Study of steel bars for use in reinforced concrete
produced by the “Tempcore” process
João Pedro Florindo Lourenço
Extended abstract
Dissertation for obtaining the degree of master in
Civil Engineering
Jury
Supervisors: Prof. Augusto Martins Gomes
Prof. Ana Paula Ferreira Pinto
October 2012
Abstract
This paper tries to achieve a better understanding of the mechanical behavior of
Tempcore steel, steel whit which almost all reinforced concrete structures are executed
nowadays.
This text contains a detailed description of what really is Tempcore steel, what are its
constituents, its characteristics and the history of steel production technologies until now, also
presenting new technologies that might become the future of steel production.
An exhaustive description of the legal documents currently in use is made, their
evolution in time and how they compare to the Eurocodes which we will soon have to follow.
A comparative general analysis of Tempcore steel behavior with cold worked and multi-
alloy high strength steel is presented, identifying potential dangers / holes both in current
legislation and Eurocodes in regards to welding and corrosion influence in Tempcore steel
behavior.
Lastly a series of experimental tests is described with the objective of better
characterizing the behavior of Tempcore steel subjected linear tensile tests (traction) when a
reduction in diameter occurs, for instance due to corrosion.
Key words: Tempcore steel; steel bars; reinforced concrete; steel production; steel bars
corrosion; steel bars welding.
1. Introduction
Nowadays, almost all the steel used in reinforced concrete is produced with Tempcore
technology or similar (hot rolled quenched tempered steels), so it is of the utmost importance to
clearly define the mechanical properties these types of steels present, while at the same time
understanding the rules and codes/standards currently in use, their differences and how they
apply.
2. Production
Tempcore steel is a technology that allows the steel mills to produce high resistance
steel without the addition of expensive metal alloys or post production mechanical treatments.
The increase in resistance of these types of steel is achieved by quenching the steel bars
surface at the end of the hot rolling process, followed by a tempering.
The quenching is achieved by exposing the steel bars, which at the end of the rolling
process are at a very high temperature, to a cold water spray, causing the bar’s surface to cool
very quickly, resulting in a martensite transformation while the core remains at the same
temperature.
The resulting untempered martensitic steel is very hard, however it is also too brittle to
use in most applications, requiring tempering in order to be used in reinforced steel.
The tempering is achieved through the residual heat present in the core of the steel bar.
This energy heats the previously quenched section of the steel to below the critical temperature,
followed by a gradual and slow cooling by exposure to air until it reaches room temperature.
From this second heat treatment results a final product which roughly consists of a high
resistance tempered martensite surface, a plain pearlite core and a transition zone in bainite.
Figure 1 – Tempcore steel section with the three different microstructure types distinguishable after chemical treatment (emeraldinsight.com)
Steel as a material was historically obtain with the improvement of iron production, in
the 18th century, due to the invention of the reverbatory furnace, which reached 1300ºC, melting
pig iron. And a technique called pudling that consists on agitating the liquid pig iron, allowing
oxygen to reduce it, creating small globes of solid steel.
These globes could then be removed from the furnace, forged in order to remove slag,
presenting a final product with a quality that can be hot or cold worked in to billets.
The process that brought mass production to steelmaking was the Bessemer process,
invented in the mid 19th century.
Figure 2 – Bessemer converter (http://belajar-engineering.blogspot.com)
This core of the process consists applying a series of hot air jets to the molten pig iron,
causing the pig iron to reduce in to low carbon steel. This also had the positive side effect that
the heat released from the oxidation reaction of the impurities was enough to even melt the
steel (1600-1650ºC).
A similar process that is still currently used in Great Britain was the Siemens-Martin
process. This process has the advantage of reducing energy consumption by using the hot
exhaustion gases to heat the hot air to be injected in the chamber.
Today there are two ways to produce steel, one is the full process of extracting,
producing pig iron (using blast furnaces) and finally reducing it to form steel. The other consists
simply of recycling scrap steel, being this the only way in which new steel is produced in
Portugal nowadays.
The transformation of scrap steel in new steel products is generally made using an
electric arc furnace. This type of furnace works by causing a high intensity electric arc to pass
through the scrap steel causing it to melt, while at the same time oxygen jets reduce applied to
the molten metal reduce it, removing any impurities.
In all steelmaking processes the steel is tested and thoroughly analysed along its
production cycle in order to guarantee that the final product posses the necessary quality.
