Tensile Strength of Steel Bar Subjected to Oxy-Acetylene Heat

Grace T. Dolloso
Flordeliza Neri-Cabang

College of Engineering and Architecture

Abstract

The purpose of this study was to determine the significant difference on the tensile strength of reinforcing steel bars of varied diameters, exposed and not exposed to oxyacetylene heat. The steel bars used were 16 mm, 20 mm and 25 mm diameters of grade 33 and exposed to different duration of heating such as thirty seconds, 1minute and thirty seconds and three minutes. The bars were tested using universal testing machine after cooling at ambient temperature. Results show that there is a significant difference on the tensile strength between heated from non-heated. The diameter of the bar has the higher effect to the tensile strength than the time of heating.

1. Introduction

Background of the Study

Deformed steel bars are commonly used as reinforcement especially to concrete and masonry and to other materials of which the tensile strength is low. Steel is known to have a high tensile strength which is one of the structural properties of a material to resist tensile force and elongation.

Oxyacetylene is widely known and used for welding and connecting metals like steel bar and metal plates; on the other hand, oxyacetylene heat is also used to aid in bending deformed steel bars to its desired form such as hooks to avoid slippage as they are embedded in the concrete to take up tensile forces. It is believed that any materials subjected to heat can deplete or reduce its strength. Steel material as reinforcement plays a significant role in structural members since this material reinforced the strength of the other weak material like concrete. The common practice of the construction industry nowadays in bending bigger diameter steel bar is the use of oxy-acetylene heating.

Review of Related Literature

Tensile Strength

Tensile strength measures the force required to pull something such as rope, wire, steel, any metal or a structural beam to the point where it breaks.

The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. Maximum load that a material can support without fracture when being stretched, divided by the original cross- sectional area of the material is an important concept in engineering, especially in the fields of material science, mechanical engineering and structural engineering. (Science Daily, 2011)

Tensile strengths have dimensions of force per unit area and in the English system of measurement are commonly expressed in units of pounds per square inch, often abbreviated to psi. When stresses less than the tensile strength are removed, a material returns either completely or partially to its original shape and size. As the stress reaches the value of the tensile strength especially when the material is ductile, it has already begun to flow plastically rapidly forms a constricted region called a neck, where it then fractures. (Encyclopedia Britannica, 2011).

Steel Bar as Reinforcement

Steel bars as reinforcement are requirements for all types of concrete and masonry work of which concrete hollow block is one. When concrete structural members must resist tensile stresses, steel reinforcement bars supply the necessary strength. Deformed or corrugated steel bars are embedded in the concrete in the form of a mesh. These deformed bars have been developed in order to force the concrete between deformations such that failure in shear will occur before slippage. The National Building Code has promulgated guidelines on how and what kind of reinforcement is appropriate for a certain type of work depending upon the purpose for which it is to serve. A bond forms between the steel and the concrete, and stresses can be transferred between both components. The size and spacing requirements for concrete and concrete hollow block reinforcement must be indicated on the plan or specifications.

Structure of Iron and Steel

Carbon steel is an alloy of iron and carbon. Alloys containing less than 0.008 percent carbon are classed as irons. Steel is an iron-carbon alloy in which the carbon content is less than 2.0 percent. These steel products, including structural steel and reinforcing steel, can be rolled and molded into a shape. However, as the carbon content goes above 2.0 percent, the material becomes increasingly hard and brittle. Cast iron has carbon content above 2.0 percent. Carbon acts as both a hardener and a strengthener, but at the same time it reduces the ductility. High strength steels are alloys containing less than 0.8 percent carbon (the eutectoid composition) and are referred to us hypoeutectoid steels. (Derucher, Kenneth; et.al.1998).

Based on the Philippine National Standard, a weldable deformed steel bars have a carbon content of 0.30% for grades 33, 40 and 60 combined with other alloying elements.

According to Bob Capudean (2003), a contributing writer on the fabricator.com, when a piece of metal is heated and cooled to a specific temperature, that metal goes through a phase change, where crystal structures change. When a piece of metal melts, the crystal structure breaks down and metal goes from solid to liquid. When it solidifies, it can also be a phase change, as the structure reforms from liquid to solid. Phase changes can take place in many metals while still in the solid state. These phase changes are directly related to temperature and take place in the metal's crystalline structure. While the temperature controls these transformations; stress, cooling rate and alloy or chemical composition can all influence the temperature at which the changes take place.

