TECH  67

REPORT ON THE QUALITY AND TENSILE 

STRENGTHS

OF STEEL REINFORCEMENTS USED 

IN SARAWAK.

TABLE OF CONTENTS

1.0         INTRODUCTION

2.0         GENERAL REQUIREMENTS

3.0         TEST PROCEDURES AND PROBLEMS                                  ENCOUNTERED

              3.1    Testing Methods

              3.2    Problems Encountered

4.0          SUMMARY OF TEST RESULTS

               4.1    General

               4.2    1997 Results

               4.3    2006 Results

               4.4    Comments

5.0          REASONS FOR FAILED STRENGTHS

6.0          CONCLUSION AND SUMMARY

7.0           REFERENCES

TECH 67 -  REPORT ON THE QUALITY AND TENSILE STRENGTHS

             OF STEEL REINFORCEMENTS USED IN SARAWAK.

By  Ir.  Wong  King  Ming, P. Eng, MIEM.

1.0        INTRODUCTION

During the 1980s, quality and tensile strengths of steel reinforcements were seldom questioned, due to the common belief that steel factories were “hi-tech” factories managed by “Professionals” and would be able to control the quality of their products within close range. The lack of testing laboratory or high cost also discouraged supervision team to test the steel materials freely.

Following the setting up of many small steel bars factories from 1990s, using second hand or inferior machineries and unknown imported raw materials from less developed countries to manufacture steel reinforcements, the quality of such products was suspected. With the availability of steel tensile test laboratories in recent years, Clients / Engineers / Site Supervisors were able to test the steel reinforcements and confirmed the abundant low quality steel reinforcements being supplied to the local construction industries.

    

This Report presents the tensile test results of the steel reinforcements that had been conducted at one Material Test Laboratory (Timur Testing Sdn Bhd) from 1997 to 2007, to highlight the large variations of steel reinforcement quality that had been encountered during the last 10 years. In order to ensure the tests were done properly and reliable, Perunding WL Sdn Bhd was engaged to oversee and witness these tests.

Altogether six Hundred eighty-eights tensile tests were executed, of which seventy-four samples (11%) failed in strengths, thirty-six samples (5%) could not achieve the required extensions (too brittle) and one-hundred and twenty samples (17%) did not possess yield points.

On the brighter side, based on the test results, it appeared that average yield stress of the high yield steel reinforcements had improved from 413 N/sq.mm. in 1997 to 564 N/sq.mm. in 2006,.  The average elongation of these steels remained at 23%. However, the ratio of failure stress to yield stress decreased from 1.50 to 1.20 during the corresponding period.

2.0        GENERAL REQUIREMENTS

Modulus of elasticity, yield stress, failure stress and ductility of steel bars are the important parameters to be considered in structural engineering design to achieve safety, serviceability, and to avoid brittle failure in the event of unexpected over-loading.

When the working strength is within elastic zone, the property of metal will not change and will therefore be safe for continuous or repeated use. However, when the metal is over-loaded, yield will first occur, leading to large permanent deformation, giving warning sign to the user of overloading, and time to rectify or escape. Only further loading will cause the breakage of the steel reinforcement and failure of the structure.

In a tensile test, a suitable shaped specimen is extended until fracture occurs. Both loadings and extensions are automatically recorded simultaneously. Thus, a typical stress / strain relationship can be obtained as shown in Sketch 1.

In order to measure ductility, elongation is measured as a percentage of extra length after fracture compared to that before fracture. Brittle materials exhibit comparatively little elongation. Brittle materials, such as cast iron exhibit on 2% elongation, while some steels give more than 25%. However, as much as 71% elongation had been measured for some steels during the tests.

Steel Reinforcements are supplied in three types:

(1)  Hot-rolled (BS 4449)

(2)  Cold-rolled (BS 4461)

         (3)  Hard drawn (BS 4482)

The minimum required tensile strengths are between 250 and 485 N/mm².

The tensile strength for hot-rolled steel shall be 15 percent greater than the yield strength.  For cold rolled and hard drawn steels, the tensile strength only needs to be 10 percent higher than yield strength.

