大多数缺陷钢筋可在工地用秤、卷尺和弯曲试验查出

6.1 Seven defects account for most problems

The table sets out the defects, how they arise, why they matter and how each can be detected. Several usually occur together: a bar re-rolled from mixed scrap is often light, irregular in its ribs, erratic in strength and unmarked.

Table 6.1 The seven common rebar defects and how to detect them

Source: Cameroon Steel synthesis from the standards and studies cited in this paper [S19][S25][S26][S28][S31][S34][S44].

6.2 The regional evidence points the same way

Published tests of market rebar in Cameroon and comparable countries are few, use different methods and are not directly comparable, but they converge on the same pattern: a significant share of the bars tested, on sale or on site, do not meet their declared grade, and the shortfall is concentrated among unmarked or weakly controlled products.

Table 6.2 Published evidence on the quality of rebar on sale or on site (selected)

Source: as listed. Methods, standards and sampling differ between studies; the table shows patterns, not comparable failure rates.

Two observations stand out. First, marking matters more than any other single attribute: the Cameroonian study found that unmarked bars failed every test [S19], and the Nigerian regulator found unregistered marks and tampering [S25]. Second, the failures are not only in strength. The Ethiopian bars mostly met their minimum strength but showed scatter and strength ratios that limit ductility [S28]; the Nigerian bars failed on diameter and mass [S25]. A site that checks only the grade printed on a label misses most of the problem.

6.3 Weigh the bar: the single most useful site test

Mass per metre is the honest measure of a bar’s size (see the standards chapter), and it can be checked with a hanging or platform scale that any builders’ merchant owns. The procedure takes ten minutes per diameter:

1. Choose three full-length bars of each diameter at random from the delivery, not from the top of the bundle.
2. Measure each bar’s length with a tape, to the nearest centimetre.
3. Weigh each bar on a scale with a resolution of 0.05 kg or better, checked against a known mass.
4. Divide mass by length to get kg/m and compare with the nominal value less the tolerance: 4.5% for bars larger than 8 mm, 6.0% for 8 mm and smaller [S31][S32].

Source: Cameroon Steel calculation from nominal diameters and a density of 7,850 kg/m³; tolerance from the European reference standards [S31][S32]. Annex A.

The differences are large enough to be unmistakable. One millimetre of undersize removes 16% of the steel from a 12 mm bar and 19% from a 10 mm bar (derived). These are not marginal failures that depend on the accuracy of a scale; they are visible on any calibrated scale. Because merchants usually sell by the bar, the buyer who weighs also protects his budget: every missing kilogram is a kilogram paid for and not delivered (see the price analysis in the next chapter).

6.4 A few bars weighed at random catch widespread problems

How many bars must be weighed? Weighing is cheap, but on a busy site three bars per diameter is a realistic routine. Elementary probability shows what such a sample can and cannot detect. If a fraction p of the bars in a delivery are light and n bars are drawn at random, the chance that at least one light bar is found is 1 minus (1 minus p) to the power n (derived).

Source: Cameroon Steel calculation (binomial sampling from a large lot). Annex A.

Two conclusions follow. First, small samples catch systematic problems reliably. When a whole rolling campaign has been rolled light, which is what happens when a mill aims below nominal or runs worn rolls, half or more of the bars are light, and three bars weighed will expose the problem in nearly nine deliveries out of ten (derived). Second, small samples catch occasional light bars less reliably in any single delivery, but repetition compensates: with one bar in five light, weighing three bars in each of four deliveries gives a 93% chance of detection (derived: 1 minus 0.8 to the power 12). The rule is therefore not to weigh many bars once, but to weigh a few bars every time and keep a record.

6.5 Read the bar: marks, tags and certificates

The second check is documentary and takes a few minutes. A conforming bar carries a rolled-in mark, a pattern of ribs or symbols that identifies the mill, repeated along its length [S31][S34]. The bundle tag should give the producer, the grade and standard, the diameter, the heat number and, for certified products, the certification mark and certificate number [S31][S32]. The delivery note and the inspection certificate should list the same heat numbers.