3. Legislation
Legislation applicable to steel used in construction, most specifically for use in
reinforced concrete, came to be with the objective of ensuring a good quality of buildings and
infrastructure while ensuring safety conditions for its use.
With the increasing exposure of markets to imported goods an ever tighter mesh of
legal documents has and is being created in order to change the scope of its action, protecting
not only national interests but also those of the community in which we are included, the
European Unity.
This is being made with the introduction of the Eurocodes (EC2) and the European
Standards (EN 10080), gradually replacing the national documents, in this case the Portuguese
Regulamento de Estruturas de Betão Armado e Pré-Esforçado (REBAP).
The first legal document applied to reinforced concrete in Portugal dates back to 1918,
called Instruções Regulamentares para o Emprego de Beton Armado (Dec. Lei 4036 of
3/4/1918), evolving and being replaced sporadically until 1983, when the legal document
currently in use, REBAP, was promulgated in the Dec. Lei n.º349-C/83 of July the 30th.
We are now experiencing a transition phase from this document to the Eurocode 2, but
in regards to the mechanical properties of steel, the demands are very similar in all documents,
the values required by current regulations fitting inside the value intervals presented by the
EC2.
A difference is however observed in the way extension at rupture is measured between
some documents, as observed in figure 3.
Figure 3 – Difference between extension at rupture in REBAP against EC2 and LNEC Specifications
4. Behaviour
Tempcore steel bars behaviour is, according to PIPA, 1993, very similar to that of
natural and multi-alloy steel, presenting a higher yield stress resistance than all of them, but a
lower maximum stress resistance than multi-alloy steel.
In terms of extension characteristics, it is observed that it also has a lower strain at
maximum stress than multi-alloy steel bars.
5. Effect of welding and corrosion in Tempcore steel
Due to the nature of Tempcore steel bars, it is currently though that the mechanical
proprieties of this type of steel bars can suffer more from the welding and corrosion than cold
worked or multi-alloy high resistance steel.
5.1. Welding
The problem with welding is the possibility that the heat generated by the welding
process may be enough to negate the effects of the heat treatments applied to the bars,
causing a significant reduction in its load bearing capabilities.
This was investigated by RIVA et al, 2001, reaching the following conclusions:
Butt-welded bars by SMAW technique possess a similar stress resistance than
unwelded bars, while at the same time present a lower ductility;
But-welded joints are most suited to larger diameter bars due to the possibility
of altering the mechanical properties of smaller diameter bars;
Fatigue testing shows that butt-welded bars possess lower ductility than non
welded bars.
Tests made on cruciform specimen show that stirrups can be welded to
longitudinal bars safely as long as the welding made is just spot welding made
bay GMAW or SMAW techniques.
5.2. Corrosion
The specific problem presented by corrosion on Temcore steel bars related to its
structurally heterogeneous section, where the superficial area, which is most susceptible to
corrosion, is also responsible for most of the tensile resistance due to its martensite structure.
Another problem is that Tempcore steel presents a greater vulnerability to corrosion, as
proved by MPATIS et al, 1999, then cold worked or multi-alloy high resistance steel.
The effect of corrosion in the mechanical characteristics of Tempcore steel bars was
studied by APOSTOLOPOULOS el al, 2005 (tensile testing) and APOSTOLOPOULOS, 2007
(fatigue resistance), by subjecting steel bars to accelerated corrosion caused by salt spray
exposure, concluding the following:
Salt spay exposure to S500s Tempcore steel results in a significant mass
reduction, increasing this reduction with the increase in exposure time.
Exposure times of 40 days or more present realistic values with which to
simulate natural corrosion on old buildings in seaside areas;
The effect of the salt spray in the mechanical characteristics of this steel is
moderate. However, when associated to the considerable mass reduction, the
resistance of the elements is reduced in such way that the security coefficients
used are not sufficient to guarantee the elements resistance to the subjected
loads;
The effect of salt spray corrosion as a significant effect in the bars ductility,
noting that for exposures of over 35 days the legal requirement in regards to
strain at rupture is not met;
The maximum load in fatigue tests reduces with the increase in the number of
cycles, reduction which is higher to corroded specimens.
6. Tensile test
6.1. Test
In order to achieve a better understanding of the mechanical behaviour of Tempcore
steel bars, specially the influence of the reduction in section caused by corrosion, a series of
tests was planned and carried out as follows.