Typical minimum required yield and failure strengths of steel reinforcements are:

(a)    Hot-rolled

R-bar,    fy  = 250 N/mm²,   ft = 288 N/mm²

Y-bar,    fy  = 410 N/mm²,   ft = 472 N/mm²

T-bar,    fy  = 465 N/mm²,   ft = 535 N/mm²

(b)   Cold-rolled

   fy  = 465 N/mm²,   ft = 472 N/mm²

(c)    Hard drawn wire mesh,

   fy = 485 N/mm²,   ft = 535 N/mm²

3.0        TESTING PROCEDURES AND  PROBLEMS ENCOUNTERED

3.1     Testing Methods

The testing procedures followed the British Standard BS 18: Method for tensile testing of metal (including aerospace materials) generally.

Geotech GT-7001LC-50 Universal Testing Machine (Equipment Serial No. 860077), purchased in 1997, was used for the tensile testing in the Studied Laboratory. The machine was calibrated yearly by SIRIM or JKR Central Laboratory to ensure the accuracy of the measured loads.

Although the maximum size of steel reinforcement allowed for this machinery was 32mm diameter, it was recommended only to test 25mm diameter bars so as not to strain the machine, thereby ensuring better accuracy and longer machinery life. The bigger sized bars were therefore required to be machined / trimmed to 25mm diameter size before the tensile testing.

The following testing procedures were observed:

(a)           machines was periodically maintained to good conditions and regularly calibrated,

(b)          as the reinforcements were mostly round, test pieces were prepared and tested as proportional test pieces i.e. length and cross sectional area were first obtained as follows:

           Original gauge length,       Lo = 5.65 √ So,   < 20mm

           Minimum parallel length   Lc = 5.5d

           Transition radius, r ≥ d, where `d' is the diameter of the bars.

    The test pieces were then cut to require lengths.

(c)           The test pieces were then inserted into the machine grips and applied with an increasing force to strain the reinforcements continuously. Force and extension were recorded automatically until the test pieces break.

(d)          The rate of stressing on steel reinforcements shall be between 180N/sq.mm. per minute and 1800N/sq.mm. per minute. About 600N/sq.mm. per minute was used for the tests.

(e)           The yield strength and tensile strength were determined from the kink and breaking strength respectively from the graph.

(f)           The elongation, e, of the test pieces was then measured manually by putting together the breaking pieces and computed in according to the following formula:

        

                         e = Lu-Lo x 100

                                  Lo

                           Where   Lu is the measured gauge length

                                         Lo is the original gauge length

(g)          The measured and computed data were summarised in a standard Test Report sheet, together with the automatically plotted graphical sheet, certified by all the parties who witnessed the tests.

3.2      Problems Encountered

           A few problems were encountered during the testing:

(a)    The grips slipped during the tests. Sometimes, the grips were not secured properly or stained with water, soils and oil, leading to slippage on higher loads, the tests had to be repeated after cleaning and securing properly,

           (b)      Some automatically plotted graphs did not start from zero (see Fig. 1), this was due to the machine preset load of 981N in order for the grip to be effective, and therefore for smaller bars such as Y10, immediate 12.5N/sq.mm would be registered. For Y25 bar, instant stress is only 2N/sq.mm and would not be noticed on the graph, further more the lower loadings generally had higher error (about 4%)  while the higher loading  has less than 1% error,

            (c)     Few plotted curves were found to have steep gradients and some gentle, and that the deformation recorded by the machine in the elastic zone was much higher than the computed elongation, as much as ten times.  After checking the grip marks, it was found that the bars slipped 1-2mm initially leading higher deformation recorded. However, such slips would not affect the final results on yield stress and failed stress, but would lead to the gentle or steep curves. This machine is therefore not suitable to obtain Modulus of Elasticity. Final elongation had to be measured manually,

            (d)    It took about half an hour to complete one test generally. However, after one maintenance service, it took more than one hour to complete one test. It was found that the rate of stressing was changed from 600 N/sq.mm. to 250 N/sq.mm. After re-adjusting the rate, the time taken was reduced to half an hour again,

            (e)     On a few occasions the samples failed at the grip, the results were generally lower than actual as the strong grips caused some damage to the steel bars thus making the gripped location the weakest point. It might be due to the smaller diameter or lower quality at this location.  The actual strengths could be much higher. The elongation could not be measured when this occurred and hence no result. Strictly speaking, the tests had to be repeated. But generally due to the cost of testing, the clients were unwilling to do another test especially the strengths had satisfied the minimum requirements.

