Monthly Archives: May 2012
Stages of Construction of New Building/Structures
- A concept plan
- Selection of appropriate materials and methodology of construction
- Freezing assumptions in structural design
- Selection of trained manpower and machinery
- Execution of the project
Expected Service Life of Structures
- Monumental Structures like temple, mosque or church etc – 500 to 1000 years
- Steel Bridges, Steel Building or similar structures – 100 to 150 years
- Concrete bridges or Highrise building or stone bridges etc – 100 years
- residential houses or general office/commercial buildings etc – 60 to 80 years
- Concrete pavements – 30 to 35 years
- Bituminous pavements – 8 to10 years
Conception and Design Process
- Concept plan of project to satisfy its functional needs,
- Architectural planning and design
- Detailed structural design
- Preparation of working/execution drawings
- Preparation of specifications and tender conditions for execution
- Modifications (if any) during construction phase.
Conception of structure / building depends on the functional need and local environment rather than purely on architectural/structural considerations. Though, the basic structural frame will share relatively small proportions (about 45%) of the total cost of the project, yet the designer has to give due emphasis on its proper planning, design and specifications of structural frame work. However, these have to conform to local environment and locally available construction resources e.g. materials, labour and plant & machinery.
Figure 1: Excessive strain & poor concreting at beam column joint
The functional utility as well as aesthetics of building are important in the architectural design process. But, durability is the most important in structural design and laying down specifications for construction for achieving service life of structure. Thus, all three aspects (viz architectural, structural and construction) are important. Yet, many a times, there are clashes between the three. Slender RCC columns, provided from architectural considerations, especially at the ground-floor level, where the trend is to make the ground floor area reserved for parking and access, but these became critical for earthquake resistance and could be a cause for slow disintegration within five to ten years. RCC columns could be observed to split, mainly due to corrosion of the steel bars and the resultant cracking. Corrosion crack in concrete is cancerous, if not treated in time, and will spread to cause the concrete structures to crumble and collapse. So, it is necessary to provide shear walls in the ground floor at suitable places in the design process itself.
The design of slender columns, shear walls and beams are provided to look slim and to increase carpet area but could become problems during construction and may not ensure the correct alignment and proper compaction of concrete. Many times columns are reduced in thickness well below the minimum 225/300mm to result in congestion of steel bars, with probability of exposure due to inadequate cover after de-shuttering. For such structure, appropriate coordinated efforts between architect, structural designer and construction team are needed.
In structural designs, emphasis is given on stress and strains on concrete and steel with a suitable factor of safety. Structures are designed on the basis of LIMIT STATE philosophy which was adopted in 1978 in India. An approach with respect to performance of structures like cracks, deflections and minimum cement content for durability and control of maximum w/c ratio are considered as per IS:456-2000 in the design. The IS code lays down following factors for durability of concrete for adoption during design / construction stages:
- Environment or exposure condition,
- Cover to embedded steel,
- Type and quality of constituent materials,
- Cement content and water/ cement ratio,
- Workmanship, and
- Shape and size of the Structural member.
Exposure to highly aggressive environment
Figure 2: Poor quality of concrete and in grace of moisture leading to corrosion
Relatively new but severely distressed buildings are observed in the locations, close to sea or creek. Corrosion of reinforcement and distress in concrete in the structural members (columns, beams, slabs) in some of them could be alarming. It is mainly due to high concentration of chlorides and/or sulphates in the ground water and saline environment around the structure. Moisture from soil could rise. The absence of damp proof course at the DPC level could allow the dampness to rise quite high. This combined with substandard materials and workmanship could get further aggravate deterioration process. Thus, extra care must be taken in materials selection and have proper control on quality of concrete in structures, close to sea.
Selection of Materials of Construction
- Concrete (Plain, Reinforced, or Pre-stressed)
- Steel of all grades: Fe 250, Fe 415, Fe500 and high tensile steel
- Bricks Work
- Fiber Reinforced Concrete
- Composite materials ferrocem- ent, FRp etc.
- Different type of plastics and polymers like Acrylic etc
Reinforced Cement Concrete
Figure 3: Sulphate attack on pipe pedestal
Concrete came into existence in last 200 years or so and it was expected that concrete should behave like stone structures. But with the introduction of steel reinforcement in concrete, its structural utility has been enhanced for taking tensile loads. Hence, reinforced cement concrete has now become one of the most important materials of construction all over the world. Thus, reinforced cement concrete has become so important that without this, there is no major building in India or anywhere in world.
