🧱 Compressive strength of masonry units (clay, calcium silicate, aggregate concrete and aerated concrete, stone) — surface preparation by grinding or capping with mortar, conditioning, loading to failure in a compression machine, PN-EN 772-1

Construction PN-EN 772-1

Overview

WHAT THE STANDARD COVERS. Scope from the PKN catalogue card of PN-EN 772-1+A1:2015-10, in full (our translation of the Polish card text): “This European Standard specifies a method for determining the compressive strength of masonry units.” We read the text of the standard in the preview of SIST EN 772-1:2011 (EN 772-1:2011) from the cover to subclause 7.3.1 (conditioning of specimens, page 7); we do not have the text of amendment A1:2015 and do not know which points it changed.

ACCORDING TO THE TEXT OF EN 772-1:2011 (foreword, clauses 1–7.3.1). The standard was prepared by CEN/TC 125 “Masonry”; it replaced EN 772-1:2000 without technical changes, adding a normative Annex B with the methods of surface preparation and conditioning for each unit type (previously given only in the product specifications). Normative references: EN 771-1 to EN 771-6 (specifications for clay, calcium silicate, aggregate concrete, autoclaved aerated concrete, manufactured stone and natural stone units), EN 772-2 (percentage of voids), EN 772-10 (moisture content of calcium silicate and aerated concrete units), EN 772-13 (density), EN 772-16 (dimensions), EN 1015-11 (strength of mortar), EN ISO 6507-1 (Vickers hardness). Principle: the specimens — after preparation, when needed — are laid and centred on the platen of a compression testing machine and a uniformly distributed load is applied and increased continuously up to failure. Symbols: fb — normalised compressive strength of the masonry unit (N/mm²), d — shape factor for converting the air-dry compressive strength of specimens to the normalised strength (Annex A, informative). Materials: sand with a maximum grain size of 1 mm, cement. Testing machine (6.1, Table 1): maximum permissible repeatability of forces 2,0 % of indicated force, maximum permissible mean error ± 2,0 %, zero error ± 0,4 % of the maximum force of the range; scale such that the failure load exceeds one-fifth of the full scale; a load-pacer (rate according to 8.2); two steel bearing platens whose deflection at failure load is less than 0,1 mm over 250 mm, through hardened or case hardened, with a surface hardness of at least 600 HV according to EN ISO 6507-1; one platen aligns freely with the specimen on contact but must not tilt during loading, the other is non-tilting; platens larger than the largest specimen, departure from planeness not more than 0,05 mm; auxiliary platens — of the same hardness, stiffness and planeness. Balance accurate to 0,1 % of the specimen mass; stiff steel strips for units bedded on mortar strips. Sampling (7.1): according to the relevant part of EN 771; at least six specimens unless the product specification requires more; representative portions, e.g. cubes, may be cut from large units at different positions. Surface preparation (7.2): specimens are tested in the specified orientation, stated in the report (for some forms of construction — in more than one); the loaded faces must be plane to 0,1 mm in 100 mm, and the top face must lie between two planes parallel to the bottom face not more than 1 mm apart per 100 mm; otherwise they are ground (7.2.4) or capped with mortar (7.2.5); tongues and grooves are removed (7.2.2); units with frogs whose net loaded area exceeds 35 % of the bed face are tested without removing or filling the frogs, and at 35 % or less — the frogs are filled with mortar as for capping (7.2.3); specimens are prepared according to Annex B, and the surface preparation method is stated in the report; clay HD units that cannot practically be ground (e.g. high strength) may be capped. Grinding (7.2.4): until planeness and parallelism are achieved; frogs, indented lettering, cavities and holes are left in place; when grinding would significantly alter the contact area — capping; when after grinding the specimen height is less than 40 mm or the height/width ratio less than 0,4 — a composite specimen of two units is made by placing one on the other, without mortar, binder or separating layer, and such a specimen gives one result (so more units than stated in EN 771 are needed). Capping (7.2.5): a cement/sand mortar which at the time of testing the specimens attains — when tested according to EN 1015-11 — a strength at least equal to the expected unit strength or 30 N/mm², whichever is the lesser; for units with high water absorption the surfaces may be moistened first; a plate of ground glass or stainless steel with planeness of 0,1 mm in 100 mm, levelled in two directions, coated with mould release oil or thin paper or film; a mortar layer of about 5 mm, about 25 mm longer and about 10 mm wider than the unit; the unit is pressed in perpendicular to the plate (checked with a square or spirit level on four faces), mortar thickness at least 3 mm over the whole area; only cavities filled in the wall are filled; after hardening and checking the layer for defects (lack of compaction, lack of adhesion, cracks), the second face is capped with mortar from the same batches of cement and sand, in the same proportions; for face-shell or strip bedded units the mortar is placed in strips under the shells or on all surfaces bedded in use. Curing (7.2.5.4): under sacks kept damp throughout or in a chamber at relative humidity above 90 %, until the mortar reaches the required strength. Conditioning (7.3.1): according to a prescribed moisture regime or to a prescribed moisture condition, according to Annex B; except for conditioning by immersion — free air circulation around each specimen. The table of contents then lists: 8 — procedure (loading rate in 8.2), 9 — calculation and expression of results, 10 — evaluation of results, 11 — test report; Annex A (informative) — conversion to normalised compressive strength, Annex B (normative) — surface preparation and conditioning of units.

