Causes Of Design And Detailing Errors In Construction Industry

Design And Detailing Errors In Construction Industry

Common design and detailing errors in construction arises due to either inadequate structural design or due to lack of attention to relatively minor design details. These types of design errors are discussed below:

Inadequate Structural Design

Due to inadequate structural design the concrete is exposed to greater stress than it can handle or strain in concrete increases more than its strain capacity and fails.

The symptoms of such kind of failures due to inadequate structural design shows either spalling of concrete or cracking of concrete. Excessively high compressive stress due to inadequate structural design results in spalling of concrete. Also, high torsion or shear stresses results in spalling or cracking of concrete. High tensile stresses also results in cracking of concrete.

To identify the inadequate design as cause of the structural damage, the structure shall be inspected and locations of the damage should be compared to the types of stresses that should be present in the concrete. For rehabilitation projects, thorough petrographic analysis and strength testing of concrete from elements to be reused will be necessary.

Prevention: Inadequate structural design can be prevented by thorough and careful review of all design calculations. Any rehabilitation method that makes use of existing concrete structural members must be carefully reviewed.

Design-Detailing-Errors-In-Concrete-Construction

Poor Design Details:

Poor design details can cause localised concentration of high stresses in structural members even if the design is adequate to meet the requirements. These high stresses may lead to cracking of concrete that allows water or chemicals to pass through the concrete. Thus poor design detail may lead to seepage through the structural members.

Poor design detail may not lead to structural failure, but it can become the cause of deterioration of concrete. These problems can be prevented by a thorough and careful review of plans and specifications for the construction work.

Types of poor design detailing and their possible effects on structures are discussed below:

Abrupt Changes In Section: 

Abrupt changes in section may cause stress concentrations that may result in cracking. Typical examples would include the use of relatively thin sections rigidly tied into massive sections or patches and replacement concrete that are not uniform in plan dimensions.

Insufficient Reinforcement At Corners And Openings: 

Corners and openings also tend to cause stress concentrations that may cause cracking. In this case, the best prevention is to provide additional reinforcement in areas where stress concentrations are expected to occur.

Inadequate Provision For Deflection: 

Deflections in excess of those anticipated may result in loading of members or sections beyond the capacities for which they were designed. Typically, these loadings will be induced in walls or partitions, resulting in cracking.

Inadequate Provision For Drainage: 

Poor attention to the details of draining a structure may result in the ponding of water. This ponding may result in leakage or saturation of concrete. Leakage may result in damage to the interior of the structure or in staining and encrustations on the structure. Saturation may result in severely damaged concrete if the structure is in an area that is subjected to freezing and thawing.

Insufficient Travel In Expansion Joints: 

Inadequately designed expansion joints may result in spalling of concrete adjacent to the joints. The full range of possible temperature differentials that a concrete may be expected to experience should be taken into account in the specification for expansion joints. There is no single expansion joint that will work for all cases of temperature differential.

Incompatibility Of Materials: 

The use of materials with different properties (modulus of elasticity or coefficient of thermal expansion) adjacent to one another may result in cracking or spalling as the structure is loaded or as it is subjected to daily or annual temperature variations.

Neglect Of Creep Effect: 

Neglect of creep may have similar effects as described for inadequate provision for deflections. Additionally, neglect of creep in prestressed concrete members may lead to excessive prestress loss that in turn results in cracking as loads are applied.

Rigid Joints Between Precast Units: 

Designs utilizing precast elements must provide for movement between adjacent precast elements or between the precast elements and the supporting frame. Failure to provide for this movement can result in cracking or spalling.

Unanticipated Shear Stresses In Piers, Columns, or Abutments: 

If, through lack of maintenance, expansion bearing assembles are allowed to become frozen, horizontal loading may be transferred to the concrete elements supporting the bearings. The result will be cracking in the concrete, usually compounded by other problems which will be caused by the entry of water into the concrete.