4.0        SUMMARY OF TEST RESULTS

             

4.1    General

         Test results of the steel reinforcement tensile tests from 1997 to 2006 are summarised in the following Table 1 to 10. These include mild steel bars(R-bars), high yield steel bars (Y- and T- bars) and wire-meshes.

         The Y-bars and T-bars were grouped together, because the Contractors often used T-bars for Y-bars.

Altogether six Hundred eighty-eights(688) tensile tests were executed, of which seventy-four samples (11%) failed in strengths, thirty-six samples (5%) could not achieve the required extensions (too brittle) and one-hundred and twenty samples (17%) did not possess yield points, i.e. did not behave in accordance to typical hot-rolled steel materials.  The number of tests, failure rates and indistinct yield points from 1997 to 2006 are summarized in Table 11.

         The number of tensile tests on mild steel, high yield steel and wire-mesh are 29(4%), 644(94%) and 15(2%) respectively. These also reflected the common practice to use high yield steel reinforcement rather than the mild steel during the period. Mild steel reinforcements are generally used for stirrups and slabs.  Wire-meshes are often used for non-suspended ground slabs and occasionally used for suspended slabs.

The high, low, average yield strengths, tensile strengths, ratio of tensile strength to yield strength and  the elongations are summarized in Table 12.

From this Table, the highest yield and failed strengths recorded were  741 N/sq.mm. and 1031 N/sq.mm. respectively. The lowest yield and failed strengths recorded were 219 N/sq.mm and 330 N/sq.mm for mild steel and 328 N/sq.mm and 437 N/sq.mm for high yield steel. The highest and lowest elongations measured were 71% and 6% respectively.

Average yield stress of the high yield steel reinforcements had improved from 413 N/sq.mm. in 1997 to 564 N/sq.mm. in 2006  The average elongation of these steels remained at 23%. However, the ratio of failure stress to yield stress decreased from 1.50 to 1.20 during the corresponding period. There was no improvement on strengths on mild steel, with yield strengths between 265 N/sqmm. and 351 N/sq.mm..There was slightly decrease in yield strength from 606 N/sq.mm. to 506 N/sq.mm..

4.2   1997 Test Results

         Out of the 98 tests conducted, 39 samples (40%) failed in yield strength, 7 samples (7%) failed in elongation and 22 samples had no distinct yield points. The average yield strength, failed strength and elongation were 413N/sq.mm, 619 N/sq.mm. and 22%.  The average ratio of failure stress to yield stress was 1.50.

 4.3   2006 Test Results

         Out of the 71 tests conducted, none failed in yield strength, 3 samples (4%) failed in elongation and 11 samples had no distinct yield points. The average yield strength, failed strength and elongation were 564N/sq.mm, 675 N/sq.mm. and 23%.  The average ratio of failure stress to yield stress was 1.20.

          4.4   Comments

It was alarming to find such large number of steel bars failed to achieve the design requirements in 1997, the first year of testing.   The average yield strengths of the high yield steel bars (Y-bars) were merely 413 N/sq.mm. compared to the designed requirement of 410N/sq.mm. It also confirmed that the rampant usage of low quality steel reinforcements during that time and before.

                    The average yield strengths of the high yield steel bars (Y- and T-bars) improved to 488 N/sq.mm. in 1998 while the failure rates reduced to 17%. It was likely that those factories which had sent the samples and failed would not send the samples again to the laboratory. It was unlikely that the factories would improve within one year.

By 1999, the average yield strength of the high yield steel bars (Y- and T-bars) remained at 488 N/sq.mm. While the failure rates reduced drastically to only 1%.

In 2006, the average yield strength of the high yield steel bars (Y- and T-bars) further strengthened to 564 N/sq.mm. and there was no failure recorded. The average failed strength of the high yield steel bars (Y- and T-bars) improved only slightly from 619N/sq.mm to 675 N/sq.mm. The yield and failed strengths had improved by 36% and 9% respectively compared to 1997.The average ratio of failure stress to yield stress was reduced to 1.20. from 1.50 during the corresponding period. Only 3 samples (4%) failed in elongation and 11 samples (15%) had no distinct yield points in 2006.

5.0   REASONS FOR VARYING STRENGTHS AND FAILURES

The large variations of steel strengths were due to various steel making techniques, heat treatments and the ability of owners to control and maintain the plants and their workers.

