Good construction practices and techniques should be followed to prevent occurrence of structural damages that may occur during occupancy and with age of the structure. A structure has to go through different stages in construction process.
Each and every stage of construction is important to make sure that the structure being constructed will not experience damage under any general circumstances. Development of cracks in structure is the first sign of damage. Structural damage does not only reduces strength of the structure but it may also make it unfit for use to the extent that the structure collapses
It is very important for the civil engineers to ensure good construction practices are followed at every stage of construction to prevent structural damage and failure of the structure.
Good Construction Practices and Techniques to Prevent Structural Damage
Following are the good construction practices and techniques that shall be followed for good quality and durable construction of structure:
1. Masonry work:
- Masonry work in a structure should be carried out in uniform levels at all parts of the structure to prevent differential settlement of foundation due to differential loading. This will prevent the cracking of masonry walls and also other structural elements. Difference in the height of masonry in different parts of a building should normally not exceed 1m any time during construction.
- Masonry work should be properly cured for a minimum period of 7 to 10 days.
- Masonry works on any RCC elements such as RCC Slab and beams should not be started till minimum of 2 weeks after striking off the shuttering.
2. Concrete work:
- In reinforced concrete members such as cantilever beams and slabs which are liable to deflect appreciably under load, removal of centering and imposition of load should be deferred at least one month so that concrete gains sufficient strengths before it bears the load.
- Curing of any concrete member should be done for a minimum period of 7 to 10 days and terminated gradually so as to avoid quick drying.
- Concrete work in very hot and windy climate should be avoided, and in case it is not avoidable then precautions shall be taken to keep the temperature of fresh concrete down and to prevent quick drying of concrete. Following steps should be taken to keep the temperature of freshly prepared concrete down:
– Aggregate and mixing water should be shaded from direct sun.
– Part of mixing water may be replaced by pounded ice.
– As far as possible concreting should be done in early hours of the day.
- Re-trowelling the concrete surface slightly, before its initial setting to mitigate plastic shrinkage cracks
3. RCC frame work:
- As far as possible frame work should be completed before starting work of panel walls for cladding and partitioning.
- Work of construction of panel walls and partition should be deferred as much as possible and should proceed from top to down ward.
- When partition walls are to be supported on floor beam or slab upward camber should be provided in floor slab/beam to counter act deflection.
- Horizontal movement joint should be provided between top of panel wall and soffit of beam and when structurally required little support to the wall should be provided at the top by using telescopic anchorage or similar arrangement. Horizontal movement joint between top of wall and soffit of beam/slab shall be filled which some compressible jointing material.
- If door opening is to be provided in partition wall a center opening is more preferable than off center opening.
- Light re-vibration of concrete shall be done, before it has set, for the member and section prone for plastic settlement cracks i.e. narrow column and walls, at change of depth in section.
- When plastering is to be done on masonry, mortar joints in masonry should be raked out to 10 mm depth while the mortar is green. Plastering should be done after masonry has been properly cured and allowed to dry so as to undergo initial shrinkage before plaster.
- For plastering on concrete background, it should be done as soon as feasible after removal of shuttering by roughing of concrete surface where necessary by hacking, and applying neat cement slurry on the concrete surface to improve the bond.
- When RCC and brick work occurs in combination and to be plastered, then sufficient time (at least 1 month) shall be allowed for RCC and brickwork to undergo initial shrinkage and creep before taking up plaster work. In such case either groove shall be provided in the plaster at the junction or 10cm wide strip of metal mesh or lathing shall also be provided over the junction to act as reinforcement.
5. Concrete and terrazzo floor:
- Control joint should be provided in the concrete and terrazzo floor either by laying floors in alternate panels or by introducing strips of glass, aluminium or some plastic material at close interval in grid pattern.
- When flooring is to be laid on RCC slab, either a base course of lime concrete should be provided between the RCC slab and the flooring or surface of slab should be well roughened, cleaned and primed with cement slurry before laying of floor.
5. RCC Lintels:
Bearing for RCC lintels should be on the liberal side when spans are large so as to avoid concentration of stress at the jambs.