Inadequate Joint Spacing In Slabs: 

This is one of the most frequent causes of cracking of slabs-on-grade.

Selection and Applications of Concrete Admixtures in Construction Field

Selection and Applications of Concrete Admixtures in Construction Field

Concrete admixtures are used to enhance the properties of concrete for applications in concrete works with special requirements. Concrete admixtures are used to modify the properties of concrete to achieve desired workability in case of low water cement ratio, and to enhance setting time of concrete for long distance transportation of concrete. So, it is of much importance for a civil site engineer to know about the properties of admixtures for better selection and application in concrete works.

Selection of Concrete Admixtures:

Concrete admixtures shall be selected carefully as per the specifications and shall be used as recommended by the manufacturer or by lab testing report. The quantity of admixtures to be used for specific application of admixtures are recommended by the manufacturers. For use in large construction projects, the quantity of the admixture to be used shall be obtained from tests reports for concrete mixed with admixtures at various percentage admixtures use. These tests are conducted to understand the behaviour of admixtures on the desired quality and strength of concrete at different quantity of admixtures used. Thus, the optimum quantity of admixtures can be selected for specific application based on results.

The selection of specific admixtures for use in concrete to alter properties of concrete should be selected carefully  as per requirement of concrete works. Concrete admixtures should be used judiciously according to specification and method of application to avoid adverse effect on concrete properties at fresh and hardened state.

After selecting the admixtures product, one should carefully choose the supplier with quality product, timely service and at competitive price. The admixture supplier should be with good history and should possess the staff with efficient and professional experience to guide on effective application/use of admixture in right way.

Concrete admixtures should be accepted with test certificate, manufacturing date and its chemical composition, should comply specifications given by the authorities.

Concrete Admixtures Selection and Applications

Applications of Concrete Admixtures:

The following chemical admixture commonly used for specific purpose and are explained in detail:

i) Accelerating Admxitures:

Admixtures which are used to speed up the initial set of concrete is called an accelerator. The advantage of these admixtures are either to increase the rate of hydration of cement or to shorten setting time results in early strength development, removal of shuttering, reduction of curing period and specially when concrete used in low temperature and marine construction.

The common accelerator is Calcium chloride and rarely Tri-ethanolamine, Carbonates, Silicates etc., used. However the use of Calcium chloride is not permitted in Pre-stressed concrete whereas in other concrete it is restricted to 1.5% by eight of cement content.

Mechanism: According to investigations, the essence of hardening intensification is CaCl2 as a catalyst acts on hydration of C S results in increase the rate of setting and strength development. It also established from the figure 1 that the CaCl2 accelerate the setting time considerably.

(ii) Retarding Admixtures:

The function of retarded is to delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance in transit mixers and helpful in placing the concrete at high temperatures, specially used as grouting admixture and water reducers results in increase of strength and durability.

The commonly known retards are Calcium Ligno-sulphonates and Carbohydrates derivatives used in fraction of percent by weight of cement.

Mechanism: The mechanism of set retards is based on absorption. The large admixture anions and molecules are absorbed on the surface of cement particles, which hinders further reactions between cement and water i.e. retards setting. Later as a result of the reaction between the organic salts and C3A for cement, the former are removed from liquid phase of system, thus eliminating further retardation. The figure 2 shows the effect of retards on setting.

(iii) Water-Reducing Admixture (WRA) :

The water reducing admixture are group of products which posses concrete of a given workability as measured by slump or compaction factor at a lower water-cement ratio than the control concrete.

The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.

