Civil Engineering Reference
In-Depth Information
(Class A, >125 N/mm², <4.5%Abs and Class B, >75 N/mm²,
<7%Abs) (Thomas, 1996).
The design assumptions should ideally be stated within a
specification, with reference to manufacturers' specific product
details for a unit if known. A specification may include:
achieve compressive strengths of up to 10 N/mm
2
. The low
density means that they are often used for internal partitions or
the inner leaf of cavity walls.
20.2.3.4 Manufactured stonework
Manufactured stonework is formed in a similar way to aggre-
gate concrete blockwork; yet units resemble natural stone.
Manufactured stonework is defined in BS EN771-5. This
code gives testing procedures along with minimum compressive
strength values, with the onus on the manufacturer providing the
stone properties. The code suggests a minimum characteristic
compressive strength of 17.5 N/mm
2
for a homogeneous unit.
Compressive strength
■
Tolerance range
■
Durability classification■■
■
Active soluble salts classification■■
■
Water absorption.
■
20.2.3.5 Natural stonework
Natural stone is used widely for masonry construction. Natural
stone buildings often reflect the stone which available within
the local geology. The specification of natural stone is covered
within BS EN771-6.
For guidance some approximate lower bound values of stone
compressive strengths are given in
Table 20.1
, along with
some guidance on the specifics of the type of stone (IStructE,
1996; Thomas, 1996).
20.2.3.2 Aggregate concrete blocks
Concrete blocks are typically 100 mm wide × 440 mm long
and 215 mm high, although various widths and lengths of
block are available.
Concrete blockwork is commonly used as the internal leaf of
masonry buildings or as internal partitions. Given the reduced
number of mortar joints, due to the larger unit size, blockwork
walling tends to give greater compressive strength when com-
pared to brickwork (Curtin
et al
., 1999).
Aggregate concrete blocks are manufactured in a range com-
pressive strengths; typically 3.5 N, 7 N, 10 N, 15 N and 20 N.
Similar to brickwork, the assumptions made in the design
must be stated in a specification. For blockwork the specifica-
tion may include:
20.2.4 Mortar types (clause 3.2)
There are several different types of mortar used for masonry
construction; these are normally batched proportions of cement,
lime, aggregates (sands) and sometimes additives (plasticisers,
pigments, etc.).
Traditional four mixes were presented within BS 5628 (and
subsequently the National Annex to BS EN1996), based on pro-
portions of the constituent materials to gain specific compres-
sive strengths. These four mixes gave compressive strengths
which were inversely proportional to the ability to tolerate
movements; i.e. stiffer stronger mortars are less tolerant to
movements and contained greater proportions of cement.
BS EN998-2 gives guidance on the specification of mortar.
The new guidance gives the manufacturers of designed mor-
tars a greater range of mortar strengths, as indicated in
Table
20.2
. Mortars to BS EN998-2 are defined as 'M' followed by
the compressive strength of the mortar in N/mm².
Compressive strength
■
Allowable density
■
Thermal conductivity
■
Freeze/thaw resistance.
■
20.2.3.3 Aerated concrete blocks
Aerated concrete blocks are similar to aggregate concrete
blocks, yet are formed using fine sand, aluminium powder and
pulverised fuel ash (Thomas, 1996).
The units are lightweight meaning that they are easily han-
dled, even with thickness in excess of 215 mm, yet can still
Unit type
Typical compressive
strength (N/mm
2-
)
Water Absorption (%)
Comments
Sandstone
25-40
2.4-10
Can delaminate, sandstones should be laid with the bed face
perpendicular to the exposed surface. Sandstone is porous and
can stain
Limestone
16-35
1-15
Similar to sandstones, yet typically harder
Granite
140-170
0.19-0.30
Impermeable and well suited to the UK environment, Very
good abrasion resistance
Based on lower bound values of natural stone.
Table 20.1
Natural stonework
