Work-ability, strength, and durability are three basics properties of concrete. Amount of useful internal work necessary to overcome the internal friction to produce full compaction is termed as Work-ability. Size, shape, surface texture and grading of aggregates, water-cement ratio, use of admixtures and mix proportion are important factors affecting work-ability. Strength is to bear the desired stresses within the permissible factor of safety in expected exposure condition. The factor influencing the strength are: quality of cement, water-cement ratio, grading of aggregates, degree of compaction, efficiency of curing, curing temperature, age at the time of testing, impact and fatigue. Durability is sustenance of shape, size and strength; resistance to exposure conditions, disintegration and wearing under adverse conditions. Variation in concrete production, loading conditions in service life and subsequent attack by the environment factors are main deteriorating factor of concrete. Properly compacted and cured concrete used in RCC continues to be substantially water tight and durable till capillary pores and micro-cracks in the interior are interconnected to form pathways up to surface.
Durability is mainly influenced by environmental exposure condition, freezing – thawing, contact to aggressive chemicals, type and quality of constituent materials, water-cement ratio, work-ability, shape and size of the member, degree of compaction, efficiency of curing, effectiveness of cover concrete, porosity and permeability. During service life of structures, penetration of water and aggressive chemicals, carbonation, chloride ingress, leaching, sulphate attack, alkali-silica reaction and freezing-thawing are resulting deterioration. Loading and weathering inter link voids and micro-cracks present in transition zone and network of same micro cracks gets connected to cracks on concrete surface which provides primary mechanism of the fluid transport to interior of concrete. Subsequent increase of penetrability leads to easy ingress of water, oxygen, carbon dioxide and acidic ions etc into concrete resulting cracking, spalling, loss at mass, strength and stiffness.
Low permeability is key to durability and it is controlled by factors like water-cement ratio, degree of hydration, curing, entrapped air voids, micro cracks due to loading and cyclic exposure to thermal variations. Admixture improves work-ability, compactibility, strength, impermeability, resistance to chemical attack, corrosion of reinforcement and freezing – thawing etc. and in turn to durability. For this study durability is interpreted in terms of porosity, moisture movement, surface strength, ultra sound pulse velocity and elasticity modulus of concrete. Optimum use of Rice Husk Ash (RHA), obtained by open field burning method, is decided for improving workability, strength and durability of concrete.
Rice Husk Ash
RHA, produced after burning of Rice husks (RH) has high reactivity and pozzolanic property. Indian Standard code of practice for plain and reinforced concrete, IS 456- 2000, recommends use of RHA in concrete but does not specify quantities. Chemical compositions of RHA are affected due to burning process and temperature. Silica content in the ash increases with higher the burning temperature. As per study by Houston, D. F. (1972) RHA produced by burning rice husk between 600 and 700°C temperatures for 2 hours, contains 90-95% SiO2, 1-3% K2O and < 5% unburnt carbon. Under controlled burning condition in industrial furnace, conducted by Mehta, P. K. (1992), RHA contains silica in amorphous and highly cellular form, with 50-1000 m2/g surface area. So use of RHA with cement improves workability and stability, reduces heat evolution, thermal cracking and plastic shrinkage. This increases strength development, impermeability and durability by strengthening transition zone, modifying the pore-structure, blocking the large voids in the hydrated cement paste through pozzolanic reaction. RHA minimizes alkali-aggregate reaction, reduces expansion, refines pore structure and hinders diffusion of alkali ions to the surface of aggregate by micro porous structure.
Portland cement contains 60 to 65% CaO and, upon hydration, a considerable portion of lime is released as free Ca(OH)2, which is primarily responsible for the poor performance of Portland cement concretes in acidic environments. Silica present in the RHA combines with the calcium hydroxide and results excellent resistance of the material to acidic environments. RHA replacing 10% Portland cement resists chloride penetration, improves capillary suction and accelerated chloride diffusivity.
Pozzolanic reaction of RHA consumes Ca(OH)2 present in a hydrated Portland cement paste, reduces susceptible to acid attack and improves resistance to chloride penetration. This reduces large pores and porosity resulting very low permeability. The pozzolanic and cementitious reaction associated with RHA reduces the free lime present in the cement paste, decreases the permeability of the system, improves overall resistance to CO2 attack and enhances resistance to corrosion of steel in concrete. Highly micro porous structure RHA mixed concrete provides escape paths for the freezing water inside the concrete, relieving internal stresses, reducing micro cracking and improving freeze-thaw resistance.
Non Destructive Tests (NDT)
NDT, systems required for assessing strength in service feature, is defined as a test which does not impair the intended performance of the element or member under investigation, carried out onsite, with ability to determine the strength and durability of critical constructions without damaging.
For conducting ND Tests, Rebound hammer, Protimeter-moisture measurement system, Porositester and pulse Ultrasonic Non-Destructive Indicating Tester (PUNDIT) equipment are used.
Rebound hammer test is conducted to asses the relative strength of concrete based on the hardness at or near its exposed surface. Concrete Rebound Test Hammer is a traditional instrument used for the non-destructive testing of hardened concrete. This provides a quick and simple test method for obtaining an immediate indication of concrete strength in various parts of a structure. Knob of this instrument is kept perpendicular to the surface (i.e. 90 degree to the surface) for measurement and is push pressed from the bottom towards the surface of the concrete, hammer like sound is produced. The button near the bottom of the instrument is pressed to lock the indicator and reading is taken.
The surface moisture is generally not seen for cleaning and restoring the structure after damage from storms, floods or fires. For exposed or unseen structural damage, the undetected moisture damages strength, durability and reliability. The Protimeter Moisture Measurement System is a powerful and versatile instrument for measuring and diagnosing surface moisture in buildings. This instrument directly displays moisture content (%) along with three conditions of material like DRY, WET and RISK condition. For using Protimeter operational modes are selected and information is presented on a large, back lit liquid crystal display. The radio frequency sensor is positioned so that large number of moisture readings is taken quickly and easily.
Porositester contains of 3 glass tube used to measure the water penetration in concrete at atmospheric pressure. Tubes are fixed with vacuum plate and vacuum pump without any subsequent cleaning. Rubber seal inserted in the tube provides a defined contact surface of dia 25 mm vertical or horizontal. Compressible seals on suction plate and tube permit secure fixing also to uneven surfaces. Current supply is provided from a 12 V rechargeable battery. Vacuum plate is pressed against the façade with a wet sponge, motor is placed on adhesion of the plate is checked. Test tubes are placed under the clip and secured firmly with screws. Test tubes are filled with water up to the zero mark and again refilled after the descent of the water level by 1 or 2 ml and readings of quantity of penetrated water are noted down for 15 minute test time. As per the manual of equipment, water absorption coefficient A is calculated by using following formula:
Effect of Rice Husk Ash on Cement Mortar and Concrete
X=Amount (level) of water penetrated in ml.
d= Dia. of test tube in mm.
t= Time of penetration in minutes.