In this article, we will investigate the use of maturity method for in-place strength measurement. We will review the concept behind the maturity technique, and the most common practices. Later, we will describe the benefits and limitations of this method for in-place strength prediction. In the last part of this article, we will review and examine some of the existing technologies for using the maturity technique.
Concrete (compressive) strength is by far the most important property of concrete. It represents the mechanical properties of concrete; for example, the 28 days compressive strength of concrete cylinders is used as the specified compressive strength for design purposes (ACI 318-14, CSA A23.3-14). Strength is also considered (at least in the old school) a key factor for durability performance.
The most common method for the evaluation of concrete compressive strength is the breaking cylindrical concrete samples in a compressive-testing machine. The force associated with the breaking point is divided by area to get the compressive strength. In a standard practice (i.e. ASTM C 39), concrete samples should be cured for 28 days in water tank, or in moist room condition.
Why In-Place Concrete Strength?
The in-place measurement of strength has several benefits. The following describes the advantages of in-place strength measurement:
1- Determination of whether a structure is capable of being put in service
2- Adequacy of curing and protection of concrete in the structure
3- Form or shoring removal time requirements
4- Termination of cold weather protection
5- Post-tensioning of tendons
6- Opening of roadways to traffic
ASTM C 31 specifies the standard test procedure for in-place measurement of concrete strength using concrete cylinders. The use of Cast-in-Place-Punch-Out-Cylncore, known as (CIPPOC) is another method used in construction practice these days.
There are certain challenges with these practices. Generally, test samples do not reflect the influence of several factors on strength, including, Temperature fluctuations within mass of concrete, Weather conditions, critical curing conditions. In addition, the improper sample preparation and testing can make interpretation of the results even more difficult. Selecting the location to take these samples can be very subjective.
The Maturity Method
“The maturity method is a technique to account for the combined effects of time and temperature on the strength development of concrete.” (Carino and Lew, 2001). Maturity method provides a simple approach for evaluating the strength of cement-based materials in real-time, i.e. during construction.
The two common procedures for using the maturity technique are standardized in the ASTM C 1074. These are:
Method 1 – Time-Temperature Factor
Method 2 – The Equivalent-Age Factor
The following video describes the concept behind the method, and how to implement it in field practice.
Benefits and Applications of Maturity Method
1- The method provides a simple field implementation
2- Real-time and continuous strength prediction in the field. In this context, the test can be considered Non-destructive.
3- Maturity method offers a cheap, yet reliable alternative to conventional method of CIPPOC’s.
4- Selecting critical locations such as joints, or edges is possible in this method.
5- The test results are not affected by cylinder or CIPPOC sample preparation, handling and testing.
These benefits make maturity method a reliable alternative for quality assurance/control in real-time. The reduces the wait-time for compressive strength results from lab, and can be used when rapid scheduling is a concern. The test also provides a reliable method for choosing the best time for the termination of cold weather protection.
Limitations of Maturity Method
Maturity method provides a simple and effective approach for evaluating in-place strength of concrete. However, the application of this method has some limitations. ASTM C 1074 summarizes some of the limitations as:
1- The concrete must be maintained in a condition that permits cement hydration;
2- The maturity method does not take into account the effect of early-age concrete temperature on the long-term strength;
3- The method needs to be supplemented by other indications of the potential strength of the concrete mixture.
In addition to the limitations mentioned in the standard, the following challenges are reported by engineers and field technicians:
4- The application of maturity method for high strength concrete needs further investigation.
5- Calibrating the strength-maturity relationship for each individual mix is challenging.
6- The selection of proper datum temperature or activation energy might be subjective. A recent research by Chang Hoon Lee and Kenneth C. Hover investigates the significance of these initial values on the strength prediction.