This chapter describes the manufacturing process for pantiles and different types of ridge tiles. Pantile production is described step-by-step. For ridge tiles, most production stages are similar to those of pantiles. Ridge tile manufacturing will therefore be described only where it differs from pantile production.
Climatic conditions may affect cement setting. Three factors are of major importance:
- air moisture;
- air temperature;
- wind speed.
A combination of all three factors is detrimental to tile quality: wind combined with a low air moisture and high temperature speed up the evaporation of mixing water. This will result in micro-cracks which impair the quality of tiles.
Excess loss of mixing water through evaporation results in:
- reduced hydration of the binder: lack of efficiency of binder and reduced strength of the end-product;
- increased porosity;
- excessive shrinkage during setting.
For a mortar of constant consistency, there exists a relationship between air temperature and the amount of water to be used for the mix (figure 34).
If tile production is to be started in an area where wind speed, temperature and air moisture are likely to impair the manufacturing process, it is essential to assess the effect of these factors on the mix water. Wind speed may be estimated as shown in table 18.
Table 18. Wind speed
Very stiff breeze
Figure 34. Approximate relationship between quantity of mixing water required and ambient temperature for constant mortar plasticity (ref.9)
Figure 35. Surface evaporation of water in mortar (ref. 21)
To calculate evaporation plot parameters following example (arrows)
The evaporation of water from the mortar as a result of the air moisture content, air temperature and wind speed is quantified in the abacus on figure 35.
In order to calculate evaporation resulting from air temperature, the parameters should be plotted on the abacus following the sequence of the arrows in the example.
Evaporation is excessive if it exceeds one litre per square metre per hour. In this case, the mortar must be covered to prevent drying.
This section is a summary of the major steps described in chapter II, section IV, and chapter III, section VII.
1. Sand, aggregate and cement
The amount of cement used in the mortar mix determines the end characteristics of the finished product. An accurate proportioning of sand and cement is critical to the economics of tilemaking and the strength of the finished product.
Two major factors directly affect the quantities required:
- moisture content (of sand and aggregate);
- the degree of compacting (sand and/or cement).
The moisture content of the sand or aggregate depends on their origin, mode of transportation, storage conditions and air moisture. The degree of compacting of the sand, the aggregate and/or cement depends on the way the batching boxes are filled.
The key to an accurate proportioning of the materials consists in measuring the sand, the aggregate and the cement in standard containers. The content of the containers is calculated on the basis of the average volumetric mass of the materials concerned (see chapter II, section IV).
In order to avoid major deviations due to excessive bulking (loose material), each batching box should be slightly shaken or tapped on the sides with a piece of wood. This will ensure that the batching boxes are filled with consistently identical quantities of materials.
Fibre presents exactly the opposite difficulty: the bulk of the raw chopped fibre is far greater than its final volume in the end-product. The bulking effect and compacting potential are such that the only acceptable unit of measurement for fibre is a weight unit (see chapter II, section IV.2).
Cement is supplied in 50 kg bags and stored in a dry place. One bag of cement is sufficient to produce approximately 100 to 125 tiles.
Sand and aggregate are generally supplied in truck-loads of "x" tons or per cubic metre. They can be stocked outside, preferably under shelter.
Fibre is supplied in bunches or bales. It should be kept in dry storage and protected against rodents.
STEP 1: Sieving of sand
Before mixing the mortar, the coarser gravel should be removed. For fibre concrete production, the sand should be sieved with a mesh-size of approximately 2mm. For micro-concrete production, sand and aggregate should be sieved with a mesh-size of approximately two-thirds of the tile thickness. The choice of mesh-size also depends frequently on what the local hardware stores have to offer.
Screening should be performed close to the storage area, on a clean surface. Sieving may be carried out in several ways: the sieve may be placed on the ground, on a transport device or slung on posts, trees, etc. (see chapter II, section X.I).
If the tests for clay and micro-organisms reveal an excessive amount of impurities, the sand should be washed out with water. This should be done carefully: excessive water pressure or flooding might result in a loss of all the finer particles and produce a coarse end-product. In such cases, a given amount of very fine sand may be added, although this technique produces very unpredictable sand grades.
