CONCRETE

Structural Optimization Techniques:

Engaging in the approaches above will identify opportunities for improved efficiencies with tangible design solutions like:

1. Utilizing voids, coffers, and fill
2. Lightening the weight of slabs
3. Optimizing strengths
4. Limiting thermal cracking, and
5. Smart approaches to reinforcement, and
6. Using structural concrete as a finish material.

1. Replace concrete with voids, coffers, and fill

Significant volumes of concrete can be replaced by voids, coffers, and fill with strategic planning early in the design phase. Concrete is inexpensive and easy to use, soit frequently gets used in non-structural or low-structural instances where another material would suffice but is not currently economically feasible. Beware of sacrificial polystyrene void formers which may contain more carbon than the displaced concrete.

In which use cases can concrete be replaced?

• In thick sections, the central portion is merely a spacer to hold tension and compression “flanges” apart. Fill can be used here in place of concrete. 
• In use cases where weight is needed for stability, like retaining wave walls, fill can be used in place of concrete. 
• Often, the tension side of the neutral axis only requires a small amount of concrete to carry shear load, hold the reinforcement position, and provide corrosion and fire protection. Voids can be carefully placed in these locations to reduce concrete volume by 30-50%².

2. Lighten the Weight of Slabs

Reducing the weight of slabs helps reduce the loads on the columns and foundations, thereby reducing their size and embodied emissions. The use of some proprietary void systems has been shown to yield slabs with an average of 35% less concrete than traditional slabs, while performing like solid reinforced concrete. Preliminary calculations should be run to balance weight versus strength and stiffness in slab systems. Consider the following approaches for reducing the weight of the slab: 

• Structural lightweight concrete (which incorporates lighter aggregates but can still be high strength) may increase GWP per unit volume, but result in a lower total volume required (thus a lower total GWP).
• Incorporate voids (such as proprietary air-filled recycled plastic spheres) to reduce the amount of concrete needed (ICE).
• Post-tensioned, pre-tensioned, and precast concrete require more cement per unit volume, but can often achieve thinner slabs and therefore less volume, thus possibly lowering embodied emissions.

3. Optimize strengths for emissions reductions

Every concrete mix is a balancing act. Find the lowest emission mix by:

• Understanding the relationship between strength, volume, and reinforcement
• Designing with strength as constructed
• Optimizing cure times
• Sizing bays appropriately, and
• Considering trade-offs between using prestressed or precast concrete and cast-in-place concrete.

Understand the relationship between strength, volume, and reinforcement: Understanding the relationship between strength, volume, and reinforcement is critical for optimizing strength to reduce carbon. Strengthening concrete increases the emissions per unit volume while lowering the total volume. Incorporating supplementary cementitious materials (SCMs) typically decreases emissions per unit volume and reinforcement required, but sometimes increases the volume required. The interplay of these relationships must be analyzed to achieve the lowest emissions possible. The following are general guidelines that may help with early decision making:

• Axial Load or Shear elements: Increasing strength tends to result in a net reduction in emissions.
• Bending elements: Increasing strength tends to result in a net increase in emissions.
• Cover Requirements: Be aware that the cement type used can influence cover requirements (the thicknesses required to protect corrosion of reinforcement)².

Design with strength as constructed: Collaborate with the design team to promote quality control measures that will reduce variability, allowing the strength of concrete as constructed to match more closely with the strength of concrete as designed, therefore reducing overdesign and emissions. Frequently, the strength of concrete – and therefore the amount of cement used – is higher than what’s specified for a variety of reasons including consistence/workability, sieve segregation resistance, reduced formwork removal/striking/demoulding time, to limit post-tension stress loss, and to ensure safety as the properties of concrete will always be variable². Strength can be optimized with maturity meters, which provide real-time sensor data that can determine precisely when concrete has reached critical strength. These technologies allow for better scheduling and strike time, minimizing overdesign.

Specify the appropriate exposure class for each member: Specify the appropriate exposure class for each concrete member to achieve considerable cost and emissions savings. Specifications often indicate the same class for all concrete on a project. This can cause problems during placing and finishing. For example, concrete protected from the environment and not subject to freezing and thawing should not have the same exposure class as concrete exposed to weather.

Optimize cure times: Where possible, schedule site works to accommodate a slower rate of strength gain. This enables more SCMs in the cement mix and less cement in the concrete mix², and therefore less emissions. Concrete with high SCMs often require longer cure times to reach the required strength. Identify building components that don’t need high early strength and plan ahead to allow for these components to have longer cure times. For example, footings and mat slabs, as well as shear walls and columns at lower levels of high-rise structures, are good targets for low cement mixes even when relatively high strengths are required. Specify strengths at 56, 72, 90, or 120 days wherever possible.

Size bays appropriately: Louis Kahn said, “the play of what may be between the columns is just an endless study.” When designing bay sizes (the distance between columns) careful studies must be conducted to find the optimum distance. There are many interrelated factors and it’s important to understand and analyze them to optimize your structure.

• Shorter bay sizes can allow for thinner slabs, but require more columns.
• Longer bay sizes increase usable floor area and reduce foundation loads.
• Deflection and creep in long bays may drive concrete volume more than strength demands.
• High-strength concrete can enable longer bay sizes with smaller columns and thinner slabs. However, high-strength concrete has higher cement content, increasing emissions per unit volume.
• Keep in mind the limitations of minimum reinforcement cage size and protective cover for the reinforcement.

Optimize structural efficiency with simple shapes: Simplified form structures are the most optimized in terms of embodied and operational emissions. Maximum structural efficiency is achieved with rectilinear shapes and minimal envelope articulation. This results in less formwork (reducing costs significantly), fewer thermal breaks, easier placement of concrete and reinforcement, fewer transfer beams and discontinuous columns, and more standardized repetitive sizes resulting in less material waste for common finishes^4,5.

Select the structure’s site based on the best soil conditions

Choosing the most favorable soil conditions upon which to build can eliminate a lot of unnecessary concrete from the project’s stabilization and foundation requirements. If the soil requires massive foundations, reconsider the site. If the structure must be placed on poor soil for construction, consider the most lightweight structural solutions possible to reduce impact and foundation size⁶.

4. Limit Thermal Cracking

Thermal cracking is a major structural threat that can be mitigated with the use of low-emissions SCMs that typically limit thermal cracking and replace some proportion of the high-emission clinker content in cement.

5. Optimize Reinforcement

Use reinforcement only when needed: As long as alternate crack control measures are taken, many slabs on grade can be cast without reinforcement. The embodied emissions of metal or polymer fibers used in standard quantities to control cracking in slabs on grade are typically lower compared to the reinforcement removed, but the emissions balance between the two should be evaluated.

If using reinforcement, consider high-strength: High-strength reinforcement often has the same embodied carbon as conventional reinforcement, since the strength is achieved using small quantities of micro-alloys with negligible embodied emissions. When the higher strength results in lower steel quantities, the embodied emissions of the concrete structure are reduced.

Avoid Corrosion in Reinforcement: In applications of concrete that are likely to see road salts (or other corrosive forces), consider the high maintenance costs (economic and environmental) of replacing corroded steel, and consider using non-corrodible reinforcement, such as Glass Fiber Reinforced Polymer rebar (GFRP), or Basalt Fiber Reinforced Polymer rebar (BFRP)².

6. Use Structural Concrete as a Finish Material

Consider using structural concrete as a finish material to eliminate the embodied emissions of additional architectural finishes.