Mechanism: The principal role on mechanism of water reductions and set retardation of admixtures are usually composed of long-chain organic molecules and that are hydrophobic (not wetting) at one end and hydrophilic (readily wet) at the other. Such molecules tend to become concentrated and form a film at the interface between two immiscible phases such as cement and water, and alter the physio-chemical forces acting at this interface. The presence of such admixture in a fresh concrete results in:

• a reduction of the interfacial tension.
• an increase in the electro kinetic potentials and
• protection sheath of water dipoles around each particles i.e. mobility of fresh mix becomes greater, partly because of reduction in inter-particle forces and partly because of water freed from the restraining influence of the highly flocculated system which is now available to lubricate the mixture. Hence less water is required to achieve given consistency.
• High range water-reducing agents (HRWA) : These are the second generation admixture and also called as Superplasticizers. These are synthetic chemical products made from organic sulphonates of type RSO3, where R is complex organic group of higher molecular weight produced under carefully controlled condition :

The commonly used superplasticizer are as follows :

• Sulphonated melamine formaldehyde condensate (S M F C)
• Sulphonated napthalene formaldehyde condensate (S N F C)
• Modified ligno-sulphonates and other sulphonic esters, acids etc.,

Mechanism: The H R W A consists off very large molecules normally anionic in nature, are thought to be absorbed on to the cement particles become negatively charged and subsequently dispersed in the water, similar to the action of WRA. However the H R W A produce a much higher degree of dispersion and do not lower the surface tension of water significantly. The figure 3 shows the mechanism of Superplasticizer with cement and water.

iv) Air Entraining Agent Admixtures :

These are generally used to improve workability, ease of placing, increased durability, better resistance to frost action and reduction in bleeding.

The common Air-Entraining agents are natural wood resins, neutralized vinsol resins, polyethelene oxide polymers and sulphonated compounds.

Mechanism: These are anionic, because the hydrocarbon structures contain negatively charged hydrophilic groups, such as COO, SO3 and OSO so that large anions are released in water. Conversely, if the hydrocarbon ion is positively charged, the compound is cation active or cationic. In other words, anionic surface active agents produce bubbles that are negatively charged, cationic charged cause bubbles to be positively charged, surface active agents of all classes can cause air entrainment in concrete, but their efficiency and characteristics of air-void system vary widely.

Applications of Steel Fiber Reinforced Concrete

Applications of Steel Fiber Reinforced Concrete

Steel fiber reinforced concrete provides superior resistance to cracking and crack propagation due to increased tensile strength in concrete structures.

Applications of Steel Fiber Reinforced Concrete:

It is known that plain cement concrete does not have good tensile properties to resist flexure in structural members. In case of concrete reinforcement steel, cracks still appear on the tension face due to bending. So, to prevent cracking of concrete, specially in the case of water retaining structures, or water transporting structures, it is advisable to design structural concrete as uncracked section. This results in heavy structural design with resulting in high cost.

Steel fiber reinforced concrete is a low cost solution for uncracked section design of concrete members. Use of steel fiber reinforcement in concrete enhances the ability of structural members to carry significant stresses. The use of fibers increases the toughness of concrete under any type of loads. Fibers in concrete has the ability absorb more energy.

As recommended by ACI Committee 544, steel fiber reinforced concrete is used as supplimentary material to prevent cracking, to improve resistance to impact or dynamic loading and to prevent material disintegration.

A guide to design of concrete structures with steel fiber reinforcement has also been published by American Concrete Institue.

The applications of Steel Fiber reinforced concrete are for so varied and so widespread, that it is difficult to categories them. Following are the common applications of steel fiber reinforced concrete constructions:

• Tunnel linings
• Manholes,
• Risers,
• Burial Vaults,
• Septic Tanks,
• Curbs,
• Pipes,
• Covers,
• Sleepers
• Roller compacted concrete with steel fibers

Steel Fiber Reinforced Concrete

Application of Steel Fiber Reinforced Concrete in other Structures:

A) Highway And Airfield Pavements:

• Repair of existing pavement.
• Reduction in pavement thickness.
• Increase in resistance to impact.
• Increase in transverse and longitudinal joint spacing
• Smooth riding surface.

B) Hydraulic Structures:

• Resistance to cavitations or erosion damage.
• Repair of spilling basin.

