This article is concerned with how masonry is built, the architectural forms used and the resulting appearance. The basic structural form of many types of masonry is expressed on the surface of buildings and other structures and can be a very attractive and reassuring aspect of these. Appearance is a synthesis of the size, shape and colour of the units, the bond pattern, the mortar colour and finish, the masonry elements – walls, piers, columns, corbels, arches, etc. – and the scale and proportion of the whole structure.
Masonry Construction and Forms
Other key aspects are the workmanship, accuracy, the detailing in relation to other features and the use of specially shaped units. The basic method of construction has hardly changed for several thousand years: units are laid one on top of another in such a way that they form an interlocking mass in at least the two horizontal dimensions.
It is not practical to achieve interlocking in a third dimension with normal rectangular prismatic units, but a degree of such interlocking is sometimes used in ashlar stonework. Most practical masonry employs a mortar interlayer to allow for small to large inaccuracies of size between units and to make walls watertight, airtight and soundproof.
Walls and Other Masonry Forms
Walls are built by laying out a plan at foundation level and bringing the masonry up layer by layer. To maximise the strength and attractiveness it is important to make sure that all the foundation levels are horizontal, are accurate to the plan and allow multiples of whole units to fit most runs between returns, openings, etc. It is also essential to maintain the verticality (plumb), the level of bed joints and the straightness of the masonry within reasonable limits.
The thickness of the mortar joints must be kept constant within a small range, otherwise the masonry will look untidy. The standard technique used is to generate reference points by building the corners (quoins) accurately using a plumb bob and line, a builder’s level, a straight edge and a rule. Any openings are then filled with either a temporary or permanent frame placed accurately in the plan position. Lines are stretched between the reference points and the intervening runs of masonry are built up to the same levels. Columns, piers and chimneys are built in the same way but need plumbing in two directions and more care because of their small dimensions.
Some masonry built with precisely sized bricks, cut stone or terracotta ware is built to a higher accuracy, usually with mortar joints on the order of 3–5 mm thick. This is termed ‘gauged masonry’ and demands a higher standard of workmanship but can look very attractive. There is also a tradition for the use of pebbles, rubble and partly-squared stones combined with wider and more variable mortar joints, particularly in East Anglia, UK.
This can also look very pleasing, especially when combined with more accurate units at corners and openings. Arches and tunnels must follow a curved shape defined by the architect or engineer and are traditionally built on timber formwork. Adjustable reusable metal formwork systems are also available. Some arches use special tapered units called voussoirs but large radius or shallow arches may be built with standard units and tapered joints.
Figure 1 illustrates the main elements of arches and their associated vocabulary. Reinforced and post-tensioned masonry are used to a limited extent in the UK, mainly for civil engineering structures, high single-storey halls, retaining walls and lintels within walls. Masonry lintels may sometimes be constructed by laying special bed-joint reinforcement in the mortar. This acts as tensile reinforcement for a masonry beam.
Most other reinforced masonry is formed by building masonry boxes in the form of hollow piers, walls with cavities or walls with slots in them, and then locking the reinforcing elements into the voids using a concrete grout. Post-tensioned masonry may be built in the same way but the reinforcement is then passed through the cavities and stressed against end plates, which removes any necessity to fill with grout. Figure 2 shows some typical reinforced masonry forms.
Most modern masonry is the thickness of a single unit breadth and is built by overlapping half the length with the next unit. This is known as stretcher bond or half bond and is shown in Fig. 3. Variations of stretcher bond may be achieved by using third or quarter bond (also shown).
Soldier courses, where all the units stand on their ends, may be incorporated as a decorative feature but reduce the strength and robustness of the masonry. Much of this stretcher bonded work is used as cladding to frame structures where the strength is less important because of the presence of the supporting structure.
In occupied structures it is widely used in the form of the cavity wall, as illustrated in Fig. 4, which comprises two such walls joined with flexible metal ties across a space that serves principally to keep out rain and keep the inner wall dry. Blockwork is almost universally built with this bond, and broader units are used to achieve thicker walls.
