HOW TO BUY GOOD STEEL

1.0 Introduction

The spate of collapsed buildings has become a matter of concern to all stakeholders in the construction industry.  The reason for the collapses have been severally adduced to poor designs, use of substandard materials, inadequate quantities of even good materials, poor workman-ship etc.  The reasons are legion.  In this write-up, emphasis is on how to identify good steel reinforcement bars, a major material used in construction.

Reinforcement bars are used to beef-up the strength of concrete in construction.  They assist concrete in load bearing capacities. Good structural designs ensure that loads are appropriately distributed as to ensure a sturdy building.  This, however, presupposes that the reinforcement bars used in the construction are within the standard used in the calculation of the parameters of the design.  Where the quality of the bars is below the standard applied, building collapse may occur. The Standard Organization of Nigeria (SON)’s standard for reinforcement bars is contained in Nigerian Industrial Standard (NIS) 117. Meanwhile, not all reinforcement bars in the market conform to this standard.  Thus, just as we have substandard drugs, so also, there are substandard reinforcement bars in the market.  The question then is, how can one tell the difference, in order to ensure a reliable and safe construction, given that all other factors are adequate?  The difference lies in the metallurgical properties and features that make for good quality bars.

2.0 Characteristic Properties/Features of Good Reinforcement Bars

  2.1  High Yield Strength

The yield strength indicates the capacity of the bar to carry load (vertical and horizontal) before plastic deformation. NIS 117 stipulates a minimum of 420N/mm²  for high tension bars.   It is impossible to tell the strength of a bar by mere looks.  One can however, rely on the fact that the product tag carries an NIS mark as an indication of conformity to strength standard. The method of steel production may give some indication of the quality of product there from.  Generally, products from the electric arc furnace, continuous casting and continuous rolling processes are better than those from induction furnace route. The arc furnace route has the advantage of removing deleterious tramp elements like sulphur and phosphorus and other inclusions unlike the induction furnace process.

2.2  Rib Configuration:

The rib configuration of the bars refers essentially to the height and type of the ribs. The ribs ensure concrete bonding and do not contribute to the load bearing function of the rebars. Therefore, very low rib height does not allow for adequate concrete bonding while excessive ribs compromise the weight bearing function of the bars. An average rib height of about7 to 8% of the rated diameter is just adequate

2.3  Standard Weight/ meter:

There is a standard weight per meter for each profile of rebar.  The heavier bar, of two pieces of rebar of same length and profile will conform better to the standard weight per meter for the profile in question.

2.4  Dimensional Specification:

This refers to the diameter and length of the bars.  The diameter of rebars can be easily measured by a vernier caliper and the length by a tape.  The diameter depends on the profile sought after, for example, 10mm should be 10mm not 9.5mm, or 12mm being 11 or 11.5mm.  Approved standard finishing length should be 12m or 40feet.

2.5  Bend-abilty

The rebar should be able to bend, and without fracture to accommodate the design of the building.  Those that break on bending when the right mandrel is used are substandard. Below is the recommended mandrel diameter for the corresponding profiles.

3.0  Summary of Characteristic Features and Benefits

4.0  Quality Assurance Plan of Producer.

It is generally better to patronize products from organizations that have adequate quality assurance plans for their products.  For such organizations, quality checks are carried out right from the raw materials consumed and at every stage of production to ensure quality conformity. For them too, finished products are subjected to various metallurgical tests to ensure conformity to standard before dispatch to dealers and end-users.  A detailed history of quality practices and billet tracking from charging to final product can therefore be of great assistance in determining the product reliability and quality.

5.0  Reinforcement bar testing centers in Nigeria

The table below shows some of the places where the properties of steel reinforcement bars can be tested

6.0  Product Identification Mark

All mills have their own product identification marks. You can check with the distributor if you need more information on the same.

7.0  Conclusion

Generally speaking, the NIS mark of quality should be looked out for in rebar tags. The mark should be enough indication that the rebar has been produced to standard.  Other checks, as itemized above can serve to erase any doubts concerning the quality of products to be purchased.  Suffice to say that the pains to ensure the purchase of good steel are worth the while, if the consequences of buying substandard product are considered.