Experiment No: 15

Determination of Splitting Tensile Strength of Cylindrical Concrete Specimens

 

Introduction

 

The splitting tension test, a method used to determine the tensile strength of concrete, originated during World War II in Rio de Janeiro. The city's rapid expansion led to the relocation of the small church of Sá´o Pedro, which was occupied by a section of the redesigned roadway system. Due to the shortage of steel rollers, concrete cylinders covered by 9mm thick steel plates were used as rollers to transport the church. Engineer Lobo Carneiro noticed uniform and consistent splitting failure in the cylinders, which led him to study Hertz's work on stress distribution for concentrated loads applied to cylinders. Carneiro concluded that this configuration was suitable for measuring the indirect tensile strength of concrete. However, plans for the church's relocation were abandoned due to weak masonry and potential collapse risks during transport. The splitting test, now known as the "Brazilian test" in rock mechanics, became popular for measuring the tensile strength of brittle materials.

This test involves applying a diametral compressive force to a cylindrical concrete specimen until failure occurs. This results in tensile stresses on the load-containing plane and high compressive stresses in the area around it. Tensile failure occurs due to triaxial compression, allowing the areas to withstand higher compressive stresses than a uniaxial compressive strength test. Thin plywood bearing strips distribute the load along the cylinder's length. The maximum load sustained is divided by geometrical factors to obtain splitting tensile strength.

Scope

This lab report covers the procedures outlined in ASTM C496 for determining the splitting tensile strength of cylindrical concrete specimens. This method is widely used in structural engineering to assess the tensile properties of concrete, which are important for design purposes.

 

Purpose

 

To determine the Splitting Tensile Strength of Cylindrical Concrete Specimens.

 

ASTM Designation

 

ASTM C496—Splitting Tensile Strength of Cylindrical Concrete Specimens.

 

Terminology

 

Concrete —

a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate; in hydraulic-cement concrete, the binder is formed from a mixture of hydraulic cement and water.

concrete, fresh —

concrete that possesses enough of its original workability so that it can be placed and consolidated by the intended methods.

concrete, hardened —

concrete that has developed sufficient strength to serve some defined purpose or resist a stipulated loading without failure.

Consistency (fresh cementitious mixture)—

the relative mobility or ability to flow.

Curing —

action taken to maintain moisture and temperature conditions in a freshly-placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.

Pozzolan —

a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.

Cementitious Material (Hydraulic) —

an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under water.

Fracture —

The failure of the specimen under compression load.

Splitting Tensile Strength —

The tensile strength of concrete determined by applying compressive forces along its length until failure occurs, indirectly inducing tension.

Significance and Use

Splitting tensile strength is typically larger than direct tensile strength and lower than flexural strength (modulus of rupture). Splitting tensile strength is utilized in the design of structural lightweight concrete elements to evaluate the shear resistance supplied by concrete and to calculate the development length of reinforcement.

Apparatus

 

Testing Machine:

The testing machine shall be of a type having sufficient capacity and capable of providing the required rates of loading.

Supplementary bearing bar or plate:

If the diameter or the largest dimension of the upper bearing face or the lower bearing block is less than the length of the cylinder to be tested, a supplementary bearing bar or plate of machined steel shall be used. The surfaces of the bar or plate shall be machined to within 6 0.001 in. [0.025 mm] of planeness, as measured on any line of contact of the bearing area. It shall have a width of at least 2 in. [50 mm], and a thickness not less than the distance from the edge of the spherical or rectangular bearing block to the end of the cylinder. The bar or plate shall be used in such manner that the load will be applied over the entire length of the specimen.

Bearing Strips:

Two bearing strips of nominal 1⁄8 in. [3.2 mm] thick plywood, free of imperfections, approximately 1 in. [25 mm] wide, and of a length equal to, or slightly longer than, that of the specimen shall be provided for each specimen. The bearing strips shall be placed between the specimen and both the upper and lower bearing blocks of the testing machine or between the specimen and supplemental bars or plates, when used. Bearing strips shall not be reused.

Specimens:

 

·        The test specimens shall conform to the size, molding, and curing requirements of ASTM C 31

·        Moist-cured specimens, during the period between their removal from the curing environment and testing, shall be kept moist by a wet burlap or blanket covering, and shall be tested in a moist condition as soon as practicable.

·        The following curing procedure shall be used for evaluations of light-weight concrete: specimens tested at 28 days shall be in an air-dry condition after 7 days moist curing followed by 21 days drying at 23.0 ± 2.0°C and 50 6 5 % relative humidity.

Procedure

Marking:

Draw diametrical lines on each end of the specimen using a suitable device that will ensure that they are in the same axial plane or as an alternative, use the aligning jig.