C) Fiber Shotcrete (FRS):

The inclusion of steel fibers in shotcrete improves many of the mechanical properties of the basic material viz the toughness, impact resistance, shear strength, flexural strength, and ductility factor.FRS has been used for

• Rock stabilization, tunnels, dams, mines.
• Bridges arches, dome structures, power-house
• Stabilization of slopes to prevent landslides repair of deteriorated concrete surface, water channel etc.

A seminar report on fiber reinforced concrete is available which provides more information and mix design of fiber reinforced concrete. The technology used for production, properties and structural use of fiber reinforced concrete is also explained.

Geopolymer Concrete - Ecofriendly Construction Material

Geopolymer Concrete - Ecofriendly Construction Material

Geopolymer concrete is an innovative and ecofriendly construction material and an alternative to Portland cement concrete. Use of geopolymer reduces the demand of Portland cement which is responsible for high CO2 emission.

What is Geopolymer Concrete:

Geopolymer was the name given by Daidovits in 1978 to materials which are characterized by chains or networks or inorganic molecules. Geopolymer cement concrete is made from utilization of waste materials such as fly ash and ground granulated blast furnace slag(GGBS). Fly ash is the waste product generated from thermal power plant and ground granulate blast furnace slag is generated as waste material in steel plant.

Both fly ash and GGBS are processed by appropriate technology and used for concrete works in the form of geopolymer concrete. The use of this concrete helps to reduce the stock of wastes and also reduces carbon emission by reducing Portland cement demand.

The main constituent of geopolymers source of silicon and aluminium which are provided by thermally activated natural materials (e.g. kaolinite) or industrial byproducts (e.g. fly ash or slab) and an alkaline activating solution which polymerizes these materials into molecular chains and networks to create hardened binder. It is also called as alkali-activated cement or inorganic polymer cement.

Composition of Geopolymer Concrete:

Following materials are required to produce this concrete:

• Fly ash – A byproduct of thermal power plant
• GGBS – A byproduct of steel plant
• Fine aggregates and coarse aggregates as required for normal concrete.
• Alkaline activator solution for GPCC as explained above. Catalytic liquid system is used as alkaline activator solution. It is a combination of solutions of alkali silicates and hydroxides, besides distilled water. The role of alkaline activator solution is to activate the geopolymeric source materials containing Si and Al such as fly ash and GGBS.

Geopolymer Concrete Materials

Mechanical Properties of Geopolymer Concrete:

Compressive strength of geopolymer concrete have been found upto 70 MPa (N/mm2). The concrete gains its compressive strength rapidly and faster than ordinary Portland cement concrete. The concrete strength after 24 hours have been found to be more than 25MPa. Compressive strength after 28 days have been found to be 60 to 70 MPa. -Ref. Paper by – James Aldred And John Day and Test results by SERC Chennai.

Other Properties of Geopolymer Concrete:

• The drying shrinkage of is much less compared to cement concrete. This makes it well suited for thick and heavily restrained concrete structural members.
• It has low heat of hydration in comparison with cement concrete.
• The fire resistance is considerably better than OPC based concrete. -Reference – Paper by – James Aldred And John Day.
• This concrete jas chloride permeability rating of ‘low’ to ‘very low’ as per ASTM 1202C. It offers better protection to reinforcement steel from corrosion as compared to traditional cement concrete.
• This concrete are found to possess very high acid resistance when tested under exposure to 2% and 10% sulphuric acids.

Applications of Geopolymer Concrete:

The applications is same as cement concrete. However, this material has not yet been popularly used for various applications.

This concrete has been used for construction of pavements, retaining walls, water tanks, precast bridge decks.

Geopolymer Concrete Building

Recently world’s first building Structural Building, The University of Queensland’s Global Change Institute (GCI) has been constructed with the use of geopolymer concrete. It is a four storey high building for public use.

