DESTRUCTIVE TEST METHODS OF HARDENED CONCRETE Strength of

DESTRUCTIVE TEST METHODS OF HARDENED CONCRETE

Strength of Hardened Concrete • Two important feature: -Strength -Durability • Strength(kgf/cm 2 or Mpa): The ability to resist strain -Compressive strength -Tensile strength -Bending strength

Strength of Hardened Concrete • Depends on -Strength of paste -Strength of aggregates -The bond between the cement paste and the aggregates. (no obstacle between cement and aggregates) • Also: -Water/Cement ratio of the concrete mix -Quality of the mixing water -Properties of the cement -Properties of the aggregates -Mixing, transportation, placing, and compaction operations applied to the concrete -Curing conditions and age of the concrete.

Strength Of Hardened Concrete

Effect of the Water/Cement Ratio The w/c ratio affects the content of the total capillary porosity of the cement paste. The higher the w/c ratio, the higher is the total capillary porosity of the paste, thus the lower concrete strength.

Effect of the Quality of Mixing Water on Strength Presence of excessive amounts of impurities in the mixing water causes not only harmful effects but also reduce the strength and durability of the hardened concrete

Effect of the Properties of Cement • Compound composition and fineness of the cement affect the rate and amount of the gel production upon hydration of the cement. • As the gel production increases, the capillary porosity in he cement paste decreases.

Effect of Aggregates Properties • • Gradation Max. aggregate size Shape of aggregate particles Amounts of deleterious materials in aggregates. • • • Affect the strength of concrete. Improper gradation Odd shaped particle Presence of excessive amount of clay lumps Increase the water requirement, decrease the strength of concrete. Presence of very fine materials as a coating on the aggregate particles reduces the bond between aggregate particles and the cement paste

Effect of Mixing, Transportation, Placing, Compaction • Inadequate mixing concrete results in a non-uniform mixture • Prolonged mixing not only causes the aggregate particles to break but also increases the concrete temperature (increases water requirement) • Improper transportation and improper placing may lead to segregation. • Inadequately compacted concrete may contain a large number of voids, which lead to reduction of strength. • Long vibration times cause segregation.

Effect of Mixing, Transportation, Placing, Compaction - • Inadequate mixing concrete results in a non-uniform mixture • Prolonged mixing not only causes the aggregate particles to break but also increases the concrete temperature (increases water requirement) • Improper transportation and improper placing may lead to segregation. • Inadequately compacted concrete may contain a large number of voids, which lead to reduction of strength. • Long vibration times cause segregation.

Compressive Strength of Concrete • The maximum resistance of the concrete to axial compressive loading. • In structural applications, concrete is employed primarily to resist compressive stresses • Calculations for the design of structures are usually based on the compressive strength of the concrete. • Strength of concrete is usually determined by the standard test methods.

Compressive Strength of Concrete Most common used test method. • Conducting compressive strength test on standard test specimens • Drilling cores from the hardened concrete and determining the compressive strength by testing the core specimens • Determination of the compressive strength by measuring the rebound hardness of the concrete’s surface.

Standard Test Method • Preparation of the standard cylinder test specimen: Specimens are 15 cm in diameter 30 cm in length. (In Turkey sometimes 15 cm concrete cube specimens are also used) • The freshly mixed concrete is placed in lightly oiled mold in three layers. Each layer is compacted manually by 25 strokes of a 16 mm steel rod, and the top surface is finished by troweling. After 24 hours, the specimen is removed from mold and stored in a moist room or saturated lime water. • The top and bottom ends of he cylinder specimen used in compressive strength test should be perfectly smooth. Mortar or stiff Portland cement can be used as capping material. • No capping is necessary when cube specimens are used.

