Enhanced maturity assessment of concrete strength for improved construction efficiency and structural safety
An enhanced approach to the assessment of concrete maturity index, developed by James Fisher Testing Services, aims to provide constructors with new insights and opportunities in this important area.
In the majority of large construction projects – including the repair of in-service structures such as highway bridges and viaducts – concrete is poured in situ and develops its full mechanical properties through a curing process measured in days and weeks. A real-time knowledge of the maturity of concrete can be particularly beneficial, both in terms of optimising the efficiency of construction operations and determining when the structure is safe for formwork removal.
Arguably the most widely used construction material worldwide, concrete is deployed in almost all types of major construction projects, from highway and railway bridges and viaducts to high rise buildings and tunnels, to the foundations of nuclear reactor buildings and of wind turbines. Other than components such as pre-stressed beams, blockwork and tunnel segments, much of the concrete used in major civil structures will be poured and cured on site. Entering the formwork in a semi-fluid composite mix of aggregates and cement paste, the concrete acquires its full mechanical properties over time as the material solidifies and cures. The rate at which this curing process occurs will be heavily dependent upon both time and the temperature of the material above the minimum below which hydration of the cementitious paste ceases to take place.
A detailed and accurate knowledge of the development of the in-situ mechanical strength of concrete structures as they cure can be extremely beneficial to constructors. The availability of such data can, for example, enable the optimal timing of formwork removal, or the post-tensioning of structural tendons. In cold weather, it can inform decisions as to the removal of protective measures, and in operational terms, it can enable highways and railways to be opened to traffic as soon as soon as can this safely be done.
The Concrete Maturity Method
The maturity method is used by the construction industry to provide an estimate of concrete strength development. The most widely used approach is defined in ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method. This code sets out a procedure for the calculation of a ‘Concrete Maturity Index’ which is expressed in one of two ways: either in terms of the temperature-time factor, or the equivalent age at a specified temperature. In each case a key assumption is that the concrete is maintained in a condition that permits cement hydration throughout the curing process.
The first of these methods is arguably more intuitive, being based on the assumption that strength development in concrete is a linear function of hydration temperature. The maturity index in this approach is calculated at any given time as a value expressed in degrees Celsius-hours or -days. It is in effect the summation of the average temperature (above the minimum datum value for hydration) for each time period of measurement, which is reflected in the commonly used alternative terminology of temperature-time factor, as defined by the Nurse-Saul equation:
M(t) = the temperature-time factor at age ‘t’ in degree-days or degree-hours,
Δt = a time interval, days or hours
Ta = average concrete temperature during time interval,
To = datum temperature, °C.
The real-time strength of concrete is linked to the maturity index or temperature-time factor, through a laboratory experimental procedure in which cylindrical or cube specimens of the intended mix are tested to compressive failure in order to measure their strength at a given curing time. These tests are carried out at ages depending on the speed of set of the selected mix, which can range typically from a few hours to up to 28 days. At each test, strength is calculated as the average of two simultaneously tested samples, or three if the range exceeds a threshold of 10 percent of the recorded average value. From these laboratory test data, the strength-maturity relationship can be expressed as a curve, using regression analysis to determine a best-fit to the data.
From the laboratory-derived data set, the instantaneous strength of the same mix of concrete on the construction site can be estimated at any time from a knowledge of its temperature and the time elapsed since the start of curing. This is achieved through the instrumentation of the ‘as poured’ installation, while taking care that the temperature probes are correctly installed – with the thermocouples fully immersed within the concrete, while not touching items such as reinforcements or formwork.
Improving on the standard
While the ASTM code for estimating concrete strength provides a good basis for construction planning and management, it has some weaknesses too. Its accuracy relies upon use of the appropriate strength-maturity function for the concrete mixture selected for a given job; where no valid calibration data exists for the intended mix, a new set of laboratory tests will be required.
More fundamentally, however, the code requires only that the calibration is carried out a single ambient temperature. This represents current industry practice, but for greater accuracy, James Fisher Testing Services offers an enhanced method that includes calibration across a range of temperatures. At the company’s Warrington test facility, multiple samples can be aged in water baths at different temperatures, and compression tests carried out for strength data.
The use of multiple temperature tests also provides for improved estimation of the true To datum threshold temperature for hydration of the mix. The additional data points available from the laboratory trials also further improves the precision of predictions. These enhancements provide a greater accuracy of calibration across a wider range of conditions, meaning that safety factors can be refined, and construction time consequently can be saved in the removal of formwork and commencement of subsequent processes.
In terms of site-based data acquisition automation, the real-time temperature data recorded from the installed thermocouples can be integrated with the Smart Asset Management System (SAMS) reporting platform. This is delivered via simple rugged, battery-powered loggers that transmit via a 3G/4G mobile telecommunications network to the internet. This enables a live data feed of on-site strength data to inform crucial downstream construction decision-making that can be delivered across multiple platforms, from computers to smartphones.
Finally, as an additional on-site physical check, the calibration process itself can be further validated through the use of on-site casting of test specimens in the same conditions as that of construction. These can be matured in parallel with the newly built infrastructure and tested in order to update the strength-maturity index calibration for the mix, based on prevailing site conditions.
A library of standard mix data
With significant construction efforts being made to maintain, repair and extend the life of concrete infrastructure built in previous decades – in particular on major highway networks – James Fisher Testing Services has begun the process of building-up a library of strength-maturity data for standard repair concrete products such as Rendero LA60, which is a popular selection for the reinstatement of reinforced concrete where low permeability characteristics are required along with high compressive strength.
More testing for greater efficiency
Far from bringing an additional cost and time burden to concrete construction and repair projects, the use of a maturity approach to the strength assessment of both virgin concrete and of major repairs can present a very positive return on investment and offer improvement in construction efficiency. By taking the guesswork out of post-pour estimation of concrete strength, formwork can be removed at the earliest practical opportunity and downstream operations carried out at an earlier stage than would otherwise be possible. Moreover, while the industry standard methods defined in ASTM C1074 provide a sound basis for establishing the strength-maturity relationship for a given mix, there are numerous enhancements both to the calibration process as well as to on-site testing and data acquisition and processing that may be advantageous, depending on the practical, technical, and commercial requirements of each construction project. By delivering this as an integrated laboratory-site process in the manner described, further site environmental and cost benefits can accrue from a reduced requirement for routine cube making and transportation.
Contact us for more information about how our new approach to the assessment of concrete maturity index can support your next construction project.