STEP 2: Chopping of fibre
Fibres are generally very long and must be cut down to pieces of approximately 15 mm. Chopping may be done with a machete and block. A chopper may also be used to chop the fibre into pieces of identical size. This operation should produce enough fibre to last the whole day.
Figure 36. Sieving of the sand
The blending of the various components of the mix is an important operation which must be carried out very carefully.
The materials are mixed in a clean area, either by hand or in a mortar mixer. If a mixer is used, several large hard spherical stones should be placed in the mixing tank. These stones will crush the aggregates formed by the moist sand, aggregate and cement. After blending, the mix should be smooth and homogenous, both in appearance (colour) and consistency.
STEP 3: Sand/aggregate plus cement mix
The sand and cement and aggregate are mixed together in the first stage. The various cement/sand ratios and tile thicknesses are given in chapter II, section IV.
Inaccurate measurement of proportions results in the following defects:
- too much cement:
- development of micro-cracks due to insufficient sand content;
- increased costs;
- too little cement:
- brittle and porous end-product.
STEP 4: Mixing in the fibre (for fibre or micro-concrete only)
Once the sand/cement mixture is homogeneous, the fibre is added to the mix. The quantity of fibre is in the range of 0.5 to 1 per cent of dry mortar weight.
Fibres should be sprinkled into the mix in order to avoid bunching. A good distribution of fibres in the mortar gives added strength and resistance to the material during handling.
Inaccurate fibre proportioning results in the following defects:
- too much fibre:
- bunching, poor distribution of fibres, development of non-reinforced, brittle and porous areas in the tile.
- too little fibre:
- breaking during demoulding, handling, transport and placing on roof;
- development of micro-cracks due to cement shrinkage during setting.
STEP 5: Adding water
The mortar must be workable and lend Itself to several operations: moulding, compacting, etc; it should therefore be soft enough, though not too wet. Water should be added gradually and sparingly.
The exact amount of water to be added to the dry mix (sand + aggregate + cement + fibre) is difficult to assess beforehand. It varies essentially with the moisture content of the sand and aggregate. A consistency and workability test should therefore be carried out: if the mortar contains too much water, it will tend to run. In theory the ideal cement: water ratio (in weight) is 0.65 (see chapter II, section IV.3). The Abrams cone slump test gives a measurement of mortar consistency. A description of the test follows.
A frequent mistake at this stage consists in not using the mortar soon enough and adding water to the mix the moment it starts hardening. The adjunction of water at this stage perturbs the ongoing chemical process. This will result in impaired tile strength after curing. This defect is particularly marked at higher temperatures.
Using the wrong proportions will result in:
- excess water
- poor shaping on the mould: the mortar slum] down from the convex part into the channel;
- increased porosity;
- reduced stress and strain strength.
- too little water
- poor blending of materials;
- presence of air pockets.
Abrams cone slump test
This test is frequently used to measure mortar consistency. A sheet metal truncated cone is used. The dimensions of the cone are: D = 20 cm, d = 10 cm, H = 30 cm (D = diameter of large base, d = diameter of small base and H = height). The cone is placed on the ground on its large base. It is filled with mortar through the small opening in four layers, each layer being spiked several times. Once the mould is completely filled and the top levelled, the metal cone is removed carefully and placed next to the mortar cone. Immediately after demoulding, mortar slump is measured with a measuring rod. Slump is expressed in centimetres. By way of an example, a mortar with good workability should produce a slump figure of 4 to 8 cm (figure 37).
Figure 37. Measuring slump with Abrams cone test
STEP 6: Preparation of materials
The first step consists in selecting and fixing the appropriate screeding frame for a normal tile (6 mm), medium tile (8 mm), thick and heavy-duty tile (10 mm), ridge tile. For normal climates, a 6 mm or 8 mm frame should be used. For hurricane or monsoon climates, 10 mm tiles will be required.
The top of the vibrating table should be clean and free of mortar residues. After each vibration, mortar projections and spillings should be removed from the table and frame. The presence of residues prevents a tight fit of the screeding frame on the table.