A typical phase diagram of iron-carbon is shown below as presented by Dr. Dmitri Kopeliovich. He disclosed the phase's compositions and their transformations occurring with the alloys during their cooling or heating. The carbon content 6.67% corresponds to the fixed composition of the iron carbide, Fe3C, which is called as cementite, has 93.33 percent iron and is very hard and brittle substance, influencing the properties of steels and cast iron.

The following phases are involved in the transformation, occurring with iron-carbon alloys: L - Liquid solution of carbon in iron; d-ferrite Solid solution of carbon in iron. Ferrite is iron that has not combined with carbon in pig iron as steel. This fact allows the steel to be cold-worked. Maximum concentration of carbon in d-ferrite is 0.09% at 2719 F (1493 C) - temperature of the peritectic transformation. The crystal structure of d-ferrite is cubic body centered. Austenite is gamma iron with carbon solution. The eutectic of the carbon-steel alloy is the combination that melts at the lowest temperature, so that the eutectic at 4.3 percent carbon melts at 2060 F (1130 C) and will contain a solid solution of austenite and cementite. Below the 1330 F (723 C) mark, the solution changes to cementite and pearlite. Pearlite is a eutectoid and changes to a solid at 1330 F (723 C), a lamellar aggregate of ferrite and cementite often occurring in carbon steels and cast irons. The eutectoid in an equilibrium diagram for a solid solution is the point at which the solution on cooling is converted to a mixture of solids (Derucher, Kenneth; et.al. 1998).

Figure 1. The Iron-Carbon Phase Diagram. (Kpeliovich, D., 1991)

According to Dr. Dmitri Kopeliovich, the critical temperatures are:

Phase compositions of the iron-carbon alloys at room temperature

From a woodworker's guide to tool steel and heat treating (Peter L. Berglund, 2006), steel exhibits different colors depending on temperature.

Figure 2. Tool Steel Color vs Temperature (Tool Steel and Heating, 2006).

Temperatures above 800 F (427 C) will produce incandescent colors, where the atoms in the steel are so energized by heat that they give off photons. Temperatures below 800F (427 C) will produce oxidation colors. As the steel is heated, an oxide layer forms on the surface; its thickness (and thus the interference color as light is reflected) is a function of temperature. These colors may be used in tempering tool steel.

Oxyacetylene

Oxy-fuel welding (commonly called oxyacetylene welding, oxy welding, or gas welding in the U.S.) and oxy-fuel cutting are processes that use fuel gases and oxygen to weld and cut metals, respectively. French engineers Edmond Fouche and Charles Picard were the first to develop an oxygen-acetylene welding machine in 1903.

Oxy-fuel is one of the oldest welding processes, though in recent years it has become less popular in industrial applications. However, it is still widely used for welding pipes and tubes, as well as repair work. It is also frequently well-suited, and favored, for fabricating some types of metal-based artwork. Oxy-fuel equipment is versatile, lending itself not only to some sorts of iron or steel welding but also to brazing, braze-welding, metal heating (for bending and forming), and also oxy-fuel cutting.

In oxy-fuel welding, a welding torch is used to weld metals. Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal. The molten pool is generally supplied with additional metal called filler. Filler material depends upon the metals to be welded.

Acetylene (C2H2) is a volatile fuel gas which stems from acetylene's triple carbon bond. This bond can hold a considerable amount of energy that releases when ignited. However, with the bond's unstable nature, it can suddenly explode unless it is kept at the proper pressure between 15 psi and 29.4 psi. Even sudden bumps and small shocks can cause an explosion. Because of the fact that oxygen is needed to sustain any flame, it also plays an essential role in the use of all blowtorches. Oxygen acts as accelerant that helps the acetylene burn at a higher temperature. The oxyacetylene combination produces hotter flames than any other gas combinations. Oxyacetylene torch produces flame ranging from 5000 ?F (2760 ?C) to 6000 ?F (3316 ?C). Adding pure oxygen to the flame increases the performance of acetylene by more than 1000 ?F (538 ?C). (Howstuffworks, 2011)

Related Studies

This research about the test of ?Oxyacetylene welded joints in steel plates? gives the results of a series of tests of the strength of oxyacetylene welded joints in mild steel plates. The joints were welded by skilled workmen in a plant especially- equipped for oxyacetylene welding.

A study conducted by Herbert L. Whittemore entitled "The Strength of Oxyacetylene Welds in Steel" gives the results of tests of strength of welds made under repair shop conditions; it also gives a detailed discussion of the technique of welding with the oxyacetylene blow torch. Tests were made under three conditions of loading: (a) static load in tension (in a testing machine), repeated load (bending), and (c) impact in tension (in a drop testing machine).

