🏗️ Making and curing concrete specimens for strength tests — moulding, compaction and curing

Construction PN-EN 12390-2

Overview

The standard specifies methods of making and curing specimens for strength tests. It covers the preparation and filling of the moulds, the compaction of the concrete, the finishing of the surface, the curing of the specimens and their transport. The standard also permits these methods to be used for making and curing specimens intended for other tests.

This is the foundation standard of the whole group of concrete strength tests. The compressive strength (PN-EN 12390-3), the flexural strength (PN-EN 12390-5) and the tensile splitting strength (PN-EN 12390-6) are measured on specimens made and cured precisely according to it — all those standards cite it in their normative references. An error made in the compaction or in the curing will falsify each of those results, and it cannot afterwards be detected in the strength test itself.

The standard contains no definitions of terms. It does, however, refer to PN-EN 12350-1 (sampling of fresh concrete), PN-EN 12390-1 (shape, dimensions and requirements for specimens and moulds) and PN-EN 206 (specification of concrete — from there come the consistence classes S1 and S2 to which the typical figure of 25 strokes per layer refers).

Self-compacting concrete is treated differently: the mould is filled in one operation and the concrete is not compacted either during or after the filling of the mould.

In Polish practice the test is a standing item in the lists of road laboratories — the list of the GDDKiA Road Laboratory in Warsaw names it under the item „Wykonywanie i pielęgnacja próbek do badań wytrzymałościowych" [Making and curing specimens for strength tests] with a reference to PN-EN 12390-2:2019-07.

Method principle

A sample of fresh concrete taken according to PN-EN 12350-1 is remixed in a remixing container or on a remixing tray and then placed in moulds conforming to PN-EN 12390-1 in as many layers as are needed for full compaction. Each layer is compacted mechanically (with an internal vibrator or a vibrating table) or by hand (with a compacting rod or a tamping bar) so as to achieve full compaction of the concrete without excessive segregation and without laitance escaping. The surface is finished, the specimen is permanently marked and then cured under strictly defined conditions of temperature and humidity until the moment of testing.

Applications

• Preparation of specimens for the compressive strength test according to PN-EN 12390-3
• Preparation of specimens for the flexural strength test according to PN-EN 12390-5
• Preparation of specimens for the tensile splitting strength test according to PN-EN 12390-6
• Conformity control of ready-mixed concrete and of pavement concrete
• Making specimens for other tests on hardened concrete
• Interlaboratory comparisons covering the sampling and moulding of specimens

Key parameters

Standard

Standard number

PN-EN 12390-2:2019-07

Title (PL)

Badania betonu — Część 2: Wykonywanie i pielęgnacja próbek do badań wytrzymałościowych

Title (EN)

Testing hardened concrete — Part 2: Making and curing specimens for strength tests

Step-by-step procedure

1. Taking and remixing the sample

   Take the sample of fresh concrete in accordance with PN-EN 12350-1. Before the moulds are filled, remix the sample in a remixing container or on a remixing tray, using a shovel. If the specimens are to be made away from the place of sampling, the concrete may be transported to the place of making, provided it is protected against drying out and can still be remixed.

2. Preparing the mould

   Where necessary, cover the internal surface of the mould before filling with a thin layer of a non-reactive release agent, so that the concrete does not adhere to the mould.

3. Filling

   Depending on the consistence of the concrete, the height of the mould and the manner of compaction, fill the mould in as many layers as are needed for full compaction. Place self-compacting concrete in the mould in one operation and do not compact it either during or after the filling. If a filling frame is used, choose the amount of concrete so that after compaction a layer with a thickness of 10 % to 20 % of the height of the specimen remains in the frame.

4. Compaction — general principle

   Compact the concrete immediately after it has been placed in the moulds, so as to achieve full compaction without excessive segregation and without laitance escaping. Compact each layer by one of the methods: mechanical vibration or by hand with a rod or a tamping bar. Full compaction by mechanical vibration has been achieved when large air bubbles cease to appear on the surface of the concrete and the surface becomes relatively smooth and takes on a glassy appearance, without excessive segregation.

5. Compaction with an internal vibrator

   Vibrate for the shortest time needed for full compaction of the concrete. Avoid over-vibration, which may cause a loss of entrained air — laboratory tests have shown that with an internal vibrator this requires particular attention. Take care not to damage the mould; guide the vibrator vertically and do not let it touch the bottom or the walls of the mould. The use of a filling frame is recommended. With beam specimens the vibrator is inserted at at least three places evenly distributed along the length of the specimen.

6. Compaction on a vibrating table

   Vibrate for the shortest time needed for full compaction. It is recommended to fasten the mould to the table or to press it firmly against it. Avoid over-vibration, which may cause a loss of entrained air.

7. Hand compaction

   Distribute the strokes of the rod or the tamping bar evenly over the whole cross-section of the mould. Make sure that the rod or the tamping bar does not strike the bottom of the mould forcefully when compacting the first layer and does not penetrate appreciably into the preceding layer. Make a sufficient number of strokes for each layer — typically 25 for concrete of a consistence corresponding to the slump classes S1 and S2 according to PN-EN 206 — so as to remove entrapped air bubbles but not the entrained air. After the compaction of each layer, tap the walls of the mould vigorously with a rubber mallet until large air bubbles cease to appear on the surface and the depressions left by the rod or the tamping bar disappear.

8. Finishing the surface

   If a filling frame was used, remove it immediately after the compaction has been completed. Remove the surplus concrete above the upper edge of the mould with a trowel and finish the surface carefully.

9. Marking

   Mark the specimens legibly and permanently, without damaging the specimen in doing so. Keep records ensuring that the path of the specimen from sampling to testing can be traced.

10. Storage in the mould

    At the place of storage leave the specimens in the mould for at least 16 h, but not longer than 3 days, at a temperature of 20 °C ± 5 °C (25 °C ± 5 °C in a hot climate), protecting them against shock, vibration and drying out. As long as the concrete can still be remixed, the specimens may be transported from the place of making to the place of storage, provided they are protected against drying out. In special cases — rapid-hardening concrete, testing at an early age — demoulding before 16 h have elapsed is permitted.

    ⏱ Time: 16 h – 3 days • 🌡 Temperature: 20 °C ± 5 °C

11. Curing after demoulding

    After demoulding, cure the specimens until the moment of testing in accordance with the procedure given in the relevant test standard: in water at a temperature of 20 °C ± 2 °C or in a chamber at a temperature of 20 °C ± 2 °C and a relative humidity of at least 95 %. Methods of curing departing from the above can be related to them by applying factors.

    🌡 Temperature: 20 °C ± 2 °C

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Fresh concrete mix is strongly alkaline — waterproof gloves, goggles, immediate rinsing of the skin on contact
• Cement causes chemical burns on prolonged contact with the skin — do not work in soaked clothing
• Moulds with concrete weigh over a dozen kilograms — carry with a straight back, a risk of crushing the feet, protective footwear
• Internal vibrator and vibrating table — vibration transmitted to the hands and to the whole body, limit the time of exposure; protect the supply cable against moisture
• The noise of the vibrating table — hearing protectors
• The curing tank at a temperature of 20 °C — a slippery floor around it, protection against slipping

Frequently asked questions