The test sample consists of 12 A500 NR SD bars 300mm long, 6 with a nominal
diameter of 20mm 6 with a nominal diameter of 16mm, collected from various construction sites
in Lisbon.
In order to simulate the section reduction caused by corrosion but in a more controlled
(section reduction caused by corrosion isn’t uniform throughout the bar, APOSTOLOPOULOS
et al, 2005) and expedite manner, a lathe was used to wear down the surface to achieve
predetermined diameters along a 100mm long central area of the bars.
The resulting test bars present the geometric characteristics described in table 1 and
are shown in figures 1 and 2.
Table 1 – Test bars geometric characteristics
Specimen ø nominal
(mm) ø (mm) Area (mm
2)
P1 20 19,8 307,9
P2 20 19,6 301,7
P3 20 18,0 254,5
P4 20 17,9 251,7
P5 20 16,0 201,1
P6 20 15,7 193,6
P7 16 15,1 179,1
P8 16 14,9 174,4
P9 16 13,9 151,8
P10 16 13,9 151,8
P11 16 12,0 113,1
P12 16 12,0 113,1
Figure 4 – Specimens P1 to P6
Figure 5 – Specimens P7 to P12
In order to measure the extension after rupture, marks with 20mm intervals were
marked in the central area of each specimen, as observed in figures 1 and 2.
All tests were executed in a universal traction test machine INSTRON Model 1343 and
data acquisition was accomplished automatically using a data logger Spider 8 HBM connected
to a computer.
Tests were executed in displacement control at a speed of 0,20mm/s, and the values for
strength and machine plate displacement were recorded at a frequency of 20Hz.
6.2. Results
As seen in table 1, test specimens P3 to P6 and P9 to P12 possess a heterogeneous
section; as such the results related to those specimens are obtained using the area
corresponding to the area of the machined section.
The results related to yield strength, tensile strength, maximum extension and extension
at maximum strength are shown in table 2 with the corresponding stress-strain graph for all
specimens represented on figure 3.
Table 2 – Test results
Specimen Ø nominal
(mm) Ø
(mm) Area (mm
2)
Yield strength (Mpa)
Tensile strength (Mpa)
εu (mm) εu (%)
P1 20 19,8 307,9 547,9 662,7 - -
P2 20 19,6 301,7 581,1 689,5 22,0 22,2
P3 20 18,0 254,5 537,4 656,6 22,9 23,4
P4 20 17,9 251,7 558,8 676,5 22,6 25,1
P5 20 16,0 201,1 488,3 617,3 22,1 24,7
P6 20 15,7 193,6 470,1 615,0 20,8 26,0
P7 16 15,1 179,1 593,2 715,7 22,3 28,4
P8 16 14,9 174,4 609,9 735,8 19,1 25,3
P9 16 13,9 151,8 511,0 639,8 19,6 26,3
P10 16 13,9 151,8 506,4 633,5 18,9 27,2
P11 16 12,0 113,1 479,6 617,7 14,5 20,9
P12 16 12,0 113,1 462,9 593,7 15,0 25,0
Figure 6 – Stress-strain graph of all specimens tested
In order to allow a better understanding of the test results, the stress-strain graphs were
divided in two groups according to the specimens’ nominal diameter. So in figure 4 the stress-
strain graph refers to specimens P1 to P6 (nominal diameter of 20mm), and figure 6 the graph
represents specimens P7 to P12 (nominal diameter of 16mm). Also shown are the load-
displacement graphs, figure 5 representing specimens P1 to P6 and figure 7 specimens P7 to
P12.
0
100
200
300
400
500
600
700
800
0,00 5,00 10,00 15,00 20,00 25,00
Ten
são
(M
Pa)
Extensão (%)
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
Figure 7 – Stress-strain graph of specimens P1 to P6
Figure 8 – Load-displacement graph of specimens P1 to P6
Figure 9– Stress-strain graph of specimens P7 to P12
Figure 10 – Load-displacement graph of specimens P1
to P6
By observing the stress-strain graphs and the results in table 2, it´s clearly noticeable a
significant degradation of the mechanical proprieties of the specimens with the increase of their
surface wear. Such results were expected as the area of the bars subject to wear is composed
of martensite, which is responsible for the bars high strength resistance.