The static tension tests give an index of the resistance of the welded joint to loads applied only a few times and without heavy impact, such as floor loads in warehouses and the dead loads on bridges. The repeated stress tests give an indication of the resisting power of the welded joint to loads repeatedly applied, such as loads carried by springs and carriage axles. The impact tests give an index of the ability of the welded joints to resist sudden heavy shock without complete rupture. High resistance to rupture under impact load represents insurance against the sudden and complete failure of a part subjected to severe bending or stretching, rather than its stress- carrying ability. High resistance to rupture under impact is of importance in material for machine parts or for railway service.

Most grades of steel used as rebar are suitable for welding, which can be used to bind several pieces of rebar together. However, welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire (Rebar, 2008).

Topcu and Karakurt (2008) of the Department of Civil Engineering, Eskisehir Osmangazi University, Turkey conducted an experimental study on the properties of reinforced concrete steel reinforcing bars (rebars) exposed to high temperatures. Samples used were 10 and 16 mm in diameter and 200 mm in length S220 and S420 reinforcement steel bars. Test specimens were subjected to 20, 100, 200, 300, 500, 800 and 950 ?C temperatures in a high furnace for 3 hours, respectively. After the heating process, steels were cooled naturally down to room temperature. Consequently, tensile tests were performed to the steel rebars using universal tensile strength test machine of which the tensile strength, yield strength and elongation were determined for elevated temperatures. Test results showed that there was no significant reducing of tensile strength for both types of steel rebars up to 500 ?C temperature. However, the tensile strength losses 51% for S220 and 85% for S420 steel rebars at 800 ?C temperature exposure. The tensile strength also decreased with the highest temperature exposure of 950 ?C, at 60% for S220 and 90% for S420. Based on the results, the remaining tensile strengths of S220 plain steel rebar is higher than S420 ribbed steel bars after high temperature exposure. The remaining strength of the steel rebars after 500 ?C exposure were seen lower than the design strengths of the steels. Possibility of complete strength loss at high temperature may happen when a structure is subjected to huge fire. On the other hand, the strength of the steel rebar in structures is influenced with the exposure time and type of fire depending on the heat transfer through concrete cover to steel parts.

It was observed that the elongation ratios were the same under elevated temperatures but S220 steel rebars have higher elongation ratios than S420 rebars. The S420 steel showed a brittle fracture behavior under elevated temperatures. This behavior is not sufficient for steel rebar in reinforced concrete. It can be presumed that after a fire inside the reinforced concrete building, the deflections of the structural members increase with the ductile behavior of the steel reinforcement at high temperatures. The elongation ratios slightly increased up to 300 ?C, however above this temperature material became brittle with decrease of the elongation values. Elongation losses for both S220 and S420 steel rebars were 1.2% and 1.6%, respectively at 800 ?C. Toughness of the material was also considered in the study, where it showed a decrease in value at elevated temperatures.

Lejano, et. al. (2006) investigated the effects of elevated temperature on the yield and tensile strengths of reinforcing bars embedded in rectangular concrete beams. Specimens used were 16mm, 20 mm and 25 mm diameter bars of 420 mm length arranged and distributed to a 200 mm x 200 mm x 500 mm concrete beams and 6 inches x 12 inches concrete cylinders. The bars were tied with a 5 mm-diameter stirrups to avoid misplaced during the pouring and rodding of concrete in the molds. The researchers had chosen three target strengths, 21, 28 and 35 MPa and the concrete cover was maintained at 40 mm. After curing for 28 days, the samples were air dried and heated in a furnace for eight (8) hours,starting from a room temperature to 900 ?C. After reaching the temperature of 900 ?C, samples were heated for 1, 2 and 3 hours at the same temperature, after which were removed from the furnace and allowed to cool to room temperature. The samples were crushed and steel specimens were retrieved and subjected to tension test using Universal Testing Machine (UTM) and elongation was measured. Results showed that the yield and tensile strengths of the bars had decreased by as much as 45%. It was found out that the smaller the size of the rebars, the faster the decrease of yield and tensile strengths. It was also revealed that the longer exposure of the rebars to an average heat of 900 ?C reduces the yield and tensile strengths even greater compared to shorter heating times of the steel bars. Larger rate of decrease were also shown in the yield and tensile strengths with concrete having lower strengths.

Definition of Terms

Acetylene is a colorless gas with a very distinctive, nauseating odor that is highly combustible when mixed with oxygen. It is very unstable if compressed to more than 15 psi.

Alloy Steel is a metal comprising mainly of iron and carbon with additional elements such as manganese, chromium, or vanadium thus exhibiting excellent properties.

