ACI Materials Journal/July-August 2005 231 ACI MATERIALS JOURNAL TECHNICAL PAPER ACI Materials Journal, V. 102, No. 4, July-August 2005. MS No. 04-069 received April 26, 2004, and reviewed under Institute publication poli- cies. Copyright © 2005, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Perti- nent discussion including authors’ closure, if any, will be published in the May-June 2006 ACI Materials Journal if the discussion is received by February 1, 2006. Although there are several procedures predicting concrete compressive strength, reliable methodologies involve either extensive testing or voluminous databases. This paper presents a simple and efficient procedure to evaluate the activation energy and the rate constant of concrete. These two parameters can be used for a rapid prediction of the mechanical properties of concrete and particularly the evolution of compressive strength. They also allow separation of effects due to physical phenomena such as humidity loss. The procedure uses an experimentally-determined parameter called “hardening time” as an indicator of equivalent maturity when comparing two hardening profiles. Test results from specimens of six concrete types validate the approach. Keywords: hydration; maturity; strength. INTRODUCTION At early age, the mechanical properties of cement-based materials are time-dependent and involve hydration. The hydration process is a thermally-activated reaction that may be described by the Arrhenius equation. This equation establishes the progression of a chemical reaction in terms of rate of reaction k. 1 The integral over time of the rate of reaction gives the degree of reaction. Two independent and parallel research areas have been generated through applying degree of reaction indexes in this research. For the purposes of this paper, they are called “predictions” and “separation of effects.” Predictions of mechanical properties of concrete are possible based on the empirical relationship between the degree of reaction (hydration) and physical properties such as compressive strength, tensile strength, and elastic modulus. 2-6 Separation of effects involves decoupling the contributions to the total deformation of a physical and chemical phenomenon during hardening. 7 Unfortunately, the separation of an effect cannot be done by direct comparison of deformation time-histories, measured in concrete pours that are hardening in different environments. The effects of the temperature after similar elapsed times of hydration change with the thermal expansion coefficient (TEC), and this coefficient depends on the degree of hydration. 8 To perform predictions and separate effects, knowledge of maturity indexes is required. Maturity indexes need to be determined experimentally for each concrete type. This article describes a new methodology to determine two common maturity indexes. These indexes lead to the prediction of the evolution of compressive strength in six different concretes. RESEARCH SIGNIFICANCE A maturity method is used to predict the compressive strength evolution of concrete. Values for the activation energy and the rate of reaction are necessary to implement this approach. Determination of these values usually requires either extensive tests or large databases. This has resulted in limited use of maturity methods. A simple and fast method- ology to determine these values and consequently predict compressive strength evolution is presented. More timely knowledge of compressive strength evolution will lead to savings during construction and improve safety. BACKGROUND The Arrhenius equation states that the rate of a chemical reaction k increases exponentially with absolute temperature, regardless of the degree of reaction already obtained (refer to Eq. (1)) (1) where A = frequency factor, s –1 ; E a = activation energy (KJ/mole); k = reaction rate; R = gas constant (KJ × mole –1 × K –1 ); and T = absolute temperature, K. The degree of reaction is calculated by integrating Eq. (1) over time. The rate of reaction k is constant when the tempera- ture of the hydration process is constant (T = T r = constant imply k = k r = constant). Equation (2) uses k r to predict the compressive strength. This empirical equation is widely used. 9 (2) where k r = rate of reaction at reference temperature T r ; S = compressive strength at age t; S u = ultimate compressive strength; t 0 = age at start of strength development, h; and t = time, h. With the exception of controlled laboratory conditions, the temperature of the hydration process changes during the reaction and then Eq. (2) becomes inapplicable. To overcome this difficulty, it is sufficient to change the time-history into a degree of reaction history. This can be done using the equation of Freisleben-Hansen and Pedersen. 10 Observing that hydration of cement is a chemical reaction, the Arrhenius k A E a – RT -------- exp = Sk r t , ( ) S u k r t t 0 – ( ) 1 k r t t 0 – ( ) + ------------------------------ = Title no. 102-M25 Three-Day Prediction of Concrete Compressive Strength Evolution by Marco Viviani, Branko Glisic, and Ian F. C. Smith