🏗️ Flexural strength of concrete (two-point loading)

Construction PN-EN 12390-5

Overview

The standard PN-EN 12390-5 specifies a method of determining the flexural strength of specimens of hardened concrete. Prismatic specimens (beams) conforming to PN-EN 12390-1 are subjected to a bending moment by loading through upper and lower rollers, the maximum load is recorded and the strength is calculated. The reference method is two-point loading; centre-point loading is set out in the normative Annex A. In the interlaboratory comparison cited in the standard, centre-point loading consistently gave results 13 % higher than two-point loading.

The flexural strength is of particular importance for concrete pavements: the WWiORB technical specifications of GDDKiA D-05.03.04 require the test according to PN-EN 12390-5 with two-point loading on beams of 150×150×600–750 mm, and the mean of three specimens required is at least 4.5 MPa (traffic categories KR1–KR4) or 5.5 MPa (KR5–KR7). The strength so determined is not strictly a material property — it depends on the support arrangement, on the dimensions of the specimens and on the spacing of the supports, and for that reason the test procedure has to be strictly specified.

The 2019 edition replaced PN-EN 12390-5:2009; the changes comprise technical corrections and the alignment of the curing with PN-EN 12390-3. It is good practice to determine the density before the test as a check on the compaction of the concrete. The fracture is also to be examined: a fracture outside the loading rollers has to be recorded in the test report.

Method principle

A prismatic specimen is bent by a load transmitted through two upper rollers, mounted in a hinged crosshead which divides the force of the machine equally between the two rollers, and two supporting rollers. The span between the outer rollers l = 3d (d — the width of the specimen), the spacing of the inner rollers = d, arranged symmetrically; the error of positioning ≤ 2.0 mm. The rollers are of steel with a diameter of 20–40 mm and at least 10 mm longer than the width of the specimen; three of them (including both upper ones) must rotate freely and tilt in the plane perpendicular to the axis of the specimen. The load is increased at a constant rate of stress s in the range 0.04–0.06 MPa/s until failure; the rate of loading required R = s·d1·d2²/l [N/s].

Applications

• Testing of pavement concrete for roads and airfields (the requirement of the WWiORB technical specifications of GDDKiA: ≥ 4.5 MPa or ≥ 5.5 MPa depending on the traffic category)
• Quality control of concrete in members in bending and in precast units
• Assessment of the conformity of the concrete with the approved mix proportions in the trial batch
• Comparative studies of loading arrangements (two-point vs centre-point)
• Research on fibre-reinforced and pavement concretes

Key parameters

Standard

Standard number

PN-EN 12390-5:2019-08

Title (PL)

Badania betonu — Część 5: Wytrzymałość na zginanie próbek do badań

Title (EN)

Testing hardened concrete — Part 5: Flexural strength of test specimens

Step-by-step procedure

1. Preparation of the specimens

   Beams cast in accordance with PN-EN 12350-1 and PN-EN 12390-2, with the direction of casting marked; cut specimens conforming to PN-EN 12390-1 are permitted. Examine the specimens and record any irregularities. Reject specimens exceeding the tolerances or correct them: the irregularities by grinding, the deviations of the angles by cutting or grinding.

2. Time and conditions of the test

   Test as soon as possible after removal from curing and no later than 10 h afterwards, at a temperature of (20 ± 5) °C or (25 ± 5) °C in a hot climate. Protect specimens kept in the laboratory for more than 4 h against loss of moisture.

   ⏱ Time: ≤ 10 h • 🌡 Temperature: 20 ± 5 °C

3. Cleaning and placing

   Wipe the bearing surfaces of the machine and remove any loose grains from the surfaces of the specimen in contact with the rollers; wipe excess moisture from specimens kept in water. Place the specimen centrally, with its longitudinal axis perpendicular to the axes of the rollers, so that the direction of loading is perpendicular to the direction of casting.

4. Preload

   Do not apply any load until all the rollers bear evenly on the specimen. The preload is to be not more than about 20 % of the failure load.

5. Loading to failure

   Increase the load without shock, continuously, at the selected constant rate of stress of 0.04–0.06 MPa/s (±10 %) until the specimen can carry no greater load. On manually controlled machines, correct any fall of the rate as failure approaches.

6. Recording of the result

   Record the maximum load. Record the fact if the fracture occurred outside the loading rollers.

7. Calculation

   Calculate the flexural strength from the maximum load and the dimensions of the specimen; the rate of loading required R = s·d1·d2²/l, where d1, d2 are the cross-sectional dimensions of the specimen [mm] and l the spacing of the lower rollers [mm].

Required equipment and apparatus

Health and safety (OHS)

• The testing machine: the crushing zone and flying fragments of concrete at failure — a guard, safety goggles, the hands outside the loading zone
• Beams of 150×150×600 mm are heavy — carry them with two people or with a trolley
• Grinding and cutting of concrete only wet or with dust extraction (silica dust)

Frequently asked questions