Brittle is applied to materials that fail in tension or when there is little or no evidence of plastic deformation before failure. The material is liable to fracture when subjected to stress.

Brittle fracture has relatively little plastic deformation and crack is unstable such as ceramics, ice and cold metals.

Concrete is an engineering material made from a mixture of cement, water, fine aggregates and coarse aggregates, and a small amount of air which can be delivered to the jobsite fresh and capable of being poured, cast or molded that hardens to form a stonelike mass.

Concrete masonry is block and brick building units molded of concrete and used in all types of masonry construction. Concrete masonry is used for load-bearing and nonload-bearing walls; piers; partitions; fire walls; backup for walls of brick, stone, and stucco facing materials; fireproofing over steel structural members; firesafe walls around stairwells, elevators, and other enclosures; retaining walls and garden walls; chimneys and fireplaces; concrete floors; and many other purposes.

Deformed Steel Bar is a steel bar, the surface of which is provided, during hot rolling, with lugs or protrusions called deformation.

Ductility is a property of a material used to describe the extent to which a material can be deformed plastically without fracture. It is the material�s ability to deform or stretch under tensile stress.

Ductile fracture has extensive plastic deformation ahead of crack and crack is stable that resists further extension unless applied stress in increased. Most metals (not too cold) have ductile fracture.

Elongation is the act of lengthening or the change in its size. It is a extension in the gage length of test specimen, measured after rupture, expressed as percentage of the original gauge length.

Ferrite is a soft and ductile material with a Brinell hardness of 90 and a diamond hardness of 170 with a 40 percent elongation in 5.08 cm specimen.

Fracture is the separation of an object or material into two, or more, pieces under the action of stress.

Impact Load is a force delivered by a blow as opposed to a force applied gradually. It is the dynamic effect on a structure, either moving or at rest, of a forcible momentary contact of another moving body.

Ingot is a mass of metal cast into some convenient shape for storage or transportation to be remelted later for casting or finished by rolling, forging, etc.

Length is a piece of straight bar without joint or weld, cut to a specified size.

Nominal Diameter of a Deformed Bar is the diameter of a deformed bar equivalent to the diameter of a plain bar having the same mass per meter.

Pearlite is harder and less ductile than ferrite but is softer and less brittle than cementite which has a Brinell hardness of 275 and a diamond hardness of 300. It has an elongation of 15 percent in a 5.08 cm specimen. In this state the steel is soft and workable.

Reinforcing Bar (Rebar) is also called deformed steel bar commonly used in reinforced concrete and reinforced concrete masonry structures. It is usually formed from carbon steel with ridges for better mechanical anchoring into the concrete.

Reinforced Concrete is a concrete in which steel reinforcement bars or fibers have been incorporated to strengthen a material that would otherwise be brittle.

Resistance is a force that tends to oppose or retard motion.

Rupture Strength is the strength of the material at rupture. It is also called the breaking strength.

Stress is a force per unit area.

Tensile strength is the ability of a material to resist a force that tends to pull it apart which is expressed as the minimum tensile stress needed to split the material apart. It is the value obtained by dividing the maximum load observed during the tensile straining until failure occurs, by the nominal specimen cross sectional area before straining, also called the ultimate strength.

Tension Test is a destructive test in the sense that the specimen is finally broken or fractured into two pieces in which a universal testing machine is capable of applying that load to break the material.

Yield Point is the point at which the material will have an appreciable elongation or yielding without any increase in load.

Yield Stress is the stress reached when a specimen is loaded beyond the elastic limit and the material starts yielding. It is the minimum stress required to create permanent deformation in metal.

Universal Testing Machine (UTM) is used to test the tensile, compressive and bending properties of materials.

Welding is a process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a stronger joint. (Welding, 2002).


Statement of the Problem

Deformed steel bar subjected to oxy-acetylene heat is believed to have a lower tensile strength compared to steel bar which had not undergone heating. The main objective of this study is to compare the tensile strength between the two steel bars; thus, the researcher tried to seek answers to the following questions:

  1. Is there a significant difference in the tensile strength of steel bar subjected to oxyacetylene heat with varied time and steel bar not exposed to heat?
  2. Is there a significant difference in the tensile strength of steel bar subjected to oxyacetylene heat with varied diameter and steel bar not exposed to heat?
  3. The variables used in this study are the varied time of oxy- acetylene heat exposure and the different sizes of bars. In order to have a basis of comparison, deformed steel bar samples not exposed to oxy-acetylene heat were used as control.

