Example 1: Avoid Specifying Mixture Proportions

The proportions of ingredients used for concrete mixtures can have a significant influence on the environmental footprint of concrete, but this determination should not be limited to the mixture composition—the impacts to constructability and performance of the structure must also be considered. For example, the mix design shown in Table 2 has 50 percent SCMs, which would generally be considered to have a low carbon footprint. Is this mixture sustainable? It is difficult to tell. This mixture may have a higher compressive strength than that required for structural design. If the concrete was being proposed for a mass concrete member, one would generally need to have 70 percent slag cement to reduce temperature rise from heat of hydration. If this concrete mixture was being proposed for post-tensioned floors, it might not gain strength at an early enough age to allow post-tensioning in a timely manner, thus prolonging the construction schedule.

In general, for a concrete mixture to be sustainable, it must be able to meet the performance requirements of the owner, designers, contractor, and producer, in addition to meeting the following criteria that support sustainable construction:

• Minimize energy and CO₂ footprint
• Minimize potable water use
• Minimize waste
• Increase use of recycled content

Suggested Specifications

The National Ready Mixed Concrete Association (NRMCA) has developed a Guide to Improving Specifications for Ready Mixed Concrete to help designers improve concrete specifications. The following are a few general recommendations for proposed specification language:

Manufacturer qualifications: Concrete shall be supplied with the following current certifications:

• NRMCA Certified Concrete Production Facility
• NRMCA Producer Quality Certification
• NRMCA Green-Star Certification
• Quality-control personnel with responsibility for concrete mixtures certified as an NRMCA Concrete Technologist Level 3

NRMCA has developed several education and certification programs to help qualify concrete producers to design, batch, and deliver concrete for performance-based products while meeting the strictest environmental requirements.

Concrete mixtures: Prepare design mixtures for each class of concrete on the basis of laboratory trial mixtures or field test data, or both according to ACI 301. Design mixtures shall meet the specified strength requirements listed below in Table 3.

This is where the designer can specify physical characteristics and mechanical properties of the concrete along with durability criteria without prescribing the mix design. NRMCA’s guide provides alternate performance tests and criteria for alkali-aggregate reaction (AAR), shrinkage, etc.

Figure 4: The Triangle Building located in Denver used 15 different types of concrete, including shotcrete, cast in place, high strength, and lightweight. Each variety serves a particular purpose and contributes to the building’s LEED Gold certification.