Author: Ashley Simons, Doka UK Formwork Technologies Ltd
UK, 27 setember, 2016.- This analysis provides an introduction to the maturity method for establishing in-place strength development of concrete during construction. The proper application of this simple procedure can result in savings by allowing construction operations to be performed safely at the earliest possible time.
Concrete is the most widely used construction material in the world > 4.8 billion cubic meters/year and also one of the oldest construction materials in the world dating back to the start of Roman Empire, with its fundamental and globally abundant ingredients being; aggregates, cement (binder) and water, when the cement and water are combined they trigger a chemical reaction known as ‘hydration’, along with a release of energy in the form of heat. The hydration process is essential in creating the glue that binds the graded aggregates (coarse & fine particles) together along with any steel reinforcement or other tensile materials and over a relatively short period of time the binder hardens and forms a solid composite material, commonly known as concrete.
The heat generation due to hydration along with the combined effects of time result in a value for maturity. This maturity index is influenced by many factors, with the key influences being:
- Cement characteristics.
- % of Cement replacements ggbs, pfa, etc.
- Modern chemical additives, accelerators, retarders, plasticers, etc.
- Placement temperature of fresh concrete.
- Temperature development during process of hydration.
- Ambient temperature.
The key physical property of concrete is; it develops high comprehensive strength over time and generally reaches its design strength at 28 days. It is this 28 day strength that is used by architects and engineers in the ultimate design of concrete structures and for specifying the hardened concrete mix performance. However, during the construction process this 28 day strength is not required and most construction activities only require up to a maximum 70% of the 28 day strength e.g. for striking vertical & horizontal formwork, climbing formwork, pre and post tensioning stages, etc.
And if the contractor can predict with certainty when to carry out these operations at the earliest time the target strength is reached, he has the greatest opportunity to leverage cost savings due to building faster, with better quality and improved levels of safety.
Modern concrete mix designs must deliver the challenges of pouring concrete in extreme climatic conditions of high and low ambient temperatures and every temperature in between. Damage to concrete structure can occur if concrete develops to high temperatures in the early age phase. These damages include reduced final compressive strength and delayed ettringite formation. And in very low temperatures the setting time will be extended, resulting in non-practical cycle times.
These mixes often include partial replacements for the Portland cement with industrial by-products from steel making and energy production; ggbs (ground granulated blast-furnace slag) and pfa (pulverised fly ash), which have an direct impact on the chemical reaction of hydration, generally, slowing it down and reducing the release of heat. Plus the addition of chemical compounds (admixtures); accelerators, retarders, plasticers and super plasticers, have the effect of reducing the water/cement ratio and impacting on the concrete workability phase of fresh concrete, allowing sufficient time for placement and consolidation of the flowable/workable fresh concrete.
Early, fast and accurate assessment of in-place concrete strength is a major challenge for the construction industry. The lack of a practical solutions will cost the industry many £billion’s of unnecessary investment, along with lack of reduction in CO2 and other emitted pollutants during the production of cement. By using the Concrete Maturity Method as the foundation for assessing early strength development in fresh concrete, major savings can be leveraged to build faster, safer and to a better quality.
The maturity method relies on the measured temperature history of the concrete to estimate strength development during the curing period. However, to realise these benefits a practical and real time measurement regime to monitor and report back on the actual strength gain in the in-place concrete must be adopted. Doka Concremote is such a product that provides the concrete mix calibration metrics, sensors, data recording & transmission with real-time, online access to accurate data monitoring in the measuring of the progressive strength development of the in-place concrete.
The ‘Concrete Maturity Method’ is an internationally recognised and normalised approach to assessing early age comprehensive strength of in-place concrete and is used in the majority of developed construction markets. The original material research was carried out in the UK in the late 1940’s, early 50’s by Nurse & Saul who established a linear relationship between maturity (summation of temperature x time) and strength of concrete. The underlying assumption is that strength development in concrete is a linear function of hydration temperature.
They developed the law for this relationship “Concrete of the same mixture at the same maturity has approximately the same strength whatever combination of temperature and time goes to make up that maturity” expressed with the following formula:
More recent research in the 1980’s has refined the equation and this new equation is the norm for many international Standards including; British & European standard EN 13670, USA ASTM C1074 and Canadian CBD-187 Non-Destructive Testing of Concrete all allow the use of Concrete Maturity for measuring concrete strength, etc. In 1987, ASTM adopted as standard practice the use of the maturity method to estimate the strength of in-place concrete. The US Standard ASTM C 1074 recommends using the modified exponential equation for modeling maturity-strength relationship of concrete.Freiesleben-Hansen and Pederson’s modified exponential equation is:
𝑆 = 𝑆𝑢 𝑒−(𝜏𝑀)𝛽 (2-5)
S = Compressive strength (psi),
Su = Limiting compressive strength (psi),
M = Maturity index (˚C×hours or hours),
τ = Characteristic time constant (hours), and
β = Shape parameter.
The procedure for estimating the in-place strength requires measuring the in-place maturity. This is achieved by installing sensors at locations in the structure that are critical in terms of exposure conditions and structural requirements. The importance of this step cannot be over emphasised when the strength estimates are being used for the timing the start of critical construction operations.
The October 5th Doka UK published the second part of this analysis:
“New But Old”: Maturity Method of Measuring Concrete Strength