Significance of the Study

Concrete is a material that is very strong in compression but weak in tension. To compensate the weakness of concrete, reinforcing bars are used to carry the tensile load. Thus, it is called as reinforced concrete. However, reinforced concrete may also faildue to inadequate strength or due to a reduction in its durability. Oxyacetylene heating of steel bar reinforcement is a common practice in some construction sites.

This study is intended to raise awareness of construction steelworkers as well as construction engineers on the effect of heating the steel bar reinforcement to its tensile strength and ductility.

Scope and Limitation of the Study

This study was limited to testing the tensile strength of steel bars which have a diameter of 16 mm, 20 mm, and 25 mm. The steel bar used in this study is grade 33 available at local construction suppliers in Ozamiz City. The strength was tested by the use of the universal testing machine only. After heating, group of bars was cooled down to ambient temperature. Testing of the tensile strength was excluded while the steel bar was in the verge of heat.

2. Methodology

Research Method

The method used in this study was purely experimental. In order to get the data needed to study the significant difference on the tensile strengths subjected to oxy-acetylene heat, experimental procedures were performed. The data gathered from the tests were used for analysis and investigation.

Research Procedure


Materials

The materials used in this study were grade 33 type of steel bar which have varied diameters such as 16 mm, 20 mm and 25 mm. The said materials were cut at a length of 400 mm.


Preparation of Specimen

Nine (9) groups of specimen were prepared with three (3) samples per specimen and these were heated at varied time, such as thirty seconds; one minute and thirty seconds; and 3 minutes. Since the researchers employed the experimental research method, they used a controlled group which was not heated and composed of three (3) samples for each diameter bar size.


The Heating Process

  1. Each diameter type consists of 9 samples.
  2. The materials were subjected to an oxy-acetylene heating in three different time variables: 30 seconds; 1 minute and 30 seconds; and 3 minutes. There were three (3) test samples for each bar diameter in three different time of heat exposure.
  3. Each bar sample was laid horizontally with metal supports at both ends and passed with oxyacetylene heat from one end to the other.
  4. The time of heating was observed to ensure that there was a proper distribution of heat to the entire length of steel sample.
  5. To maintain the heat given off by the oxy-acetylene flame, the pressure gauge of the acetylene was kept at 14 psi while that of the oxygen was maintained at 75 psi throughout the heating process.
  6. A cutting tip was used, and attached to the regulators with the oxyacetylene fuel, to heat the bars.
  7. After cooling, the samples were labeled and marked with securely attached tag before tensile strengths were determined.

Tension Testing of Steel Samples

  1. Procedure for tension test of steel bar shall conform to AASHTO Designation T68-86.
  2. Once the preliminary procedures were finished, the test samples will be placed at the UTM (universal testing machine) one by one and its tensile strength and elongation were measured. A continuous tensile load was applied by the Universal Testing Machine (UTM) until the specimen breaks while the load-deformation diagram of the test can be observed from the computer monitor.
  3. After the tension test, the broken steel samples were placed back together and thier elongation was measured.
  4. Results obtained in the test were collated and tabulated.
  5. Data of test results were treated statistically.

Statistical Analysis

  1. Tensile Strength
  2. Statistical Treatment

3. Results and Discussion

Tensile Strength of Unheated and Heated Steel Samples Cooled to Ambient Temperature

Table 4.1.1 to Table 4.1.3 list the mean tensile strength of the various bar diameter considered in this study, unheated and heated for 30 seconds, one minute - 30 seconds and three minutes, air cooled to ambient temperature. These values were arrived by getting the average of the tensile stresses for each specimen.

Table 4.1.1 Tensile strength of 16 mm diameter bar heated with varied time and cooled at ambient temperature

Specimen No. Tensile Strength, MPa
0-minute heat exposure 30-second heat exposure 1-minute & 30-second heat exposure 3-minute heat exposure
1 528.40 537.60 544.40 517.60
2 528.00 540.70 516.70 529.80
3 521.90 499.5 521.70 519.20
X 526.10 525.93 527.60 522.20

The tensile strength of the 16 mm deformed bar decreases by about 0.74 % during the three minutes time of oxyacetylene heat exposure.

Table 4.1.2 Tensile strength of 20 mm bar diameter heated with varied time and cooled at ambient temperature

Specimen No. Tensile Strength, MPa
0-minute heat exposure 30-second heat exposure 1-minute & 30-second heat exposure 3-minute heat exposure
1 537.50 646.20 646.80 649.60
2 648.90 645.50 536.10 649.30
3 648.00 534.70 533.70 644.70
X 611.47 608.80 572.20 647.87

It can be shown also that there was a 6% increase in the tensile strength of the 20 mm heated for three minutes but a decrease by 0.44% for the 30 seconds time of heating and 6.4 % decrease after heating for one minute and 30 seconds compared to the unheated bar.

