What is a rebar?
A reinforcing steel bar (rebar) is a round steel bar with protrusions called deformation (see DFORMATION in our Technical Specifications) used as a tension device in reinforced concrete to strengthen and hold the concrete together.
What is PNS 49?
Steel bars are covered by a mandatory standard called the Philippine National Standard 49 or PNS 49, as formulated by the Bureau of Product Standards (BPS) of the Department of Trade and Industry (DTI) with the help of the Steel Industry, the Association of Structural Engineers of the Philippines, and the Philippine Construction Association.
What is ASTM A615? ASTM A706? How do they differ from PNS 49?
The American Society for Testing and Materials (ASTM) has developed various standards agreed upon and adopted internationally by a multitude of industries. For rebars, ASTM A615 (Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement) and ASTM A706 (Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement) are the prevailing standards. PNS 49 has been aligned to these standards as much as possible, however, the seismic conditions of our country as well as the requirements of the key stakeholders has introduced some distinctions between the standards.
How do I know if the steel bars I buy conform to PNS 49?
Check if the manufacturer is accredited by the Bureau of Product Standards (BPS). All manufacturers accredited by the BPS are required to place their distinguishing logo on every meter of their steel bars. The logo is registered with the BPS. However, a manufacturer can still attempt to circumvent PNS 49 despite its accreditation. Always have two or three specimen steel bars tested in a reputable laboratory. The laboratories will measure, weigh and test the yield point, tensile strength, elongation, and bending of the steel bars. One cannot ascertain the quality of a steel bar simply through visual inspection.
Where can I have my steel bars tested?
Your sample can be tested at ISO/IEC 17025 accredited laboratories such as the Metals Industry Research and Development Center (MIRDC) in Bicutan or the Philippine Geoanalytics in Quezon City to see if it conforms to PNS 49 specifications.
What is Tension Testing?
Tension testing, also called tensile testing, is material testing test in which a sample’s properties such as yield strength, ultimate tensile strength, fracture strength, elongation etc are measured directly. A tensile (stretching) force is applied to a sample of certain area until it fractures.
The maximum pressure points wherein the sample can withstand permanent uniform deformation (yield strength), permanent localized deformation (ultimate tensile strength), and inevitable fracture (fracture strength) are accounted for, usually with a graph that plots the pressure against the strain experienced by the sample.
The percent elongation and area reduction, both measures of a sample’s ductility, can be calculated by comparing the dimensions of the sample after the fracture from its original dimensions.
The fracture type of the sample can also give indications of the ductility/brittleness of a sample. Flat, grainy fracture surfaces imply a brittle rebar while cup-cone surfaces imply a ductile rebar. Rebars with irregular fractures fall in between ductile and brittle materials.
A typical stress-strain diagram as well the different fracture surfaces of a rebar are depicted below.
What is Mass Variation Testing?
As the name implies, mass variation testing is a type of materials testing wherein the unit weight (weight per length) of a sample is compared with its theoretical unit weight. PNS 49 specifies the theoretical unit mass requirement of each bar size. See WEIGHT TABLE our Technical Specifications. A mass variation of ±6.0% is allowed by PNS 49, IT can be computed by the following equation:
Why is it important to have standard steel bars?
In an earthquake prone country like the Philippines, the use of substandard steel bars can be life-threatening. The yield point and tensile strength ratio ensures that there is enough “reserve energy” for the steel you use to withstand the sustained swaying motions and the subsequent aftershocks of an earthquake.
What are some practices that should concern a buyer?
One common practice is to maximize the 6% weight tolerance allowed by PNS 49 for each steel bar. Since weight deviations of 2% – 3% occur during the rolling process, undersized steel bars are included in the production batch. Another practice is the use of unsuitable raw materials to produce steel bars that are too brittle and cannot pass bending tests under laboratory conditions. Still another practice is passing a lower grade off as a higher grade by switching the color coding of the steel bar. A rule of thumb is never to buy from a manufacturer whose selling price is more than 2% lower than other competitors. The savings involved are not commensurate to the risk.
How do you measure the diameter of a rebar?
The diameter of a bar, say 16 mm, refers to the mean nominal diameter of the cross sectional area of the bar, core and deformations included. It is a theoretical diameter which cannot be directly measured using a ruler, caliper or micrometer. This theoretical diameter is controlled by the mass variation and the height of lugs of a rebar. See MASS VARIATION and DEFORMATION in our Technical Specifications.
What is rusting and how does it affect the performance of rebars?
Due to the reactivity of iron with oxygen, mild steel products are inherently sensitive to the atmosphere. The interaction between the iron in mild steel and the oxygen in atmospheric air causes the oxidation process more commonly referred to as rusting or atmospheric corrosion in the bars.
Superficial rusting of rebars does not affect its performance. In fact, surface rust can increase the bond of the bar to the concrete. However, prolonged surface rusting can eventually lead to pitting of the steel and this may lead to a weakening of the steel section. Suspect steel can be checked by weighing a cleaned sample to check that the section is not below the lower unit mass tolerance and tensile testing undertaken to ensure that the physical properties are still above the minimum requirements.
You can find more information about rusting and its effect on rebars here.
What is the smallest bend radius that we can bend a bar?
PNS 49 specifies the minimum pin diameter for the inside bend diameters of the various rebar sizes and grades. See PIN DIAMETER in our Technical Specifications.
What is quenched and tempered (QT) rebars?
QT rebars are manufactured through a thermomechanical process called quenching and tempering. This process involves the rapid, superficial cooling of a hot-rolled rebar (quenching) and then the subsequent reheating by the residual heat (tempering). QT rebars have higher strength and toughness than simple hot-rolled bars.
The higher strength is derived from the tempered martensite layer formed by the quenching and tempering process while the improved toughness is due to its bainite transition layer. The ductility of the bars is kept by its pearlite-ferrite core.