Types of structure with significant tension members
Thermo-Mechanically Treated or TMT steel reinforcing bar (rebar) is widely used in the reinforced concrete construction industry in India for the past two decades. Technically, the TMT (jargon in India) refers to QST or Quenched and Self-Tempered steel.
The TMT/QST steel rebars are manufactured by a rapid cooling (quenching) and slow reheating (self-tempering) of hot-rolled bars. The combination of high strength and ductility is its specialty, which is attributed to the special Cross-Sectional Phase Distribution (CSPD) with two phases. An ideal/adequate CSPD is defined as that in Figure 1a with a continuous, concentric, and uniform ‘tempered martensite’ (TM) ring around the ‘ferrite-pearlite’ (FP) core. The step-by-step procedure on how to obtain these images on a steel rebar is provided in Nair and Pillai (2017).
The Reality a poor quality QST steel (PQ-QST)
A recent research (Sooraj 2017) on multiple steels in the Indian market revealed that many products exhibit inadequate CSPD and poor quality, as in Figure 1b, due to inadequate quenching or manufacturing process.
The “quality” in the current context is defined based on the CSPD. Herein, (i) Good Quality and (ii) Poor Quality are denoted as GQ and PQ, respectively. The PQ-QST/TMT steel rebars could exhibit ‘discontinuous’ (top image in Figure 1b) or ‘eccentric’ (bottom image in Figure 1b) distribution of TM phase. The CSPD tested in 13 brands of 8, 12, and 16 mm bars revealed that this issue is profound in 8 and 12 mm rebars. A few more photographs are shown in Figure 2.
Why should we be concerned about the CSPD? Because, the effect of these defects are reflected in the corrosion resistance and mechanical response of TMT/QST steel bars.
PQ-QST rebars exhibit poor corrosion resistance
At times, TMT/QST rebars might seem to rust at an earlier stage or faster than expected at site. This could be attributed to the presence of FP phase at the periphery of poor quality TMT/QST steel rebars.
Sooraj (2017) found that the FP phase has less corrosion resistance than the TM phase; and starts corroding at a relatively lower concentration of chlorides (say, at nearly 20% less chlorides). As a result, TMT/QST steel rebars with inadequate CSPD can corrode much earlier than those with ideal/adequate CSPD (with complete TM ring).
Also, the PQ-QST/TMT rebars are prone to bending-induced cracks. This becomes worse in the case of bent rebars used as stirrups with the least clear cover in concrete (See Figure 3a). Overbent rebars could have crevices through which corrosion could propagate faster, even in absence of chlorides (See Figure 3b). This corrosion could result in eventual spalling of concrete at the corners of beams and columns. A GQ-QST rebar with a complete TM ring on its periphery will bend without the formation of such cracks.
PQ-QST rebars show a relatively higher variation in tensile properties
GQ-QST steel rebar specimens from same product could exhibit very similar stress-strain behavior as shown by the black curves in Figure 4. However, a PQ-QST specimens from the same product may exhibit significantvariation in the yield strength (and ultimate strength), as shown by the red curves in Figure 4. This could be due to the inadequate CSPD (as shown in Figure 1b).
It is evident that there is a serious issue in the quality of QST/TMT steel rebars produced by some of the local brands in India. Although this is not an unknown phenomenon to the steel industry, literature does not provide adequate documentation on defective CSPDs. As an initial step towards a better quality control in the production of TMT/QST steel rebars, a ‘TM-ring’ test setup, procedure, and a 2-level acceptance criterion (snapshot shown in Figure 5) were developed (Nair and Pillai 2017). This test involves cutting of rebar with a coolant, and immersing the cut surface in an etching solution (Nital solution) to form the darker ring (TM phase) and lighter core (FP phase). Visual observation and photography can help in assessing the quality of TMT/QST rebars in the laboratories at steel plant and construction sites. It is expected that the stakeholders would become aware of this quality issue and use the “TM-Ring” test to monitor/maintain good quality of steel rebars.
For more details on the test procedure for assessing the CSPD and classification, see Nair & Pillai (2017). Further details on the effect of inadequate CSPD on the mechanical and corrosion properties of TMT/QST steel rebars is available in the thesis by Sooraj (2017).
- Sooraj A. O. Nair and Radhakrishna G. Pillai (2017), ‘TM-ring test’ – A quality control test for TMT (or QST) steel reinforcing bars used in reinforced concrete systems, Indian Concrete Institute Journal, 18 (1), 27-35, July
- Sooraj Kumar A. O. (2017), ‘A study of corrosion and mechanical characteristics of Quenched and Self-Tempered (QST/TMT) steel reinforcing bars used in concrete structures, Master of Science Thesis, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, July.