Top 5 Construction Failures in Civil Engineering

When we see amazing and efficient structures around us but we often forget about some of the biggest failures in construction. We can learn a lot as civil engineers from these failures. Here is a list of Top 5 construction failures 

5) Leaning Tower of Pisa

Tower of Pisa in Italy is one of the best examples of settlement failure. Settlement of Pisa tower started in 1174 AD.

Pisa tower foundations were built on layers of compressible clay overlaying dense sands. These clay layers effect were not accounted for in soil investigation.

Tower was designed to be 185 feet tall having 8 stories. It started to lean when construction reached the third story. Engineers tried to build rest of the stories away from lean, but it made the tower more heavy resulting in increased lean.

No one is sure why it leans, but following theories were made

• Tower leans because of difference in soil compressibility on each side of tower
• Some Geotechnical Engineers say that lowering in water table is the cause
• Tower eccentric stress is responsible for tilting

At one stage Pisa Tower was on the verge of collapse in 1992. A lot of technical and ground work was done to stop lean.

4) Lotus River Side Complex China

Another failure related to Soil. You can see clearly in above picture that body moved as a rigid body which clearly states that structural design engineers and site engineers are not at fault.

This failure was human negligence more than anything else. Excavation was started near the building foundations as depicted in picture below which increased lateral earth pressure in the direction of fall resulting in collapse

3) Tacoma Narrows Bridge

Tacoma Narrows Bridge was a suspension bridge designed in the U.S state of Washington spanned between Tacoma and Kitsap Peninsula.

Construction of the bridge started in 1938 and it was opened for traffic in 1940. Bridge collapsed dramatically on 7th November 1940.

As soon as the deck of the bridge was built, it started moving in the direction of the wind. The motion was observed even before public opening. Many different measures were adopted to mitigate the movement were in vain and it finally collapsed under 64km/hr wind conditions.

2) Teton Dam

Geotechnical Engineers have learnt a lot from Teton Dam Failure especially in the field of slope stability. After this failure regulations were forced on similar projects by US government.

On the morning of 5th June 1976, the worker on dam noticed leakage which they thought was not alarming. Leakage hole grew larger and larger with time and after some time it burst at 11.55 am sending 2,000,000 cubic meters per second of water into Teton river.

Investigation revealed that Soil type used to construct the slope was permeable and caused water to seep through which increased with time.

1 )  Hyatt Regency Walkway Collapse

Hyatt Regency Failure comes in the category of one of the most deadliest structure design failure in US history until attacks on world Trade Center.

In 1981, two walkways at Hyatt Regency hotel located in Kansas city  fell on lobby below where dance competition was being held. In this tragedy almost 114 people died and more than 200 were left injured.

Initial Investigation revealed that changes in the design of the walkway restricted the weight distribution to be held by tie rods and support beams.  Whole walkway plus the weight of the people proved too much for ties and beams in the end.

Advantages & Limitations of Prestressed Concrete

The prestressing of concrete has several advantages as compared to traditional reinforced concrete (RC) without prestressing. A fully prestressed concrete member is usually subjected to compression during service life. This rectifies several deficiencies of concrete. The following text broadly mentions the advantages of a pre-stressed concrete member with an equivalent RC member. For each effect, the benefits are listed.

A) Section remains un-cracked under service loads

1. Reduction of steel corrosion
   • Increase in durability.
2. Full section is utilized
   • Higher moment of inertia (higher stiffness)
3. Less deformations (improved serviceability).
4. Increase in shear capacity
5. Suitable for use in pressure vessels, liquid retaining structures.
• Improved performance (resilience) under dynamic and fatigue loading.

B) High span-to-depth ratios

1. Larger spans possible with prestressing (bridges, buildings with large column-free spaces)
2. Typical values of span-to-depth ratios in slabs are given below.

For the same span, less depth compared to RC member.

• Reduction in self weight
• More esthetic appeal due to slender sections
• More economical sections.

C) Suitable for precast construction The advantages of precast construction are as follows.

• Rapid construction
• Better quality control
• Reduced maintenance
• Suitable for repetitive construction
• Multiple use of formwork

1. Reduction of formwork

• Availability of standard shapes.

Limitations of Prestressing

Although prestressing has advantages, some aspects need to be carefully addressed.

• Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete.
• The use of high strength materials is costly.
• There is additional cost in auxiliary equipments.
• There is need for quality control and inspection.

Minimum Steel Reinforcement in Concrete and Clear Cover Requirements

The minimum amount of steel reinforcement is defined as that for which "peak load at first concrete cracking" and "ultimate load after steel yielding" are equal. In this way, any brittle behavior is avoided as well as any localized failure, if the member is not over-reinforced.

In other words, there is a reinforcement percentage range, depending on the size-scale, within which the plastic limit analysis may be applied with its static and kinematic theorems.

A minimum area of reinforcement is required to control the cracking, which occurs in the concrete due to temperature, shrinkage and creep. It enables cracking to be uniformly distributed and therefore minimizes individual crack width.

The following criteria have been used to determine the cross-section area of temperature or minimum reinforcement required in hydraulic structures. The percentages indicated are based on the gross cross-sectional area of the concrete to be reinforced. Where the thickness of the section exceeds fifteen (15) inches (380 mm), a thickness of fifteen (15) inches (380 mm) should be used in determining the temperature or minimum reinforcement.

The minimum ratio of required reinforcement is;

FOR SLABS:

f_min = 0.002 (for f_y = 40,000 psi)

S_min = 0.0018 (for f_y = 60,000 psi)

FOR WALLS:

For Vertical Steel

f_min = 0.0015

For Horizontal Steel

f_min = 0.0025

The temperature reinforcement shall not be less than ½ inch at nine 9 inch center to center. All concrete stilling basins, glacis and floors and all concrete aprons structures (with slab thickness > 15 inch ) shall be reinforced in the exposed (top) face with ¾ inch bars at twelve (12) inch center to center, both ways, placed three (3) inch clear from concrete face, unless otherwise designed.

Nominal reinforcement of concrete chute blocks, baffle blocks and sills for stilling basins, aprons and other portion of structures shall consist of ¾ inch bars at twelve (12) inch center to center, both.

Temperature and shrinkage reinforcement should be uniformly distributed alongside faces of structure elements to control cracking due to temperature changes, creep, and shrinkage.

Depending on the thickness of the structural element, it is preferred that the centre-to-centre spacing of the primary and secondary reinforcement be equal to or less than 300 mm; however, in no case should it exceed 450 mm. The minimum clear distance between bars should not be less than 1.4 times the bar diameter or 1.4 times the nominal maximum size of the coarse aggregate, whichever is greater. This requirement also applies to the clear distance between a contact lap splice and adjacent splices and bars.

The minimum thickness of concrete cover over the reinforcement has been determined by considerations of adequate fire resistance and durability. The cover for the reinforcement to meet specified period of fire resistance is detailed as follows:

Fire Resistance (hours)	Beams		slabs		columns
	Simply supported	continuous	Simply supported	continuous
0.5	20	20	20	20	20
1.0	20	20	20	20	20
1.5	20	20	25	20	20
2.0	40	30	35	25	25
3.0	60	40	45	35	25
4.0	70	50	55	45	25

Cover in excess of 40 mm (1.57 inch) may require additional measure to reduce risk of spalling.