Table 4.1.3 Tensile strength of 25 mm bar diameter heated with varied time and cooled at ambient temperature

Specimen No. Tensile Strength, MPa
0-minute heat exposure 30-second heat exposure 1-minute & 30-second heat exposure 3-minute heat exposure
1 569.50 574.40 577.40 576.50
2 568.20 581.20 574.80 579.50
3 568.00 569.40 579.20 579.70
X 568.57 575.00 577.13 578.57

For the 25 mm steel bar, it can be shown that there was an increase in tensile strength of about 1.13% within the 30 seconds time of heating and continued to increase by as much as 1.5 % and 1.75% when heated for 1.5 minutes and 3 minutes respectively.

Analysis of Variance of Tensile Strength with Varied Diameter and Heating Time, Cooled at Ambient Temperature

Table 4.2.1 Tests of between-subjects effects in the ambient temperature

Source Type III Sum of Squares df Mean Square F Sig. Partial Eta Squared
Corrected Model 52187.456(a) 11 4744.314 4.297 .001 .663
Intercept 11701302.5 1 11701302.51 10597.374 .000 .998
diameter 43365.494 2 21682.747 19.637 .000 .621
Time-heat 2600.448 3 866.816 0.785 .514 .089
diameter * time-heat 6221.515 6 1036.919 .939 .486 .190
Error 26500.080 24 1104.170
Total 11779990.0 36

R Squared = .509

The table above showed that the tensile strength has a significant difference with respect to the three diameters and also the tensile strength has significant difference to the time of heating. However, the diameter of the steel bars has the higher effect to the tensile strength than the time of heating. This is indicated in the sig. column which is less than 0.05.

Table 4.2.2 Multiple comparisons between diameters in the ambient temperature

Diameter, mm (I) Diameter, mm (J) Mean Difference (I - J) Sig.
16 20 -84.625(*) .001
25 -49.358(*) .000
20 16 84.625(*) .001
25 35.267 .140
25 16 49.358(*) .000
20 -35.267 .140

* The mean difference is significant at the .05 level

In the multiple comparison table, there is a significant difference in the tensile strength of those with respect 16 mm, 20 mm and 25 mm bar diameter. But there is no significant difference between the 20 mm and 25 mm bar diameter.

Table 4.2.2 Multiple comparisons with respect to time heat and cooled in the ambient temperature

Diameter, mm (I) Time heat (J) Mean Difference (I - J) Sig.
unheated 30 seconds-heat -1.200 1.000
1minute- 30seconds-heat 9.733 .998
3 minutes-heat -14.167 .994
30 seconds-heat unheated 1.200 1.000
1minute- 30seconds-heat 10.933 .997
3 minutes-heat -12.967 .996
1minute- 30second-heat unheated -9.733 .998
30seconds-heat -10.933 .997
3 minutes-heat -23.900 .892
3 minutes-heat unheated 14.167 .994
30seconds-heat 12.967 .996
1minute- 30seconds-heat 23.900 .892

* The mean difference is significant at the .05 level

Under the ambient cooling, different temperature seems to have same effect to the tensile strength. The tensile strength for 16 mm bar diameter was observed to be lower when heated at 3 minutes while it increases for bars of 20 and 25 mm diameter. This shows that the tensile strength of the steel bar is influenced by the time of exposure to heat and the heat transfer or absorption with respect to the size of the bar.


Estimated Marginal Means of Tensile Strength

Figure 3. Graph of tensile strength against time of exposure to oxyacetylene air cooled to ambient temperature

The graph showed the tensile strengths of the various diameters of steel bars with respect to time of heating compared to the unheated bars.

Tensile Strength of Unheated Steel Sample and Steel bar Subjected to Oxyacetylene Heat with Varied Diameter

Table 4.4.1 Average tensile strength of unheated and heated steel sample cooled at ambient temperature

Bar diameter, mm Tensile Strength, MPa
0-minute heat exposure 30 seconds heat exposure 1.5 -minute heat exposure 3-minute heat exposure
16 526.1 525.93 527.6 522.2
20 611.5 608.8 572.2 647.9
25 568.6 575.0 577.13 578.6

Table 4.4.1 summarized the average tensile strengths of the various bar diameters exposed to varied time of heating cooled to ambient temperature and not exposed to heat. Statistical data showed that there is no significant difference in the tensile strength for 16 mm, 20mm and 25 mm bar diameter not exposed to heat with those bars exposed to heat with varied time. There is a significant difference in the tensile strength for steel bars with varied diameter between the unheated and those exposed to oxyacetylene heat for 30 seconds, 1.5 and 3 minutes. It was also observed that the steel bars have a brittle fracture with the increase of exposure time to oxyacetylene heat.