STATUS (cards as of 02.10.2026). The card of PN-EN 772-1+A1:2015-10 (English version) has no “Withdrawn” header — the standard is CURRENT: published 07-10-2015, 15 pages, price group H, Construction Sector, KT 233 for Masonry Structures, ICS 91.100.25, “Introduces: EN 772-1:2011+A1:2015 [IDT]”, “Replaces: PN-EN 772-1:2011 - English version, PN-EN 772-1:2011 - Polish version, PN-EN 772-1:2011 - German version”. There is no Polish version of the edition with A1 in the catalogue: the PKN search for “PN-EN 772-1” (02.10.2026) shows PN-EN 772-1+A1:2015-10 (English), PN-EN 772-1:2011 (Polish, English, German) and PN-EN 772-1:2001 (Polish). Cards of PN-EN 772-1:2011 (Polish version — published 27-08-2014, 16 pages; English version — 26-08-2011, 13 pages): “Withdrawn and replaced by PN-EN 772-1+A1:2015-10 - English version”, withdrawal date 07-10-2015; they had replaced PN-EN 772-1:2001 with corrigendum Ap1:2002. What A1 changes for the result is NOT GIVEN: we have not read the amendment text, and the “Scope” field of the card is the same as for the 2011 edition. Masonry units are construction products under the EN 771 standards; whether and in which wording the standard is cited by legislation we have not checked.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read 02.10.2026). The number 772-1 appears in 18 records of 8 laboratories: AB 054 and AB 251 (5 each), AB 011, AB 687 (2 each), AB 023, AB 535, AB 619, AB 1669 (1 each). Current scopes on the PCA website (read 02.10.2026, scope issues from 16.07.2025 to 13.08.2026): 19 different lines with the number (the AB 251 document is repeated twice in the PCA file — 8 pages instead of 4 — so a search gives 24 lines). Forms: “PN-EN 772-1+A1:2015-10” — AB 054 (3 items), AB 251 (5), AB 535, AB 619, AB 687 (2), AB 1669; “PN-EN 772-1:2015-10” without “+A1” — AB 054 (2 items: aggregate concrete units, autoclaved aerated concrete) — there is no document with that designation in the PKN catalogue; “PN-EN 772-1:2011” (withdrawn) — AB 011 in one item; without date — AB 011 in the other item and AB 023 (2). None of these items is suspended (at AB 054 the footnotes on the suspension of part of the scope from 25.06.2026 to 22.12.2026 concern lines in bold italics — the lines with 772-1 are not set that way; at AB 023 the word “suspension” refers to a product). The “Laboratories” tab searches for entries BEGINNING WITH “PN-EN 772-1” and shows all 8 laboratories — checked with the tab query on the production server labcoda.pl on 02.10.2026: 8 (under “EN 772-1” — 0). Under this prefix, however, the tab also catches items with PN-EN 772-11, -13, -16, -18, -20 and -21 and shows their total number next to each laboratory: 66 items in all (AB 054 — 21, AB 251 — 18, AB 687 — 7, AB 011 — 6, AB 023 — 5, AB 535 — 4, AB 619 — 3, AB 1669 — 2), compared with 18 records with the number 772-1.