Cover Against Spalling

Concrete Element	Minimum Concrete Cover
	(in)	(mm)
Face in contact with earth	3	75

Concept of Demand & Capacity in Structures

Definition of Demand (Loads on structures) Demand refers to all external actions (self weight) gravity, wind, earthquake, snow all are external actions. If differential settlement occurs stresses are induced in structure. Internal stresses are also called load effects. Definition of Capacity (Load capacity of structures) Overall ability of a structure to carry an imposed demand is called capacity. Analysis of Capacity and Demand To find out whether capacity is enough to carry demands or not. To enhance capacity, either change the material or its geometry. Failure occurs when capacity is less than demand. To avoid failure, capacity to demand ratio should be kept greater than one or at least equal to one. This can be done by increasing the area or reducing the loads or increase the strength of the material by replacing the material Steel Reinforcement area should be opposite to the direction of demand e.g. in slabs the demand direction is in the direction of compression and tension i.e. along the length of bending. So steel is placed along the length and along the width.

Reinforced Concrete Building Elements

Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete. 1st Floor It is the floor that has 1 storey height above ground. Basement Floor The floor of the basement of the building. It is also called cellar. The basement floor is either completely or partially below the ground floor. A basement can be used in almost exactly the same manner as an additional above-ground floor of a house or other building. However, the use of basements depends largely on factors specific to a particular geographical area such as climate, soil, seismic activity, building technology, and real estate economics. The concrete floor in most basements is structurally not part of the foundation; only the basement walls are. Since warm air rises, basements are typically cooler than the rest of the house. In summer, this makes basements damp, due to the higher relative humidity. Basement Wall The wall surrounding the basement floor is called the basement wall. The basement walls can be regarded structurally as part of the foundation. The basement walls are shear walls which can resist lateral laods as well. Moreover, these walls are meant to be higly non-porous and water resistant. Column Bracket Column Bracket is protrusion from the column also used for hanging or attaching lamps, bulbs or other accessories to it like road signs. Column Capital Column capital is an architectural element used for aesthetic purposeswhich forms the topmost member of a column. Drop Panels Drop panels are used to thicken the slab around the column in flat slabs to avoid punching shear. Since flat slabs have no stirrups shear is resisted by thickening the slab around the column to increase the concrete in shear. Beams can also be used, but generally drop panels are preferred to avoid conflicts with the electromechanical works of the structure. Exterior Columns The columns supporting the main structure of the building. Usually in frame structures the exterior columns are of extreme importance and bear the load of the building as well as resist environmental factors like wind, rain, and other physical factors. Flat Plate Slabs connecting to columns directly. Flat plate system is widely adopted by engineers as it provides many advantages . The system can reduce the height of the building, provide more flexible spatial planning due to no beams present, and further reduce the material cost. However, the main problem in practice is the brittle failure of flat plate under punching shear. Due to the relatively small floor loading and the close column spacing, flat plate construction is preferred. For heavier loading and larger column spacing, column capitals are required, and for even larger spans to reduce the self-weight, waffle slabs are used. Flat Slab The flat plate is a two-way reinforced concrete framing system utilizing a slab of uniform thickness, the simplest of structural shapes. The flat slab is a two-way reinforced structural system that includes either drop panels or column capitals at columns to resist heavier loads and thus permit longer spans. Construction of flat slabs is one of the quickest methods available. Lead times are very short as this is one of the most common forms of construction. Interior Columns Interior columns in a frame structure suppor the slab and beams internally. They are not as susceptible to buckling and environmental effects as the external ones but still are extremely important considering the safety and stability of the building. Interior columns can also serve aesthetic and architectural purposes. Pedestal An architectural support or base, as for a column or statue. Roof A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather and the invasion of animals. Structures that require roofs range from a letter box to a cathedral or stadium, house buildings being the most numerous. The elements in the design of a roof are: the material the construction the durability & Serviceability Spread Footing To distribute the load of the foundation on the soil, spread footings are installed below the building's foundation. This type of footing is continous below the perimeter of the house walls and may be thickened or widened at the points where concentrated loads are applied e.g. columns. These components are constructed from concrete and are often reinforced with rebar or steel to add additional support. Depending on the size and configuration of the building, the footers can be buried just below ground level or several feet below the surface. In cold climates, they are always placed below the frost line to minimize problems with concrete heaving that occurs during freeze/thaw cycles. This type of footer design is highly beneficial to builders and homeowners. Since they transfer the weight of the building over a large area, they have little risk of failure Upturned Beam Through the use of upturned concrete T-beams, designers created a naturally ventilated workspace that employs the thermal mass of an exposed concrete ceiling. This concrete absorbs heat during the day and is purged at night by cool breezes. The term is usually used in concrete construction, in parking structures, but here is how it works: The beam is above the floor it supports, or a combination. Take a parking structure, there is the required barrier wall, so if you turn the beam up it acts as support and the barrier. Think of your simple beam diagram with a uniform load on it. The beam supports this load, so it doesnt matter if the load is applied at the bottom (simply, other than there are the compression/tension face) This also works well in buildings, rather than have a large beam under the floor, the beam is cast above and below the floors, acts as bearing and shear tension and compression face reinforcement will be some what different, but beam cross section area will stay the same.