 

Measurements:

[1] Determine the diameter of the test specimen to the nearest 0.25 mm (0.01in.) by averaging three diameters measured near the ends and the middle of the specimen and lying in the plane containing the lines marked on the two ends.

[2] Determine the length of the specimen to the nearest 2.5 mm (0.1 in.) by averaging at least two length measurements taken in the plane containing the lines marked on the two ends.

 

Positioning Using Marked Diametral Lines:

Center one of the plywood strips along the center of the lower bearing block. Place the specimen on the plywood strip and align so that the lines marked on the ends of the specimen are vertical and centered over the plywood strip. Place a second plywood strip lengthwise on the cylinder, centered on the lines marked on the ends of the cylinder. Position the assembly to ensure the following conditions:

·        The projection of the plane of the two lines marked on the ends of the specimen intersects the center of the upper bearing plate, and

·        The supplementary bearing bar or plate, when used, and the center of the specimen are directly beneath the center of thrust of the spherical bearing block

Positioning by Use of Aligning Jig:

Position the bearing strips, test cylinder, and supplementary bearing bar by means of the aligning jig, and center the jig so that the supplementary bearing bar and the center of the specimen are directly beneath the center of thrust of the spherical bearing block.

 

Figure: Schematic Diagram of Cylinder Splitting Test

 

Rate of Loading:

 

Apply the load continuously and without shock, at a constant rate within the range 100 to 200 psi/min [0.7 to 1.4 MPa/min] splitting tensile stress until failure of the specimen. Record the maximum applied load indicated by the testing machine at failure. Note the type of failure and the appearance of the concrete.

 

The relationship between splitting tensile stress and applied load is shown in formula. The required loading range in splitting tensile stress corresponds to applied total load in the range of 11 300 to 22 600 lbf [50 to 100 kN]/min for 6 by 12-in. [150 by 300-mm] cylinders.

Calculation

Calculate the splitting tensile strength of the specimen as follows:

T = 2P/pld

where:

T = splitting tensile strength, psi [MPa],

P = maximum applied load indicated by the testing machine, lbf [N],

l = length, in. [mm], and

d = diameter, in. [mm].

Report:

·        Identification number,

·        Diameter and length, in. [mm],

·        Maximum load, lb. [N],

·        Splitting tensile strength calculated to the nearest 5 psi [0.05 MPa],

·        Estimated proportion of coarse aggregate fractured during test,

·        Age of specimen,

·        Curing history,

·        Defects in specimen,

·        Type of fracture, and

·        Type of specimen

 

Lab Questions:

Basic Understanding

1.      What is splitting tensile strength, and why is it important for concrete?

2.      How does the splitting tensile strength differ from the compressive strength of concrete?

3.      Why is the tensile strength of concrete tested indirectly through the splitting method?

Procedure and Equipment

4.      Can you describe the procedure for determining the splitting tensile strength of concrete using cylindrical specimens?

5.      What is the significance of the loading strip used in the splitting tensile strength test?

6.      Why are cylindrical specimens placed horizontally between the loading surfaces during the test?

7.      How is the load applied in this test, and why is it important to apply it gradually?

8.      What type of machine is used for the splitting tensile strength test?

9.      What are the dimensions of the cylindrical specimens used in this test?

10.  Why is it important to cure the concrete cylinders before testing their tensile strength?

Calculation and Interpretation

11.  How do you calculate the splitting tensile strength from the test data?

12.  What factors can influence the splitting tensile strength of concrete?

13.  How do the dimensions of the specimen affect the tensile strength calculation?

14.  What would you infer if the specimen fails prematurely or shows unusually low tensile strength?

15.  How does the water-cement ratio affect the splitting tensile strength of concrete?

16.  What is the significance of the failure pattern observed in the concrete specimen after testing?

Standards and Specifications

17.  Which standards (e.g., ASTM, IS) are followed for determining the splitting tensile strength of concrete?

18.  What are the typical values of splitting tensile strength for normal-strength concrete?

19.  How does the curing duration affect the splitting tensile strength?

20.  What are the acceptable tolerances for splitting tensile strength in various construction applications?

Application and Practical Considerations

21.  How does the splitting tensile strength of concrete relate to its performance in structures under tension?

22.  What are the practical implications of having a low splitting tensile strength in concrete?

23.  What are the possible reasons for an unexpectedly low or high splitting tensile strength result?

24.  How can the splitting tensile strength of concrete be improved?

25.  What role does aggregate size and type play in the tensile strength of concrete?

26.  How do admixtures affect the splitting tensile strength of concrete?