Permanent Form-work Method For Beam Construction - Composite Beams

Permanent formwork construction method is used to:

• Speed construction, especially of downstand beams
• Provide high-quality finishes
• Promote low tolerances

The units can act solely as permanent formwork, i.e. they may be designed for construction loads only without contributing to the strength of the completed beams.

More efficiently, they can act compositely with the in-situ concrete. Indeed some beams, particularly in seismic areas are designed to act both compositely and non-compositely in the same span.

Non-Composite Permanent Formwork:

Non-Composite Permanent Formwork

Non-composite precast permanent formwork is used to speed the erection process and assure quality finish. This is particularly useful on exposed spandrel beams which require specialized finishes or profiles. Polystyrene void formers can be introduced into rectangular sections to reduce self weight.

Non-Composite Permanent Formwork

Non-Composite Permanent Formwork

Non-Composite Permanent Formwork

The above units can be used in the main elevations supported by trestles and column form work used in the curved elevation.

Composite Permanent Formwork:

Beam shells perform number of functions:

• They are generally designed to support precast floor units and construction loads
• To act as formwork for infill in-situ concrete
• To act compositely with the in-situ infill to support permanent loads.

Their most common use is to form downstand beams. The units are usually in a U-form. The units may be reinforced or prestressed and once concreted, may be post-tensioned. Thin plain sections may incorporate lattice girders for temporary rigidity and to support construction loads.

Melbourne Cricket Ground, Southern Stand

Precast beam shells were chosen for speed of construction and quality of finish. The shells were made from grade 60 concrete and were placed at 7m centres to support hollowcore floor units.

Composite Permanent Formwork

Once supplementary reinforcement and tendons had been fixed, the shells and floor units were concreted with a grade 40 mix. The composite beams and floors were subsequently post-tensioned to obviate cracks. A four-day cycle was achieved.

Transparent Light Transmitting Concrete - Pure Transparent Concrete Technology

Transparent Light Transmitting Concrete - Pure Transparent Concrete Technology

Transparent concrete also called as translucent concrete or light transmitting concrete is achieved by replacing aggregates with transparent alternate materials. The bonding material in transparent concrete may be able to transmit light by using clear resins the concrete mix. Use of optical fibers and fine concrete also used as transparent concrete.

Transparent concrete was originally developed in 2001 by a Hungarian architect Aronlosonzi by using glass fibers. Transparent concrete is produced by mixing 4% to 5% (by volume) optical fibers in the concrete mixture. This concrete has less weight compared to original concrete.

Materials for Transparent Concrete:

Transparent concrete is manufactured by using combination of fiber optics and fine concrete. These fibers blend into the concrete like any other aggregates. These optical fibers can transmit light from natural and artificial sources into spaces enclosed by the translucent concrete panels. The main reason for using optical fiber in concrete is that it can transmit light even an incident angle greater than 600.

Optical fiber consists of three layers called as core, cladding and buffer coating or jacket. The light is transmitted through the core of the optical fiber.

Cross-Section of Optical Fiber

Cross-Section of Optical Fiber

Transparent concrete is manufactured using fine materials only. It does not contain coarse aggregates. This concrete can have the compressive strength of that of high strength concrete around 70 MPa ( 10,000 psi).

Light Transmitting Concrete

Light Transmitting Concrete

Materials used:

Cement: As the optical fiber is only responsible for transmission of light, there is no special cement required. So, ordinary Portland cement is used for transparent concrete.

Sand: Since the transparent concrete is manufactured only using fine materials, the size of sand should pass through 1.18mm sieve. The sand should be free from any impurities such as vegetation, large stones etc.

Water: Water to be used for transparent concrete should be of drinking water quality, free from any impurities.

Optical fibers: Optical fibers in the range of 4 to 5% by volume is used for transparent concrete. Thickness of the optical fibers can be varied between 2 µm and 2 mm to suit the particular requirements of light transmission.