Stretcherbonded walls may be built thicker by linking two or more layers with strong metal ties and filling the vertical ‘collar’ joint with mortar, as shown by Fig. 5. In thicker walls built in multiples of a single unit breadth there are a large number of possible two dimensional bonding patterns available, known by their traditional names. A few of the widely practised bonds are shown in Fig. 6.
More are given in BS 5628 Part 3(2005), BS EN 1996-1-2 (2006) and Handisyde and Haseltine (1980).
Use of Specials
It has always been possible to make structures more interesting by using specially shaped units to vary angles from 90° to generate tapers, plinths, curves, etc. In recent years such features have, if anything, become more popular. A very large range of shapes is available on a regular basis called ‘standard specials’.
Additionally, it is possible to get almost any shape manufactured to order, although it is inevitably quite expensive for small quantities. As an alternative, some specials can be made by gluing cut pieces of standard bricks with high performance adhesives.
It is often not realised how much the joint colour and shape influence the appearance and performance of masonry. Obviously the colour contrast between the mortar and the units must have a profound effect on the appearance but so does the shape of the finished joint. The common joint styles are shown in Fig. 7. Recessed and weathered joints cast shadows and increase the contrast between mortars and light-coloured bricks in most lighting conditions.
Workmanship and Accuracy
Standards of good workmanship in terms of how to lay out work and avoid weather problems by protection of new work against rain, wind and frost are covered in BS EN 1996-2 (2005) and BS 5628-3(2005). Realistic tolerances for position on plan, straightness, level, height and plumb are given in BS EN1996-2 and in BS8000: Part 3 (2005), both based on the principles of BS5606 (1990).
Buildability, Site Efficiency and Productivity
The process of constructing masonry has traditionally been regarded as difficult to mechanise and ‘bring into the 21st century’. This is partly because it is a skilled craft and often has to be adapted to compensate for inaccuracies in other components. There is also a tendency to a high wastage rate because of the use of mortar that has a limited life and because of poor handling and storage conditions on site and loss and damage of materials between stores and the workpoint.
Many of these problems have been reduced by innovations such as shrink wrapping of materials, crane delivery of packs direct to the work points and the use of retarded ready-to-use mortar with a long shelf life. Despite many attempts it has not yet been possible to economically automate the site construction of masonry. Bricklaying machines have been developed but are only suitable for building the simplest of walls. For a brief period in the 1960/70s prefabricated brickwork cladding panels became popular in the USA and some degree of mechanised construction was feasible for relatively simple factory-built panels.
Recent continental innovations are the use of very large but precisely sized masonry units, with cranes to lift them if necessary, and the use of thin-joint mortar to bond them. The larger size and simple technique necessary can improve productivity and speed to similar levels to that of timber frame for building the structural core of small to medium-sized buildings.
They can then be clad with conventional brickwork or other finishes. Since the cladding is off the critical path it is possible to simultaneously finish the outside and the inside in the same way as framed buildings.
This is very much a matter of taste and expectation but there are some general rules to follow. Precisely shaped bricks with sharp arrises demand accurate layout with perpends lined up vertically and evenly sized mortar joints throughout, otherwise they tend to look untidy.
Less accurate uneven bricks will tolerate some variation in joint size and position without looking ugly. Except with very accurate bricks, walls can only be made fair faced on one side while the other side has to suffer from any variability in thickness or length of the units. If a solid 220 mm thick wall is required to have two fair faces it should be built as a collar jointed wall (Fig. 5).
If an internal half-brick wall is exposed on both faces it is probably best to use a recessed joint that is tolerant of some inaccuracy. Exposed external walls should be protected as much as possible from run-off of rain.
Any detail which causes large amounts of rain water to course down the wall in one spot or leach out through mortar joints at damp-proof membranes will eventually lead to discoloration due to staining by biological growths, e.g. lichen, lime or efflorescence.