174 working hours
25 unskilled labourers, 6 carpenters, 4 steel fixers, 2 concrete workers, 1 mixer driver, 1 superintending officer
One of the privileges of collecting contemporary military manuals (see the full list of publications in my library here), is that every now and again I stumble across a snippet of information that enhances my knowledge and understanding. In this instance, it was the penultimate table in the back of Military Engineering, Volume XIV – Concrete, Part I, Practical Work, dated 1944. This modest pamphlet was one of a series issued to Royal Engineer field units, as well as port and railway construction units. It details the practicalities of working with cement, including design constraints, safety limitations, and ways of ensuring the quality of cement prior to pouring.
For this article, I will be focussing on the information contained in Table 24 of this pamphlet. As I work through, I will use images from the Imperial War Museum (IWM) photographic archive as well as some from my own collection, and information from other historic technical manuals in my collection in an attempt to project manage the construction of a pillbox.
Table of Contents
Schedule of operations for a typical concrete pillbox
The following table is taken from Military Engineering, Volume XIV – Concrete, Part I, Practical Work, 1944, and is a suggested schedule of operations, labour and time for 100 cu. yd. typical pillbox built in good weather. A table of this type would have helped to inform Royal Engineer officers when planning construction tasks such as this. Of course there are many other factors that have to be taken into consideration, but as a guide this table proves very informative.
|Operation||Labour||Time||Tools and plant||Materials|
|1. Fixing steel reinforcement to floor level||2 steel fixers|
|15 hours||Wire nippers|
2 steel rules
3 ½ lb. hammers
|Reinforcement binding wire, 16 S.W.G.|
Chalk for marking steel
Timber for templets (sic) and spacers
|2. Concreting to floor level||1 mixer driver|
|12 hours||10/7 mixer|
R.E. measuring boxes
6ft x 6ft timber platform
Spading and ramming tools
|Coarse and fine aggregate|
Rough timber for shutters
Nails, wire for ties
|3. Fixing first lift steel, erecting entire internal shuttering and external shuttering for first lift||6 carpenters|
4 steel fixers
|15 hours||Carpenters tools|
|Timber for shuttering|
Nails, 3 ins., 4 ins., or 5 ins.
4 ins. x 2 ins. timber for templets (sic) and spacers
Binding wire, 9 S.W.G.
Wire for shutter ties
|4. Concreting first lift (including staging)||1 mixer driver|
|8||Scaffolding and planks|
Remainder as operation 2
|As operation 2|
|5. Fixing steel and erecting external shutters for second lift||6 carpenters|
4 steel fixers
|24 hours||As operations 1 and 3||As operation 3|
|6. Concreting second lift (including staging)||1 mixer driver|
|8 hours||As operation 4||As operation 2|
|7. Fixing roof shutters and reinforcement||6 carpenters|
4 steel fixers
|24 hours||As operations 1 and 3||5 ins. pip for periscope|
Remainder as operation 2
|8. Concreting roof||1 mixer driver|
|20 hours||As operations 2 and 4||As operation 2|
|9. Curing||Unskilled||24 hours||Hose spray|
Roses (for the water cans)
Sacking, hessian, etc
|Earth, sand, clay|
|10. Striking and cleaning shutters||5 skilled|
- Curing: Concrete becomes hard by chemical combination of cement and water, during which the process it is necessary to prevent, as far as possible, evaporation of the water from the surface of the concrete. This process is called curing, and is accomplished by covering the concrete as soon as it can be done without damaging the surface, with damp cloths, wet straw, sand, etc., kept wet by sprinkling, or by immersing in water. Concrete made with ordinary Portland cement, should be cured for at least 10 days when normal methods are employed.
- Shuttering: (The same as formwork) Moulds, formed of timber, metal sheets, etc., into which concrete is placed immediately after mixing, and left to set and harden. There is reference to templets in Table 24, this is an alternative and rare spelling of template. Also another word for shuttering or formwork.
Some notes on materials
For estimating the total quantity of water required, allow about 20 to 27 gallons (123 litres) of water per cubic yard of finished concrete1Manual of Field Engineering, Volume II (Royal Engineers) 1936. That’s up to 12,300 litres of suitable water required for the construction of a pillbox using 100 cubic yards of concrete.
The guidance issued in the WD Schedule of Prices (1939) 2War Department schedule of prices for works and repairs to buildings, 1939states that the contractor can obtain water free of charge from WD sources, however where no mains supply is convenient, the contractor must supply all water required at their own expense, including transporting it to site. Certainly a logistical challenge during the construction of remote pillboxes.
This of course is only water required for the production of the concrete, and does not include the surplus for the washing and scrubbing of any aggregate prior to use.
Sand / aggregate
Sand, or a fine aggregate, used in concrete production is tightly specified3War Department schedule of prices for works and repairs to buildings, 1939 in that is must be “clean, sharp, coarse grit, pit or freshwater, containing less than 3% by weight of silt, loam or clays, and be less than ½% organic or other foreign matter reckoned as tannic acid.”
The use sea sand is not allowed unless approved by the Superintending Officer in writing, and in that case the sea sand must be washed in fresh water before use.
Aggregate can be any granite, gravel, crushed ballast or other sound and hard material, free from impurities. Pit gravel or river ballast can be used as a substitution for a sand and stone mixture, but must also be free of impurities.
Contractors are responsible for the supply, design, stability, striking and removal of shuttering. The curing period for normal Portland cement is sufficiently long to require the formwork to be left in position. For walls under 6 inches in thickness, this must be left in place for 5 days, or 10 days if over 6 inches thick. This time can be increased if the weather is bad or there is a frost.
