the horizontal curve with spiral transitions, as well as the existing horizontal alignment and the existing bridge. Figure: Ric Maggenti.

32 | ASPIRE Fall 2019 Plan view of the two asymmetrical arch ribs. Located on a steep hillside, the ribs, which span 366 and 400 ft, have lateral kinks near their apexes, and their ends are supported on thrust blocks at different elevations. Figure: Ric Maggenti. for all LWC components except the deck, to dynamic behavior under pressure which was modified to include fibers. and the viscosity of the mixture as measured by slump. Slump is directly The bridge structure is located in an related to water content and admixtures. area prone to freezing and thawing, so However, slump gained by admixtures entrained air was to be no less than 5%. changes the rheology of a mixture, The concrete needed the capability of making the “water slump” (the slump being pumped at a minimum rate of 60 before admixtures are added) important. yd3/hr. Therefore, a maximum ¾-in.- All things being equal, a higher slump diameter lightweight aggregate (LWA) enhances pumping as long as the sourced from Frazier Park, Calif., was mixture remains cohesive. The cement selected. This LWA could be delivered paste is the transport fluid that moves fully saturated, which is a necessary the concrete through the pump. The condition for pumping the LWC. Proof mortar conveys the coarse aggregate, pumping to a height of 100 ft was done keeping it in suspension and making the at the bridge site. concrete pumpable, which is why the sand gradation is an important factor Trial Batching for pumpability. The coarse aggregate Iterative trial batches revealed that affects the plastic and cohesive qualities properly air-entrained mixtures with a of fresh concrete, as well as the water maximum water–cementitious materials demand to achieve slump values. ratio (w/cm) of 0.35 achieved adequate strengths and met the 120 lb/ft3 An early pumpability trial was a failure. equilibrium density criterion. Because The pump was overly strained before even the batch plant was located more than a 30 yd3/hr rate was achieved, and then an hour away from the bridge site, the pump plugged. For a subsequent trial, chemical admixture types and dosage the amount of cementitious material was rates were adjusted to provide a stable increased from approximately 850 to slump and, more importantly, consistent 900 lb/yd3, and the aggregate proportions air content, which affects both strength were adjusted. Because of the high and density. cementitious materials content, with 50% slag and no fly ash, the mixture Some of the first trial batches could required mass concrete cooling tubes not be pumped at even a 30 yd3/hr at the bent caps. Although the LWC rate. However, the pumping issues were was prone to some countertendencies, eventually solved, and the final mixture such as a hotter or stickier mixture, could be pumped at a rate exceeding 100 increasing the cementitious content yd3/hr and to a height of 100 ft. provided several pumpability benefits. First, it allowed an increase in water Pumpable Lightweight Concrete content without exceeding 0.35 LWA must be fully saturated prior to w/cm. This increased water slump. Second, pumping, which may be accomplished it increased the paste content, which is the by vacuum or thermal methods for some fluid transporting the aggregate. Third, it LWAs. Aside from that requirement, indirectly enhanced aggregate gradation Lightweight concrete being pumped from fresh concrete properties necessary regarding pumping because the extra a working platform on a work trestle up to for pumping LWC are the same as for cementitious material could be treated as the bridge superstructure. Photo: California pumping NWC. Pumpability is related part of the fine aggregate. Department of Transportation.

ASPIRE Fall 2019 | 33 Proprietary tables and graphs for to stem, were greater than 50 yd3/hr. optimizing aggregate gradations for The last deck placement rate exceeded pumpable mixtures were developed back 75 yd3/hr, despite some logistical in the days when a 700 lb/yd3 cement problems that led to production delays. content was considered very high. Such The concrete mixture for the deck mixture resources were used as guidance included 4 lb/yd3 of polyfibers, one of for developing a pumpable mixture the provisions for California Department with a cementitious content of 900 lb/ of Transportation (Caltrans) “CRACK- yd3 by considering 200 lb/yd3 of the Less” concrete deck mixtures. cementitious materials as fine aggregate. The gradation of the local natural Conclusion sand was within the gradation band of The LWC met or exceeded all ASTM C33, Standard Specifications for expectations, including the capability Concrete Aggregate.1 By adjusting the to be pumped over 100 ft high. The Placing lightweight concrete for bent caps by sand gradation to include a portion of pumping. Because of the high cementitious Caltrans standard design life for a the cementitious materials as sand, as materials content, active (cooling tubes) and highway bridge is 75 years. Many design suggested by ASTM C33, the fineness passive measures were used to control the features and upgrades were included with modulus dropped from 2.91 to 2.51. concrete temperature to avoid thermal cracking service life in mind, such as stainless A lower fineness modulus allows for and the potential for later cracking caused by steel deck reinforcement, polyfibers in the a larger volume of coarse aggregate, delayed ettringite formation. deck concrete mixture, and inclusion of a optimizing a pumpable mixture without 1-in.-thick protective polyester concrete adding water. deck overlay. With the reduced load resulting from the use of LWC for the Production Rates— superstructure, the support components Pump Them Up! should also see improved longevity. The On the Shasta Viaduct Arch Bridge resulting structure is anticipated to last construction project, work shifts began as long and be as durable as the initially in the evening and continued until the planned NWC bridge. early morning. After 1600 yd3 of concrete for the box girders and bent caps were Reference pumped in four shifts during three 1. ASTM International. 2018. Standard 3 weekends in August 2018, 900 yd of Specifications for Concrete Aggregate deck concrete were pumped in two shifts (ASTM C33-18). West Conshohocken, during one weekend in September. PA: ASTM International. ______There were no issues with workability and pumpability. Inspection after form Ric Maggenti was a senior bridge/materials removal showed good consolidation. The and research engineer for the California pump rates at times exceeded 100 yd3/ Department of Transportation (stationed in hr. The net placement rates for the stem Sacramento at the time of his retirement). Nighttime placement of lightweight concrete and soffits, which included stopping Sonny Fereira is a senior area bridge with polyfibers for the bridge deck. Photo: to move the pump and intermittent construction engineer for the California Ric Maggenti. placement as the crews moved from stem Department of Transportation in Redding.

The completed Shasta Viaduct Arch Bridge. Photo: California Department of Transportation.

34 | ASPIRE Fall 2019