Hardened concrete and methods of testing.

Hardened concrete and methods of testing.Research by shepherd nhangaThere are destructive tests(DT) and non-destructive tests (NDT).The main tests are,Compressive strength test DT Modulus of Elasticity NDT Split-tension test DT Flexural strength test DT Rebound hammer test NDT Penetration resistance test NDT Ultrasonic pulse velocity test NDTMaturity tests

Compressive strength tests Compressive testing is carried out using a compression test machine at a constant rate of 0.25 0.05 MPa for cylinder moulds. Compressive strength is equal to maximum load divided by the average cross sectional area of the concrete to the nearest 0.5N/mm Fig1.1 compression testing machine

SPLIT TENSION TESTIt measures the tensile strength on concrete, the cylinder is subjected to compressive load along vertical diameter at a constant rate until fatigue. Failure occurs along vertical diameter due to tension developed in transverse direction .Split tension is computed by T = 2p/LD T = tensile strength, MPa; p= load at failure, N ; L = length of specimen, mm; D = diameter of specimen, mm Tensile strength varies from 2.5 MPa to 3.1 MPa (360 psi to 450 psi), about 10% of compressive strengthFig 1.2 split tension test

Flexure tension testA prismatic beam of concrete is supported on a steel roller bearing Each end is loaded through similar steel bearings placed at the third points on the top surface. The flexural strength (the max tensile stress at the bottom surface )is FL/BD where F = load ,L = the distance between the lower supporting rollers; B and D are the breath and depth of the beam. Important test for road and airport concrete pavements

Fig 1.3 Flexure strength test

Maturity testMaturity is the degree of cement hydrationVaries with time and temperatureIt is assumed that the strength is a function of maturity for a particular concrete mixture

PermeabilityPermeability affects the durability of hardened concrete Air voids that affect permeability are obtained from ;-incomplete consolidation of fresh concrete evaporation of mixing water that is not used for hydration of cement

Non-destructive testing methods

Physical properties of concrete can be related to strength and can be measured by non-destructive methods.Such properties include hardness, resistance to penetration by projectiles, rebound capacity and ability to transmit ultrasonic pulses and X- and Y-rays.Penetration testsWindsor probeEquipment consists of a powder-actuated gun or driver, hardened alloy probes, loaded cartridges, a depth gauge for measuring penetration of probes Depth of penetration provides an indication of the compressive strength of the concreteadvantages-Is a quick method of checking quality and maturity of in-situ concrete-is non -destructive, with only minor patching of holes on exposed faces-provides a means of assessing strength development with curing.

disadvantagesThe test produces quite variable results and should not be expected to give accurate values of concrete strength .Rebound Hammer Test (Schmidt Hammer Test) used to test the uniformity of the concrete

consists of a spring controlled hammer mass that slides on a plunger within a tubular housing. The hammer is forced against the surface of the concrete by the spring and the distance of rebound is measured on a scale. The higher the rebound, the harder the concretes surface, and the greater the concretes strengthRebound hammer test

disadvantagesThe results are affected smoothness of surface, size and shape of specimen, moisture condition of the concrete, type of cement and coarse aggregate, and extent of carbonation of surfaceadvantagesThe Schmidt hammer provides an inexpensive, simple and quick method of obtaining an indication of concrete strength, accuracy of 15 to 20 % is possible only for specimens cast, cured and tested under conditions for which calibration curves have been established. Pull out testsUses a special ram, the force required to pull from the concrete a specially shaped steel rod whose enlarged end has been cast into the concrete to a depth of 3 in. (7.6 cm). The concrete is simultaneously in tension and in shear, but the force required to pull the concrete out can be related to its compressive strengthFig 1.4 pull out test

advantages- measures quantitatively the strength of concrete in place- give information on the maturity and development of strengthdisadvantages- they have to be planned in advance and pull-out assemblies set into the formwork before the concrete is placedULTRA PULSE VELOCITY METHOD is used to test concrete strength in-situ by detecting discontinuities, cracks and internal deterioration in the structure of concrete. It measures the time of travel of an ultrasonic pulse passing through the concrete. Length between transducers/ travel time = average velocity of wave propagation.

Fig 1.6 (upvm)

advantagesis an ideal tool for establishing whether concrete is uniform-can be used on both existing structures and those under construction-cracks can be detected

disadvantagesbeyond a strength of 13.6 to 20.4 MPa accuracy in determining strength is less than 20%. Accuracy depends on careful calibration and use of the same concrete mix proportions and aggregate in the test samples used for calibration as in the structure.-Its use for predicting strength is much more limited, owing to the large number of variables affecting the relation between strength and pulse velocity