Practical Installation Applications for Wireless Maturity Sensors
Once the calibration and validation are complete, maturity and temperature sensors can be installed on site to monitor in-place strength. None of the standards provide any guidelines for installation and ASTM C1074 only provides the following note: “In building construction, exposed portions of slabs and slab-column connections are typically critical locations. The advice of the engineer should be sought for critical locations in the particular structure under construction.”
The following section discusses the practical points for the selection of locations and the number of maturity sensors required based on the applications in the field. When choosing a location, one must always keep in mind that colder temperatures will lead to decreased strength and vice-versa. Cylinders are made to represent the entire pour, but by using maturity, it is possible to target specific locations of the structure for temperature and in- place strength monitoring.
ACI 228. 1R, Chapter 5 - Implementation of in-place testing does propose a guideline in terms of maturity monitoring frequency in different elements. On average, it is recommended that one maturity meter be installed for every 15 m3 (20 yd3) which, based on the author’s experience in the field, is somewhat overly conservative, unrealistic, and too expensive. Since there are no standards on the number of maturity probes that need to be placed in a concrete pour, a general practical guideline is proposed. Furthermore, as the maturity method is meant to eliminate the use of field-cured specimens, it is recommended to follow the same minimum requirement of one maturity reader per 100 m3 (1/150yd3). However, one maturity meter per 75 m3 (1/100yd3) is a typical guideline for maturity monitoring followed in the industry. It is also recommended to record maturity at a minimum of two locations in a pour. The number of measurements required is always dependent on the specific application and volume of the pour.
In terms of where to install the sensors, there are three main questions that the engineer or the general contractor should answer:
Where are the critical weather locations on the structural element?
Critical weather location can be defined as the coldest and most exposed area to environmental conditions (e.g. wind, rain, etc.). Concrete will typically be colder at the corners or edges of a structure where there is less mass effect and more exposition to the ambient temperature.
What is the pour schedule?
In most cases, a pour can take several hours, which means that the concrete poured at the beginning and at the end does not possess the same strength as the concrete poured at the beginning since it will have started to hydrate earlier than the concrete poured at a later time. In general, a distribution of maturity sensors over the pour schedule will provide a good indication of the average strength gain. The end of the pour is a typical location for maturity monitoring since it will most likely be the location where lower in-place strength is observed.
Where are the structurally critical locations?
It is important for the concrete to gain sufficient strength at some critical locations in the structure before the project can move to the next phase. The critical locations can change depending on the type of structural element being monitored. More specific guidelines for typical critical locations are described in Section 1.1.
Depending on the type of structure, maturity might need to be monitored at different locations. This section covers the principal behind the placement of maturity meters in slabs, post-tension decks, mass concrete, and vertical elements.
In one-way or two-way slab systems, the structurally critical areas are located at the larger negative and positive design moments. Typically, the maximum positive moment is located at a mid-span and maximum negative moment is located at a slab-column interface.
To illustrate a simple example for a two-way or one-way slab with unsupported span length (where L1 = L2) and uniformly distributed load, the moment distribution is usually represented as shown in Figure 1-1. In the case of this image, which is specific for a two-way slab system without beams, the maximum positive moment is located in the middle of the exterior span, while the maximum negative moment is located at the exterior slab-column interface for the first interior column. For a different slab configuration (slab with drop panels, beams, bands), span lengths, load patterns, and connections, the location of the maximum positive and negative moment will change. The structural engineer will be able to identify those critical locations.
The moment distribution shown in Figure 1-1 can also be visualized on a floor plan, as shown in Figure 1-2. In this case the highlighted locations (red circles) represent the locations of the maximum positive and negative moments. The maturity should be monitored in those locations considering the pour schedule and the possible colder areas.
For the negative moment location, the part of the concrete that is under compression is located at the bottom of the slab (or beam), and vice versa for positive moment location. Maturity should be monitored at critical compression locations as shown in Figure 1-3. Installing the sensor at the location of the nearest rebar from the top or the bottom would make the installation easier for any type of monitoring equipment.
Post tensioning is one of the most popular applications where the maturity method is used in North America. Tendons are usually tensioned as early as possible. Knowing the actual in-place strength at any given time is more cost effective than using cylinders since no time is wasted waiting for the results from the breaks. As a result, the decision to tension can be made more quickly. More importantly, maturity allows for the monitoring of the strength at the center and the edges of the slab or girder. When implementing post tensioning, the mid-span is usually warmer, meaning that it will reach the required strength faster.
Often the critical location becomes the edges toward the ends of the structural element (anchor location) where the temperature is cooler and the cables are located closer to the surface (stress concentration zones). Figure 1-4 represents damages to a post tensioning slab where cylinders were used for strength monitoring. In these cases, maturity could have prevented such damages. Figure 1-5 schematically represents the typical locations where maturity should be monitored on a post-tensioned deck, focusing on the edges and the maximum interior moment.
