Sustainable
Precast Concrete
Material Selection

Material selection and resource use is one of the key areas assessed in all rating systems. The choice of materials and systems within a material category can have dramatic effect in terms of the environmental burdens captured in a full life cycle assessment. Material selection impacts a number of more specific concerns, such as design efficiency and related material use, recycled content, recyclability, and the potential for reuse. All these considerations are either directly or indirectly taken into account in green building assessment and rating systems. While the systems do not now deal explicitly or well with building durability issues, material selection can also be a key aspect of ensuring a building service life exceeds the desired or planned service life. This is especially true for materials that will be exposed to the elements or subjected to wear and tear from occupants.

LCA Comparison of Precast versus Cast-in-place Systems
Floors and roofs are significant in terms of the environmental burden during the production of the load bearing components of a structure. Optimization of floor and roof components can significantly reduce the environmental impact of the overall structure construction. The table below shows that a precast concrete hollow core slab system can offer clear embodied energy advantages relative to a cast-in-place floor slab.

Energy consumption required to produce concrete floors

Item Hollow core slab (MJ/m²) Cast-in-place slab (MJ/m²)
Cement 186 389
Steel 45 60
Other raw materials 15 23
Manufacturing process 128 32
Transportation 28 42
Total 401 560

Source: fib Environmental issues in prefabrication

Distances from a precast plant and a ready-mix plant to the building site are assumed to be the same. The higher energy consumption for the cast-in-place slab is due to the larger volume of cement and concrete needed per square meter of floor.

In the table below, the comparison is broadened to cover a number of physical characteristics and a range of environmental effects. The precast concrete hollow core floor slab is less environmentally intensive than its cast-in-place counterpart, with the exception of greenhouse gas emissions where the precast system was found to have slightly higher impacts.

Comparison of different floor types from a study in the Netherlands where an extensive investigation compared a precast hollow core floor with a cast-in-place floor.

Item Hollow core slab (per m²) Cast-in-place slab (per m²)
Concrete (kg) 263.7 423.0
Reinforcement (kg) 3.2 6.1
Total mass (kg) 266.9 429.1
 
Eutrophication (kg P04-3 eq.) 0.0356 0.0410
Exhaustion (x 10-12) 0.0468 0.0707
Ecotoxicity (xl03 m3) 2.78 5.81
Greenhouse effect (kg CO2 eq.) 55.2 53.4
Acidification (kg SO2 eq.) 0.252 0.306
Summer smog (kg C2H4 eq.) 0.0297 0.0460
Human toxicity (kg) 0.318 0.411
 
Use of primary energy (MJ) 461 643
Solid waste (kg) 36.3 58.8

*Note: "eq." = equivalents
Source: fib Environmental issues in prefabrication

These results reflect the efficiency of material use in a precast system with advanced production processes using high strength concrete reinforced with prestressing steel. Precast structures use less material to achieve the same load bearing capacity compared with cast-in-place structures. Smaller precast cross sections result in less dead load to carry. CSA A23.3 allows the use of a higher material resistance factor for certified plant manufactured precast concrete elements. The choice of one precast load bearing system over another does not have a major impact on the environmental burden created by the construction.

Prestressing done by pretensioning prestressing strands in the forms at a precast plant is highly efficient; reducing the amount of embedded steel and improving structural performance.

Example: Comparing a prestressed and rebar reinforced concrete beam
A beam spanning 7.0 m is required to carry a dead load of 30 kN/m and a live load of 25 kN/m. A cross section of 300x600 mm is chosen with top reinforcement of 2-No.20 bars. The main reinforcement required for the reinforced beam is 4-No.30 bars to carry the load, while a prestressed beam needs 8-13 mm strands. The reduction in shear reinforcement in the prestressed beam is neglected. The reinforcement required is 2800 mm2 for the reinforced beam compared with 792 mm2 for the prestressed beam - a reduction in the area of the main reinforcement of about 70%.

Summary

  • precasting allows optimized concrete mix designs. Mixes are usually designed to reach 25-30 MPa in 12-16 hours for precasting a new element each day.
  • prestressing can significantly reduce the amount of steel used; and
  • post-tensioned concrete can be employed at a precast plant and is often used at the site to join elements as moment connections, shear walls and for stabilizing elevator and stair shafts in tall buildings.
PreCAST CONCRETE... SUSTAINABLE STRUCTURES FOR TOMORROW!
Canadian Precast/Prestressed Concrete Institute
PO Box 24058 Hazeldean, Ottawa Ontario, Canada
Tel: 613.232.2619
Toll Free: 1.877.937.2724
© 2018 All rights reserved.