The rule that follows from the regional evidence is blunt: bars without a legible rolled-in mill mark should not be used in structural members. A printed tag can be produced by anyone; a rolled-in mark requires a set of rolls and is the producer’s signature on every metre of steel. Even a rolled-in mark can be imitated by a rogue mill, which is why the mark, the tag, the certificate and, where applicable, the certificate’s current validity with ANOR or the certification body should all be cross-checked. A merchant who cannot produce a certificate matching the heat numbers on the tags is offering an untraceable product.

6.6 Bend the bar: a ten-minute ductility check

No site tool measures yield strength. Ductility, however, reveals itself to a simple bend. The site bend test is an adaptation of the standard bend test [S44]:

1. Cut a sample about 0.5 m long from a bar of each diameter in the delivery.
2. Using a bar-bending machine or a manual bender with a former of about four bar diameters for sizes up to 16 mm, and larger for bigger bars, bend the sample cold through 180°.
3. Inspect the outside of the bend in good light. Any crack, split or tearing at the root of the ribs is a failure.
4. For bars that will be bent and straightened in service (starter bars, for example), partially straighten the sample and inspect again.

A brittle bar, whether from excess carbon or phosphorus, nitrogen ageing, a seam or a laminated re-rolled section, usually cracks or splits in this test. A bar that passes may still be weak, but it is unlikely to be dangerously brittle. The test should be performed cold: heating a bar to bend it changes its properties, and quenched and self-tempered bars, whose strength comes from their thermal treatment, must never be heated.

While a sample is in hand, the ribs can be checked: lay a ruler along the bar and measure the length of ten rib spacings; measure the rib height at mid-rib with a depth gauge or vernier caliper. Comparing these with the producer’s data, or with a known good bar, exposes worn rolls and shallow ribs. Cut ends deserve a look too: a lamination, split or dark inclusion in the cross-section of a cut end is a sign of a poor billet or of re-rolled scrap.

Source: Cameroon Steel recommended practice, based on the product requirements in [S31][S34][S38][S44] and the test methods in [S44][S45].

6.7 Look at the bar: corrosion and contamination

Bars stored for months on bare ground in Douala or Kribi will rust. Light, uniform surface rust that wipes off is not harmful and does not reduce bond. What matters is loss of section: flaking rust, pits and reduced rib height. American practice provides a practical rule: rusty bars are acceptable if a hand-wire-brushed sample still meets the minimum dimensions, mass and rib height of the specification [S34]. The same rule works with European-family bars: brush a sample, weigh it, measure its ribs. Oil, paint, mud and release agents reduce bond and should be cleaned off before placing.

Prevention is cheaper. Bars should be stored on timber bearers off the ground, covered against rain, segregated by diameter and grade so that bundles are not mixed, and, near the sea, sheltered from spray. A site that buys a month’s steel at a time rarely has a corrosion problem; one that buys a year’s supply when prices are low often does.

6.8 When in doubt, test in a laboratory, and report

Any failure in the site checks, any missing or inconsistent document, or any doubt on an important structure justifies laboratory testing. The practical rule is to quarantine the lot, mark it clearly so that no bar is used, and send three samples per diameter, long enough for tensile and bend testing, to a competent laboratory, preferably one accredited to ISO/IEC 17025 [S55]. The laboratory should report Re, Rm, Rm/Re, Agt and the bend result, and the mass per metre; where bars will be welded, a chemical analysis gives the carbon equivalent.

The cost of these tests is small against the cost of the steel in a single floor slab, and negligible against the cost of demolishing and recasting a column. If a lot fails, it should be rejected, the supplier and the supervising engineer informed in writing, and the case reported to ANOR, which operates a channel for reporting non-conforming products [S74]. Reports from sites are the raw material of market surveillance: an agency that never receives complaints cannot target its sampling.