A Fast Approach for Estimating In-Situ Compressive Strength of Concrete
A Global Challenge
Early, rapid and accurate in-situ estimation of the compressive strength of concrete is one of the major challenges for the concrete industry. A practical solution for such challenges can prevent multi-million dollars of extra annual investment for construction industry and owners of civil structures since annual global production of concrete is estimated at 3.8 billion cubic meters. An accurate and reasonable in-situ estimation of the compressive strength provides the opportunity to optimize the concrete mix design as well as optimizing the formwork removal time. The optimization of mix design affects the consumption of raw materials (e.g. cement and aggregates) and alternative materials (e.g. natural pozzolans and supplementary cementitious materials like fly-ash and silica-fume). Considering the high volume global consumption of concrete, this could, in turn, effectively optimize the consumption of resources and reduce a great extent of CO2 and toxic materials (emitted during the cement production) into the atmosphere.
The maturity method is a convenient approach to predict the early age strength gain of concrete, using the principle that the concrete strength is directly related to the hydration temperature history of cementitious paste. The maturity concept for estimating the strength gain of concrete is described in ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method. This method can potentially address many immediate challenges facing the concrete industry such as predicting appropriate time for formwork stripping and post-tensioning, especially at low temperatures while the strength development of concrete is hindered; and optimizing concrete mix design and concrete curing conditions (e.g. concrete heating at low temperatures or surface protection in hot-dry weathers). Lack of an accurate estimation of strength at early ages of construction is twofold: contractors either wait too long for next action (e.g. stripping formwork) which is costly due to delays in completing the project, or they act prematurely which could cause the concrete structure to crack - that would lead to future durability and performance issues - or even structural collapse.
Such approaches to compressive strength evaluation may cause concrete contractors to make conservative decisions, face more complicated technical problem (e.g. delay in formwork stripping, and unnecessary long-term curing and surface protection), and spend more financial resources.
As a non-destructive testing, the maturity method may be a reasonable candidate to fill this gap. In comparison to most on-site non-destructive technologies (e.g. Schmidt Hammer or Ultrasonic Pulse Velocity), the privilege that the maturity method stands on is that, the procedure for estimating the compressive strength would be objective and qualitative once the maturity curve is developed and adopted.
This technical article describes how the maturity curve is developed, and how it is applied for on-site estimation of the compressive strength of concrete.
What is the principle of the Maturity Method?
Maturity method is a relatively simple approach for estimating the in-place compressive strength of concrete, specifically at early ages less than 14 days. Once the maturity curve is developed in the laboratory for a specific project, it can be used for on-site estimation of compressive strength of concrete in real-time.
The maturity method is governed by the fundamental assumption that a given concrete mix design poured during course of a specific project has the same compressive strength when it has the same “maturity index”. It means that a given concrete mix design, for example, may reach the same compressive strength after 7 days of curing at 10 °C when it is cured at 25 °C for 3 days.
The maturity method based on the ASTM C1074 is the most commonly used method to estimate the in-situ strength of concrete. ASTM C1074 provides two maturity functions: 1) Nurse-Saul function; and 2) Arrhenius function. Based on the Nurse-Saul method, there is a linear relationship between the maturity and the temperature in real time. The underlying assumption is that the strength development in concrete is a linear function of hydration temperature. Equation 1 shows the relationship between maturity and hydration temperature history.