Due to the mass effect, the concrete core of a mass element has a high temperature compared to the surface, which is greatly affected by ambient temperature, and the bottom where heat is absorbed by the soil. Project specifications must be followed to measure the temperature at various locations. This is usually done at the center and surface, while additional measurements can be specified at the bottom and on the edges. If the difference in temperature between the core and the other locations are too large, it can cause internal thermal stress and, as a result, thermal cracking. If the tensile strength of the concrete is not high enough to withstand the thermal stress, it can cause significant cracking. The location of the temperature/maturity measurement is summarized in Figure 1-6.
The ACI 207- Mass Concrete Guideline states that the difference in temperature between the center of the element and the surface must remain smaller than 20ºC (35ºF) during curing. As the concrete hardens, this requirement can become conservative since it would gain enough strength to withstand larger stresses. In other cases, specifically at early age, it can lead to an overestimation of the allowable gradient. As concrete hardens, the tensile strength increases, which means that the concrete is actually able to withstand a higher temperature gradient differential as it cures. By measuring the in-place strength based on maturity, it is possible to determine the actual temperature differential allowed in order to prevent cracking. Equation 1-1 shows the simplified equation proposed by Bamforth and Crook (2006) to determine the allowable temperature difference limit.
f’t represents the tensile strength, which can be measured at the surface of the mass element (see Figure 1-7) using the maturity method. E represents the modulus of elasticity (modulus of elasticity can also be determined through the maturity method, see section 1.2.2), R the degree of restrain, and C the creep factor, which can be taken as 1 to be conservative. The coefficient of thermal expansion (CTE) can be obtained by performing the AASHTO T336 test.
Additional information on how to obtain these factors are provided in ACI 207.2R. Using maturity to determine the allowable variation in temperature in a mass pour can reduce the amount of heating or the cooling required as well as optimize the curing time.
Critical paths for the completion of a project are highly dependent on the completion of vertical elements such as columns and walls. A common practice is to leave walls to cure for 7 days. This approach is overly conservative. Important time savings can be gained by quickly moving on to the next construction phase. The colder area in a vertical element is at the top exposed surface for both walls and columns. Additionally, maturity monitoring needs to be done at critical locations in shear walls or columns which are located at connection areas (identified in Figure 1-7). Sometimes, large vertical elements can also be considered as mass concrete. In those cases, temperature differentials between the core and surface need to be monitored.
One main application which can use maturity to its advantage is the paving industry. In most pavement repair cases, opening the road earlier to traffic can be beneficial. Additionally, in a lot of cases, pavement projects can be located at a far distance from the lab, which makes the transportation of the cylinders time consuming and expensive. Pavement jobs usually require high early-strength concrete where time is of the essence. Making a calibration curve to target strength in a very short period of time can allow for more precise results when compared to cylinder break tests meaning contractors are able to open the roads to traffic in a shorter time. Most departments of transportation currently approve of the maturity method to determine in- place strength.
For pavement, the location at which maturity must be monitored varies for typical structures. The location of the sensor must be based on the targeted area for early opening to traffic. Typically, maturity must be monitored in the last segment of pavement (within a certain distance from the end of the paving operation). Other monitoring locations must be defined by the engineer and should follow the specifications from the DOT in terms of frequency of the measurement.
Modulus of Elasticity (MoE)
The maturity method is well known and applied for compressive strength, and also frequently used to determine the flexural strength of the in-place concrete in accordance to the ASTM C1074 test method. One other application which is not standardized is the use of the maturity method to monitor the concrete modulus of elasticity. Researchers have established that the maturity method could be applied to determine the concrete MoE at early age.
The precast industry constitutes a very large amount of the concrete produced every year. Efficiency and quality control are very important factors. The National Precast Association (NPAC) and Canadian Standard Association (CSA A23.4) both require temperature monitoring to be done in precast elements. Since temperature monitoring is the base of the maturity method, using maturity would not come at a significant extra cost as it would only require calibration. Knowing the in-place strength can greatly enhance quality control and optimize the curing time.
A specific precast industry that can also benefit from the use of maturity is the tilt-up industry. In addition to compressive strength measurement, it might also be required to measure flexural strength and modulus of elasticity to avoid damages during the lifting process. Maturity calibration can be used to monitor all three properties. In other words, by monitoring the temperature and maturity of the tilt-up element, the flexural and compressive strength, and MoE can easily be obtained without the need for testing additional cylinders and beams. It would also provide additional benefits to the quality control practice and the amount of curing required. Maturity in tilt-up application is gaining more and more popularity in the industry.
Shotcrete is usually used in hard-to-access areas, where having the appropriate strength is crucial, such as mining applications. In this case, maturity can be used to improve quality control and potentially reduce the cost of the core-drilled cylinder breaks taken from the panel test. The same step as described in the calibration must be done. The only difference is that instead of casting cylinders for the calibration, cores from panels must be taken according to ASTM D1140 and the temperature measurement for calibration must be taken from the center of the panel. Special attention would need to be directed to the dry-batch shotcrete as the nozzle operator is the element in charge of controlling the amount of water in the mix. Additionally, larger variance could be expected between the calibrated mix and the in-place concrete.