Masonry Materials: Brick, Stone, and Concrete Block

Masonry is one of the oldest continuous traditions in building construction, traceable to the fourth millennium BCE. Modern masonry covers four principal material classes: fired-clay or calcium-silicate brick; concrete masonry units (CMU); cut, dressed, or rubble stone; and the mortar that bonds them. Each class is governed by material specifications — predominantly ASTM in North America and EN in Europe — that define composition, dimensional tolerance, strength, and durability, with structural design rules consolidated in TMS 402/602 and Eurocode 6.

History and Development

The earliest fired bricks appeared in Mesopotamia around 4000 BCE, alongside the still earlier tradition of sun-dried mud brick. Vitruvius, in De Architectura (c. 30 BCE), catalogued the lime and pozzolan mortars used in Roman construction and prescribed proportions for brick acceptable in public works.[1] Surviving Roman maritime structures — the harbour moles at Caesarea Maritima and Portus among them — demonstrate the durability of pozzolanic lime mortars in saline environments.[2]

The Industrial Revolution mechanised brick manufacture: the Hoffmann continuous-firing kiln, patented in 1858, raised throughput by orders of magnitude and enabled the urban brick housing of nineteenth-century Europe and North America. The concrete masonry unit emerged at the turn of the twentieth century with Harmon Palmer's 1900 U.S. patent for a hollow-block machine, with the eight-inch nominal block standardised by the 1940s. Reinforced masonry was codified in parallel and is now the dominant structural form in seismic regions.

Brick: Composition, Types, and Specifications

Building brick is classified by raw material and by intended exposure. Fired-clay brick — the dominant form — is manufactured from clay and shale extruded or moulded to shape and fired at approximately 900–1,200 °C. Calcium silicate (sand-lime) brick is autoclaved from sand and hydrated lime to produce a uniform off-white unit of comparable strength. Adobe and mud brick remain in regional vernacular use but fall outside structural masonry codes.

Three principal ASTM specifications govern building brick in the United States:

Standard United States modular brick has nominal dimensions of 8 × 4 × 2⅔ in. (203 × 102 × 68 mm), with actual unit dimensions of 7⅝ × 3⅝ × 2¼ in. once a ⅜-in. mortar joint allowance is removed. Compressive strength requirements scale with exposure: grade SW requires a minimum 3,000 psi average to resist freeze-thaw cycling under saturated conditions, while grade NW is permitted to 1,500 psi for interior or sheltered applications.[3] Cold-water absorption and the saturation coefficient are tightened with increasing exposure severity, since brick durability under freeze-thaw is governed by capillary structure rather than strength alone. The Brick Industry Association's Technical Notes on Brick Construction — TN 9A on classification and TN 10 on dimensioning — serve as the industry interpretation of the underlying ASTM provisions.[4]

Concrete Masonry Units

Concrete masonry units are hollow or solid prefabricated blocks manufactured from a low-slump concrete mixture compacted into steel moulds and cured under controlled humidity. ASTM C90 governs loadbearing units, C129 nonloadbearing units, and C55 concrete brick of CMU manufacture.[5]

The standard "eight-inch block" has nominal dimensions of 8 × 8 × 16 in. and actual dimensions of 7⅝ × 7⅝ × 15⅝ in., the difference accommodating a ⅜-in. mortar joint. ASTM C90 classifies units by oven-dry density into three categories: lightweight (≤ 105 pcf), medium-weight (105–125 pcf), and normal-weight (> 125 pcf). Lightweight units are typically produced with expanded-shale, clay, or slate aggregate in place of natural sand and gravel.

Minimum net-area compressive strength under ASTM C90 is 2,000 psi for individual units. The assembly design strength, f'm, is determined under TMS 402 either from tabulated values keyed to unit and mortar combination or by prism testing under ASTM C1314, with typical f'm values for residential and light-commercial CMU walls falling between 1,900 and 4,000 psi. For reinforced construction, open cells receive coarse grout conforming to ASTM C476 around vertical and horizontal reinforcement.