Post Tensioning in Concrete Slab

Bonded Slab Post Tensioning

This can be defined as the method of applying compression force on concrete after it has been poured and cured.

Introduction to Post tensioning and benefits of post tensioning in concrete slab.

Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands rebars, typically referred to as tendons. It has the following benefits as compared to unbonded post tensioning.

Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element.

A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete.

The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-tensioning is also used in the construction of various bridges; both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.

The advantages of this system over unbonded post-tensioning are:

1. Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
2. Tendons can be easily 'weaved' allowing a more efficient design approach.
3. Higher ultimate strength due to bond generated between the strand and concrete.
4. No long term issues with maintaining the integrity of the anchor/dead end.

Advantages of Post-tensioning

1. Allows longer clear spans, thinner slabs
2. Lower overall building height for the same floor-to-floor height.
3. Allows a high degree of flexibility in the column layout, span lengths and ramp configurations
4. The use of traditional reinforcement requirement can be reduced as tendons cannot de-stress in accidents.
5. Increases the ultimate strength due to strong bond between the strand and concrete.
6. Significant savings in costs can be achieved by post tensioning in concrete slabs because of:
1. Reduced cracking and deflections
2. Reduced storey height
3. Better Water tightness

Placement of Tendons

• Positioning and fixing of casting and block-outs to the edge formwork or construction joint form work
• The support bars shall be prepared in advance.
• Lay tendons according to tendon layout in accordance with the drawings.
• Fix tendons to correct profiles with support bars and chairs and the tendons are made with provisions for grouting using grout using grout vents and grout hoses
• Prepare installation report for every installation as per the enclosed format.
• Tolerance of tendon profiles is recommended as follows:
  • vertical: + 5 mm (at lowest and highest points)
  • Horizontal: + 100 mm

Structure - Types of Structures - Definition & Classification

Definition:

A combination of members connected together in such a way to serve a useful purpose is called structure.

Types of Structure Rigid Frame Its is that type of structure in which the members are joined together by rigid joints e.g. welded joints.

Truss (Pin connected joints) A type of structure formed by members in triangular form, the resulting figure is called a truss. In truss joints are pin connected and loads are applied at joints. No shear force & bending moment are produced. Only axial compression and axial tension is to be determined while analyzing a truss Structural Members Those members that are interconnected in such a way so as to constitute a structure are called structural members. Beam Beam is a flexure member of the structure. It is subjected to transverse loading such as vertical loads, and gravity loads. These loads create shear and bending within the beam. Columns A long vertical member mostly subjected to compressive loads is called column Strut A compressive member of a structure is called strut. Beam-Column A structural member subjected to compression as well as flexure is called beam column Grid A network of beam intersecting each other at right angles and subjected to vertical loads is called grid. Cables and Arches Cables are usually suspended at their ends and are allowed to sag. The forces are then pure tension and are directed along the axis of the cable. Arches are similar to cables except hath they are inverted. They carry compressive loads that are directed along the axis of the arch. Plates and Slabs Plates are three dimensional flat structural components usually made of metal that are often found in floors and roofs of structures. Slabs are similar to plates except that they are usually made of concrete.