However, because of its heterogeneous character, the durability of structure has assumed much importance in last three decades. Durability is affected because of poor quality of its constituent materials and workmanship could lead to early deterioration. Chlorides may also be present in these materials which lead to early and faster corrosion of reinforcement. Poor quality of concrete materials also affects quality of concrete:
- Quality of cement
- Type and quality of course aggregates
- Fineness of sand and silt contents therein
- Concrete mix design etc.
Quality of other associated construction materials should also be looked into:
- Poor burnt clay Bricks (its crushing strength should be more than 3.5 MPa),
- Weathered Stone
- Rusted Steel
- Good quality of construction material
- Appropriate methodology
- Trained manpower and
- Appropriate machinery.
Figure 4: Early corrosion due to inadequate cement content in concrete
Engineers in India are well trained in the academic and technical institutions in theories. Our engineers and construction managers can match their counter- parts in the developed countries in producing sophisticated design calculations and drawings, including the use of computers so that our structures are designed safe, elegant and slender. But how about executing them to specifications at the construction site? In India, concrete structures are made at construction site itself and hence, quality largely depends on artisans like mason, bar bender etc. The construction industry has virtually no practical training facilities to enable our basic artisans to produce good, cohesive, workable and durable concrete economically. They are either not trained or do not make efforts in the correct way of concreting like placing and compacting (vibrating) the concrete to obtain a dense and impervious watertight concrete which protects the reinforcement against corrosion. Result is poor quality and unsound structures.
Even after the construction supervision by site staff, corrosion of the steel bars and spalling/splitting of concrete continues at an early stage, leading eventually many times to the total collapse of the building. Generally, in India owners feel that the consulting fees to an Architect or a consulting Engineer is a waste of money and they feel happier, if the Architects and Engineers extend their services free of cost. It is recommended that site construction services (Architects and Engineers) should be engaged on payment and the owner must utilize their full expertise and experience. The site engineers should be paid by the owner, as is done all over the world. Then only site engineers shall be answerable for quality to the owner of structure. Therefore, to get good construction, we must have trained manpower and quality supervision throughout the construction period by a competent, experienced and strict Resident Engineer / site supervisor.
Causes of Early Deterioration of Concrete Structures
Poor workmanship: Need for certified Artisans & quality of construction
Figure 5: Shrinkage cracks in concrete pavement due to concreting in hot weather
Substandard workmanship in RCC can be in the form of honey combing (insufficient compaction of concrete) or inadequate cover to reinforcement (improper placement of bars) or both. These lead to early corrosion of reinforcement especially in thin structural member. In masonry and plaster, poor workmanship could be in the form of loosely fitted masonry joints (within walls or between external walls and beams / columns), poor lines and levels and hollow plaster etc. They lead to excessive seepage. Some of these deficiencies may become evident only after the full loading of structure has been put to use like 5 to 7 years. Therefore, construction workers employed should be trained artisans, who know the job well. To get trained manpower, there should be training schools to cater to the need of artisans like bar bender, concreting mason etc. Training schools should be opened and look into the following:
- Construction workers to be trained and certified for a number of trades e.g. concrete, bar bending, masonry/plaster, carpenter, etc.
- Assess the availability and demand of the certified personnel and then fill the gap with certified trained construction personnel
- Run simple & short training courses with minimal loss of wages for personnel on job
- Practical sessions with hands on experience on works itself
Further, necessary steps must be taken at construction site which calls for increase in number and proper mix of knowledge, skills and attitudes.
Effect of Climate
Inadequate cement quantity
Excessive water cement ratio
Figure 6: Beam member with inadequate cover
For concrete to be durable there is a maximum w/c ratio specified for mixing concrete, as well as to give proper workability and concrete strength. Again the IS 456:2000 has laid down upper limits of water cement ratio. Normally, it needs about 11.5 liters of water (say w/c = 0.23) per 50 kg bag of cement for hydration process. However, the concrete will be stiff and un-compactable with this quantity of water. Therefore, additional water and super-plasticizer is added to make the concrete workable. This extra water (not needed for Hydration) eventually evaporates and leaves minute capillary pores which permit the ingress of moisture and pollutants which lead to slow corrosion of steel bars and ultimate disintegration of the concrete. The objective is to add only the water as per concrete mix design.
Many a times, the concrete is made at site and contractors use more than the permissible water to make concrete workable because of following:
- The contractors may not have or like to use vibrators.
- Masons want fluid concrete for easy placing and compaction of the concrete.
- The sand with excessive silt (clay) demands more water to maintain workability (high slump) desired by the workers.
- The coarse aggregates (stone metal) are excessively flaky instead of cubical, due to bad crushers. The extra surface area of the flaky stone particles, demand extra water to maintain easy workability desired by workers.
