🏗️ Assessment of in-situ compressive strength of concrete — direct and indirect methods, PN-EN 13791

Construction PN-EN 13791

Overview

A DIFFERENT QUESTION FROM THE TESTING OF SPECIMENS. The classic strength test concerns specimens made during casting and cured under standardised conditions. PN-EN 13791 answers a different question: what is the strength of concrete that is already standing in a structure — it cured under real conditions, may have been compacted differently, and often nobody kept any specimens. It is therefore not an alternative method of the same test but a separate procedure with its own rules and its own way of reasoning.

THE SCOPE FROM THE PKN CATALOGUE CARD — THE OPENING, QUOTED VERBATIM: "This document: — specifies methods and procedures for the assessment of the in-situ compressive strength of concrete and of the characteristic in-situ compressive strength in structures and precast concrete components, using direct methods (testing of cores) and indirect ones, e.g. ultrasonic pulse velocity, rebound number [...]". The quotation is cut within the list of indirect methods — after ultrasonic pulse velocity the card also names the rebound number, that is the sclerometric method.

DIRECT AND INDIRECT METHODS — A DIFFERENCE THAT MUST NOT BE BLURRED. A direct method means drilling a core and destroying it in a press: strength is measured, at the price of damage to the structure and of labour. Indirect methods — ultrasonic pulse velocity, the rebound number of a sclerometer — measure a quantity CORRELATED with strength, not strength itself. The correlation depends on the composition of the concrete, its moisture, age and carbonation, and so is not universal: using the factory curve from the instrument’s manual instead of a correlation established for the concrete in question is the commonest error in this field. The standard does not replace cores with indirect methods — it orders their combined use.

IN-SITU STRENGTH IS NOT THE SAME FIGURE AS THE STRENGTH CLASS OF THE CONCRETE. Concrete in a structure usually has a strength different from that of standardised specimens from the same batch — depending on compaction, curing and the conditions of hardening. Comparing an in-situ result directly with the requirement of a strength class, without applying the procedures provided by this standard, leads to false conclusions in both directions: to the rejection of a sound structure or to the acceptance of a defective one.

A TRAP IN THE NUMBER — TWO DIFFERENT STANDARDS BEARING THE NUMBER 13791. Alongside PN-EN 13791 the PKN catalogue holds PN-EN ISO 13791 — "Thermal performance of buildings — Calculation of internal temperatures of a room in summer without mechanical cooling — General criteria and validation procedures", a document of TC 179 for the Thermal Insulation of Buildings, whose 2012 edition was withdrawn on 29-09-2017. The numbers differ only by the element "ISO", and the fields have nothing in common: concrete as against the thermal physics of a building. A reference to "13791" without the full designation is therefore ambiguous — and this is the same class of mistake as the pair PN-G-04584 against PN-G-04582 noted earlier in this knowledge base, only more dangerous, because here both documents belong to construction.

EDITION AND LANGUAGE VERSIONS. The edition in force, PN-EN 13791:2019-12, is available BOTH in Polish and in English; the Polish version has a publication date of 17-01-2022 and 42 pages. It superseded PN-EN 13791:2008.

WHAT WE DO NOT GIVE. We do not have the text of the standard — PKN supplies it for a fee (TC 274, Concrete). We therefore DO NOT GIVE the number of cores required for an assessment, the way of calculating the characteristic strength, conversion factors or the requirements for correlations in indirect methods.

Method principle

The strength of concrete in a structure is estimated from tests carried out on the object: directly — by drilling cores and destroying them in a press, or indirectly — by measuring a quantity correlated with strength, such as the velocity of an ultrasonic pulse. Indirect methods require a correlation established for the concrete under test, because the relation between the measured quantity and strength depends on composition, moisture and age. From the results the characteristic in-situ strength is derived according to the procedures given in the standard — a quantity different from the strength class determined on standardised specimens.

Applications

• Assessment of existing concrete structures — on a change of use, on reconstruction, or where there is doubt about the quality of the work
• Assessment of precast concrete components
• Structural expert opinions and disputes concerning the quality of concrete
• Supplementing core testing with indirect methods where the number of drillings must be limited

Key parameters

Standard

Standard number

PN-EN 13791:2019-12 (dostępna w wersji POLSKIEJ i angielskiej; wprowadza EN 13791:2019 [IDT])

Title (PL)

Ocena wytrzymałości betonu na ściskanie w konstrukcjach i prefabrykowanych wyrobach betonowych

Title (EN)

Assessment of in-situ compressive strength in structures and precast concrete components

Step-by-step procedure

1. Establish the question and the extent of the assessment

   Define which element and which area of the structure is being assessed. A result from one place does not describe the whole structure.

2. Choose the methods

   Direct (cores), indirect, or a combination. Indirect methods do not replace cores — the standard orders their combined use.

3. Establish the correlation for indirect methods

   The correlation must be established for the concrete under test. Using the instrument’s factory curve is the commonest error in this field.

4. Take and test the cores

   Taking to PN-EN 12504-1, compression testing to PN-EN 12390-3. WE DO NOT GIVE THE NUMBER OF CORES — it is in the text of the standard.

5. Estimate the characteristic strength

   According to the procedures of the standard. Do not compare the result directly with the strength class — they are different quantities.

6. Record in the report

   Give the FULL designation "PN-EN 13791:2019-12" — "13791" alone does not distinguish this standard from PN-EN ISO 13791 on the thermal performance of buildings.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Drilling cores in a load-bearing structure may impair its capacity — the drilling locations are agreed with the designer or an expert before starting
• Water-cooled drilling near electrical installations — risk of electric shock; locate reinforcement and services before drilling
• TECHNICAL NOTE: the result of an indirect method without a correlation established for the concrete under test is not an estimate of strength but a reading of an instrument

Frequently asked questions