For easier comparisons the specimens were grouped in pars as shown in table 3 and 4
where are shown variations in percentage of area, yield and maximum load and stress and
extension between the pairs.
0
200
400
600
800
0,00 10,00 20,00
Stre
ss (
MP
a)
Strain (%)
P1 P2 P3
P4 P5 P6
0
50
100
150
200
250
0,00 15,00 30,00 45,00
Load
(K
N)
Displacement (mm)
P1 P2 P3
P4 P5 P6
0
200
400
600
800
0,00 10,00 20,00
Stre
ss (
MP
a)
Strain (%)
P7 P8 P9
P10 P11 P12
0
30
60
90
120
150
0,00 10,00 20,00 30,00
Load
(K
N)
Displacement (mm)
P7 P8 P9
P10 P11 P12
Table 3 – Variation of mechanical and geometric proprieties of specimens with 20mm nominal diameter (P1 to P6)
P1/P2 P3/P4 P5/P6 Δ1 (%)
(P1/P2-P3/P4)
Δ2 (%)
(P3/P4-P5/P6)
Δ3 (%)
(P1/P2-P5/P6)
Ø (mm) 19,7 18,0 15,9 - - -
A (mm2) 304,8 253,1 197,3 -17,0 -22,0 -35,3
Fy (KN) 172,0 138,7 94,6 -19,4 -31,8 -45,0
σy (MPa) 564,5 548,1 479,2 -2,9 -12,6 -15,1
Fu (KN) 206,1 168,7 121,6 -18,1 -27,9 -41,0
σu (MPa) 676,1 666,5 616,1 -1,4 -7,6 -8,9
εu (%) 22,2 24.2 25,3 9,0 4,5 14,0
Table 4 – Variation of mechanical and geometric proprieties of specimens with 16mm nominal diameter (P7 to P6)
P7/P8 P9/P10 P11/P12 Δ1 (%)
(P7/P8-P9/P10)
Δ2 (%)
(P9/P10-P11/P12)
Δ3 (%)
(P7/P8-P11/P12)
Ø (mm) 15,0 13,9 12,0 - - -
A (mm2) 176,7 151,8 113,1 -14,1 -25,5 -36,0
Fy (KN) 160,3 77,2 53,3 -27,4 -31,0 -49,9
σy (MPa) 601,6 508,7 471,2 -15,4 -7,4 -21,7
Fu (KN) 128,2 96,6 68,5 -24,7 -29,1 -46,6
σu (MPa) 725,7 636,7 605,7 -12,3 -4,9 -16,5
εu (%) 26,8 26,8 22,9 0,0 -14,6 -14,6
In all compared pairs the yield and maximum load reduction was greater than the
corresponding decrease in area.
This reduction was less noticeable when comparing pairs P1/P2 – P3/P4, and should
indicate some problems with specimens P1/P2 given that if the area of the identification marks
is taken in to account, the reduction in area is actually greater than the reduction in yield and
maximum loads, which means an actual increase in yield and maximum stress strength.
Due to the likelihood of there being a problem with specimens P1 and P2, the analysis
should focus on bars with a nominal diameter of 16mm.
Figure 11 – Relation between load resistance and area for specimens with nominal 16mm nominal diameter
40
60
80
100
120
140
100 120 140 160 180 200
Load
(K
N)
Area (mm2)
Yield Load
Maximum Load
Figure 12 – Relation between stress resistance and area for specimens with nominal 16mm nominal diameter
As observed in figures 8 and 9, the relation between load resistances, both yield and
maximum, and the corresponding stresses, and the specimens’ area are not linear, the
decrease in both types of load resistance being greater than the corresponding decrease in
area.
Analysing yield stress of pairs P5/P6 and P11/P12 and comparing them to pairs P1/P2
and P7/P8 respectively. A decrease of 15.1% for the first and 21.7% for the latter is shown,
meaning that the specimens P5, P6, P11 and P12 posses yield stress resistance values inferior
to those legally established for this type of steel. This reduction is due to the fact that the
microstructure of the central area of the specimens possesses an essentially ferric + perlitic
nature, similar to that found in hot rolled steel bars.
The behaviour similarity of the bars steel core is also show by the behaviour in the
graphs relating fu/fy with the area of the specimens as represented in figure 13.
Figure 13 – Relation between fu/fy and the specimens’ area
As expected the value of fu/fy increases with the decrease in area, showing that the core
material is similar to hot rolled steel in that it has a higher relation between fu and fy than the
Tempcore steel bar as a whole.
