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Structure Repair & Rehabilitation
Concreting Works, Defects & Remedies
Honey combings, irregular finished concrete surfaces, (Fig-4) after the removal of formwork on the concrete surfaces are the most common developments that need repairing from both durability & strength point of view. Also, concreting at the beam-column joints may be loosely packed due to congestion of steel rebars and difficulty in applying needle vibrators if the concrete made is harsh one.
Fig-4 (Rough/ Honey combings concrete surface formed at beam's side face due to poor concreting works : Affects durability & strength badly)
Beam-Column joints are more critical while concreting because normally column levels are pre-casted at the lower level than the beam bottom level; thereby increasing the depth of concreting. It is done in the fulfillment of development length. Loose concreting and even a throughout gap could be formed at such joints. Cement grouting or grouting along with the gravel packing could be a solution as a measure of repair/rehabilitation in such cases.
Proper quality control at the site in the production of quality concrete that fullfills both the criteria of workability and strength is needed to prevent such defects. Concreting in difficult conditions as mentioned above needs good workable concrete possessing required degree of flowability.
Fig-5 (Cone Slump of 135mm achieved with water-cement ratio of 0.52 and chemical admixture of 200ml/cement bag. Well graded 20mm down aggregate with cement-sand-aggregate ratio of 1:3:3 was used)
Use of well graded, with a maximum size aggregate not exceeding 20mm and proper water-cement ratio are crucial determining factors in producing concrete with required degree of flowability that produce well finished, void free surface. Slump test by using slump-cone could be performed at site to measure the flowability and to detect any variations in mix. Higher the slump height, (Fig-5), higher will be the flowability of concrete. Similarly low slump concrete makes harsh concrete that may result in bad finished surface and honey combings
Variation in water-cement ratio, and ratio of cement-sand-aggregate are readily detected by the change in slump height.
Fig-6 (Smooth finished surface of slope roof slab. Honey combings are common in such slabs due to difficulty in compaction. Concrete with slump of 135mm as shown in Fig-5 was used for casting)