Mortar: Types, Standards, and Historical Context

Mortar for unit masonry is specified in the United States under ASTM C270, which defines five types by compressive strength under either a property specification (laboratory-confirmed minimum strength) or a proportion specification (volumetric mix ratios):[6]

The five C270 types are formulated from three commercial cement options: portland-lime mortar (proportioned Portland cement, hydrated lime, and sand); masonry cement (ASTM C91, a blend of portland-cement clinker, finely-ground filler, and air-entraining agents); and mortar cement (ASTM C1329, similar to masonry cement but with tightened bond and air-content limits for high-seismic applications).

Lime mortars predate portland-cement mortars by approximately two millennia and remain in use for historic restoration and where vapour permeability is required. Natural hydraulic lime (NHL) mortars are classified under EN 459-1 as NHL 2, NHL 3.5, and NHL 5 by minimum 28-day strength.[7] Pozzolanic mortars — lime combined with a reactive aluminosilicate such as volcanic ash, calcined clay, or pulverised brick — have re-emerged as a lower-carbon alternative where early-age strength is not critical. In-service performance is governed more by bond strength to the unit than by compressive strength alone, as reflected in ASTM C1072 and C952.

Structural Design, Reinforcement, and Modern Developments

The principal United States design code for masonry is TMS 402/602-22 Building Code Requirements and Specification for Masonry Structures, adopted by reference in Chapter 21 of the International Building Code.[8] It succeeds the Masonry Standards Joint Committee (MSJC) document formerly issued jointly as ACI 530 / ACI 530.1 / ASCE 5 / ASCE 6, with transition to sole TMS sponsorship completed in the 2013 cycle.

Reinforced masonry is distinguished from unreinforced (or plain) masonry by deformed steel reinforcement embedded in grouted cells or bed joints to resist tensile stresses. In a hollow CMU wall, vertical reinforcement is placed in cells subsequently filled with coarse grout, with bond beams at wall plates and openings. TMS 402 specifies cover requirements protecting reinforcement from carbonation and chloride ingress. Allowable stress design and strength design provisions appear in parallel within the code, with strength design favoured for seismic applications.

European masonry design follows Eurocode 6 (EN 1996), structured around limit-state design and partial-factor analysis. EN 1996-1-1 covers general rules; EN 1996-2 addresses execution; EN 1996-3 provides simplified methods for low-rise unreinforced masonry.[9]

Recent developments include autoclaved aerated concrete (AAC), a closed-cell precast lightweight material with density of approximately 25–50 pcf governed by ASTM C1693 and TMS 402 Chapter 9; fibre-reinforced masonry; and prefabricated panelised assemblies. The durability of any masonry assembly remains governed by the combined performance of unit, mortar, and detailing — flashing, weep design, and movement joints — rather than by any single material property.

References

1. Vitruvius. De Architectura. Book II, Chapter 3 (brick) and Chapter 5 (lime). c. 30 BCE.
2. Brandon, C. J., Hohlfelder, R. L., Jackson, M. D., and Oleson, J. P. Building for Eternity: The History and Technology of Roman Concrete Engineering in the Sea. Oxford: Oxbow Books, 2014.
3. ASTM International. ASTM C62-17(2023): Standard Specification for Building Brick (Solid Masonry Units Made From Clay or Shale). West Conshohocken, PA.
4. Brick Industry Association. Technical Notes on Brick Construction — TN 9A Specifications for and Classification of Brick and TN 10 Dimensioning and Estimating Brick Masonry. Reston, VA.
5. ASTM International. ASTM C90-22a: Standard Specification for Loadbearing Concrete Masonry Units. West Conshohocken, PA, 2022.
6. ASTM International. ASTM C270-19ae1: Standard Specification for Mortar for Unit Masonry. West Conshohocken, PA, 2019.
7. European Committee for Standardization. EN 459-1:2015: Building lime — Part 1: Definitions, specifications and conformity criteria. Brussels, 2015.
8. The Masonry Society. TMS 402/602-22: Building Code Requirements and Specification for Masonry Structures. Longmont, CO, 2022. Adopted by reference in International Code Council, 2021 International Building Code, Chapter 21: Masonry.
9. European Committee for Standardization. EN 1996-1-1:2005+A1:2012: Eurocode 6 — Design of masonry structures — Part 1-1: General rules for reinforced and unreinforced masonry structures. Brussels.

Related slabcalc resources

• Mortar (glossary)
• CMU block wall calculator