Monitoring Thermal Aspects Of Concrete
What if we get to know a complete profile of strength gain in concrete in early days such as from day one to a month of casting? Additionally, knowing the temperature history of the fresh concrete up to its 28 days or more based on our requirements at every 30 minutes interval? We can forecast the strength gain after specific days of casting by using it’s profile, by extrapolation. This makes the construction process faster and uninterrupted as planning can be done based on the current required strength level of concrete at specific day. Also, it notifies well in advance in case required strength is not achieved in the concrete mix. The reason could be anything, but if the result is not as per the requirement that we are able to derive in early days and hence the remedial/corrective actions can be planned out accordingly. Let us consider a case of high-rise building, where the column near the ground floor is either of enormous size or the size is reduced using high strength concrete material. Generally, the rate of strength gain is also higher in such concrete. If we are relying on the cube testing at prescribed days, it would not provide continuity of data to generate a profile in graphical format to realize and extrapolate. If due to unforeseen reasons the strength of column has not achieved its design requirement and meanwhile the construction cycle continues up to upper floors, it results in a complex situation where it is cumbersome to take remedial measures. If the column is to be demolished, it would be difficult to break the concrete due to its early strength gain rate. Also, it requires to provide alternative load path during recasting of this column so that on-going construction and the existing floors above this column are least affected. This would be a costlier affair. Instead of this, if we choose smarter way and use the sensor that indicates strength gain profile, we can get the forecasting earlier than the method of cube testing. This would make it easier to rectify with, as appropriate decision can be taken in time when less number of floors are constructed above this column and less strength of concrete is achieved which is comparatively easier to break.
For further information, contact:We can continuously monitor temperatures at every 30 minutes. It also detects difference in temperature of concrete in day and night. This would provide a complete temperature history profile that helps to study the effects of temperature on various properties of concrete. This results in the in-time treatment to the fresh concrete, like: when to start curing to get optimum results, up to what extent the curing should be continued, which type of curing shall be proposed, whether ice flakes needed in the mix or not, temperature gradient in case of mass concrete in thick elements like raft foundations. There are several ways where this data can be utilized to get most optimized alternative in terms of ease of construction methodologies, time saving and overall cost effective. Even though strength development of concrete in lab testing is usually determined with time as the single independent variable as the temperature is more or less controlled, it is well known that actual strength of in-place concrete is dependent on both time and temperature. The product of time-temperature history of concrete determines its maturity. Because time-temperature history of in-place concrete can differ considerably from the time-temperature history of similar laboratory-cured specimens, strength development of the laboratorycured specimens is often a poor indicator of strength development of the in-place concrete. Field curing of specimens is an established practice [ASTM C 31(ASTM 1992a), ASTM C 873 (ASTM 1992g)] that seeks to reduce this discrepancy and these methods provide a reasonable solution to this problem when the dimensions of the concrete structure are such that ambient temperature is the principal factor controlling the temperature of both the in-place concrete and the test specimens. In the case of mass concrete, the ambient temperature is only one of several factors controlling concrete temperature, with size of placement, rate of heat evolution, and location in the placement being others. Therefore, the time-temperature history of mass concrete, and consequently the strength development, may vary considerably from that of field-cured specimens and may vary throughout the structure. This complexity makes use of laboratory-cured or field cured specimens essentially useless in predicting in-place strength. However, if temperature history can be determined and a reasonable functional relationship between strength and this time-temperature history can be developed, then strength development can be estimated from this temperature history.
Mr. Shashank Duraphe
POST TENSION SERVICES (GUJARAT) LLP
(A Group Company of Post Tension Services India Pvt. Ltd.)
E: firstname.lastname@example.org, email@example.com
O: +91 265 2341298, +91 756 787 8798
M: +91 756 783 3351
To check the Temperature, Maturity & Strength in Mass Concrete.
- Trial done in 1m x 1m x 1.5m
- Ambient Temperature while concrete is placed: 24oC at 1:44 pm Noida, UP
- G-81 Concrete Temperature at 500mm below concrete top surface at initial stage: 23.7oC.
- G-80 Concrete Temperature at 1500mm below concrete top surface at initial stage: 37.0oC.
- G-82 Concrete Temperature in 150mm x 50mm x 150mm Cube mould at initial state: 19.2oC.
- Observation– To check the Heat of Hydration generated at different stages of the concrete placed in 2400m3 raft.
- Cracking due to volumetric changes.
- Shrinkage Maturity
- Mass Concrete is a concrete where thermal stress is a concern. Strength
- Stage 1 Generation of heat due to cement hydration (Graph 2 and 3).
- Stage 2 after the hydration process, there is a decrease in temperature (Graph 1).
Temperature – Time Relationship
Cement: 260kg (OPC 53) + Flyash 260kg
Raft Size: 30m x 40m x 2m=2400m3
RMC Producers: UltraTech Concrete, Noida
Grade of Concrete: M-40
Temperature & Strength Measurements of In – Place Concrete
- Two specimens were instrumented with Smart Sensors to measure the Temperature, Maturity and Strength. These were in monoliths (Figure 1) placed on 11th December 2017.
- Two Smart Sensors were placed in one monolith at depth of 500mm (G-81 as shown in Figure 1) from top surface of the concrete and 1500mm (G-80 as shown in Figure 1) from the top surface of concrete.
- Temperature data logged every 30min from the initial concrete placement through 28 days.
- Temperature data for laboratory-cured specimens used for calibration were collected every 30minutes through 28 days.
Strength – Time Relationship
If concrete temperature is below 20oC during the initial day (within 7 days) then it is compulsory to perform steam curing at where the mass concrete is done.
Strength – Time History
- The strength history of the laboratory cured calibration mixture is illustrated in Graph 2 - sensor embedded in cube specimen G-82.
- The strength history of Mock up 500mm G-81, which was placed in December 2017 is illustrated in Graph 2 –mock up 500mm.
- The strength history of Mock up 1500mm G-80, which was placed in December 2017 is illustrated in Graph 2 –mock up 1500mm.
- The strength history of this Mock up 1500mm G-80 at initail stage was more than all of them because heat of hydration was high in core area, later strength measured by sensor in cube specimen G-82 was more than all three because it was cured under controlled temperature of 25oC to 27oC Hence, it is suggested to use the HVFA (High Volume Fly Ash) in mass concrete to reduce the heat of hydration generated in concrete, to avoid internal defects.
Concrete temperature rises faster in depth compared to surface temperature. In addition, the cracks resulting from thermal stresses are important because thermal cracks can decrease the structural integrity of concrete and can cause serviceability problems. Such serviceability problems are excessive deflections due to a decreased section, and decreased durability resulting from the corrosion of reinforcing steel, freeze-thaw damage, and increased sulphate attack resulting from the ease of penetration of sulphates through the cracks. The smart sensor works on the maturity of concrete and it derives strength gain by measuring temperature history. There are several other benefits that make it smarter than other available devices and methodologies. It is connected through mobile phones. The data is shared on cloud and it can be shared globally. A person can monitor the concrete material sitting in office, without going to the site. When a site person connects his mobile through wireless connection to the sensor, it receives all the data till the current time and it instantaneously displays strength as well as temperature data and the maturity gained till date. Simultaneously, it syncs the data with cloud, so that it can be shared with more number of users. This is how it makes it easy to access at any time. Instructions can be given to site persons for remedial measures and continuous monitoring of changes can be done after remedial measures are implemented. This way instantaneous data can be received and concluded in better manner without any delays in getting the tests at laboratories and receiving the test results. It works as a NDT in a new construction to take preventive action for any adverse effect of concrete poured.
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