Transparent Concrete Wall

Transparent Concrete Wall

Advantages of Transparent Concrete:

The main advantage of transparent concrete is that it can transmit light. There, it can be used to make green buildings. Since it can transmit light from natural as well as artificial sources, the building can have fewer lights to meet its demand for lighting. Thus saving huge energy cost.

Transparent concrete uses sunlight as source of light instead of electrical energy and reduces power consumption. This concrete can also be used cold countries to transmit heat with sunlight.

Long Term Shrinkage Cracks of Concrete - Concrete Shrinkage Cracks

Long Term Shrinkage Cracks of Concrete - Concrete Shrinkage Cracks

Shrinkage is the inherent property of cement paste, which in pure form may shrink up to 1%. Aggregate provides internal restraint that reduces the magnitude of this volume change to about 0.06%. Shrinkage also occurs partly due to hydration of cement. On wetting concrete tends to expand but to a lower extent as compared to original shrinkage. Concrete would continue to shrink during its lifetime albeit at a very reduced rate.

If the shrinkage of concrete could take place without restraint, the concrete would not crack. The combination of shrinkage and restraint cause tensile stresses to develop in the concrete, leading to cracking. In thicker section of concrete, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface

causes cracks to develop that may, with time, penetrate deeper into the concrete.

Long term measurements on some large reinforced concrete bridge structures have shown that the strain due to drying shrinkage after 5 years was about 30 x 10-6. As the tensile strain capacity of hardened concrete is in the range 80 to 150 x 10-6, it is clear that long term drying shrinkage alone could not initiate the no-load-induced cracks. However it certainly plays an important role.

The shrinkage of a particular concrete mix is also affected by additional factors such as temperature history, curing, relative humidity and ratio of volume to exposed surface. Sound aggregates for concrete have low shrinkage and the more quantity of it is present in concrete smaller would be the shrinkage. Fig. 1 & 2 shows shrinkage cracks in concrete tunnel and Pre-Stressed Concrete girder respectively.

Lateral Cracks in Tunnel Ceiling

Lateral Cracks in Tunnel Ceiling

Cracks in Girder

Cracks in Girder

Preventive Measures for Long Term Shrinkage Cracks:

Minimum water content, use of plasticizer for compensating workability due to lesser water, use of highest possible aggregate content and hence smaller quantity of cements, eliminate external restrains (e.g. smooth polythene sheet on the sub grade for base slab), sufficiently close spaced reinforcement (e.g. generally 15 cm in slabs & walls).

Repair Method:

Sealing & grouting as necessary depending on the width of crack.

Concrete durability problems in RCC Structures - Durability of Concrete

Concrete durability problems in structures can be due to environment to which the concrete structure is exposure or due to internal causes within the concrete.

The following conditions causes the concrete durability problems in structures:

• Temperature
• Moisture
• Physical factors
• Chemical factors
• Biological factors

Durability of concrete in structure occurs due to above factors which cause weathering in concrete, abrasion or chemical reaction with concrete or reinforcement.

Durability problems in concrete structures related to environmental causes include the following: steel corrosion, delamination, cracking, carbonation, sulphate attack, chemical attack, scaling, spalling, abrasion and cavitations.

Durability problems due to Temperature

Concrete contracts and expands due to change in temperature. Concrete expands when temperature increase and contracts when temperature decreases. The effect of these expansion and contraction will not be in unrestrained concrete member. But when a concrete is restrained by connecting members such as columns, beams, slabs, foundations etc, these changes produces significant stresses in concrete which lead to development of cracks.

Warping of Concrete due to Temperature Change

Warping of Concrete due to Temperature Change

Concrete exposed to temperatures greater than 95°C (203°F) can have significant effects. These effects are caused due to change in volume of cement paste and aggregates. Cement paste shrinks at high temperature due to dehydration while aggregates expands. The net result of high temperature on concrete is expansion. Therefore, exposure to very high temperatures (i.e. fire) will result in concrete spalling, particularly when the concrete is exposed to high temperatures for a long time.

