The practice of writing prescriptive specifications originated decades ago when concrete quality control standards were not as robust as they are today. However, prescription does not assure performance and can actually result in poorer concrete quality, harsher sustainability impacts, and higher costs.
In a recent webinar, Dr. Michael Thomas, cement and concrete expert and professor at the Department of Civil Engineering of the University of New Brunswick, makes the case for performance-based concrete.
Understanding Prescriptive-Based and Performance-Based Concrete Specifications
Traditionally, the AEC community relied on prescriptive specifications for concrete mix designs. Prescriptive specifications include clauses for the methods of construction and impose restrictions on the compositional parameters of concrete mix. For example, prescriptive specifications may prescribe a minimum cement content of 700 lb/yd3 (415 kg/m3), a maximum fly ash content of 25%, etc. In some cases, prescriptive specifications may entirely disallow certain ingredients.
“Performance-based specifications make more sense in the modern construction landscape as they do not put parameters around the components or proportions of the mixture,” said Dr. Thomas. “Instead, performance specifications are based on performance indicators like strength, permeability, shrinkage, sulfate resistance, resistance to alkali silica reaction, etc. These indicators are measured by standard test methods with defined acceptance criteria such as chloride permeability no greater than 1500 Coulombs at 56 days.”
The ready mix industry has been moving to performance-based specifications from prescriptive specifications for a decade or more. This transition represents one of the most significant embodied carbon reduction strategies available that can be implemented today.
Prescriptive Specifications Impede Sustainability Efforts
Prescriptive specifications are often overly conservative, which can lead to higher costs, negative results, and poorer sustainability. Examples of prescriptive requirements include:
- Limitations on source and composition of materials
- Minimum cement factors
- Limitations on supplementary cementitious materials (e.g. quality, type, composition)
- Water to cement ratio limits (when durability doesn’t apply)
- Aggregate grading requirements
- Requirement to use potable water
- Limitations on the composition of mixtures
- Restrictive requirements for slump or air content
- Restrictions on concrete temperature outside standards
Variations of historical prescriptive specifications persist in the construction industry as they are copied and amended for new projects. Below is an example of a prescriptive specification from Canada. This is from a specification that was originally published in 1942 — yet variants of it are still widely used today. One of the clauses states that admixtures are not recommended. In 1942, admixtures were new and there was a general mistrust of them in the industry.
“Imagine if this specification still existed in this form today?” said Dr. Thomas. “It would mean that the use of water reducers, superplasticizers, etc. would be limited.”
While this particular specification has evolved to prescribe the use of admixtures, it still imposes restrictions that impede innovation in other areas of concrete mix production — the same way that admixture innovation was impeded in 1942.
The True Extent of Overly Prescriptive Specifications
Recently, the National Ready Mix Concrete Association (NRMCA) surveyed 102 project specifications to identify the true extent of prescriptive limitations. They found the most common prescriptive requirement to be a restriction on the amount of supplementary cementitious material (SCM) that can be used in concrete — 85% of specifications contain this restriction.
While limitations in SCM make sense in certain exposures (e.g. where concrete is exposed to icing salts, freezing, and thawing), in other exposures the restriction is redundant and may even lead to lower quality concrete. Despite this, specifiers often copy existing prescriptive specifications and, therefore, unnecessarily restrict the use of SCMs.
The NRMCA also found prescriptive water to cement ratios were applied in 73% of specifications where the limitations were not even applicable.
These prescriptive specifications result in overly concrete mixes that are not optimized for the performance requirements of the specific use case.
The Shift to Performance-Based Specifications
Dr. Thomas described his first experience convincing a client to move from prescription-based to performance-based specifications in the mid-1980s. He worked on a cooling tower project where the prescriptive specification for the concrete had a minimum Portland cement content and a limitation that fly ash and slag could not be used.
“There was a concern that, in a thin-shell tower, adequate one-day strength to jump the forms could not be achieved with fly ash and slag,” said Dr. Thomas. “We managed to convince the owners of the power stations to permit fly ash while guaranteeing 10 MPa (1500 psi) in one day demonstrated by temperature-matched curing. As a result, we reduced Portland cement usage by 30%, reduced the risk of temperature rise, and reduced the risk of cracking — proving that performance-based specifications are good for sustainability and technically sound.”
More Efficient, Low Carbon Concrete with Performance-Based Specifications
According to Dr. Thomas, prescriptive specifications are outdated and prescribed for scenarios that are no longer relevant today. Instead, he recommends the following ways to minimize prescriptive-based specifications and drive innovation in concrete mix design:
1. Don’t Include Minimum Cement Quantities
“There is a perception that more cement makes better concrete,” said Dr. Thomas. “However, research over the past 100 years has shown that it’s not cement content that controls the strength or permeability of concrete, it's water to cementing materials ratio.”
In some cases, more cement can actually result in poorer concrete. Studies have shown that going from 500 lb/yd3 to 700 lb/yd3 (296 kg/m3 to 415 kg/m3) of cement results in no increase in strength. Instead, it results in an increase in heat and cracking, an increased risk of alkali–silica reaction (ASR), and an increase in the embodied carbon dioxide (CO2) of the concrete.
Minimum cement contents are no longer an effective way to ensure durability. As such, they have disappeared from many – but not all – national specs.
2. Replace Water to Cement Ratio Criteria
Higher strengths does not equal better durability. Today there are so many different types of cementing materials and producers can discover very different relationships between strength and permeability or other durability factors.
With Portland cement, as strength increases, permeability decreases. However, concrete with 50% fly ash, at the same strength as Portland cement, is almost 10 times less permeable. As such, water to cement ratio starts to lose meaning as we change the nature and type of cement. Despite this, maximum water to cement ratio values are still imposed in most specifications.
Consider ASTM C1202 (a standard test method for resistance to chloride ion penetration) to replace the water to cement ratio with the following alternative criteria:
- w/cm = 0.50 → 2500 coulombs
- w/cm = 0.45 → 2000 coulombs
- w/cm = 0.40 → 1500 coulombs
3. Don’t Limit SCM Quantity
Dr. Thomas recommends that, other than on assigned exposure classes, concrete producers should not limit SCM quantity as it increases the risk of ASR, delayed ettringite formation (DEF), sulfate resistance, chloride resistance, and reduces later-age strength and durability. Restrictions on SCMs also limit the reduction in cement content, contributing to greater embodied CO2 in the concrete.
4. Consider Low-Carbon Cement Alternatives
To tackle the embodied carbon dilemma, producers can adopt technologies like CarbonCure. CarbonCure injects CO2 into the concrete during mixing, similarly to an admixture. Once introduced to the concrete mix, the CO2 chemically converts to a mineral and is permanently trapped in the concrete. This process actually improves the concrete’s strength, which allows greater opportunity for mix optimization.
If you’d like to chat about how you can deliver on performance-based specs with CarbonCure and benefit from more sustainable concrete, please get in touch with a CarbonCure representative.