Que3.9. What do you mean by Rheology of fresh concrete? Explain the parameters of Rheology. Answer 1. Rheology It may be defined as the wisdom of the distortion and inflow of accoutrements , and concerned with connections between stress, strain, rate of strain, and time. 2. The term rheology deals with the accoutrements whose inflow parcels are more complicated than of simple fluids( liquids or feasts). Parameters of Rheology Following are the parameter of rheology 1. Stability It’s defined as a condition in which the aggregate patches are held in homogeneous dissipation by matrix, and arbitrary slice shows the same flyspeck size distribution during transportation, placing and contraction. ii. The stability of concrete is measured by its isolation and bleeding characteristics. 2. Mobility i. The mobility of fresh concrete is its capability to flow under instigation transfer, i.e., under mechanical stresses. The inflow is confined by cohesive, thick and frictional forces. ii. The cohesive force develops due to adhesion between the matrix and aggregate patches. It provides tensile strength of fresh concrete that resists isolation. iii. The density of the matrix contributes to the ease with which the aggregate patches can move and rearrange themselves within the matrix. iv. The internal disunion occurs when a admixture is displaced and the aggregate patches restate and rotate. v. The resistance to distortion depends on the shape and texture of the total, the uproariousness of the admixture, the water- cement rate, and the type of cement used. 3. Compactability i. It measures the ease with which fresh concrete is compacted. ii. Compacting correspond of expelling entangled air and displacing the aggregate patches in a thick mass without causing isolation. iii. Compactability is measured by the compacting factor test. Que3.10. Describe the Bingham model of Rheology of fresh concrete. Answer 1. The inflow geste
of fresh concrete doesn’t conform to Newtonian liquid. 2. The rate of shear stress to shear rate isn’t constant but depends upon the shear rate at which it’s measured, and may also depend on the shear history of the concrete sample being delved . 3. still, at low shear rates that are important in practice, the geste
can be represented by a straight line which doesn’t pass through- the origin, i.e., which has an intercept on the stress axis. 4. The intercept indicates the minimum stress below which no inflow occurs. 5. The fact that concrete can stand in a pile( as in the case of the depression test) suggests that there’s some minimum stress necessary for inflow to do at all. 6. The minimum stress is called yield stress and designated by the symbol 0. therefore the simplest inflow equation of concrete illustrated in 3.10.1 can be written as Rate of shear, trial point Alternate trial point needed to fix the line 2 1 = 0 pitch = 1/ 0 Shear stress, Fig.3.10.1. Bingham model. = 0 where, 0 = Yield value indicating the cohesion of the material. = Constant having the confines of density and nominated plastic density. 7. This fine relationship is called the Bingham model. 8. Bingham model relates the shear stress of the material expressed in terms of its cohesion to plastic density, and the rate at which the shear cargo is applied. 9. To establish a straight line, at least two points are needed. Consequently, the plasticity of concrete can not be defined by the single- point tests that determine only one parameter, i.e., produce only single point, and thus have to be used in combination with other tests to achieve a better understanding of concrete rheology. 10. For illustration, the Vee- Bee test can be used with compacting factor test to measure mobility and compactability. Que3.11. Explain the affective factors of rheological parcels of concrete. Answer Following are the affecting factors of rheological parcels of concrete 1. Mix Proportion A concrete blend having an redundant quantum of coarse total will warrant sufficient mortar to fill the void system, performing in a loss of cohesion and mobility. 2. thickness The thickness of concrete, as measured by the depression test, is an index of the relative water content in the concrete blend. 3. Hardening and harshening Elevated temperature, use of rapid-fire- hardening cement, cement deficient in gypsum, and use of accelerating cocktails, increase the rate of hardening which reduces the mobility of concrete. ii. The dry and pervious total will fleetly reduce plasticity by gripping water from the admixture or adding the face area to be bathe. 4. Aggregate Shape and Texture i. The rough and largely angular total patches will affect in advanced chance of voids being filled by mortar, taking advanced fine aggregate contents and similarly advanced water content. ii. also, an angular fine total will increase internal disunion in the concrete admixture and bear advanced water content than well- rounded natural beach. 5. Aggregate Grading A well- graded total gives good plasticity. These goods are lesser in the fine total than in coarse total. 6. Maximum Aggregate Size An increase in the maximum size of total will reduce the fine total content needed to maintain a given plasticity, and will thereby reduce the face area to be bathe. 