Types of Arches based on Construction Materials
Arches may be constructed with bricks, stones or concrete. Classification of arches by materials is given below:
- Brick Arches:
Depending upon the nature of workmanship and the quality of bricks used, brick arches may be classified as rough arches, axed brick arches, gauged brick arches, bonds in brick arches.
1. Rough Arches: These arches are built with ordinary bricks which are not cut to wedge shape. In order that all the bed joints may be normal to the curve of arch, the joints are made wedge shaped. Thus the joints at the extra-dos are wider than those at the intra-dos. The wedged shaped joints spoil the appearance of the arch and as such rough arches are not recommended for exposed brick work. These arches are generally used over wooden lintels in order to relieve the latter of the weight of the masonry above. Hence they are sometimes termed as “relieving arches”
2. Axed Brick Arches: In this type of construction, the brick are cut to wedge shape. Thus the joints pf the arch are of uniform thickness. Since the wedge shaped units are not finely dressed, the arch does not present an attractive appearance. It is therefore common to finish the brick work by plastering.
3. Gauged Brick Arches: As the name suggests, gauged bricks are those which have been accurately prepared to a wedge shape for the arch construction. Hard bricks can not be used in this type of construction as it is difficult to cut them to true wedge shape accurately. Hence specially made bricks known as “rubber bricks” are used in the arch construction. The bed joints are very thin usually 16 mm., thick. Cement mortar is considered too coarse for such joints. Hence lime putty is used for binding the blocks. The thin white joints add to the appearance for the finished work.
4. Bonds in Brick Arches: For ordinary work, bricks in concentric rings of half bricks are laid with non continuous radial joints. Each ring is fully completed before constructing the ring immediately above it. For spans above 1.6 m, the concentric rings of brick work have a tendency to separate and there is a continuous danger of the whole pressure being thrown to one ring, which might get crushed and thus collapse the entire arch structure. This danger is avoided either by laying bricks alternately as headers and stretchers in section with continuous radial joints or by providing key stones at intervals.
- Stone Arches:
Considering the nature of workmanship and dressing of the material, stone arches may be divided in the following two categories:
1. Rubble Arches: They are constructed with rubble stones which have been hammer dressed roughly to required shape and size of the voussoirs. In arch ring up-to 38 cm., in thickness, all stones should be of the full depth of the arch. In arches above 38 cm., in thickness, the stones are laid in two rings in alternate coarse of headers and stretchers. This type of construction is weak in strength and is not attractive in appearance and is as such provided for small openings only.
2. Ashlar Arches: They consists of arch stones which have been dressed full and true to their proper shapes. In all cases where the depth of the arch is 60 cm., the voussoirs are made of full thickness of the arch and are set in fine mortar with their radial joints. The stones are laid as headers and stretchers alternately, all headers being of full depth of the ring and are not more than two stretchers going to make the full depth of the arch ring. In case where the arch thickness has to be more than 90 cm., only the quoins and the key stones are made of the full thickness of the arch ring.In order that the voussoirs may be dressed to wedge shape, a full size arch is first set out on a level platform. The size of voussoirs and that of the keystone are carefully marked on the platform after leaving gap for the mortar joints between them. This method of setting out requires slight adjustment and thereafter the required shape of the voussoirs and key stones is cut with help of arch set out on the platform. Finally the stones are cut true to the shape of the template and the voussoirs thus formed satisfy specification of the ashlar arch construction.
- Concrete Arches:
They are of two different types namely, concrete block arches and monolithic concrete arches:
1. Concrete Block Arches: For small opening in a building, concrete block arches can be provided with advantage. The voussoirs, in the form of cement concrete blocks, are prepared in specially made molds for the purpose. The methods of concrete block for the key or skew backs are of slightly different dimensions. The concrete mix usually adopted for block construction consists of one part of cement, two parts of sand and four parts of aggregates. After the concrete in the mold is fully set, the blocks of concrete are removed for curing in a water pond, constructed for the purpose. Generally, the concrete blocks are used without reinforcement. The arch construction with concrete block is similar to that for stone or brick arch construction.
2. Monolithic Concrete Arches: Monolithic concrete arching is frequently used for roofing of buildings, culverts and bridges. The construction for smaller span with ordinary loading conditions requires no reinforcements. For arching can be done in lime concrete as well as in cement concrete. For roofing arches, a rise of 5 cm., for every 30 cm., of span is usually allowed when lime concrete is used. The minimum thickness of arch should not be less than 15 cm ., for spans up-to 3 m. As a general rule, add 38 mm., for each additional 30 cm., of span. For longer spans arches have to be reinforced.