Block & Brick

What Block and Brick Are

Block and brick are modular masonry units: discrete, mass-produced elements bonded with mortar to form walls, piers, foundations, and partitions. They are the oldest manufactured building products still in mainstream use, and they remain the dominant wall system in much of the world because they combine structural capacity, fire resistance, thermal mass, acoustic isolation, and durability in a single component that an unskilled crew can install with simple tools. A masonry wall is an assembly of three parts: the units, the mortar that beds and bonds them, and, where reinforcement such as deformed rebar is needed, grout poured into hollow cores or cavities.

The word "brick" conventionally denotes a small unit sized for one-handed laying. A standard modular clay brick has a face roughly 194 by 57 mm (about 8 by 2.25 inches) with a 90 mm bed depth, and weighs around 2 to 3 kg. The word "block" denotes a larger molded unit. The reference concrete masonry unit has a nominal size of 200 by 200 by 400 mm (8 by 8 by 16 inches); its actual size is 190 by 190 by 390 mm, the 10 mm shortfall leaving room for a standard mortar joint so the nominal module stays on a clean grid. The size distinction is practical, not legal: standards bodies separate the families by document and material rather than by a single cutoff dimension.

Fired clay brick and molded concrete or aerated block are produced by fundamentally different routes. Clay brick is shaped from clay or shale, dried, and fired in a kiln at roughly 900 to 1,200 degrees Celsius, where the clay vitrifies into a hard, weather-resistant ceramic. Concrete masonry units are cast from a dry, no-slump mix of hydraulic cement, aggregate, and water, vibrated and compacted in a block machine, then cured in air or steam. AAC is cast from a slurry of cement, lime, fine silica sand, and an aluminum powder gassing agent, which liberates hydrogen and aerates the mass before it is wire-cut and cured in a high-pressure steam autoclave. These three processes produce units with very different density, strength, and thermal behavior, which is why selection always begins with the unit family.

The market scale is enormous and regionally fragmented. Clay brick is traded over long distances because its value-to-weight ratio justifies freight, so national brands exist. Concrete block is rarely shipped far: a dense CMU is mostly aggregate and water, and freight quickly exceeds the unit value, so the CMU supply chain is local, organized around hundreds of regional plants serving a roughly 200 to 300 km radius. AAC, lighter per unit volume, ships somewhat farther but still favors regional production. Any global selection exercise therefore separates the specification (what the unit must achieve) from sourcing (which local plant can supply a compliant product).

Four engineering properties dominate masonry unit selection: compressive strength, density, durability against weathering, and thermal conductivity. These are partly in tension. Raising density and strength lowers thermal performance; lowering density for insulation sacrifices load capacity. The art of masonry specification is matching the unit family and grade to the dominant demand on the wall, whether that is axial load, fire separation, thermal envelope, or exposed appearance, rather than chasing a single peak number.

Unit Families and Classification

Masonry units divide into four mainstream families by material and manufacturing route: fired clay brick, concrete masonry units, autoclaved aerated concrete, and calcium silicate (sand-lime) units. Each family carries its own governing standard, its own typical strength and density envelope, and its own best-fit application. Choosing the wrong family is the costliest beginner error, because no single grade adjustment can recover, for example, the insulation an AAC wall offers or the axial capacity a dense CMU offers. The table below sets out the core differences.

Fired clay brick is the appearance and durability leader. Vitrified clay resists freeze-thaw, abrasion, and chemical attack better than most concrete units, which is why facing brick dominates exposed exterior veneers and why hard-fired engineering brick is used at grade and in paving. Clay brick subdivides by use: building brick (ASTM C62) where appearance does not matter, facing brick (ASTM C216) where it does, and hollow brick (ASTM C652) with larger void areas. Clay units are further split by void: solid units have 25 percent or less coring, cored brick a bit more, and hollow brick up to 60 to 75 percent void.

Concrete masonry units are the structural workhorse. A standard hollow CMU carries axial load efficiently, accepts grout and reinforcement in its cores, and is fast to lay because of its size. ASTM C90 sorts CMU into three density classes: lightweight (less than 105 lb/ft3, about 1,680 kg/m3), medium weight (105 to 125 lb/ft3), and normal weight (more than 125 lb/ft3, about 2,000 kg/m3). Density class is independent of strength: a lightweight high-strength mix can outperform a normal-weight regular mix, so the two properties are specified separately. Lightweight CMU is preferred for its better fire rating per inch and lower wall dead load.

Autoclaved aerated concrete is the insulation specialist. Its closed pore structure gives it roughly one tenth the density and one tenth the thermal conductivity of dense concrete, so an AAC block wall can be both structure and insulation in one layer. ASTM C1693 sorts AAC into strength classes AAC-2, AAC-4, and AAC-6. AAC saws and routes like soft wood, accepts thin-bed adhesive mortar joints of 2 to 3 mm instead of 10 mm, and is non-combustible. Its limits are low compression strength, the need to protect the exposed surface from rain, and lower point-load fixing capacity than solid masonry. The same autoclaved aerated material is also produced as the reinforced ALC panel for floors and partitions.

Calcium silicate units, made from sand and lime cured under steam pressure, sit between clay and concrete. They offer high dimensional accuracy, a clean fair-faced finish, good acoustic mass, and consistent color, and are common in Northern Europe. They are less freeze-thaw durable in saturated ground contact than hard-fired clay, so exposure detailing matters. Across all four families, the EN 771 series provides the harmonized European specifications and the CE-marked Declaration of Performance that European projects require.

Grades, Types, and Strength Classes

Within each family, units are sorted by grade or strength class, which is the number that actually governs whether a unit suits a given wall. Confusing grade (durability) with type (appearance tolerance) or with strength class (load capacity) is a frequent specification error. The two clay-brick parameters are independent: grade describes weathering resistance, type describes dimensional and finish tolerance. The table below decodes the key clay-brick and AAC designations.

Clay-brick grades classify resistance to freeze-thaw weathering, the dominant durability threat for fired brick. Grade SW (severe weathering) is mandatory where brick is in contact with the ground or is likely to be saturated when freezing occurs, such as paving, retaining walls, and sills. It requires an average compressive strength of 3,000 psi, maximum 5-hour boil water absorption of 17 percent, and a saturation coefficient of 0.78 or less. Grade MW (moderate weathering) suits vertical above-grade exposure in freezing climates and requires 2,500 psi, 22 percent absorption, and a 0.88 coefficient. Grade NW (negligible weathering) is interior or protected-backup only, requiring 1,500 psi with no absorption limit. The weathering index, computed from annual freeze cycles and winter rainfall, tells the designer which grade a location demands.

The saturation coefficient deserves attention because it, not strength alone, predicts freeze durability. It is the ratio of water absorbed in 24 hours of cold immersion to that absorbed in 5 hours of boiling, and it estimates how much of the pore volume stays air-filled. A low coefficient means reserve pore space for ice to expand into without spalling the face. A brick can be strong yet fail freeze-thaw if its saturation coefficient is high, which is why ASTM C62 caps both properties.

Clay-brick types under ASTM C216 facing brick classify dimensional and finish tolerance, not strength or durability. Type FBS (face brick standard) is general use with normal tolerances. Type FBX (extra) carries tighter size and warpage tolerances for crisp, uniform coursing such as commercial fair-face work. Type FBA (architectural) deliberately allows wider variation in size, color, and texture to produce a rustic or handmade effect. Hollow brick under ASTM C652 uses the parallel designations HBS, HBX, and HBA. Choosing FBX where FBS suffices simply raises cost without structural benefit.

AAC strength classes under ASTM C1693 are labeled by their minimum compressive strength in megapascals: AAC-2, AAC-4, and AAC-6 correspond to roughly 2.0, 4.0, and 6.0 MPa (290, 580, and 870 psi). Each class also pairs with a nominal dry density band, since strength and density rise together in aerated concrete. AAC-2 is the lightest and most insulating, suiting non-loadbearing partitions; AAC-4 is the common loadbearing residential choice; AAC-6 carries the heaviest loads AAC is asked to take. Even AAC-6 is far below the 13.8 MPa net minimum of a structural CMU, which is the trade-off for AAC very low weight and high insulation.

Concrete masonry units are not graded for weathering the way clay is; their key sorts are net-area compressive strength and density class. ASTM C90 sets a single strength floor (2,000 psi net since 2014) and then lets the purchaser specify higher strength, density class, and any special features such as architectural finish, integral water repellent, or sound-absorbing profile separately. A specifier ordering a structural CMU wall therefore states both the required net strength (for example f'm or unit strength) and the density class, because the two drive load capacity and fire rating respectively.

Materials, Mortar, and Standards

A masonry wall is only as good as the weakest of its three materials: the unit, the mortar, and, where present, the grout. Specifying a strong unit and then bonding it with the wrong mortar is a common mistake that produces cracked units, water penetration, or poor bond. Mortar is governed in North America by ASTM C270, which defines four types by descending compressive strength, and the governing design principle is counterintuitive to beginners: the mortar should be weaker than the unit, not stronger.

The reason is movement accommodation. Mortar joints are sacrificial. When a wall expands, contracts, or settles slightly, a softer mortar yields and cracks along the bed joint, where the crack is fine, harmless, and easily repointed. An over-strong mortar refuses to yield, so the accumulated stress instead fractures the units themselves, which cannot be repointed. The specifier therefore chooses the weakest mortar that still meets the structural and exposure demand. The table below summarizes the four ASTM C270 mortar types and their fit.

Type N is the default for most above-grade exterior and interior walls, balancing bond strength, workability, and movement tolerance. Type S steps up for at-grade and below-grade walls, retaining walls, and reinforced masonry that must resist higher flexural and lateral loads. Type M is reserved for the most severe load and deep foundation conditions, but its hardness limits it where movement accommodation matters. Type O is a low-strength mortar for interior non-load partitions and for repointing soft, hand-made historic brick, where a hard modern mortar would spall the original units. Each type can be batched to the proportion specification or the property specification, but never both at once for one project.

Grout, governed by ASTM C476, is a fluid concrete, supplied much like ready-mix concrete, poured into CMU cores or cavity walls to bond reinforcing bars and add mass. It is distinct from mortar: grout is high-slump so it flows around the steel, and it is intended to be as strong as or stronger than the surrounding masonry. Reinforced loadbearing masonry, common in seismic regions, relies on grouted cells acting compositely with the unit.

The umbrella design and construction standards tie the materials together. In North America, the TMS 402/602 Building Code Requirements and Specification for Masonry Structures (formerly ACI 530 / ASCE 5) is the design code that sets allowable stresses, reinforcement rules, and the f'm prism strength that links unit and mortar strength to wall capacity. In Europe, Eurocode 6 (EN 1996) plays the equivalent role, with the EN 771 unit standards and EN 998 mortar standards feeding it. The table-driven fire ratings come from NCMA TEK 07-01D in North America, based on the ASTM E119 fire test, which expresses rating by equivalent thickness and aggregate type rather than by nominal unit size.

Wetted-surface and exposure material choices also matter. Clay brick used in chimneys or flues needs higher fire and acid resistance; brick in paving needs Grade SW plus abrasion resistance; CMU exposed to de-icing salts or marine spray needs an integral water repellent admixture and often a higher-strength, lower-absorption mix. For AAC, the soft surface must be protected by render, cladding, or a specified finish coat because bare AAC absorbs and wicks water rapidly.

Key Specification Parameters

A masonry unit datasheet or test report lists many properties, but only a handful drive the selection. The recurring trap is comparing two units on a single number, such as compressive strength, while ignoring the property that actually governs the wall, such as fire rating or absorption. The parameters below are the ones a procurement or design engineer should read off every datasheet and Declaration of Performance.

Compressive strength is the headline number, but how it is measured matters as much as its value. ASTM C90 quotes CMU strength on the net area, the actual mortar-bearing cross-section after the hollow cores are subtracted, so a 2,000 psi net block carries that stress only on its solid webs and face shells. ASTM C62 quotes clay-brick strength on the gross area. Comparing a net-area block figure to a gross-area brick figure directly is invalid. The design value the structural code uses is f'm, the specified compressive strength of the assembled masonry, which is derived from unit strength plus mortar type through the code tables, not from the unit alone.

Density and density class determine wall dead load, fire performance, thermal mass, and handling weight. ASTM C90 names three CMU classes: lightweight below 105 lb/ft3, medium weight 105 to 125 lb/ft3, and normal weight above 125 lb/ft3. Lower density usually means better fire rating per inch and lower thermal conductivity, at some cost in strength and sound mass. AAC pushes density far lower (300 to 800 kg/m3) which is precisely what gives it its insulation, while clay brick clusters around 1,600 to 2,000 kg/m3.

Water absorption and the saturation coefficient predict durability and rain resistance. ASTM C62 limits clay-brick 5-hour boil absorption (17 percent for Grade SW) and saturation coefficient (0.78 for SW). The initial rate of absorption, or suction, also affects mortar bond: a very high-suction brick pulls water out of the mortar before it can cure, so such brick is wetted before laying. For CMU, low absorption plus an integral water repellent reduces efflorescence and freeze-thaw damage in exposed walls.

Thermal conductivity is decisive when the wall is part of the insulating envelope. Dense concrete conducts heat at roughly 1.0 to 1.7 W/m.K, clay brick around 0.5 to 1.0, lightweight CMU lower, and AAC at roughly 0.10 to 0.16 W/m.K, an order of magnitude better. That is why AAC can serve as a single-layer structural-and-insulating wall in climates where a dense block wall would need a separate insulation layer to meet energy code.

Fire resistance rating is expressed by equivalent thickness, the solid thickness a hollow unit would have if recast as a solid slab of the same material. The NCMA fire tables give the equivalent thickness needed for 1, 2, 3, and 4-hour ratings by aggregate type. The remaining datasheet parameters that round out a comparison include:

• Dimensional tolerance: ASTM C90 permits plus or minus 1/8 inch (3 mm) on any face; tight-tolerance clay (Type FBX) is closer. Tolerance drives joint consistency and labor speed.
• Void area and configuration: hollow CMU is typically 50 to 55 percent solid; void area sets grout demand, weight, and which cells take reinforcement.
• Sound transmission class (STC): rises with wall mass; a grouted CMU wall reaches STC 50 or more for party-wall acoustic separation.
• Moisture movement and shrinkage: concrete units shrink as they dry and clay units expand with moisture over time, so control-joint spacing differs between the two materials.
• Initial rate of absorption (IRA): brick suction in grams per minute over 30 square inches, governing whether the brick must be wetted before laying for proper mortar bond.

Reaction to fire and combustibility are a class apart: all fired clay, concrete, AAC, and calcium silicate units are non-combustible and classified Euroclass A1 under EN 13501-1, which is why masonry is the default for compartment and party walls. Reinforced and grouted assemblies also contribute to the structural fire rating, not just the barrier rating.

Selection Decision Factors

To turn the preceding five chapters into a specific product order, follow the decision sequence below. Most selection failures come not from one wrong number but from deciding the wrong thing first, such as fixing a unit family before the wall function is clear. These eight steps can serve as a fixed specification and RFQ template for any masonry wall.

1. Wall function and load: Decide first whether the wall is loadbearing, a non-load partition, a facing veneer, or an insulating envelope. Loadbearing favors CMU or hard-fired brick; insulating envelope favors AAC; appearance favors clay facing brick. The function fixes the family before any number is chosen.
2. Compressive strength and f'm: Derive the required masonry assembly strength (f'm) from the structural design, then back out the unit net-area strength plus mortar type using the TMS 402 tables. State whether the strength is net (CMU) or gross (clay) so suppliers quote the right basis.
3. Exposure and durability grade: For clay, select Grade SW, MW, or NW from the weathering index and ground-contact condition. For CMU and AAC in wet or freeze exposure, specify low absorption, integral water repellent, and a protective finish. Saturation coefficient, not strength alone, governs freeze durability.
4. Density class and thermal target: Choose lightweight, medium, or normal-weight CMU, or an AAC class, against the energy-code thermal target and the allowable wall dead load. Lower density buys fire rating and insulation at some cost in strength and sound mass.
5. Fire rating: Read the required hourly rating from the building code occupancy and separation requirements, then size the unit by equivalent thickness using the NCMA or local fire tables for the chosen aggregate. Confirm the rating is for the actual assembly, not a bare unit.
6. Mortar and grout system: Select the weakest ASTM C270 mortar type (M, S, N, or O) that meets the load and exposure, keeping mortar weaker than the unit. Specify ASTM C476 grout and reinforcement for any loadbearing or seismic-reinforced wall.
7. Dimensional system and finish: Fix nominal size and module (200 mm CMU, 194 mm brick), tolerance type (FBS or FBX for clay), and any architectural finish, color, or texture. Tighter tolerance and special finish raise cost, so specify them only where they earn their place.
8. Sourcing and total installed cost: Because dense block ships poorly, confirm a compliant local plant within economic freight range, then compare total installed cost: unit price plus mortar, labor laying rate (block lays faster than brick per square meter), waste, and any render or finish. The cheapest unit rarely wins once labor and finishing are counted.

One last, commonly overlooked dimension is documentation and conformity: a current manufacturer technical datasheet, the test report for the specific product line, and, for European projects, the CE-marked Declaration of Performance under the relevant EN 771 part. Grade and strength class vary between a maker different product lines, so a brand name alone is never a specification. Pull the datasheet, confirm the standard designation and the exact grade or strength class, and verify the plant has current third-party certification before the order is placed. Acme Brick, General Shale, Glen-Gery, Ibstock, Wienerberger, Forterra, and Oldcastle for fired and concrete units, and Xella (Hebel, Ytong), AERCON, and Solbet for AAC, all publish the datasheets and conformity documents a defensible specification requires.