Formwork had to be oiled, to precent the absorption of water from the concrete, it also helped when removing the boards after the concrete has cured. The engineering manual suggests that one gallon of mould oil would cover approximately 20 sq. yards of shuttering.
Not all pillboxes were constructed using wooden shuttering, brick was often utilised. By using brick interior and exterior walls, it was possible to reduce the volume of concrete required, and also simplify the stages of erecting and removing wooden shuttering. The brick could be left in place while the concrete cured.
For large works, where it is not possible to work continuously, the WD issued rules that should be followed:
- Walls can be left at a convenient height, but the last layer should be the same level all round.
- If a floor or roof is too large to finish in one day, it should be poured up to a girder or other intersection.
- In all cases where work is stopped, there must be a sufficient number of continuity reinforcing bars in position to link up the new work. And before restarting the work, the joining surfaces should be cleaned and hacked, and rubbed down with a stiff wire brush.
Royal Engineers Construct a Pillbox
A very informative photo series exists in the Imperial War Museum (IWM) collection, entitled “Royal Engineers building a Pill-box.” It contains 8 black and white images of a pill-box (sic) being constructed, IWM image references O 330 to O 337 (link to the first image), and provides some more detail to the steps in Table 24.
The first image in the series (O 330) has a caption describing the engineers pumping out water when preparing a position for a pillbox. However, the pillbox is already well under construction nearby at this stage. I believe this water was being pumped in to the pit in order to wash and rinse the sand or gravel for use as an aggregate in the construction of the pillbox.
In the image below (O 332) we are shown the concrete mixing stage. A mechanical mixer is being used, we can see the bags of cement and also that boxes for the aggregate are being emptied into the mixer.
The final image to examine (O 335) below, gives us a real impression of what is involved when constructing a pillbox. Without any mechanical machinery, a scaffold frame is constructed around the pillbox shuttering, and wheel barrows of concrete are brought to the top of the wall and poured in what needed to be a near continuous cycle.
How much would a pillbox have cost?
Without a bill of works from a contractor, it is hard to know for sure. But it is possible to estimate a cost based on an original document; the War Department schedule of prices for works and repairs to buildings, 1939. The prices in the schedule are “intended to apply to every description of work that may be required in the construction and maintenance of buildings of all kinds belonging to, or hired, or occupied by the WD; also fortifications, roads, drainage, water supply, together with engineering works such as sea walls, groynes, harbour and dock work, bridges, railways, etc, required by the several maintenance or special Contracts.”
It is therefore reasonable to apply this schedule to the construction of this pillbox. All prices below are taken from the 1939 edition, without adjustments for any later price increases, and assumed that civil contractors are carrying out the construction and not Royal Engineers.
For the purposes of this exercise, I have used a Type 28 concrete, anti-tank pillbox. With a lot of assumptions made, I have come close to the 100 cubic yard pillbox in Table 24. My volume calculation is shown in the diagram below.
I have used to War Department schedule of prices to obtain the cost, in 1939, for the items and materials detailed in the schedule of operations. Disclaimer: This is my first time working with some complex pre-decimal £/s/d calculations, I am sure I have made some mistakes somewhere!
|Item||Week 1||Subsequent Week||Estimated Quantity||Estimated Duration||Sub-Total|
|3901 Barrows, wheel, navvy pattern||1s 1d||5d||4||1 week||4s 4d|
|3906 Planks, wheeling, per dozen||3s 6d||1s 3d||6||1 week||£1 11s|
|3908 Boards, scaffold, per dozen||2s 3d||10d||6||2 weeks||£1 10s|
|3917 Poles, stout, scaffold, not exceeding 22ft, per dozen||4s 3d||1s 9d||6||2 weeks||£2 14s|
|3944 Pump, with suction pipe up to 20ft in length||2s 6d||1s 1d||1||1 week||2s 6d|
|164 Reinforced concrete, exceeding 12 inches total thickness, with aggregate of ballast or gravel, mixed 1:5 cement with ballast, per yard cube||37s 3d||75 cu yd|
(walls and floor)
|£139 13s 9d|
|170 Reinforced concrete in roof slabs, 5 inches thick, with aggregate of ballast or gravel, mixed 1:5 cement with ballast, per yard super||5s 9d||14 cu yd|
1.94 yard super
1 yard super, 5 inches thick = 5/36 cubic yards
|171 Additional inch thickness, for reinforced roof slabs, add per inch||1s 1d||x7|
(The roof is assumed 12 inches thick)
|193 Ballast, 2 inch gauge, per yard cube||11s 0d||For reference|
|193 Ballast, ½ inch gauge, per yard cube||13s 9d||For reference|
|359 Cement, Portland, in non-returnable paper bags, per bag of 112 lbs||3s 0d||For reference|
|2104 Mild steel, round bars, cut to length and bent to shape and wired with 14 S.W.G. iron wire, per CWT, supplied and fixed||22s 6d||6|
- The steel reinforcement, casings and centering is to be measured in addition to the prices above.
£153 11d (1939)
This is a very conservative cost as I am sure I have not calculated the volumes or quantities correctly, and my multiplication of pre-decimal currency was painful.
From handwritten notes in a 1939 copy of the War Department Schedule of Prices, a steady increase in the cost of works was seen. Below I have roughly calculated the cost of constructing our pillbox based on the price increases as the war came to an end.
|Date||Increase from 1939 costs||New Pillbox Cost|
|June 1944||Concrete works +24%||£201 (6/1944)|
|March 1945||Concrete works +26%||£207 (3/1945)|
|July 1945||Concrete works +29%||£216 (7/1945)|
|November 1945||Concrete works +26%||£207 (11/1945)|