4. Conclusions and Recommendations

Conclusions

Based on the test results and with respect to the analysis of all the data gathered, the researchers arrived at the following conclusions:

  1. Reinforcing steel bars loose strength when subjected to oxyacetylene heat. The size or diameter of the steel bar has a higher effect on the tensile strength than the time of heating.
  2. The tensile strength of the heated steel bars, air cooled to ambient temperature decreases when the diameter is small. The 16 mm steel bar suffers a larger percentage of decrease than the 20 and 25 mm bar during the three-minute heating.
  3. Reinforcing steel bars when heated with oxyacetylene torch up to its bending condition (cherry red or very slight red from fig. 2.2), though found to be lower in tensile strength compared to the non-heated steel bars, have a tensile strength within acceptable limits based on the Philippine National Standard Specification for concrete reinforcing bars. However, steel bars exposed to heat or overheated steel bars may become brittle and if used to reinforce structures may break immediately without warning or yielding when the structure is overloaded.

Recommendations

Based on the results and conclusions, the following recommendations were drawn:

  1. Chemical analysis may be conducted too or product analysis to check the carbon content of the material for field engineers to know if the steel bar is weldable or heating at site is tolerable.
  2. Testing of reinforcing steel bars prior to use should be conducted to verify the actual yield and tensile strengths of the steel bars, and adjustment on the design computation can be done based on the actual yield and ultimate strengths.
  3. Oxyacetylene heating of reinforcing steel bars should be avoided as much as possible since it lowers the tensile strengths of the steel bars. If oxyacetylene heating of steel bars cannot be avoided due to time limitation of project implementation, it should not be exposed to heat for a longer period of time to avoid brittleness of the steel bars.
  4. Hot bending should be approved by the designer before bending and be fully supervised to ensure that there is no overexposure to heat and it is not cooled by dowsing in water.
  5. Further studies using bigger diameter of bars with same grade should be done on this area to verify and refine the findings of the researcher.
  6. Temperature of oxyacetylene flame should be taken and noted in the next experiment.
  7. Higher grade of steel bars should be used for further study since these bars are usually used in high rise structures, bridges and other major structures and the ultimate strengths should be checked whether steels of higher grade such as 40 and 60 will also give similar results.
  8. The behavior/deformation of steel bar such as yield and shrinkage or expansion when exposed to high temperature maybe considered for further studies.
  9. Increase the time of heating to more than 3 minutes or until the steel bar reached its bending condition depending on the steel bar diameter.

List of References

Bauerlein, B. (1998). The Beginners Guide to Oxyacetylene Welding Equipment. Retreived: June 18, 2009 from http://www.metalwebnews.com/howto/weld/weld.html

Beer, F.P., Johnston, E. Russell, Jr. and DeWolf, J.T. (2009). Mechanics of Deformable Bodies, 4th ed., Philippines: McGraw-Hill.

Capudean, B. (2003). Phases, structures, and the influences of temperature. Retrieved on June 2009 from http://www.thefabricator.com/Metallurgy/Metallurgy_Articl e.cfm?ID=631

Chen, B. and Liu, J. (2004). Residual strength of hybrid-fiber- reinforced high-strength concrete after exposure to high temperatures, Cement and Concrete Research, vol. 34, no. 6, pp. 1065-1069. Amsterdam: Elsevier Science Ltd.

Derucher, K; Korfiatis, G. & Ezeldin, S. (1999); Materials for Civil and Highway Engineers, 4th ed., New Jersey: Prentice Hall.

Ding, J., Li, G.-Q. and Sakumoto, Y. (2004). ?Parametric studies on fire resistance of fire-resistant steel members,? Journal of Constructional Steel Research, vol. 60, no. 7, pp. 1007- 1027. Amsterdan: Elsevier, Ltd.

Introduction to Materials Science, Chapter 8, Failure: How Do Materials Break? University of Virginia, Department of Materials Science and Engineering. Retreived: September 17, 2009 from http://people.virginia.edu/~lz2n/mse209/Chapter8.pdf

Jastrzebski, Z.D. (1977). The Nature and Properties of Engineering Materials, 2nd ed. SI Version. USA: John Wiley & Sons, Inc.

