Haghbin & Associates Ltd. was retained by EllisDon to develop the maturity calibration curves for various concrete mix designs (35 MPa, 50 MPa and 80 MPa) for CIBC Square project at 81 Bay Street, Toronto.
Estimate In-Place Concrete Strength
Maturity method—a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of the maturity index (If M1=M2 then F1=F2)
Maturity Method provides a real-time, in-place indication of the strength of the concrete. A calibration curve is developed to establish at what level of maturity the desired strength is reached.
The development of a maturity-strength relationship requires three steps. These include:
1) Developing the maturity-strength curve in the laboratory or in the field,
2) Estimating the in-place strength in the field, and
3) Verifying the strength-maturity relationship in the field.
This procedure utilizes the Nurse-Saul method for developing strength-maturity curves, as described in ASTM C 1074. The Nurse-Saul method uses a specific datum temperature to calculate the time-temperature factor (TTF) and to relate this to the measured concrete flexural or compressive strength at the particular TTF value.
The general form of the Nurse-Saul method is shown below:
TTF (M) = ∑ (Ta −T0) ∆t
TTF = the time-temperature factor at age t, degree-days or degree-hours,
∆t = time interval, days or hours,
Ta = average concrete temperature during time interval, ∆t, °C, and
T0 = datum temperature, 0°C.
Track internal temperatures during curing
Use the tracked internal temperature history to calculate maturity
Use the strength-maturity relationship to estimate strength
Exact Online portal provides real-time, online monitoring of temperature, maturity and strength, alerts and reporting, accessible from any computer or mobile device.
Verify Strength-Maturity Relationship
In order to assure that the concrete being placed in the field has not deviated from the original mix design, a process called verification is employed.
Significance and Use
This practice can be used to estimate the in-place strength of concrete to allow the start of critical construction activities such as:
Removal of formwork and reshoring;
Post-tensioning of tendons;
Termination of cold weather protection;
Opening of roadways to traffic.