Large factories normally are able to provide good steel making and heat treatment machineries, as well as skilled workers due to adequate funding. They are able to maintain the machineries and set up quality control program to ensure the steel properties are achieved and maintained.

Small factories usually bought second-hand machineries and parts, engaged cheap and unskilled laborers with little quality control.

As there was no tensile testing facility available in the past, these low quality products managed to penetrate the construction industry even though the strengths were below the required strengths. This can be seen from the frequent failed strengths during the early years of testing (see 1997 and 1998 results). The failure rates were 47% and 29% in 1997 and 1998 respectively. In 2006, the failure rate was only 4%. The above rates did not include those test results which have indistinct yield points, i.e. did not achieve the desired hot-rolled steel behaviors.

Following the setting-up of local private testing laboratory, low quality steels were discovered and rejected. Although, these steel producers were initially angry and attempted to bribe the tester, the Engineer and even threatened them, but with time, these small steel producers had to upgrade their facilities as they found that their products could not be sold. Although these steels were much cheaper, the contractors also did not want to take unnecessary risks as one test would expose the low quality products. Furthermore, these steel producers could not sell their products overseas, thereby restricting their market.

6.0        CONCLUSION AND SUMMARY

Numerous tensile tests of steel reinforcements had been executed in a private Laboratory, and the results obtained were summarized and studied.

It was concluded the variation of strengths of the reinforcements used in local construction industries was wide, signifying the problems of quality control in the steel factories. Many far exceeded the requirements, while some failed. Some passed the strengths but failed on the elongations, i.e. too brittle. Others had no distinct yield point, which means the desired steel behaviours could not be achieved.  This proves the importance of supervision and checking the materials when implementing any project. By checking and controlling the quality of steel products through random testing, can prevent low quality products being used for the structures.

The failed results also showed "irresponsibility and selfishness" of some steel reinforcements producers, who only thought of make fast profit by selling the low quality products without due regard on the safety of the occupants of the reinforced concrete structures, which may collapse and cause deaths.  

It is fortunate that many private laboratories are now available in the market, where the tensile tests can be done speedily, efficiently, cheaply and most importantly; all concerned parties can witness the tests together, reducing doubts and suspicions. However, honesty is still required on the Contractor or Suppliers and the supervision team, who might switch the samples or use the good samples during testing, but use the low quality steel reinforcements at the site. It is important that the supervision team should take random samples from the site whenever suspicion arises, to prevent any mal-practice.

Independent and regular testing had actually improved the standard of the steel factories after low quality products were rejected. This could be seen from the recent years testing results, where less failures of steel were recorded. Of course, it might be also due to the fear of test failures, these manufacturers deliberately did not send the samples to this laboratory or switch samples for testing. But with fears of being rejected and/or being sued, together with good supervision team, low quality steel products would be more difficult to penetrate the market.

One Local Government Department in Sarawak had extensively sent the steel reinforcement to the Studied Laboratory and had rejected a large number of steel supplied to the sites. Some high yield steel bars which were supposed to have yield strength of 410 N/mm², failed miserably at 294 N/mmm². It was dreadful to think how many of these steels had slipped through the supervision team and used in high rise multi-storey buildings or long span structures.

7.0   REFERENCES

(a) Jackson N, "Civil Engineering Materials" ELBS & Macmillan 1976, PP3-64

(b) BS 18: 1987, Tensile Testing of Metals: (including aerospace materials)

(c) BS 4449:1988, "Carbon steel bars for reinforcement of concrete"

(d) BS 4461:1969, "Cold work steel bars for the reinforcement of concrete"

(e) BS 4482:1969,  "Hard Drawn Mild Steel wire for the reinforcement of concrete"

 (f) Numerous Test Results from Timur Testing Sdn Bhd

BIOGRAPHY

Wong KM, B.E. (Hons), P. Eng, MIEM, and former C. Eng (UK), MICE (UK), MIStructE (UK), M.I.H.T. (UK), MASCE (USA), MIE (Aust), CPEng (Aust), is a Director of Perunding WL Sdn Bhd. ( a Consulting Engineering Firm ), with  working experiences of 26 years. He had been extensively involved in the investigation, design, research and construction of Civil, Structural and Geotechnical Engineering.