Kopeliovich, D. (1991). Iron-Carbon Phase Diagram. R & D Positions in Materials Engineering. Israel. Retreived: June 07, 2009 from http://www.substech.com/dokuwiki/doku.php?id=iron- carbon_phase_diagram

Lejano, B.A., Martinez, B. C., Obina, M. I. and Ong, J.Q. Jr. (2006). Effects of Elevated Temperature with Varying Exposure Time, Concrete Strength and Bar Diameter on the Strength of Reinforcing Steel Bars. Symposium on Infrastructure Development and the Environment. Philippines: University of the Philippines, Diliman, Quezon City. Available: Retreived: Aug. 07, 2009 from http://www.engg.upd.edu.ph/~side/pdf/STR-005.pdf

McGrath, J. (1998-2009). How Blowtorches Work. Retreived: June 07, 2009 from http://home.howstuffworks.com/blowtorch.htm/printable

McKinnon, G. (1986). Bending, Welding and Coating Steel Reinforcement to Assure Compliance with AS1480-1982. Technical Policy Note 1 (TPN). Standard Bulletin No. S1- 024. Retrieved on June 2009 from http://www.powerwater.com.au/__data/assets/pdf_file/0017/ 2519/S1-024.pdf

Nasseri, S. Prof. (2008). Oxyacetylene Welding. Ppt. Manufacturing Process Lab (MET 1321. Southern Polytechnic State University. University System of Georgia. Retreived: June 10, 2009 from http://met.spsu.edu/snasseri/welding-oxy.pps

Oxyacetylene Welding. (2003). Illinois. Advance Fabricated Metals, Inc. Retreived: August 10, 2009 from http://www.advantagefabricatedmetals.com/oxyacetylene- welding.html

Oxy-fuel Welding and cutting. Wikipedia. The free encyclopedia. Retreived: June 10, 2009 from http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting Philippine National Standards. (2001). Steel Bars for Concrete Reinforcement Specification. CDPNS49. Retreived: October 23, 2009 from http://www.capitolsteel.com.ph/PNS49.pdf

Rebar. Wikipedia (2008). The free encyclopedia. Retreived: June 08, 2009 from http://en.wikipedia.org/wiki/Rebar

Reinforced Concrete. Wikipedia. (1935). The free encyclopedia. Retreived: June 07, 2009 from http://en.wikipedia.org/wiki/Reinforced_concrete

Reinforcing Steel. (1999). Retreived: July 23, 2009 from http://www.globalsecurity.org/military/library/policy/navy/n rtc/14251_ch7.pdf

Ronald E. Walpole. (1982). Introduction to Statistics, 3rd ed. Philippines: Macmillan Publishing Co.

Steel Reinforcement Properties. (2000). Thermex QST Rebars. Retreived: June 01, 2009 from http://handk- india.tripod.com/id14.html

Suprenant, B.A. (1996). Evaluating Fire Damaged Concrete. Concrete and reinforcing steel properties can be compromised at elevated temperatures. Publication #R970020. The Aberdeen Group. Retreived: October 16, 2009 from http://www.concretees.com/people/bruce/pubs/R970020.pdf

Teach Yourself Phase Diagram. The Iron - Carbon Phase Diagram. (2003). University of Cambridge. Department of Engineering. Retreived: Sept. 27, 2009 http://wwwg.eng.cam.ac.uk/mmg/teaching/typd/addenda/eut ectoidreaction1.html

sTensile Strength, Science Daily. (1995). Retreived: June 07, 2009 from www.sciencedaily.com/articles/t/tensile_strength.htm)

Tensile Strength. (2011). Encyclopedia Britannica. Retreived: July 27, 2009 from http://www.britannica.com/EBchecked/topic/587505/tensile- strength

Topcu, I.B. and Karakurt, C. (2008). Properties of Reinforced Concrete Steel Rebars Exposed to High Temperatures. Research Letter. Department of Civil Engineering, Eskisehir Osmangazi University, 26480 Eskisehir, Turkey. Retreived: June 17, 2009 from http://www.hindawi.com/journals/rlms/2008/814137.html

Wald, F., Da Silva, L. S., Moore, D. B. et al. (2006). ?Experimental behaviour of a steel structure under natural fire,? Fire Safety Journal, vol. 41, no. 7, pp. 509-522, 2006. Amsterdan: Elsevier, B.V.

The Welding Institute. Job knowledge for welders 27. Health, safety and accident prevention. Oxyacetylene welding, cutting and heating. (2002). Retreived: June 07, 2009 from http://www.twi.co.uk/content/jk1.html