Pillboxes were invariably constructed in four stages:
Lay the concrete floor
Erect shuttering and complete ‘First lift’ (i.e first pouring of concrete)
Erect shuttering and complete ‘Second lift’ – including embrasures (often pre-cast and placed in situ or sometimes constructed using formwork)
The table below details the schedule of operations, labour and time for a typical pillbox built in good weather.
The following notes expand the detail of some of the aspects of pillbox construction and strength to attack.
Shuttering (also often referred to as Formwork)
Shuttering is often constructed of timber, but any material strong enough to retain the fluid concrete without deformation may be used provided it is non-absorbent. Ideally shuttering should be designed for rapid construction and removal. In Suffolk a variety of shuttering can be seen in the construction of pillboxes. Often pre-cast concrete blocks or bricks were used due to the national shortage of timber. Bricks were in plentiful supply in East Anglia due to a large brick industry in the area. The weight that shuttering has to support is:
Weight of the concrete (this can be taken as 144 lb per cu. ft).
The shuttering itself (negligible and for practical purposes can be ignored)
A ‘ live’ load – allowing for impact, wheeling over etc – approx 75 lb per sq foot of flooring.
These factors determined the maximum unsupported length of shuttering for a given height – for example if using 2 inch shuttering, for a 6 ft high wall the maximum unsupported length was 35 inches.
The model below shows the first lift of a pillbox. The shuttering comprises of supported wood shuttering for the outside and brick shuttering for the inside (as can be found in pillboxes on the Corps Line). The bottom two images below are of pillboxes where faults have developed in the concrete which clearly show the stages of construction. The left hand image is of a Type 24 showing the concrete base / raft, and faults in the concrete between the first and second lift and the roof, as well as the reinforcing. The right hand image is of a Type 22, again a fault has developed in the concrete between the first and second lift. The image also shows clearly the precast embrasures and formwork used to construct the low level embrasure for the Boys anti-tank rifle.
Striking is the term used to describe the removal of shuttering. If brick shuttering was used this was simply just left in situ (leading to the common but incorrect belief that many pillboxes were brick built). Wood shuttering, if treated prior to erection with form oil, soap solution or whitewash, could be removed after three to eight days and after being thoroughly cleaned could be reused.
Mass (I.e. unreinforced) concrete, even of high quality, is extensively shattered by explosives detonated on contact. The resistance to shattering of highly reinforced concrete is markedly superior to that of unreinforced concrete. Reinforcement in pillboxes utilized round mild or high tensile steel bars. The thickness of concrete required to resist attack could be reduced by the level of reinforcement. Bars were fixed in place using templates / spacers and wired together. To ensure the best resistance to explosives, the ends of bars should be hooked round other bars and not simply lapped as customary in civil engineering.
Concrete hardens due to chemical reactions between the cement and water and the process continues as long as moisture is present. Curing is the measure taken to ensure the retention of moisture. In the construction of pillboxes curing would be difficult but could be done for example by leaving wood shuttering in place for as long as possible and keeping it damp.
Resistance to projectiles:
The penetration of projectiles into a concrete pillbox is dependent on:
Type of projectile
Angle of incidence
Type of fuse.
Unreinforced concrete of six inch thickness was considered to be proof against small arms ammunition at 100 yards and 15 inches proof against light anti-tank weapons up to 20mm (many so called ‘bullet proof’ pillboxes were approx 15 to 18 inches thick). So called ‘shell proof’ pillboxes (reinforced) were up to 3 ft 6 inches thick.
The resistance of concrete to penetration by projectiles is affected by:
Strength of concrete
Strength of aggregate
Size of aggregate
Thickness of concrete
The strength of concrete could be tested using a slump test or in laboratories but in the days of 1940 this would hardly have been relevant – the need was to get the job done! Strength could generally be improved however by good compaction / tamping. The quality of aggregate (the particles of sand, stone and gravel added to cement) can markedly improve the resistance to penetration. In Suffolk, shingle from the beaches was frequently used in pillboxes sited for beach defence. Shingle should be taken from the below high water level and ideally be washed in fresh water . In unreinforced concrete pillboxes the maximum size of aggregate was not supposed to exceed 1 ½ inches. In reinforced concrete the maximum size was in relation to the spaces between the bars. The increased resistance to penetration by reinforcement has already been outlined above.
Pillboxes were built by Royal Engineer Field Companies or civilian contractors under the supervision of the Field Companies. In 1940 55 Div constructed:
245 "Suffolk Square" pillboxes
243 Hexagonal (Type 22) pillboxes
134 pillboxes on Search Light sites
11pillboxes for machine guns (pairs)
For the construction of the Eastern Command Line the construction was carried out by contractors appointed by the Federation of Building Contractors through Group Contractors under the supervision of RE Field Companies.
The Royal Engineers Pocket Book 1936 ( Indian Reprint 1941), Government of India Press, 1943
Military Engineering Volume XIV – Concrete: part 1 Practical Work, War Office, 1944