Prestressed Concrete Beam Test - Lab Report Example

 Prestressed Concrete Beam Test 1 Overview This technical note template is the entire submission for the PSBlab. Complete it by following the prompts and instructions, then submit it on VITAL under the CIVE343module.This technical note contains the following sections, each worth the percentages to their right: Aims of the Lab Class. Description of the test procedure 10 Description and results of concrete properties tests: Cube Strength Flexural Tensile Strength Young’s Modulus 10 Strain distributions showing position of neutral axis at all stages 10 Deflections and crack formations: Calculation of theoretical deflections Theoretical and practical results plotted 10 Calculations leading to effective prestressed force: Elastic shortening losses Shrinkage losses Creep losses Relaxation losses Final prestressed force 10 Calculations of: Design working load (i.e. load at 1st crack) Deflection at working load Ultimate load 10 Discussion of results: Concrete properties Comparison of experimental and theoretical loads Stress Distributions Position of neutral axis Mode of failure Safety factors 20 Conclusions 10 Presentation (not a section but for overall layout and quality of figures) 10 Total 100 2 Aims of the lab &description of the test procedure The aim was to determine the pre-stressed concrete beams’ shear strength. The beam was loaded to about 80% of the ultimate flexibility capacity. This is then followed by the test beams that will be subjected to repetitive loads with the magnitude that varies for approximately 2000000 cycles that is about 20-40 percent flexibility capacity. The load magnitude will be increased from this point and continued until at the point failure. The results achieved are related to flexural fatigue and sheer fatigue. The results will as well indicate the fatigue resistance in the given prestressed concrete beams as well as any advanced warnings related to the failure. Basically, it is recognized that warnings are indicated and provided by the increasing crack widening and deflection before the failure happens. 3 Description and results of concrete property tests 3.1 Cube Strength      1239 MPa (70% off). 3.2 Flexural Tensile Strength      1770 MPa 3.3 Young’s Modulus      205 GPa 4 Calculations leading to effective prestressed force 4.1 Elastic shortening losses      fES = (Ep/Eci)cup or 44(0.153)[0.75(270)][733,320 +31.382 (1,085)]-31.38(20,142)(1,085) 44(0.153)[733,320 +31.382(1,085)] + 1’085(733,320)(4,200) = 13.7ksi 28500 Where fpES is the sum of concrete stresses at the gravity center of the prestressing tendons as a result of the forces of prestressing at the self-weight and transfer of the membrane at the maximum moment sections Eci represents concrete modulus elasticity at transfer Ep represents the prestressing steel modulus of elasticity When this equation is applied it needs an estimation of the stress in the strands after transfer. 4.2 Shrinkage Losses       The prestress loss as a result of shrinkage is known to be an average annual ambient humidity function, H, hence it is represented by the below equation fpSR = (17.0 – 0.15H) Where H represents annual ambient relative humidity(percent) These annual ambient relative humidity could be got from local weather statistics. =17.0-0.15(70) =6.5ksi 4.3 Creep losses      Prestress losses expression as s result of creep is known to be a concrete stress function at the prestressing steel centroid at the transfer, cup, as well as the concrete stress change at the prestressing steel centroid as a result of all permanent loads apart from the transfer, fcgp, hence it is represented as below PCR = 12.0fcgp – 7.0fcdp 0 Where PCR represents the stress of the concrete at the prestressing steel center of gravity at transfer CDP represents a change in concrete stress at the prestressing steel center of gravity as a result of permanent loads CDP =[1.256(2.457)-0.862]/1.103 = 2.016ksi 4.4 Relaxation losses      The total relaxation after transfer at any given time consists of 2 components that include relaxation after transfer and relaxation at transfer. Equation that can estimate the relaxation after transfer for pretension is as below fpR2 = 20.0 – 0.4fpES – 0.2(fpSR + PCR) (ksi) Where fES is the loss as a result of elastic shortening PCR) Loss due to shrinkage fpR2 loss as a result of concrete creep Loss as a result of relaxation after transfer =20-0.4(13.7) -0.2(6.5 +16.49) =9.92 ksi 4.5 Final prestressed force       Pe = Nstrands(Aps)(fpe) =44(0.153)(162.83) = 1,096kips (total loss =19.59%) 5 Strain Distribution Diagrams showing position of neutral axis at all stages       6 Deflections and crack formations 6.1 Calculation and theoretical deflections       Calculated 9.562790698mm Theoretical deflection 12.74746 6.2 Theoretical and practical results plotted       The Practical result plot Theoretical result plot 7 Further Calculations 7.1 Design working load (i.e. load at 1st crack)       7.2 Deflection at working load       Load class 1 2 3 4 5 6 Working load > 2 kN > 3 kN > 4 kN > 4.5 kN > 5 kN > 6 kN 7.3 Ultimate load       Load class 1 2 3 4 5 6 Ultimate load > 4 kN > 6 kN > 8 kN > 9 kN > 10 kN > 12 kN 7.4 Neutral axis at failure       8 Discussion of results 8.1 Concrete properties All the beams were pretension concrete beams and they were cast through a precast beam that was manufactured on 6 September 2014. The tension stress was big enough but it seems to have reduced from the initial one and this could be due to elastic loss or friction. The flexural tensile strength was less than the tensile stress. 8.2 Comparison of experimental and theoretical loads      Basing on the results the experimental loads were a bit lower than the theoretical loads, this could be due to experimental errors during the experiment that includes wrong measurement, inaccurate equipment etc 8.3 Stress Distributions      The stress is zero on the neutral axis and this is seen when t pass through the centroid. 8.4 Position of neutral axis      After adding compressive stress to the bending stress, the stress is decreased everywhere and the neutral axis tends to move away from the centroid, the movement will be towards the tensile edge. There is a possibility for the neutral axis to extend beyond the edge. 8.5 Mode of failure      The results will as well indicate the fatigue resistance in the given prestressed concrete beams as well as any advanced warnings related to the failure. Basically, it is recognized that warnings are indicated and provided by the increasing crack widening and deflection before failure happens 8.6 Safety factors       Risk of falling and moving parts Snapping of stressed tendons One should never stand at the end of the beam, Protective screens and warnings signs need to be in position 9 Conclusions      The experiment objective was met despite a big difference in values between the practical and theoretical values. The properties of the concrete were obtained from the standard sample tests. The bending performance of a pretension concrete beam was examined Read More