Standard Test Method • • • Testing and Determination of the Compressive Strength: The amount of load which is applied by test machine to specimen is P The area of specimen is already known. (A) So σc= Pmax/A σc= Compressive Strength Pmax= Magnitude of the load that causes breaking. After 28 day the obtained strength is used in concrete designs. (calculations) We can also test 7 days strength. σ7= 0. 65 -0. 70 x σ28 For cube specimens σcylinder= 0. 80 x σcube (at least 3 specimens should be tested)

Standard Test Method

Standard Test Method • • • 1 N/mm 2= 0. 10197 kgf/mm 2 1 MPa = 10, 197 kgf/cm 2 1 MPa = 1 N/mm 2 1 Pa= 1 N/m 2 1 kgf/mm 2= 100 kgf/cm 2 1 kgf/mm 2= 9. 807 MPa

Standard Test Method • Drilled Core Specimens: This test is conducted on cylindrical core specimens removed from the hardened concrete by drilling operation. • The diameter of the concrete core specimen diameters is 10 -15 cm. Length can be different. But l/d ratio should be between 1. 0 and 2. 0 • Before compression test the ends should be smoothed by capping. • After applying compression loads Pmax can be obtained and according to the cross section area σc can be calculated.

L/Dratio Correction. Factor 1. 75 0. 98 1. 50 0. 96 1. 25 0. 93 1. 0 0. 87 Drilled Core Specimens: This test is used to find strength of concrete that is present in a structure. The strength of the concrete in the structure may be different from the strength found by standard test methods. -Placing Errors -Insufficient Consolidation -Insufficient Curing -Chemical attacks -Freezing and thawing -Fire

Tensile Strength of Concrete • Generally the concrete elements in a structure are not subjected to direct tensile load but indirectly because of shear, bending and shrinkage tensile stresses are occurred • The tensile strength of concrete is approximately 10% of its compressive strength. • In concrete designs it is considered as 0

Flexural Strength of Concrete • Concrete beam specimens(15 x 60 cm) are subjected third point loading

Flexural Strength Of Concrete • The flexure strength is calculated by using the following relationship: • σf= M x c/I • σf= Flexure Strength • Pmax= Maximum load • M= Maximum moment = Pmaxx L / 6 • L = Span length • h = Height of the specimen cross-section • c = h/2 • b = Width of the specimen cross-section • I = moment of inertia = bh 3/12

Durability of Hardened Concrete Durability of concrete is its ability to resist • • -Sulfate attack -Acid attack -Carbonation -Alkali-aggregate reaction -Corrosion of reinforcement in concrete -Freezing and thawing -Abrasion

Sulphate Attack Sulfate are often present • • -in ground waters -in rain water -in sea water -sewage water When sulphate-containing water seeps into hardened concrete the sulphates react with calcium hydroxide and lead to formation of gypsum. When more gypsum is present in the medium, this product reacts with gypsum to form ettringite The formation of ettringite in the hardened cement paste leads to very large volume expansion

Sulphate Attack • In order to reduce the sulphate attack, these precautions should be taken: • Using a low water/cement ratio: • A decrease in the w/c ratio will decrease the capillary porosity of the cement paste and decrease the penetration of sulphatecontaining water • Using the proper type of cement • Type II, Type IV and Type V cements cause relatively smaller amounts of gypsum to be formed due to the sulphate attack. Because these cements contain relatively smaller C 3 S, the compound that produces the highest amount of calcium hydroxide. • Using finely divided pozzolanic admixture: • The concrete which have less portland cement contain less calcium hydroxide. So the possibility of gypsum formation as the result of a sulphate attack is low.

Carbonation • Rainwater and snow water absorb some carbon dioxide from the atmosphere and form a weak carbonic acid. Due to the penetration of such acidic waters into the ground, the ground water may also contain some amount of carbon dioxide in a dissolved form. • When concrete is exposed to the atmosphere or ground water, a reaction takes place between the carbon dioxide and calcium hydroxide and leading to the formation of calcium carbonate and water. The water produced by this reaction gets eventually lost by evaporation. • So carbonation is accompanied by shrinkage of the concrete

Acid Attack The presence of high concentrations of acid in water or in the environment that is in contact with the hardened concrete leads to the deterioration of the concrete • • -Industrial water may contain or form some acids -Peaty soils and shale may contain iron sulphid -Mountain streams are sometimes mildly acidic -Organic acids can come from farm silage, or from manufacturing or processing industries

Thank You Vipul Arora Roll No. 140392