Crack Investigation and Residual Strength Check of Beams in an Existing Six-Story Reinforced Concrete Industrial Building in Bangladesh

Abstract

Old industrial buildings in Bangladesh often stay in full production use for many years after they are built, and cracks in their beams are a common concern. This paper looks at one such case: a six-story reinforced concrete (RC) factory building, where cracks were found in beams and slabs on four roof levels (2nd to 5th floor). The investigation had three simple steps. First, every crack was marked, photographed, and compared with the original drawings. Second, four basic non-destructive tests were used at the crack locations: ultrasonic pulse velocity (UPV) to check the crack depth and concrete quality, a carbonation test to see whether the crack had reached the steel bars, a half-cell test to check for corrosion, and a cover-meter survey to check the concrete cover. Third, for one critical beam on each floor, the load-carrying capacity was recalculated using the reduced beam depth caused by the crack, following the Bangladesh National Building Code (BNBC-2006). In total, 35 cracked beam and slab spots were surveyed across the four floors. Of these, 22 cracks went deeper than the main steel bar, 6 stopped within the concrete cover, and 7 were only surface plaster cracks. At the 14 spots where both crack depth and width were measured, depth ranged from 2.4 to 6.5 inches (mean 4.4 in., SD 1.2 in.) and width ranged from 0.03 to 0.20 mm (mean 0.09 mm, SD 0.05 mm); the two showed only a weak relationship (Pearson r = 0.18), so a deeper crack did not necessarily mean a wider one. Concrete quality from the UPV test was “good” or “excellent” everywhere it was tested, and the half-cell tests found no active corrosion. Concrete cover measured 35–60 mm (mean 44 mm), comfortably above the usual 25 mm minimum. The strength checks on the four critical beams showed that, even after subtracting the crack depth from the beam’s effective depth, the steel already in place was still enough to carry the code-required bending moment in every case. The most likely cause of the cracking is poor compaction and curing during the original construction, not corrosion or overloading. A step-by-step repair plan is given — heavier repair (remove damaged concrete, clean the bar, patch and re-plaster) for cracks that reached the bar, and a lighter wire-mesh plaster repair for shallower cracks — together with a crack-width monitoring schedule and a temporary 2 kPa live-load limit until repairs are finished. The results give a simple, data-based approach that other engineers can reuse to check similar older factory buildings.