CONTAMINATED CONSTRUCTION SITE INVESTIGATION

Contaminated construction sites are those which possess risk to human health and environment. With the increase in demand for infrastructure development around the world and shortage of land available for the same, contaminated sites may be used for new construction projects.

The Contaminated construction site can be the result of:

Any industry in the past on current site

Wastes being dumped at site

Contamination through chemicals used for agricultural purposes

Contamination in site filling materials

Soil contamination through demolition of existing structure.

The presence of chemical contamination of soil or ground about to subside creates risk of health hazards for construction workmen.

Following are the objectives for contaminated construction site investigation:

1. To identify the types of hazards, their extent and importance for assessment of potential risks to human and environment.

2. To identify suitable remedial measures for the existing contamination hazards.

The hazards that may occur at contaminated sites are:

1. Settlement problems of ground such as ground subsidence due to decomposition, weathering and natural compaction soil, leaching and sudden collapse.

2. Obstructions from existing remains of old foundation, buried walls, pile foundations etc.

3. Radioactive substances, biological contaminations, toxic powders, asbestos, fibres, liquids, explosives etc. which can attack construction workers.

4. Fire, smoke, gases, volcanic areas, microbial reaction of organic matter, explosions from combustible materials etc. posses greater risks for construction personnel.

5. Contamination affecting the construction materials with chemical reactions, contaminated ground and ground water can affect the health of humans.

6. Polluted streams of water, aquifers, wind action on contaminated dusts etc. which may affect the health.

The site investigation for contaminated soil is not sufficient just by observation of ground surface conditions and the investigation needs to be carried out for more details on the ground conditions below the site. The extent and intensity of this investigation depends on the type and project and its magnitude, conditions of construction site and its variations.

The code of practices for contaminated soil investigation for construction should be followed as per the local applicable standards. The investigation should be properly planned and executed sufficiently to get all the data pertaining to ground conditions to minimize the risk of health hazards to workmen and damage to the environment.

Too little investigation of contaminated sites may not reveal potential hazards of construction site and extra expenditure will be required for safety, while in-depth site investigation more than required may prove to be uneconomical for construction project.

REPAIR OF REINFORCEMENT IN CONCRETE

Repair of Reinforcement in Concrete

The reinforcement repair techniques are different for mild steel and prestressing steel.

1. Mild reinforcing steel

The damaged bars may either be replaced or supplemented by additional reinforcement based on engineering judgment, the purpose of the reinforcement and the required structural strength of the member.

a) Replacement: In case it is decided to replace the bars, splicing of reinforcement with the remaining steel must be done. The lap length must be according to the provision of ACI 318 and the welding (if used) must satisfy ACI 318 and American Welding Society (AWS) D1.4 (or the codal provisions of the respective country). Butt welding is usually avoided due to the high degree of skill required to perform a full penetration weld because the back side of a bar is not usually accessible. Welding of bars larger than 25 mm may cause problems because the embedded bars may get hot enough to expand and crack the surrounding concrete. Mechanical connectors may also be used according to the code requirements.

b) Supplemental reinforcement: This alternative is selected when the reinforcement has lost cross section, the original reinforcement was inadequate, or the existing member needs to be strengthened. The allowable loss of cross-sectional area of the existing reinforcing steel and the decision to add supplemental reinforcement must be evaluated on a case-by-case basis and is the responsibility of the engineer. The damaged reinforcing bar must be cleaned and extra space is to be created by removing concrete to allow placement of the supplemental bar beside the old bar. The length of the supplemental bar must be equal to the length of the deteriorated segment of the existing bar plus a lap-splice length for smaller diameter bar on each end.