WHAT THE LABORATORIES TEST (items in PCA). Object: masonry units in general (AB 011, AB 535, AB 619, AB 1669), clay (AB 054, AB 251, AB 687), calcium silicate (AB 054, AB 251), aggregate concrete (AB 054 — “small-format wall units”, AB 251), autoclaved aerated concrete (AB 054 — also precast units), clay, aggregate concrete and aerated concrete together (AB 023), ventilation blocks of calcium silicate and clay (AB 251, AB 687). Force ranges: up to 2000 kN (AB 054), (2–200) kN (AB 054, aerated concrete), (5–3000) kN and “up to 10 MN” with sides up to 1200 mm (AB 023), (10–3000) kN with specimens up to 500 × 310 × 345 mm (AB 619), (50–3000) kN (AB 535), (60–3000) kN (AB 1669), (100–3000) kN (AB 054, AB 687), (100–5000) kN (AB 011, AB 687), (150–2500) kN (AB 251). Other parts of the series stand alongside in the scopes (dimensions, density, voids, water absorption, freeze-thaw resistance) — according to other standards, which we do not describe here.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. In preparation: the method of preparation (grinding or mortar) and the orientation of the specimen during the test must be stated in the report — a result without them is ambiguous; capping mortar weaker than required by 7.2.5.1 (less than the expected unit strength or 30 N/mm²) or insufficiently cured does not meet the condition of the standard; the 35 % net area threshold decides whether frogs are filled with mortar; only cavities filled in the wall may be filled; a composite specimen of two units (after grinding below 40 mm or with height/width below 0,4) gives one result, not two. With the machine: the failure load must exceed one-fifth of the full scale — a machine range too large for weak units (e.g. aerated concrete) gives a reading outside the permitted region; platens with hardness below 600 HV, deflection of 0,1 mm over 250 mm or more, or planeness deviation above 0,05 mm do not meet the table and 6.1. In conditioning: the moisture state of the specimen is set by Annex B — different for unit types; the scopes rarely show which regime the laboratory uses. With the result: the compressive strength of the specimen and the normalised strength fb (conversion with the shape factor d according to informative Annex A) are two different numbers — it must be stated which. With the designation: “PN-EN 772-1:2015-10” (AB 054) is not the designation of an existing PKN document — the current document is PN-EN 772-1+A1:2015-10; AB 011 cites the withdrawn PN-EN 772-1:2011 in one item.

WHAT WE DO NOT GIVE. We do not have subclauses 7.3.2 onwards (conditioning methods), Clause 8 (positioning of the specimen, loading rate), Clauses 9–11 (calculations, evaluation of results, report) or Annexes A and B (shape factors d, assignment of preparation and conditioning methods to unit types) — so we do not give the loading rate, the conditioning time, the values of d or the formula for strength. We do not know the content of amendment A1:2015.

Method principle

The specimen — a whole masonry unit or a representative piece cut from it — is prepared so that the loaded faces are plane (0,1 mm in 100 mm) and parallel (1 mm in 100 mm): by grinding or by capping with cement/sand mortar, conditioned according to Annex B, centred on the platen of the compression machine and loaded with a uniformly distributed load increased continuously up to failure; the result is the compressive strength and, if required, the normalised strength fb converted with the shape factor.

Applications

• Clay, calcium silicate, aggregate concrete and autoclaved aerated concrete masonry units — compressive strength in the scopes of 8 laboratories
• Calcium silicate and clay ventilation blocks (AB 251, AB 687)
• Large units — testing of cut pieces, e.g. cubes (7.1); machines in the scopes from 200 kN to 10 MN

Key parameters

Standard

Standard number

PN-EN 772-1+A1:2015-10 (wersja angielska; EN 772-1:2011+A1:2015 IDT)

Title (PL)

Metody badań elementów murowych — Część 1: Określenie wytrzymałości na ściskanie

Title (EN)

Methods of test for masonry units — Part 1: Determination of compressive strength

Step-by-step procedure

1. Sampling

   According to the relevant part of EN 771; at least 6 specimens (or more according to the product specification); from large units — representative pieces, e.g. cubes. PN-EN 772-1+A1:2015-10 current (card as of 02.10.2026).

2. Surface preparation

   Removal of flashing, tongues and grooves; if planeness 0,1 mm/100 mm and parallelism 1 mm/100 mm are not met — grinding or capping with mortar (≥ 3 mm); frogs filled when net area ≤ 35 %; according to Annex B; method and orientation of the specimen — in the report.

3. Curing and conditioning

   Mortar under damp sacks or at RH > 90 % until the required strength is reached; conditioning according to Annex B, with free air circulation (except immersion) — details NOT GIVEN.

4. Compression

   The specimen is centred on the machine platen and loaded with a uniformly distributed, continuously increasing load to failure; loading rate (8.2) and positioning (8.1) — NOT GIVEN.

5. Result

   Compressive strength (Clause 9 — NOT GIVEN) and if required normalised fb with shape factor d (Annex A); evaluation of results and report — Clauses 10 and 11.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Crushing of units in the machine at high forces (in the scopes up to 10 MN) — guards, flying fragments; cement mortar irritates the skin; the read text of the standard contains no health and safety warnings
• TECHNICAL NOTE: the surface preparation method, the orientation of the specimen and the type of result (specimen strength or normalised fb) change the number — they must be stated; “PN-EN 772-1:2015-10” is not the designation of an existing document

Frequently asked questions