Mini_Unit 6 (Pt 2): CONCRETE Strand Testing (Part 2)Presentations By PV ESWAR29-Sep-14 1Eswar/ BTech2/ BMCT/ U6(Pt1)_CCStrTestetcFACTORS INFLUENCING STR OF CONCRETE29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 2FACTORS FACTORS affecting the Str of Concrete can be broadly grouped into those Depending upon the Testing Methods and the others Independent of the Testing Methods. FACTORS Depending on Testing Methods are Size of Test Specimen, Size of Specimen relative to Max Size of Agg, Moisture Condition of Specimen, Rate of Loading adopted, and type of Testing Machine used; FACTORS Independent of Testing Method are Type of Cement and Age of Cement, Type and Size of Agg, Degree of Compaction, W-C ratio, Agg-Cement ratio, Air Voids, Curing Method and Curing Temp, and Type of Stress Situation that may exist (Uniaxial, Biaxial and Triaxial).29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 3Influence on Str: SIZE of TEST SPECIMEN The form of Specimen which is most commonly adopted is Cube. The Concrete Cubes of side 150 mm, cured for 28 days are tested in the Moist Saturated Condition in Compr. For Brittle Materials such as Concrete which Rupture on Planes inclined > 45 with the Horizontal, Cube Test Specimen is NOT suitable; since the Str is INCREASED by Frictional Restraint acting at the Surfaces under Pressure. Frictional Restraint from the Plates OPPOSES the Lateral Expansion of the Specimen, and subjects its ends to Inward Compressive Forces. These Inward Compressive Forces DIMINISH with distance from the ends, but a Cube is SO SHORT that their Effect extends throughout its Volume. A Cube is therefore tested under Non-Uniform Tri-Axial Compr29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 429-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 5Testing PlatfromConcrete SpecimenCompressive Load1.Frictional Restraint From the PLATESDevelops along theSURFACE here2.Under Compressive Load,Specimen tries to expand Laterally.3.However, the FrictionalRestraint developed alongthe contact with the PLATES OPPOSES this Lateral Expansion creating INWARD COMPRESSIVE FORCES Prisms or Cylinders with a Ht equal to Twice the least Lateral Dimension are more suitable for such materials. End Effects have Less Influence in these Specimens, and the Cylinder Str of Concrete is a good estimate of the Mono-Axial Compr Str A Cube of Concrete is expected to have a Str 15% > a Cylindrical Specimen. Ht/Side ratio If Ht/Side ratio is changed, the Compr Str of the Prisms relative to Cube changes and is given in Table 10.4. For Ht/Side > 4, it STABILISES. The effect of Ht/ lateral Dimension ratio of Specimen on Compr Str is shown in Fig. 10.20. IfSize of Cube is DECREASED the Compr Str tends to INCREASE (Table 10.5), whereas Modulus of Elasticity DECREASES29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 6Influence on Str: SIZE OF TEST SPECIMEN29-Sep-14Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 7f Fig 10.2029-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 8Influence on Str: SIZE OF SPECIMEN The Test Specimen, as regards the Size and Shape, is different in different countries. Gen.. 150 mm Cubes are specified irrespective of size of Aggregate. However, for Aggregate Optimum Water required for Max Str, the Concrete becomes Permeable and Susceptible to Deterioration. The REDUCTION in Str with an INCREASE in % Air Voids is shown in Fig. 10.21.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 12Loss of Str (vs) % Air Voids29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 13Influence on Str: RATE OF LOADING Str of Concrete INCREASES with the INCREASE in Rate of Loading. The influence of Rate of Loading on the ComprStr is shown in Fig. 10.22. Normally, in a Static Loading Test, rate is such that the test is completed in 2 - 3 Min. Within this range, the Effect is found to be NEGLIGIBLE At LOW Rates of Loading there is MORE Time for Creep to occur, so that the increase of Str with Rate of Loading provides EVIDENCE for the theory that Failure occurs at Limiting Values of Strain, rather than Stress. 29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 1429-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 15The relation between Observed Str and Rate of Loading is given byRate of appln of Load vs Str29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 16Influence on Str: TYPE OF TESTING MACHINE Compr Str Tests of Concrete are made in Various Machines either of the Lever-Arm type or Adoptions of Lever Presses as well as Computerized. Considerable Discrepancies have been found to occur between the Results of Compr Str Test made with different types of Machines. It may be up to even 20%. It may be because of Errors in Centering the Cubes, Wear of the Lower Compr Plate, Friction in the Spherical Seating of the Upper Compr Plate, and Inaccurate Calibration of the Machine itself.