Reinforcing bars, having corrosion of their original deformations, give less bond and this factor must be considered while designing the repair of the reinforcement.

c) Coating of reinforcement: New and existing bars that have been cleaned may be coated with epoxy, polymer cement slurry, or a zinc-rich coating for protection against corrosion. The coating must have a thickness less than 0.3 mm to minimize loss of bond development at the deformations.

2. Prestressing steel

Deterioration or damage to the strands or bars can result from impact, design error, overload, corrosion, or fire. Fire may anneal cold-worked, high-strength prestressing steel. The unbonded high-strength strands may need to be detensioned before repair and retensioned after repair to restore the initial structural integrity of the member.

a) Bonded strands: Because the prestressed strand is bonded, only the exposed and damaged section is restressed following repairs. The repair procedure requires replacing the damaged section with the new section of strand connected to the existing ends of the undamaged strands. The new strand section and the exposed lengths of the existing strand must be post-tensioned to match the stress level of the bonded strand.

b) Unbonded tendons: The strands are protected against corrosion by the sheathing, corrosion-inhibiting material (commonly grease), or both. Corrosion of the end connections and the strand has been the primary cause of failure of unbonded tendons. A deteriorated portion of a strand can be exposed by excavating the concrete and cutting the sheathing. Unbonded tendons can be tested to verify their ability to carry the design load. This can be done by attaching a chuck and coupler to the exposed end of the strand and performing a lift-off test. This usually requires at least 20 mm of free strand beyond the bulkhead. If there is excessive corrosion in the strand, failure occurs and the strand must be replaced or spliced. Shoring of the span being repaired and adjacent spans up to several bays away may be required before removing or retensioning unbonded prestressed strands.

The strand is cut on both sides of the deterioration and the removed portion of the strand is replaced with a new section. The new strand is spliced to the existing strand at the location of the cuts. The repaired strand is then prestressed. Carbon fiber or equivalent systems are available to supplement the reinforcement in prestressed, post-tensioned, and mild steel reinforced structures. This system is normally glued onto the exterior surface. Unless the component being reinforced is unloaded, the strengthening system only provides reinforcement for future loadings. Fiber wrapping is commonly used for reinforcing columns, especially in earthquake zones. There are systems available that recover the dried and damaged protective barrier within the sheathing.

POOR CONSTRUCTION METHODS AND WORKMANSHIP TO AVOID

Poor construction methods and workmanship is responsible for the failure of buildings and structure. The poor construction methods and workmanship is caused due to negligence and inadequate quality control at construction site. The effects of some of the poor construction methods are discussed below:

(a) Incorrect placement of steel

Incorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in  the wrong position, collapse can occur when the element is fully loaded.

(b) Inadequate cover to reinforcement

Inadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.

(c) Incorrectly made construction joints

The main faults in construction joints are lack of preparation and  poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor joints allow ingress of moisture and staining of the concrete face.

(d) Grout leakage

Grout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.

(e) Poor compaction

If concrete is not properly compacted by ramming or vibration the result is a  portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction is essential to give a dense, impermeable concrete.

(f) Segregation

Segregation occurs when the mix ingredients become separated. It is the result of

1. dropping the mix through too great a height in placing (chutes or pipes should be used in such cases)

2. using a harsh mix with high coarse aggregate content

3. large aggregate sinking due to over-vibration or use of too much plasticizer

Fig: Seggregation of concrete

Segregation results in uneven concrete texture, or porous concrete in some cases.

(g) Poor curing

A poor curing procedure can result in loss of water through evaporation. This can cause a reduction in strength if there is not sufficient water for complete hydration of the cement. Loss of water can cause shrinkage cracking. During curing the  concrete should be kept damp and covered.

(h) Too high a water content

Excess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete,which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.

PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

Plastic Shrinkage Cracks and Its Prevention in Concrete

Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both.

Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water, the surface concrete shrinks. Due to the restraint provided by the concrete on the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually not short and run in all directions. In most cases, these cracks are wide at the surface. They range from a few millimeters to many meters in length and are spaced from a few centimeters to as much as 3 m apart.