7. cocktails The cocktails which have significant effect on the rheology of concrete are plasticizers andsuper-plasticizers, airentraining agents, accelerators and retarders. Que3.12. Describe the effect of rheological parcels on different types of concrete. Answer Effect of Rheological parcels on Different Types of Concrete. 1. Different types of concrete Fig.3.12.1 shows the three- dimensional relationship between different types of concrete and the rheological parameters. For illustration Compared to reference concrete, a wet concrete can be produced by dwindling both yield stress and plastic density, whereas a stiffer concrete can be produced by adding the yield stress. ii. originally, addition of silica cloud decreases density( fine flyspeck content increases the inflow), whereas advanced lozenge increases the yield stress as well as density. 2. Different Complements The effect of air, water, and other mineral cocktails on rheological parameters of concrete is shown in 3. Rheological parcels of Different Concretes Shear yield stress of tone- compacting concrete is in the range of 0- 50 Pa, whereas for normal concrete it’s high in the range of 100- 300 Pa as shown inFig.3.12.3. ii. Plastic density of normal concrete is in the range of 0- 40 Pa-s. iii. still, plastic density for tone- compacting concrete is fairly high having a range of 50- 90 Pa-s. Answer 1. Target Mean Strength Characteristic strength fck = 45 Target mean strength, f ck = fck1.65 × = 451.65 × 5 = 53.25 N/ mm2 Where is the standard divagation taken as 5 N/ mm2. 2. Water/ Cement rate Water/ Cement rate is taken from the experience of the blend developer grounded on his experience of analogous work away. W/ C rate = 0.42 ii. This water cement rate is to be named both from strength consideration and maximum w c denoted in Table 5 of IS 456 and lower of the two is to be espoused continuity demand. iii. W/ C proposed is0.42. This being lower than0.45, we should borrow W/ C rate as0.42. 3. Selection of Water Content Maximum water content as per table3.8.3 is 186 litre. This is for 50 mm depression. ii. Estimated water content for 125 mm depression = 186 × 9 100 186 203 litre 3 increase for every 25 mm depression over and above 50 mm depression) iii. Really speaking separate trials are needed to be done to find out the effectiveness of plasticizers. iv. In the absence of similar trial, it’s assumed that the effectiveness of super plasticizer used 25 percent. thus factual water to be used = 203 ×0.75 152 litre. 4. computation of Cement Content W/ C rate = 0.42 Water used = 152 litre ii. Cement content = W C = 0.42 C = 152 = 362 kg/ m3 iii. This cement content is to be checked against minimal cement content given in table 5 of IS 456 for continuity demand. Mix Design & Rhealogy of Concrete 3 – 20 D( CE- Sem- 5) iv. As the calculated cement 362 kg/ m3 is further than minimal cement mentioned in table 5 of IS 456 i.e., 320 kg/ m3, the cement content of kg/ m3 should be accepted. Borrow cement content of 362 kg/ m3. 5. computation of Coarse and Fine Aggregate Content i. From Table3.8.5 volume of coarse total corresponding to 20 mm size total and fine total zone II, for w/ c rate of0.50 is set up out to be0.62. ii. In the present case w/ c0.42 i.e., it’s lower by0.08. As the w/ c is reduced it is desirable to increase the coarse total proportion to reduce the fine total content. iii. The coarse total is increased at the rate of0.01 for every drop in w/ c rate of0.05. ×0.08 = 0.016 Volume of CA = 0.62 = iv. Corrected proportion of volume of CA = 0.636 Since it’s angular total and the concrete is to be pumped, the coarse total can be reduced by 10. Final volume of coarse total = 0.636 ×0.9 = 0.572 say0.57 vi. Volume of fine total = 0.43 6. computation of Mix Proportions Volume of concrete = 1 m3 ii. Absolute volume of cement = 362 1 1000 m3 = 0.115 m3 iii. Volume of water = 152 litre = 0.152 m3 iv. Volume of chemical amalgamation = 362 1 3621.2 1001.1 10001101000 Assuming lozenge of1.2 by weight of cementitious material and assuming specific graveness of amalgamation as1.1.) Absolute volume of all the accoutrements except total summations = 0.1150.1520.004 = 0.271 m3 vi. Absolute volume of total aggregate = 1 –0.271 = 0.729 m3 vii. Weight of coarse total = 0.729 ×0.57 ×2.80 × 1000 1163 kg/ m3 viii. Weight of fine total = 0.729 ×0.43 ×2.70 x 1000 846 kg/ m3 7. Mix Proportions for Trial Number 1 Cement 362 kg/ m3 Water 152 kg/ m3 Fine total 846 kg/ m3 Coarse total 1163 kg/ m3 Concrete Technology 3 – 21 D( CE- Sem- 5) Chemical amalgamation 4 kg/ m3 Wet viscosity of concrete 2527 kg/ m3 w/ c rate0.42 8. point Correction immersion of fine total = 1.0 iii. Total immersion = 14.28 litres iv. factual quantum of water to be used = 15214.28 = 166.28 litres factual weight of FA to be used = 846 –8.46 = 837.5 kg/ m3 vi. factual weight of CA to be used = 1163 –5.82 = 1157.20 kg/ m3 vii. Proportion of accoutrements at the point Cement 362 kg/ m3 Water166.28 kg/ m2 CA1157.2 kg/ m3 FA837.5 kg/ m3 amalgamation4.0 kg/ m3 viii. With the below proportion of accoutrements carry out trial blend number- 1 and see the quality of concrete. , 3 and 4 as indicated If it isn’t satisfactory carry out trial blend number 2. before under trial mixes. Arrive at the final proportions of concrete blend to satisfy the needed parameters.