Ensuring sustainable concrete
Sustainable concrete meets the performance requirements of the owner, designer, contractors, and the material supplier while minimizing energy consumption, GHG emissions, virgin material extraction, and waste generation. It will also take advantage of local and recycled materials and have high durability and a long service life. Specifications for concrete should not restrict the concrete mixtures being supplied to the jobsite from being more sustainable.
During a project, several parties will be involved in the production and construction process. The custody of the concrete will change hands several times, with each party having the ability to affect the material’s final performance. Problems can arise when conflicts exist between the specification and performance requirements. Fundamentally, these issues arise due to unfamiliarity with the most current and appropriate standards and lack of knowledge regarding local materials, communication, and planning.
The options for specifying concrete are prescriptive and performance-based specifications or a combination of the two.
With prescriptive specifications, the owner stipulates material types, sources, quantities, air content, slump, and construction processes, while the contractor plans construction methods around those parameters. The concrete supplier then verifies the concrete complies with those criteria. The specifier assumes responsibility for concrete performance.
Under the performance option, the owner specifies the structural and durability requirements and other performance criteria. The contractor is responsible for procuring concrete and working with the material supplier who will establish mix proportions to meet the plastic and hardened requirements. With performance-based requirements, the concrete supplier assumes responsibility for material as delivered. The contractor is responsible for the concrete as placed. This option provides an advantage as the involved parties are free to use their expertise to innovate and ensure the most efficient, economical, and sustainable product is finally used.
There are a number of obstacles to achieving sustainable construction, but the two most significant and easily managed issues are:
- removing barriers in specifications and opening them up to allow more sustainable options; and
- avoiding poor construction practices onsite.
It is important to ensure sufficient time is allowed for project bidding so material suppliers and contractors can discuss requirements, evaluate options, develop and test mix designs, and conduct any necessary pre-qualification testing and optimization.
Prescribing mixture proportions or specific materials should be avoided as restrictions on material types or source often result in the use of unfamiliar products, greater overdesign (i.e. material inefficiency), potential material incompatibilities, and increased transportation distances.
Outlining the details of construction methods should also be avoided as this falls within the contractor’s realm of expertise and experience. Slump specifications, in particular, are best addressed by the designer, supplier, and contractor discussing construction requirements and working together.
It is also important to not insist on faster construction schedules than required unless there is a tangible and measurable benefit to the project. Sometimes, the more sustainable option may be using a concrete with slower strength development.
Restrictions on reasonable changes to concrete mixture proportions should also be avoided as slight adjustments throughout a project might be necessary to maintain performance as material and environmental conditions change over time.
Planning and communication are keys to success. It is important to discuss concrete performance requirements with the producer and contractor to allow them to optimize mix designs. It is advisable to ask them how they can contribute to the project’s sustainability as they can often see the whole picture related to material efficiency and can come up with solutions providing a win for all the involved parties.
Best practices onsite
Onsite testing should be carried out by a competent agency following proper ASTM testing procedures. False negatives in test results lead to greater overdesign and material waste. It is important test results are shared with all parties in a timely fashion to allow control and optimization of concrete mixes for performance and sustainability.
Improper scheduling and estimates or insufficient labor and resources on the project can lead to delays onsite and excess waste generation, while undersized loads increase vehicular emissions. It is critical the site is adequately prepared, access and traffic plans are in place, and designated staging and washout areas are established to avoid delays and potential safety impacts or site damage.
Concrete must be properly protected and cured to reach its potential. Curing is necessary for the hydration reaction between the cement and water to continue. A lack of curing significantly reduces the durability and service life of concrete. Additionally, pouring during extreme weather conditions without proper precaution can result in delays and increased energy consumption to maintain favorable concrete conditions and risk of failure of an element.
It is important to follow the American Concrete Institute (ACI) specifications and guidelines to ensure the proper design is followed, reinforcement is properly spaced, jointing is timely, and excessive water addition beyond the design parameters is avoided.
Pre-placement and routine update meetings, site-specific plans, risk assessments, and procedures for document control and inspection and verification practices must be established to ensure all parties contributing to a project are on the same page.
The final word
Sustainable design in construction is motivating designers and builders to re-evaluate the materials, methods, and metrics used in constructing greener communities. A focus on the material’s performance allows each of the parties involved to bring their knowledge, expertise, and innovation to the table. Concrete offers a robust, reliable, and sustainable solution in many applications as it consumes minimal materials and energy, and when designed and placed correctly, is very durable.
Michael Stanzel is the tech sales representative for GCP Applied Technologies for cement additives in Canada. Stanzel holds a bachelor’s degree in chemical engineering from Queen’s University, with more than 18 years of experience in both cement and concrete quality and operations. He is a member on the Canadian Standards Association (CSA) A3000, Cementitious Materials Compendium, and an associate member on CSA A23.1, Concrete materials and methods of concrete construction. Stanzel can be reached at firstname.lastname@example.org.
Natalee Sembrick is a marketing specialist at Lehigh Hanson in Irving, Texas. She holds a bachelor’s degree in marketing from the University of Arkansas, with a minor in supply chain management. Sembrick can be reached at email@example.com.