The different ways in which the 20 and 16mm initial diameter bars evolve can also be
explained by a possibly thicker layer of martensite in the bars with 20mm initial diameter.
450
500
550
600
650
700
750
100 120 140 160 180 200
Stre
ss (
MP
a)
Area (mm2)
Yield Stress
Maximum Stress
1,15
1,20
1,25
1,30
100 150 200 250 300 350
Fu/F
y
Area (mm2)
20mm initial Ø
16mm initial Ø
This similarity is also evident by the look of the fracture area of the specimens witch, as
observed in figure 14, is different between a machined and non-machined specimens.
In non-machined specimens this area presents very angular structures, where as
machined specimens present a smoother, more regular look.
Figure 14 – Comparison between the rupture area in specimens P6 (machined) and P7 (non-machined)
In regards to extension values after rupture (table 2), it was measured in an 5Ø
extension centred on the rupture section using the 20mm marks etched in the specimens.
It wasn’t possible to derive any conclusion from those extension values because they
show different tendencies in specimens P1 to P6 and P7 to P12 as seen on table 3 and 4.
Specimen P1 rupture occurred outside the marked area, making it impossible to
measure this value.
An interesting phenomenon shown in figures 4 to 7, is the behaviour specimens
maximum extension. Contrary to the expected, which is that the steel the is present in the
central section of the specimens would have a greater ductility, the machined specimens
present less ability to resist extension.
This is easily observed comparing pairs P5/P6 (machined) to P7/P8 (non-machined),
which possess similar areas, and as such similar striction abilities, but the last pair shows an
extension at rupture 35,5% higher than the first.
In general terms, as observed in table 5, this propriety shows a greater reduction in the
specimens with a nominal diameter of 16mm. This was due to the decrease in striction ability
that results of the smaller section these specimens have.
Table 1 – Reduction in strain resistance (measured from the stress-strain diagrams)
Pairs Reduction in strain resistance (%)
P1/P2 - P3/P4 17,87
P1/P2 - P5/P6 22,51
P7/P8 - P9/P10 26,56
P7/P8 - P11/P12 41,58
6.3. Conclusion
Finally, and although the small number of test is insufficient to allow any definitive
conclusion, I can conclude that:
The central area of the specimens presents a similar behaviour to that of natural steel;
The superficial layer of Tempcore steel bars is responsible for a stress resistance much
higher than its proportional area.
7. Bibliography
[1] - APPLETON, Júlio, “Constructions in concrete – Historic note about their evolution” in
Portuguese.
[2] - APOSTOLOPOULOS, Ch. Alk.; PAPADOPOULOS, M. P.; PANTELAKIS, Sp. G.,
Mechanical behavior of corroded reinforcing steel bars S500s tempcore under low cycle fatigue,
Construction and Building Materials 21 (2007), www.sciencedirect.com.
[3] - APOSTOLOPOULOS, Ch. Alk., Tensile behavior of corroded reinforcing steel bars
BSt500s, Construction and Building Materials 20 (2005), www.sciencedirect.com.
[4] - MPATIS, G.; RAKANTA, E.; TSAMPRAS, L.; MOUYIAKOS, S.; AGNANTIARI, G.,
Corrosion of steel used in concrete reinforcement, in various corrosive environments, Technical
Chamber of Greece, 13th Hellenic Convention for Concrete. vol. II, Rethymnon, Crete; 1999. p.
497–505.
[5] - PIPA, José de Andrade Loureiro, Doctorate thesis with the title – “Ductility of Reinforced
Concrete Elements Subjected to Cyclical Actions, Influence of the Mechanical Characteristics of
the Rebar” in Portuguese, Universidade Técnica de Lisboa – Instituto Superior Técnico.
[6] - RIVA, P.; FRAMCHI, A.; TABENI, D., “Welded Tempcore reinforcement behaviour for
seismic applications”, Materials and Structures/Matériaux et Constructions, Vol. 34, May 2001,
pp240-7.
[7] - Decreto-lei n.º349-C/83, de 30 de Julho, “Standards for Reinforced and Pre-stressed
Concrete Structures” in Portuguese.
[8] - prEN 1992-1 (final draft), October 2001
[9] - prEN 10080, January 2005, Steel for the reinforcment of concrete – Weldable reinforcing
steel – General.