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 1729-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 18Lower Compr PlateConcrete SpecimenCompressive Load1.Frictional Restraint From the PLATESDevelops along theSURFACE here2.Under Compressive Load,Specimen tries to expand Laterally.3.However, the FrictionalRestraint developed alongthe contact with the PLATES OPPOSES this Lateral Expansion creating INWARD COMPRESSIVE FORCESINCORRECT,ECCENTRIC LOADINGUPPER COMPRPLATEInfluence on Str: TYPE & AGE OF CEMENT The effect of Type of Cement on the Rate of Str Gain is shown in Fig. 10.23. With Age,the Str of Cement REDUCES since it will set by Absorption of Moisture from the Atmosphere. The Effect of Storage of Cement under different conditions with Age is shown in Fig. 10.24.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 19Different Types of Cement & Str29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 20Effect of Storage of Cement on Str29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 2129-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 22GAIN IN STR WITH AGE* Concrete GAINS Str with Age. Initially Strength developed is MORE. * However, the RATE of gain in Str DIMINISHES with Age.* It is customary to assume the 28 days Str as Full Str of CC.STR at EARLY AGES 28 day Str is known as Characteristic Str of Concrete. Formulae for determining the Concrete Str at 1 day or 7 day are: f28 = k2 (f7)k1Where f7, f28 are strengths at 7 and 28 days,respectivelyk1 = 0.30.8 (for different Cements)k2 = 34 (for different Curing Conditions)29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 23Factors affecting Str: CEMENT-AGG RATIO Provided other factors are kept constant, Cement-Aggregate RATIO will greatly influence Concrete Str With an INCREASE in Cement - Aggregate ratio the Ultimate Str will INCREASE to some extent as shown in Fig. 10.25.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 24Str (vs) Aggregate-Cement Ratio29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 25Factors affecting Str: TYPE AND SIZE OF AGG The extent to which an Aggregate will Pack Down and produce a Min Void Content is DEPENDENT on its Particle Shape. Crushed Stone and Gravels give HIGHER Str. REDUCTION in Void Content of the CA by BETTER PACKING means amount of Mortar REDUCED and hence FA andCement REDUCED. Thus the CA to FA ratio is INCREASED; and although the Overall Mix may be Leaner, the Mortar may be Richer, and by virtue of REDUCTION in W/C ratio (which may thereby be permitted), the Str of Concrete may be INCREASED. A rounded Spherical or Cubical shaped Aggregate when Compacted contains LESS Voids than an Irregular and Flaky Aggregate of the same Nominal Size. Therefore, the FORMER givesHIGHER Str.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 26Factors affecting Str: TYPE AND SIZE OF AGG A Common Belief has been that the LARGER the Max Size of Agg the Denser and Stronger will be the Concrete. The Large Agg have LOWER Total Surface Area and require Lower W/C ratio resulting in Higher Str. In practice it is not so. It varies due to following reasons: The Larger Agg give Lower Surface Area for development of Gel Bonds leading to Lower Str. Also, Large Agg give Heterogeneous Concrete causing Non-Uniform Distribution of Load when Stressed. The INFLUENCE of Max size of Aggregate on Compr Str of Concrete with different Cement Contents is shown in Fig. 10.26, and that with different W/C ratios is shown in Fig. 10.27. The Str of Aggregate does not affect that of Concrete greatly as long as it is HIGHER than the Design Grade ofConcrete.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 2729-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 28* RICHER Concrete(HIGH Cement Content)* Str DECREASES asAgg size INCREASES* LEANER Concrete(LOWER Cement Content)Str INCREASES asAgg size INCREASES 29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 29Factors affecting Str: DEGREE OF COMPACTION Inadequate Compaction leading to Air Void Contents leads to LOSS of Str 5% 30% 10%55% Following equation may be used to find the Variation in Str.29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 30 The effect of Deg of Compaction on Str is shown in Fig. 10.2829-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 31Factors affecting Str: MIXING TIME Str of Concrete (vs) Time of Mixing Str of Concrete INCREASES with INCREASE in Time of Mixing, up to 2 Minutes beyond which NO significant improvement is observed. Fig 10.29 shows the RELATION29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 32Factors affecting Str:CURING METHOD & CURING TEMP The effect of Method of Curing on Str has been discussed in Sec. 10.3. The HIGHER the Temp the GREATER is the Rate of Hardening of Concrete. 10 hrs curing at Temp of about 90C, Concrete may attain 70% of its 28 day Str29-Sep-14 Eswar/ BTech2/ BMCT/ U6_CCStrTestetc 33