Fat vs. Thin MPE Engineering
Precast or Cast-in-Place Concrete Why should we consider Precast Concrete for Control Structures? Do we want to potentially save money? Do we have project time constraints? Is the project in a remote location where concrete is not readily available? Is the expected lifespan of the concrete structure long term or short term? MPE Engineering
History of Precast Concrete Precast has been with us for a long time and are manufactured in various configurations Waffle Panel structures have been in use since the late 1960 s / early 1970 s Districts such as the EID and the MID (probably others) had their own precast plants for Waffle Panel production Consolidated Concrete (Lafarge) also produced Waffle Panels These two structures in the EID were installed in the early 1990 s just before the EID quit producing precast panels
History of Precast Concrete This Check Structure located in the SMRID near Medicine Hat was installed in the 1970 s and is still in use today
History of Precast Concrete The BRID produced a different type of precast structure These 4 or 6 solid wall panels were developed by the Water Resources Branch of Alberta Agriculture. Panels were manufactured in Enchant by BRID staff These panels were very simple and could be bolted together in a variety of configurations Production of these panels stopped in the 1990 s
History of Precast Concrete Lateral J Drop Structure installed in the late 1980 HB Wasteway Structure (note the concrete spreader bar) MPE Engineering
History of Precast Concrete In the early 1980 s PRECON started the production of precast structures for the Irrigation Districts Early structures were mainly turnout structures Production expanded to include a variety of structures; Check, Check/Drops, Vertical Drops, Pipeline Inlets, Baffled Outlets etc. The structure shown here was installed in the early 1990 s and is still in use today SMRID West Medicine Hat Lateral 10
Precast Concrete vs Cast in Place Concrete What are the major differences between the two types of structures Precast Concrete Structures Typically used for smaller structures Wall thickness is 100 mm or 150 mm The trend has been toward 200 mm thick walls on larger structures Panel dimensions are limited to what can be hauled and lifted with conventional equipment Structure components are typically bolted together Suppliers prefer to fabricate standard structures to control costs Cast-in Place Concrete Structures Typically used for large structures Wall thickness is unlimited Minimum wall thickness is typically 250 mm or 300 mm Structure size and configuration is only limited by design requirement
A23.1 Concrete - Materials and Construction Governing Codes A23.4 Precast Concrete - Materials and Construction A23.3 Concrete Structure Design
Major Differences in Design Codes Cast-in Place Precast Concrete Resistance Factor Minimum Reinforcement Ratio 0.0020 0.0016 (20% reduction in minimum steel) Reinforcement Yield Strength (Typical) Rebar = 400MPa Wire Mesh = 550MPa (27% Reduction in Steel Area) Structural Concrete Strength (Typical) Concrete Cover (Typical) 30MPa @ 28 Days 50-75mm 45MPa @ 28 Days (40% increase in strength) 25-50mm
Benefits of Cast-in Place Concrete Benefits Different structure configurations can be accommodated Thicker walls and floors will extend the expected life of the structure No bolted connections between structural members which can fail
Disadvantages of Cast-in Place Concrete Disadvantages Construction period is quite long (2-3 months) A limited number of Contractors have the experience required to construct these structures Irrigation structures are constructed in the winter so hoarding and heating is required, increasing project costs
Benefits Structures can be installed quickly in the winter months General Contractors or District Staff can install these structures Quality of concrete is better controlled Overall structure costs are generally lower than cast in place structures Benefits of Precast Concrete
Disadvantages of Precast Concrete Disadvantages Structure life expectancy is less than cast in place structures Seepage and piping control is more difficult Size and configuration of structures are limited Bolted connections are the weak link and typically the cause of failures
Very large structures can be constructed with precast Larger structures are typically a combination of precast sections tied together with cast in place concrete Precast Concrete Successes Mike O'Callaghan Pat Tillman Memorial Bridge (Hoover Dam Bypass) Calgary Olympic Oval
Precast Concrete Failures Designers must remain cognizant of the limitations of precast Use the right type of structure for the project. Do not let cost be the driving factor Increased inspections are required to insure all connections are installed properly Pittsburg International Airport Collapse of Precast Concrete Beams San Diego Parkade Collapse
Comparison of Typical Costs Precast Concrete: Keep in mind that the concrete volume of a precast structure is typically 30% to 50% of a cast in place structure Substantial costs involved in setting up a precast plant Fabrication of form work can be substantial Cost vary greatly depending on the complexity of the structure Cost for a typical structure on the truck leaving the plant is $3,000 to $6,000 per m 3
Comparison of Typical Costs Cast-in Place Concrete: Forming costs are the same regardless of wall thickness Hoarding and heating costs in the winter can be substantial Concrete can be hard to obtain in remote locations Construction is often in harsh weather conditions which is hard on workers and slows project completion Typical costs are $1,800 to $2,400 per m 3
Comparison of Typical Costs Impact Baffle Outlet Concrete Volume - 4.7 m 3 Supply Cost: $27,000.00 Cost Per m 3 - $ 5,744.00 Potential cost savings are achieved with a shorter construction period and the use of District resources A typical cast in place baffle outlet would be approx. $40,000.00 (complete)
Comparison of Typical Costs Small Check Structure Concrete volume: 2.1 m 3 Supply Cost: $9,000.00 Cost Per m 3 - $ 4,285.00 Cost savings are achieved with a simple standard design Approximately 2/3 the cost of a similar sized cast in place structure
Comparison of Typical Costs Large Check Structure Concrete volume: 26.9 m 3 Supply Cost: $80,000.00 Cost Per m 3 - $ 2,973.00 Cast in place floor installed prior to backfill Backfilling is somewhat complicated (Tie Rods) Approximately 3/4 the cost of a similar sized cast in place structure
Conclusions The precast industry is evolving and providing different solutions to todays problems, and generally structure are becoming larger and more customizable. Size is still the limiting factor Precast concrete offers an economical solution for the Irrigation Industry Cost savings of precast are not that significant over cast in place Life expectancy of precast structures is generally less than that of cast in place structures Do not forget that you are getting structures with approximately 50% of the concrete volume Precast and Cast in Place concrete structures will always have a place in our Industry Choose the right structure for the right application
Who Wins? Questions March 2018 Technical Conference