- Concreting during the hot weather in day time at temperature more than 35°C without any precautions (cooling of concrete ingredients by shading or using ice flakes.) accelerates the rate of hydration (setting) of the concrete. The concrete becomes stiff and unworkable, demanding excess water in the concrete. If this condition exists then, it is preferable to do concreting at night time when it is cooler.
The shuttering and staging is too flimsy and rickety as it may result in the collapse of the formwork and false work or have excessive leakage of cement slurry/ grout through the big gaps in the shuttering.
Inadequate concrete cover
Honeycombed or Un-vibrated concrete
Cold joints or bad construction joints
Figure 7: Poor Concreting in column
Sometimes, chemical reaction occurs between reactive siliceous minerals or carbonates, present in aggregate and the alkaline hydroxides derived from hydrated cement. The result of reaction is formation of alkali-silicate gel of ’unlimited swelling’ type. These reactions are called i) Alkali-silica reaction, and ii) Alkali-carbonate reaction. Because the gel is confined by the surrounding cement paste, so the internal pressure develops and causes cracking and disruption of concrete. Under most conditions, this very slow reaction causes excessive expansion and cracking of concrete after few years. Aggregates containing particular varieties of silica are susceptible to attack by alkalis (Na2O and K2O) originating from cement, admixtures or other sources, producing an expansive reaction.
Aggregates petro-graphically known reactive type or aggregates which, on the basis of past history or laboratory experiments are suspected to have reactive tendency are needed to be avoided in concrete or used only with cements of low alkalis [not more than 0.6 percent as sodium oxide (Na2O)]. Use of pozzolanic cement and certain pozzolanic admixtures may be helpful in controlling alkali aggregate reaction. Alternately use non-reactive aggregate from alternate sources.
Initially rust steel bars
Figure 8: Water tank: Less cover in soffit of slab and week concrete
Often steel bars are stored in open areas, exposed to rain and atmospheric moisture resulting in rusting of them. The corrosion process starts rapidly in the presence of moisture (especially in coastal areas). The steel bars are rarely wire-brushed and cleaned thoroughly before being placed in shuttering prior to concreting. In other cases, due to suspension of work due to reasons whatsoever, structural frame remains exposed to sun, rain and misuse for a long duration. Such prolonged exposure to weather can cause rusting of rebars and carbonation to adversely affecting durability of the building frame.
Congested reinforcement bars
Figure 9: Less cover in slab bottom and cold joint
Sometimes design engineer provides too many steel bars in the narrow and slender RCC columns, walls or beams which lead to practically no cover in concrete or even space for inserting a needle vibrator to ensure full compaction. This results in honeycombed concrete. Through honeycombing, the moisture and atmospheric pollution enters the steel bars and thus starting the corrosion process.
Porous cover blocks
Effect of Cracking on the Life or Durability of Structure
Figure 10: Poor quality of concrete in cover and ingress of moisture
A good understanding of cracks in concrete will help us avoid failures of concrete on one hand and avoidable worries and expenditure on repairs on the other hand. Cracks in concrete are rarely symptoms of disease by itself. Cracks in concrete are more prevalent than imagination. But not all the cracks are dangerous. Cracks could lead to any of the following effects:
- Reduce loading capacity of structure
- Progressive failure (Cracks propagate at smaller stress than that required to initiate it).
- Loss of appearance
- Leakages (effects serviceability)
- Apprehension of failure in mind (Psychological)
- All above reasons reduce durability of concrete. Some typical cases of cracks are shown in figures above.
- Better Concrete mix design.
- Least possible w/c ratio.
- Optimum quantity of cement content.
- Use mineral admixtures or have better fineness of cement.
- Have friendly environmental conditions like wind, ambient temperature, and moisture etc at the time of concreting at site.
- Have dense concrete
- Use low heat or pozzolanic cement in mass concreting.
Figure 11: Initially rusted reinforcement bars and less cover in soffit of slabs
Requirements for Construction Engineers/Workers
- Gupta Y. P. “Use of MALWA Recycled Aggregate in Concrete Construction: a Need of Society for Better Environment,” Journal of Indian Concrete Institute, Vol. 10, No. 4, Jan 2010.
- Performance of Structures-Real Estate August 1989.
- Development Of Light Weight Concrete (cpwdeablogging.wordpress.com)
- Kolkata Underground Car Park – Perfect Example of Using Sheet Piles as a Permanent Structure and With Top Down Construction Methodology (cpwdeablogging.wordpress.com)
- Composite Concrete Beam Analysis (newtonexcelbach.wordpress.com)
- Sustainable Waterproofing Technology : Protecting Concrete and Environment (cpwdeablogging.wordpress.com)
- Joints in Concrete Structures – Part I (civilconstructionresourcez.wordpress.com)