6. RCC roof slab:
- The top of the slab should be provided with adequate insulation or protective cover together with some high reflectivity finish cover to check the thermal movement of the slab and consequent cracking in supporting wall and panel/partition wall.
- In load bearing structure, slip joint should be introduced between the slab and supporting/cross walls. Further either the slab should project for some length from the supporting wall or the slab should rest only on part width of the wall as shown in figure below:
Fig.1: Constructional detail of bearing of RCC roof slab over a masonry wall
On the inside, wall plaster and ceiling plaster should be made discontinuous by a groove of about 10 mm.
For introducing the slip joint, the bearing portion of supporting wall is rendered smooth with plaster (preferably with neat cement finish), which is then allowed to set and partly dry. Thereafter either it is given thick coat of whitewash, or 2 to 3 layers of tarred paper is placed over the plaster surface, before casting of slab.
7. Provision of glazed, terrazzo or marble tile on vertical surface:
Before fixing of these tiles on vertical surface background component should be allowed to undergo movement due to elastic deformation, shrinkage & creep otherwise tiles are likely to crack and dislodged.
8. RCC work in exposed condition:
For RCC work in exposed condition i.e. sunshades, balconies, canopies, open verandah etc., to prevent shrinkage cum contraction cracks, adequate quantity of temperature reinforcement shall be provided. In such condition quantity shall be increase by 50 to 100 % of the minimum amount prescribed.
9. Finish on wall:
Finishing items i.e. distemper and painting etc. should be carried out after the plaster has dried and has under gone drying shrinkage.
10. Pace of construction:
The construction schedule and the pace of construction should be regulated to ensure :
- All items of masonry are properly cured and allowed to dry before plastering work is done, thus concealing the cracks in masonry in plaster work. Similarly plaster work should be cured and allowed to dry before applying finishing coat. So as to conceals the cracks in plaster under finish coat.
- In case of concrete work before taking masonry work either over it or by its side, the most of the drying shrinkage, creep and elastic deformation of concrete should be allowed to take place, so as to avoid cracks in masonry or a the junction of masonry and concrete.
11. Provision of reinforcement for thermal stresses:
To control the cracks in concrete due to shrinkage as well as temperature effect, adequate temperature reinforcement shall be provided. This temperature reinforcement is more effective if smaller diameter bars and the deformed steel is used than plain reinforcement.
12. Extension of existing building:
(a) Horizontal extension: Since foundation of an existing building undergoes some settlement as load comes on the foundation, it is necessary to ensure that new construction is not bonded with the old construction and the two parts are separated by a slip or expansion joint right from bottom to top. Otherwise, when the newly constructed portion undergoes settlement, an unsightly crack may occur at the junction.
Care should also be taken that in the vicinity of the old building, no excavation below the foundation level of that building is carried out.
When plastering the new work, a deep groove should be formed separating the new work from the old.
When it is intended to make horizontal extension to a framed structure later on than the twin column with combined footing shall be provided at the time of original construction itself as under:
b) Vertical extension: When making vertical extension to an existing building (that is adding one or more additional floors) work should be proceeded at a uniform level all round so as to avoid differential load on the foundation. In spite of this precaution, however, sometimes cracks appear in the lower floors (old portion) at the junction of RCC columns carrying heavy loads and lightly loaded brick masonry walls due to increase in elastic deformation and creep in RCC columns. Such cracks cannot be avoided.
Renewal of finishing coats on old walls of old portion should be deferred for 2 or 3 months after the imposition of additional load due to new construction so that most of the likely cracking should take place before finish coat is applied thus concealing the cracks.
13. Rich cement treatment on external walls:
When it is proposed to give some treatment on external walls of some rich cement based material i.e. artificial stone finish, terrazzo etc., the finish should be laid in small panels with deep grooves in both directions.
14. Movement joints:
To mitigate/relieve the magnitude of stresses due to thermal movement and shrinkage movement joints i.e. Expansion joint, Control joint and Slip joint shall be provided in the structure.
15. Filling in plinth:
Filling in plinth should be done with good soil free from organic matter, brickbats and debris etc. It should be laid in 25 cm thick layers, well watered and compacted to avoid possibility of subsequent subsidence and cracking of floors.