Moulds and polythene interface sheets should also be kept scrupulously clean. Before each moulding the interface sheet should be checked for holes by transparency. The use of punctured interfaces causes the cement milk to leak during vibrating. Punctured polythene sheets should therefore be rejected.
When the screeding frame is fixed on its hinges, it is in an upright position. An interface sheet is placed on top of the vibrating table (figure 38). The frame is lowered on the mould and locked. The interface sheet remains clamped between the frame and the table.
Figure 38. Placing the interface sheet on the vibrating table
STEP 7: Vibration: compacting
The standard mortar scoop gives the exact quantity of mortar for one tile. Excess mortar is levelled off with the trowel. The mortar is placed on the interface covering the screeding frame (figure 39).
The following step consists in vibrating and compacting the mortar. The motor is switched on (or manual vibration started by turning the handle). During vibration, the mortar is evened out with a trowel (figure 40). The trowel is used to:
- push the mortar into the corners of the frame, which tend to fill up more slowly;
- even out the surface of the screed.
Figure 39. Transfer of standard quantity of mortar on to the vibrating table
Figure 40. Screeding the mortar in the frame
The layer of mortar is shallow enough for any fibre lump to show up during vibration. The fibre may tend to bunch up and form small balls which run through the thickness of the mortar, thereby making the tile brittle and porous. These lumps should be picked out and replaced by an appropriate quantity of homogeneous mortar. Vibration should be continued for a few seconds in order to blend the extra mortar with the screed.
Duration of vibration
The main objective of vibrating-compacting is to remove the air bubbles trapped in the mortar. The compressive resistance of the dry mortar is in inverse relation to the amount of voids or air bubbles (figure 41).
In view of this relationship there seems to be an argument for vibrating the screed until all the air has been removed and maximum compactness has been obtained. Two limiting factors, however, make it necessary to time the vibration phase accurately:
- the components of mortar have varying densities: when vibrated, the various components will segregate, with the lighter materials (fibres) rising to the surface, and aggregates falling to the bottom;
- the frame does not lock with the vibrating table with a watertight fit. During vibration, the mortar milk will tend to seep through the cracks.
For adequate compacting, the screed should be vibrated between 45 and 60 seconds, depending on the materials used for the mortar (sand grading, water ratio, quantity of fibre). For correct timing of vibrating, the various components of the mortar should be monitored:
- the cement milk should not leak under the frame;
- the screed surface must remain smooth;
- the number of air bubbles breaking out at the surface should be less than when vibration is initiated;
- the fibres should not come up to the surface.
Figure 41. Relationship between compressive strength and compactness of screed (ref. 29)
STEP 8-a: Nib making
Once the tile surface is even and smooth the nib may be formed on the tile. The nib box is filled with mortar. This can be done either during vibrating if the operator is sufficiently skilled, or after vibrating (with motor switched off).
Extra mortar should be added on the top of the nib moulds. The vibration should be switched on again for a few seconds for good adhesion of the nib with the rest of the screed. During the second vibration the extra mortar will settle. If the second vibration is too long the base of the nib will run into the screed.
The motor is switched off and the top and side of the nib are evened out with the trowel until smooth surfaces are obtained (figure 42). This will ensure a tight fit of the tiles on the roofing laths.
Figure 42. Making the nib
Figure 43. Making the second nib
STEP 8-b: Making the second nib
A second nib should be moulded on the tiles for heavy-duty roof cladding. With sufficient speed and precision, a skilled operator can mould both nibs in one operation.
The second nib jig is placed on the frame (figure 43). A four-arm clamp holds together the flat part of the pantile frame.
STEP 9: Opening screeding frame
The locked screeding frame is opened. The mortar in the nib jigs will be wrenched off from the rest of the screed if the frame is lifted without precaution. To obviate this the frame should be lifted whilst holding down the nib mortar with one finger (figure 44).
Figure 44: Opening screeding frame
STEP 10: Moulding
There are two ways of positioning the screed on the mould. If the mould is close to the vibrating table, the transfer sheet is transferred on the mould by holding it diagonally at opposite corners. An appropriate balance should be found between the weight to be lifted and the shape to be retained. The transfer sheet is placed on the mould and positioned correctly along the marker lines.
