The 4083-foot-long westbound I-70 Blanchette Bridge spanning the Missouri River between St Louis and St Charles counties was built in the late 1950s using 20 beam spans and three main steel-truss river spans. The project scope entailed removal and replacement of all driving surfaces and barrier walls, the existing center steel trusses, and the structural steel in the first nine spans on the west side. The scope also included converting three of the east spans to roadway on embankment, repairing or replacement of existing concrete substructure units, and painting all structural steel. Since the original structure was built with a lightweight concrete deck, and most of the original substructure was to remain in-place, the new bridge deck and barriers required lightweight concrete. Concrete work began in the spring of 2012 and the bridge reopened in late 2013.
Removal of existing structures included hydro- and shaped-charge demolition. IN the pier repairs, over 1500 cubic yards of self-consolidating concrete were used. Approximately 5000 cubic yards of lightweight concrete were pumped to the bridge deck from dry river-bank land below. For the 2300 cubic yards of river-span deck concrete, a concrete conveyor was used—by over 1000 feet for the center-truss-span deck placement, and by over 600 feet for the other truss-deck placements. Over 8000 linear feet of lightweight concrete safety barrier walls were slip formed.
Concrete mix designs: Several standard MoDOT mixes (eg., PCCP, Classes B, B1, and B2HE) were used. Other unique mixes included the following:
- A mass concrete mix designed for low-heat of hydration with 60% slag cement for large mass concrete structures like the pier bases.
- A self-consolidating concrete ternary mix with shrinkage-reducing admixture requiring a spread range of 24” to 28” for substructure column and pier-cap repair. Self-consolidating concrete placements began in the summer of 2012 and continued into the spring of 2014.
- Three related ternary lightweight concrete mixes—one for each kind bridge-deck placement
- a) By pump to the deck
- b) By conveyor to the deck, and
- c) By slip-form (low-slump) for the safety barrier wall.
These were distinguished by slump and admixtures content; used the same 60:25:15 combination of Type I/II low-alkali portland cement:slag cement: Class C fly ash; with natural lightweight coarse aggregate, natural fine aggregate, and as lightweight-aggregate unit weight allowed, a minor variable amount of limestone coarse aggregate to adjust to a target fresh unit weight.
The lightweight concrete needed meet the specified maximum equilibrium density of 110.0 pounds per cubic foot. During the project the lightweight aggregate density changed erratically, necessitating adjustments to the mix design aggregate proportioning weights during placements. These adjustments, using a mathematical program, reconciled the at-plant measured concrete air-content (using roll-o-meters), the fresh concrete unit weight, and known proportioning information to model the matured-concrete maximum equilibrium density. To accomplish this, considerable at-plant and on-site testing was conducted during each lightweight-concrete placement. This testing also provided a thick feedback loop toward making air and slump adjustments to the concrete before leaving the concrete yard. Remarkably, there were no rejected loads of the lightweight concrete during the project.
Lightweight and the self-consolidating mixes could not exceed 1750 Coulombs per AASHTO T 277 “Standard Method of Test for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration”, as modified by the Virginia Method. The project average was 1339 Coulombs with no results exceeding the maximum limit. Lightweight concrete was limited to a maximum equilibrium density of 110 pounds per cubic foot when tested per ASTM C567 “Standard Test Method for Determining Density of Structural Lightweight Concrete”. The project average was 107 pounds per cubic foot with no results exceeding the maximum. Lightweight concrete also had to meet or exceed the 4000 psi minimum average 28-day specified compressive strength and the minimum average 28-day splitting tensile strength of 330 psi. The project average 28-day compressive strength exceeded 5500 psi with no results under the specified strength, and all split-tensile results exceeded 330 psi. The self-consolidating concrete mix also had to meet ASTM C1581 “Standard Test Method for Determining Age at Cracking and Induced Tensile Stress Characteristics of Mortar and Concrete under Restrained Shrinkage” for a minimum of 14 days.