Preventing Plastic Shrinkage Cracks in Concrete

Plastic shrinkage cracks may extend the full depth of elevated structural slabs. Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. There are many methods and techniques to prevent this type of crack in case of rapid loss of moisture due to hot weather and dry winds. These methods include the use of fog nozzles to saturate the air above the surface and using plastic sheeting to cover the surface between the final finishing operations. In many cases, during construction it is preferable to use wind breakers to reduce the wind velocity; sunshades to reduce the surface temperature are also helpful. Additionally, it is good practice to schedule flat work after the walls have been erected.

FACTORS AFFECTING CONCRETE MIX DESIGN STRENGTH

Factors that affects the concrete mix design strengths are:

Variables in Mix Design

A. Water/cement ratio

B. Cement content

C. Relative proportion of fine & coarse aggregates

D. Use of admixtures

A. Water/cement ratio

Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete. Strength of concretedepends upon W/C ratio rather than the cement content. Abram’s law states that higher the water/cement ratio, lower is the strength of concrete. As a thumb rule every 1% increase in quantity of water added, reduces the strength of concrete by 5%. A water/cement ratio of only 0.38 is required for complete hydration of cement. (Although this is the theoretical limit, water cement ratio lower than 0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of 0.38, evaporates leaving cavities in the concrete. These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by 2 to 3 N/mm2and increase the workability by 25 mm. As stated earlier, the water/cement ratio strongly influences the permeability of concrete and durability of concrete. Revised IS 456-2000 has restricted the maximum water/cement ratios for durability considerations by clause 8.2.4.1, table 5.

B. Cement content

Cement is the core material in concrete, which acts as a binding agent and imparts strength to the concrete. From durability considerations cement content should not be reduced below 300Kg/m3 for RCC. IS 456 –2000 recommends higher cement contents for more severe conditions of exposure of weathering agents to the concrete. It is not necessary that higher cement content would result in higher strength. In fact latest findings show that for the same water/cement ratio, a leaner mix will give better strength. However, this does not mean that we can achieve higher grades of concrete by just lowering the water/cement ratio. This is because lower water/cement ratios will mean lower water contents and result in lower workability. In fact for achieving a given workability, a certain quantity of water will be required. If lower water/cement ratio is to be achieved without disturbing the workability,cement content will have to be increased. Higher cement content helps us in getting the desired workability at a lower water/cement ratio. In most of the mix design methods, the water contents to achieve different workability levels are given in form of empirical relations.

Water/cement ratios required to achieve target mean strengths are interpolated from graphs given in IS 10262 Clause 3.1 and 3.2 fig 2. The cement content is found as follows: –

Thus, we see that higher the workability of concrete, greater is cement content required and vice versa. Also, greater the water/cement ratio, lower is the cement content required and vice versa.

C. Relative proportion of fine, coarse aggregates gradation of aggregates

Aggregates are of two types as below:

a. Coarse aggregate (Metal): These are particles retained on standard IS 4.75mm sieve.

b. Fine aggregate(Sand): These are particles passing standard IS 4.75mm sieve.

Proportion of fine aggregates to coarse aggregate depends on following:

i. Fineness of sand: Generally, when the sand is fine, smaller proportion of it is enough to get a cohesive mix; while coarser the sand, greater has to be its proportion with respect to coarse aggregate.

ii. Size& shape of coarse aggregates: Greater the size of coarse aggregate lesser is the surface area and lesser is the proportion of fine aggregate required and vice versa. Flaky aggregates have more surface area and require greater proportion of fine aggregates to get cohesive mix. Similarly, rounded aggregate have lesser surface area and require lesser proportion of fine aggregate to get a cohesive mix.

iii. Cement content: Leaner mixes require more proportion of fine aggregates than richer mixes. This is because cement particles also contribute to the fines in concrete.