Hardened concrete and methods of testing

The tests of hardened concrete may either be destructive test (DT) or non-destructive test(NDT)

The main tests that are done on hardened concrete include;

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 NDT Maturity test NDT

DESTRUCTIVE METHOD OF TESTING

COMPRESSIVE STRENGTH TEST

In situ compressive strength (fc) for normal weight range from 21-34 Mpa.Large specimens have a high probability of weaker elements, reducing strength. The weakest location of hardened concrete is the interface between cement paste and aggregate. Compressive testing is carried out using a compression test machine at a constant rate of 0.25 0.05 MPa for cylinder moulds. The compressive strength is applied by the machine is obtained by dividing the maximum load by the average cross-sectional area and expressing the result to the nearest 0.5N/mm.

Fig 1.1 compression testing machine

SPLIT TENSION TEST

It 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 (psi); p= load at failure, N (psi); L = length of specimen, mm (in.); D = diameter of specimen, mm (in.) Tensile strength varies from 2.5 MPa to 3.1 MPa (360 psi to 450 psi), about 10% of compressive strength

Fig 1.2 Split tension test

FLEXURE TENSION TEST

A prismatic beam of concrete is supported on a steel roller bearing near 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 is the load ,L IS the distance between the lower supporting

rollers and B and D are the breath and death of the beam. Important test for road and airport concrete pavements .

Fig. 1.3. Measuring Modulus of Rupture (MR)

Calculation of MR

If the fracture occurs in the middle 1/3 of the span

R = PL/(bd^2)

Where: R = flexure strength, MPa ; P = maximum load applied, N ; L = span length, mm ; b = average width of specimen, mm ; a = average depth of specimen, mm

If fracture occurs further outside the middle third, the results should not be used. Useful relationships:

R = (0.62 to 0.83)fc^0.5 (S.I. units)

R = (7.5 to 10)fc^0.5 (English units)

Maturity Test

Maturity is the degree of cement hydration .Varies with time and temperature

It is assumed that the strength is a function of maturity for a particular concrete mixture .Devices are used to measure the temperature of concrete with time .

Permeability

Permeability affects the durability of hardened concrete

Allows water and chemicals to penetrate its surface

Cause reduced resistance to frost

-alkali-aggregate reactivity and other chemicals and corrosion of steel bars

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

Increasing the water/cement ratio has strong effects on permeability, age of concrete, fineness of cement particles, air entraining agents also affects permeability

Creep Properties

Creep is the gradual increase in strain, with time, under sustained load and may take several years. Increased deflection and increased stress in steel . Gradual transfer of load from concrete to steel . Losing some of the pre-stress force in pre-stressed concrete

NON-DESTRUCTIVE TESTING OF CONCRETE

The standard method of evaluating the quality of concrete in buildings or structures is to test specimens cast simultaneously for compressive, flexural and tensile strengths. The main disadvantages are that results are not obtained immediately; that concrete in specimens may differ from that in the actual structure as a result of different curing and compaction conditions; and that strength properties of a concrete specimen depend on its size and shape. 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. These non-destructive methods may be categorized as penetration tests, rebound tests, pull-out techniques, dynamic tests, radioactive tests, maturity concept.

PENETRATION TESTS

The Windsor probe

Equipment consists of a powder-actuated gun or driver, hardened alloy probes, loaded cartridges, a depth gauge for measuring penetration of probes and other related equipment. A probe, diameter 6.5 mm and length 8.0 cm, is driven into the concrete by means of a precision powder charge. Depth of penetration provides an indication of the compressive strength of the concrete. Although calibration charts are provided by the manufacturer, the instrument should be calibrated for type of concrete and type and size of aggregate used

ADVANTAGES

-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.

DISADVANTAGES

The 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, is a surface hardness tester for which an empirical correlation has been established between strength and rebound number.

. It 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 test surface can be horizontal, vertical or at any angle but the instrument must be calibrated for each of these positions. The higher the rebound, the harder the concretes surface, and the greater the concretes strength

Disadvantages -The 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 surface

Advantages- The Schmidt hammer provides an inexpensive, simple and quick method of obtaining an indication of concrete strength, but 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 Tests

Uses 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 strength.

Advantages-

- measures quantitatively the strength of concrete in place

- give information on the maturity and development of strength

Disadvantages-

- they have to be planned in advance and pull-out assemblies set into the formwork before the concrete is placed.

ULTRA 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. Pulses are generated by shock-exciting piezo-electric crystals, with similar crystals used in the receiver. The time taken for the pulse to pass through the concrete is measured by electronic measuring circuits.

ADVANTAGES

- is an ideal tool for establishing whether concrete is uniform

-can be used on both existing structures and those under construction

-cracks can be detected

DISADVANTAGES

beyond 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