Fig.1: Flexural Test on Concrete (Three Point Loading Test)
Following topics regarding flexural tests on concrete is discussed:
- What is flexural test on concrete?
- What is the application of flexural test on concrete?
- What are the factors that cause variability in flexural test results?
- Size of concrete specimen for flexural test
- Apparatus required for flexural test on concrete
- Sample preparation
- Test procedure
- Computation of modulus of rupture
What is Flexural Test on Concrete?
Flexural test evaluates the tensile strength of concrete indirectly. It tests the ability of unreinforced concrete beam or slab to withstand failure in bending. The results of flexural test on concrete expressed as a modulus of rupture which denotes as (MR) in MPa or psi. The flexural test on concrete can be conducted using either three point load test (ASTM C78) or center point load test (ASTM C293). The configuration of each test is shown in Figure-2 and Figure-3, respectively. Test method described in this article is according to ASTM C78.
Fig.2: Three-Point Load Test (ASTM C78)
Fig.3: Center Point Load Test (ASTM C293)
It should be noticed that, the modulus of rupture value obtained by center point load test arrangement is smaller than three-point load test configuration by around 15 percent. Moreover, it is observed that low modulus of rupture is achieved when larger size concrete specimen is considered. Furthermore, modulus of rupture is about 10 to 15 percent of compressive strength of concrete. It is influenced by mixture proportions, size and coarse aggregate volume used for specimen construction. Finally, the following equation can be used to compute modulus of rupture, but it must be determined through laboratory test if it is significant for the design:
Fig.4: Flexural Test Machine and Concrete Specimen (ASTM C78)
What are the Applications of Flexural Test on Concrete?
Following are the applications of flexural test:- Specifying compliance with standards
- It is an essential requirement for concrete mix design
- It is employed in testing concrete for slab and pavement construction
What Factors Cause Variability in Flexural Test Results?
- Concrete specimen preparation
- Specimen size
- Moisture condition of the concrete specimen
- Curing of the concrete specimen
- And whether the specimen is molded or sawed to the required size
Size of Concrete Specimen for Flexural Test
According to ASTM the size of the specimen is 150mm width, 150mm depth and the length should not be at least three times the depth of the specimen. Indian standard determined the size of the concrete specimen as 150mm width, 150mm depth, and span of 700mm. It also states that a size of 100mm width, 100mm depth, and span of 500mm can be used if the maximum aggregate size used is not greater than 19mm. British standard specifies square specimen cross section with 100mm or 150mm dimension and the span ranges from four to five times specimen depth. However, it preferred 150mm width, 150mm depth, and span of 750mm for the specimen.Apparatus for Flexural Test on Concrete
- Steel, iron cast, or other nonabsorbent material molds with size of (150mmX150mmX 750mm)
- Tamping rods: ASTM specify large rode (16mm diameter and 600mm long) and small rode (10mm diameter and 300mm long)
- Testing machine capable of applying loads at a uniform rate without interruption of shocks
- Scoop
- Trowel
- Balance with accuracy of 1g
- Power driven concrete mixer
- Table vibration in the case of using vibration to compact concrete in molds
Sample Preparation of Concrete
- Determine proportions of materials including cement, sand, aggregate and water.
- Mix the materials using either by hand or using suitable mixing machine in batches with size of 10 percent greater than molding test specimen.
- Measure the slump of each concrete batch after blending.
- Place molds on horizontal surface and lubricate inside surface with proper lubricant material and excessive lubrication should be prevented.
- Pour fresh concrete into the molds in three layers.
- Compact each layer with 16mm rode and apply 25 strokes for each layer or fill the mold completely and compact concrete using vibration table.
- Remove excess concrete from the top of the mold and smoothen it without imposing pressure on it.
- Cover top of specimens in the molds and store them in a temperature room for 24 hours.
- Remove the molds and moist cure specimens at 23+/-2 o C till the time of testing.
- The age of the test is 14 days and 28 days and three specimens for each test should be prepared (according to Indian Code, the specimen is stored in water at 24-30oC for 48hours and then tested)
Procedure of Flexural Test on Concrete
- The test should be conducted on the specimen immediately after taken out of the curing condition so as to prevent surface drying which decline flexural strength.
- Place the specimen on the loading points. The hand finished surface of the specimen should not be in contact with loading points. This will ensure an acceptable contact between the specimen and loading points.
- Center the loading system in relation to the applied force.
- Bring the block applying force in contact with the specimen surface at the loading points.
- Applying loads between 2 to 6 percent of the computed ultimate load.
- Employing 0.10 mm and 0.38 mm leaf-type feeler gages, specify whether any space between the specimen and the load-applying or support blocks is greater or less than each of the gages over a length of 25 mm or more.
- Eliminate any gap greater than 0.10mm using leather shims (6.4mm thick and 25 to 50mm long) and it should extend the full width of the specimen.
- Capping or grinding should be considered to remove gaps in excess of 0.38mm.
- Load the specimen continuously without shock till the point of failure at a constant rate (Indian standard specified loading rate of 400 Kg/min for 150mm specimen and 180kg/min for 100mm specimen, stress increase rate 0.06+/-0.04N/mm2.s according to British standard).
- The loading rate as per ASTM standard can be computed based on the following equation:
- Finally, measure the cross section of the tested specimen at each end and at center to calculate average depth and height.
Computation of Modulus of Rupture
The following expression is used for estimation of modulus of rupture:
