Masonry materials and units

The second half of the course turns to masonry, the oldest structural material still in everyday use. The pump house above the lift station and the wash bay at the end of the garage are both concrete block buildings, chosen because block is durable, resists impact from vehicles and equipment, needs no fire protection and is built by local crews quickly. Their design follows CSA S304, Design of masonry structures, with materials and construction governed by CSA A165 for units, A179 for mortar and grout, A371 for construction and A370 for connectors. This lesson covers the materials and how the specified compressive strength of masonry, \(f'_m\), is established.

Masonry units

Concrete block, or concrete masonry units (CMU), is made of a dry mix of cement, aggregate and water compacted in moulds and cured. The standard unit in Canada is called 20 cm block and measures 390 mm long, 190 mm high and 190 mm thick, so that with 10 mm mortar joints it occupies a 400 mm by 200 mm module. Hollow units have two or three cores, with face shells about 32 mm thick and webs between them. Units are specified by compressive strength on the net area, commonly 15 or 20 MPa, and by density.

Clay brick is made of shale or clay fired in a kiln. It is used mostly as veneer, and its durability in a cold climate depends on its absorption and its resistance to freezing and thawing, classified in CSA A82 by grade. Clay brick expands slowly and permanently after firing as it absorbs moisture from the air, while concrete block shrinks as it dries: the two materials move in opposite directions, which matters when they are built together.

Mortar

Mortar bonds the units together, seals the joints against water and accommodates small differences in unit size. It is a mix of cement, lime or masonry cement, sand and water. Four types are in common use, named from the letters of MaSoN wOrK: Type M is the strongest, Type S is the usual choice for structural and below-grade masonry, Type N is a general-purpose mortar for above-grade walls and veneer, and Type O is a weak mortar for interior non-loadbearing work.

Stronger is not always better. A strong, stiff mortar makes a wall that cracks through the units when it moves, while a weaker, more flexible mortar lets the wall crack along the joints, where cracks are easy to repoint. Mortar should be just strong enough for the job. Lime improves workability and water retention, which helps the bond between mortar and unit, and bond, not compressive strength, governs the water tightness and the flexural tension strength of the wall. The pump house and wash bay use Type S portland cement-lime mortar because they are reinforced, loadbearing walls exposed to splash and de-icing salt.

Grout and reinforcement

Grout is a fluid concrete mix that fills the cores of hollow units around reinforcing bars. It is poured or pumped into the cells after the units are laid, in lifts with consolidation, and it bonds the bars to the masonry so the wall acts as reinforced masonry. Fine grout has sand only; coarse grout includes pea gravel and is used in larger spaces. Grout must be fluid enough to fill every void, so it has a slump of 200 to 280 mm (8 to 11 in.); the absorbent units draw out excess water, leaving a strong grout behind.

Vertical reinforcement is placed in grouted cores, typically 10M or 15M bars, centred in the wall or at one face. Horizontal reinforcement goes in bond beams, special units with a channel cut out of the webs and filled with grout, or as joint reinforcement, ladder or truss-shaped wire laid in the bed joints. A wall can be fully grouted, with every core filled, or partially grouted, with only the reinforced cores filled. Partially grouted walls are lighter and cheaper and are common in low-rise buildings; fully grouted walls are stronger and better at resisting water and impact.

Section properties of hollow and grouted walls

Hollow block walls are normally laid with face-shell bedding: mortar goes on the two face shells only, not on the webs. The effective area that carries load is therefore the two face shells plus any grouted cores, and section properties for bending normal to the wall are those of two thin plates separated by the core. A partially grouted wall is a series of T-sections, each grouted core acting with the face shells on either side.

# Lesson 7: pump house masonry, 190 mm concrete block (390 x 190 x 190), properties and f'm
import statistics as st
t, fs_t, L_unit = 190.0, 32.0, 400.0       # thickness, face shell, unit length incl. joint (mm)

# Areas per metre of wall (face-shell bedding for hollow units)
A_hollow = 2 * fs_t * 1000                  # mm2/m, mortar on face shells only
A_solid = t * 1000                          # fully grouted
cells_per_m = 1000 / (L_unit / 2)           # two cells per unit
for name, grout_every in [("hollow", None), ("grouted at 800 mm", 800), ("grouted at 400 mm", 400), ("fully grouted", 200)]:
    if grout_every is None:
        A = A_hollow
    elif grout_every == 200:
        A = A_solid
    else:
        share = 200 / grout_every                    # fraction of cells grouted
        A = A_hollow + share * (t - 2 * fs_t) * 1000 * 0.9   # webs take about 10 % of the core width
    print(f"{name:18s}: effective area {A/1e3:6.0f} x 10^3 mm2/m")

# Wall self-weight (normal-weight units), kPa of wall face
w = {"hollow": 2.1, "grouted at 800 mm": 2.6, "grouted at 400 mm": 3.0, "fully grouted": 4.0}
print("Self-weight (kPa):", ", ".join(f"{k} {v}" for k, v in w.items()))

# Prism tests: five grouted prisms, 390 mm high (two units plus one joint), t = 190 mm
loads_kN = [890, 950, 855, 930, 910]
A_net = 390 * 190
h_t = 390 / 190
corr = 1.0 if h_t >= 2.0 else 0.75 + (h_t - 1.3) / (2.0 - 1.3) * 0.25     # approximate correction
f = [P * 1e3 / A_net * corr for P in loads_kN]
mean, sd = st.mean(f), st.stdev(f)
print(f"\nPrism h/t = {h_t:.2f}, correction {corr:.2f}")
print(f"Strengths: {', '.join(f'{x:.1f}' for x in f)} MPa")
print(f"Mean {mean:.1f} MPa, standard deviation {sd:.2f}, coefficient of variation {sd/mean*100:.1f} %")
print(f"Mean less 1.64 standard deviations: {mean - 1.64*sd:.1f} MPa (compare with the 10 MPa assumed in design)")

hollow            : effective area     64 x 10^3 mm2/m
grouted at 800 mm : effective area     92 x 10^3 mm2/m
grouted at 400 mm : effective area    121 x 10^3 mm2/m
fully grouted     : effective area    190 x 10^3 mm2/m
Self-weight (kPa): hollow 2.1, grouted at 800 mm 2.6, grouted at 400 mm 3.0, fully grouted 4.0

Prism h/t = 2.05, correction 1.00
Strengths: 12.0, 12.8, 11.5, 12.6, 12.3 MPa
Mean 12.2 MPa, standard deviation 0.49, coefficient of variation 4.0 %
Mean less 1.64 standard deviations: 11.4 MPa (compare with the 10 MPa assumed in design)

The hollow wall has an effective area of only 64 000 mm² per metre, a third of the gross section, while a fully grouted wall has the full 190 000 mm². Weight follows the same pattern. The pump house walls, grouted at 800 mm, weigh about 2.6 kPa of wall area.

The specified compressive strength of masonry

Masonry is a composite of units, mortar and, in reinforced work, grout. Its compressive strength is much lower than the strength of the units on their own, because the softer mortar in the joints tries to expand sideways under load and splits the units in tension. Strength depends on the unit strength, the mortar type, the grouting and the workmanship. Designers specify \(f'_m\), and the contractor shows that the masonry as built reaches it.

S304 offers two routes. The first uses tables that give \(f'_m\) from the unit strength, mortar type and whether the wall is hollow or grouted, with the hollow and grouted values given separately because grouted masonry fails differently. The second is prism testing: small assemblies, typically two units high, built with the same units, mortar and grout as the job, cured and crushed. The prism strengths are corrected for their height-to-thickness ratio, because squat prisms appear stronger, and \(f'_m\) is derived from the test results with an allowance for their scatter. For the pump house, the team specifies 20 MPa units and Type S mortar and designs for \(f'_m = 10\) MPa, which the tables support for grouted construction.

Modulus of elasticity and other properties

The modulus of elasticity of masonry is taken in S304 as \(E_m = 850f'_m\), or 8500 MPa for the pump house, well below that of concrete. Masonry creeps under sustained load and, for concrete block, shrinks as it dries. Grout and units shrink together, but bond beams, lintels and walls restrained by footings crack if movement joints are not provided; lesson 11 deals with them.

In flexure, unreinforced masonry relies on the tensile bond between mortar and units. Tension normal to the bed joints, as when a wall spans vertically between floor and roof, is especially weak, because it depends only on the bond across the horizontal joints. Tension parallel to the bed joints, when a wall spans horizontally between columns or cross walls, is stronger, because the units are interlocked in running bond. The standards give these flexural tensile strengths, which are small and variable, and unreinforced masonry is therefore used only where the loads are light.

Choosing the wall system for the pump house

Priya compares three options for the pump house walls. A hollow, unreinforced wall is the cheapest but cannot resist the wind on a 5 m wall, as lesson 8 will show. A partially grouted wall with 15M bars at 800 mm is the usual answer for a small loadbearing building and is the base case. A fully grouted wall would be stronger, more resistant to water and to a vehicle striking it, and better at containing a chemical spill, but heavier and costlier. Because the pump house sits beside a driveway used by plows and the city's maintenance trucks, the team adds a fully grouted course at the bottom and steel bollards at the door, and keeps the rest partially grouted.

Inside, the pump house is humid, and hydrogen sulphide from the wet well can attack cement paste. The interior face will receive a breathable coating and the block will be specified with a low absorption. Outside, splash from plowed snow carries de-icing salt onto the bottom of the wall, so the first courses use units of higher strength and lower absorption, and the flashing and weep holes at the base are kept clear of grade. These choices are about durability rather than strength, but they decide how long the building lasts.

Construction and quality

Masonry is built by hand, and its quality depends on the mason. Units must be laid in full face-shell joints, with head joints filled, cores must be clean of mortar droppings before grouting, bars must be held in position, and grout lifts must be consolidated. In cold weather, mortar and grout must be protected from freezing until they gain strength, which in Thunder Bay and Duluth means heated enclosures for much of the year. Inspection and testing, more or less extensive depending on the importance of the structure, are part of the design.

S304 and A371 set out the requirements for construction, and the designer's field reviews confirm that the walls are built as designed. The pump house is a critical facility for the city's wastewater system, so the team will review the reinforcement before each grout lift.