Factors such as moisture condition of concrete, types of aggregates and their stability, cement content, duration of exposure to high temperature, rate of change in temperature, age of concrete and support conditions etc affects the durability of concrete at high temperature.

Concrete Durability Problems due to Moisture:

Concrete expands or swells due to increase in moisture and contracts when moisture reduces. The effects of moisture gains and losses on the volume of concrete are illustrated in Figure.

Concrete Durability Problems due to Moisture

These changes in moisture in concrete causes it to swell and shrink. When concrete starts to dry, shrinkage first occurs at the surface of concrete. This shrinkage of concrete at the surface will develop tensile stresses on concrete surface which leads to cracks.

If a section of the concrete is restrained, and if concrete joints are not provided, major random cracks may develop.

Shrinkage of reinforced concrete is less than the shrinkage of plain concrete. The difference depends on the amount of reinforcing steel used. Steel reinforcement restricts but does not prevent drying shrinkage. The concrete will crack if the shrinkage strain of the concrete exceeds the limiting tensile strain of the concrete.

Problems in Concrete due to Moisture:

The three main problems with moisture and concrete are as follows: ­

• Carbonation
• The moisture cycle
• Contaminants

Physical Factors Affecting Concrete Durability:

Many times with the age of concrete, concrete surface is subjected to wear due to sliding, impact, scraping etc. In case of hydraulic structures, the action of the abrasive materials carried by flowing water generally leads to erosion of the concrete. Another cause of damage to concrete in flowing water is cavitation .

Abrasion in concrete is caused by the sliding or scraping of equipment across the concrete. Abrasion damage to concrete may also be caused by subjecting the concrete to abrasive materials (such as sand) that are carried by wind or water.

Tests on concrete results indicate the following facts:

• That abrasion resistance is clearly related to the compressive strength of the concrete.
• Strong concrete has more resistance than weak concrete.
• Since compressive strength depends on the water-cement ratio and adequate curing, a low water-cement ratio and proper curing of the concrete are necessary for abrasion resistance.
• Hard aggregates are more abrasion resistant than soft aggregates.
• Steel-trowelled surfaces resist abrasion more than a surface that is not trowelled.
• Cavitations in concrete occurs when a high-velocity, flow of water (or any other fluid) suffers an abrupt change in direction or velocity.

Biological Factors affecting durability:

Concrete may be damaged by live organisms such as plants, sponges, boring shells, or marine borers. ­

Mosses on Concrete

Rotting seaweed has been known to produce sulfur. Sulfur can be easily converted to sulfuric acid. The presence of sulfuric acid on concrete leads to concrete disintegration.

Durability Problems due to Seaweed on concrete

Chemical Factors affecting concrete durability:

Durability of concrete is affected by chemical reaction due to chemical interactions between aggressive agents present in the external environment and the constituents of the cement paste.

Among the exceptions are alkali-aggregate reactions which occur between the alkalies in cement paste and certain reactive materials when present in aggregate, delayed hydration of crystalline CaO and MgO if present in excessive amounts in Portland cement, and electrochemical corrosion of embedded steel in concrete.

Chemical reactions in concrete results into increase in porosity and permeability, decrease in strength, and cracking and spalling. Sulfate attack, alkali-aggregate attack, and corrosion of embedded steel etc due to chemical reactions in concrete are responsible for deterioration of a large number of concrete structures. Concrete structures in coastal and offshore structures are exposed to chemical and physical processes of deterioration, which aptly demonstrate the complexities of concrete durability problems in practice.

Deteroration of concrete by chemical reaction

Salt in the surrounding ground, ground water, or air diffuses into the concrete. Steel corrosion results in an increase in the volume of the corroded portion of the reinforcing steel bar. This increase in steel volume causes the concrete to crack and to disintegrate.