If the mould can be positioned forward of the vibrating table, either on a board or on protruding brackets, the transfer sheet may be simply slipped from the frame to the mould (figure 45).
Figure 45: Transferring the screed on the mould
Utmost care should be taken when positioning the tile on the mould. Once the mortar starts setting on the mould this determines the final shape of the tile. For perfect overlapping of the tiles on the roof, the transfer sheet should be carefully positioned on the mould so that the long side of the tile is flush with the ribbed mark on the mould. The tile must also be laterally centred on the mould in order to keep the comers from collapsing and setting in the wrong position.
In order to facilitate correct positioning of the screed, the mould bears two marks (figure 24).
- a preformed rectangular mark in the mould shows the correct position for tile nib.
- a raised line on the long side of the mould shows the exact positioning of the near edge of the tile.
Correct tile moulding has a twofold influence on profitability: the quality of the end-product has a direct impact on the reputation of the production unit and the percentage of rejects at the quality control stage is a determining factor of cost-effectiveness and pricing.
If the tile is not correctly positioned with regard to these marks, three defects will prevent a correct fit of the tile on the roofing structure:
- if the edge of the tile is not correctly positioned along the ribbed line, the tiles will not overlap, the outside curve of the tile not being identical;
- incorrect positioning of the nib will result in irregular tile/batten interface, hence irregular overlap;
- poor centering will result in collapsed edges and/or comers.
Figure 46. Finishing the nib
STEP 12: Tile fixing methods
At this stage the method of fixing will have been decided. Several fixing methods are possible (see chapter VIII, section V).
- the nib is left without fixing device. The tile will then be placed directly on the roof laths. The tiles are kept in place and relatively stable by the fit and overlap;
- the tile may be nailed to the laths through the nib. Allow the tile to dry for one hour and pierce the nib before the mortar has set completely. This cannot be done when the mortar is fresh since the mortar would slump and block the hole. The nib is pierced with a roofing nail by passing it through the tile nib several times (figure 47). The nib should be kept in place on the tile by pushing it down gently with a finger during piercing;
- the tile may be fixed with a galvanized wire loop or plastic string sunk in the nib mortar. A loop is formed with a segment of galvanized wire. The loop is closed by a twist. The twist should be 15 mm to 20 mm long. It should be bent at a right angle at a 3 mm to 4 mm distance from the base of the loop. The twist is sunk into the mortar in the centre of the nib, from the inside to the outside of the tile. The nib should be kept down with a finger. The gap created by piercing the mortar with the wire twist should be carefully closed with extra mortar. The base of the loop should be level with the upper surface of the nib (figure 48).
Figure 47. Piercing a hole in the nib
Figure 48. Wire loop cast in the nib
STEP 13: First curing
During an initial 24-hour period, the moulds are stored for initial mortar setting. Curing should under no circumstances be carried out in inappropriate external conditions, i.e. excessive heat or dryness (see section I. 1 in this chapter). Setting should be slow in order to avoid the development of micro-cracks due to quick cement setting.
If the air is not sufficiently humid or a strong wind dries the mortar too quickly, the tiles should be watered regularly on their moulds. Alternatively, the stacked moulds may be covered with plastic sheets in order to retain the evaporation water and create an appropriately damp environment.
STEP 14: Demoulding of tiles
After 24 hours cement setting time, the tiles are ready for demoulding. The mould should be held on either side with both thumbs pressing upward (figure 49). The mould is rotated towards the operator to a vertical position. The tile is placed flush against the demoulding jig (figure 50). The jig and the mould are rotated by another 90º. The tile is then in horizontal position between the jig and the mould. The mould is removed and stored for cleaning. The transfer sheet on top of the tile is lifted or peeled off and also stored for cleaning (figure 51).
Figure 49. Carrying the mould to demoulding jig
Figure 50. Positioning of mould against demoulding jig
Figure 51. Peeling off the interface
STEP 15: Initial quality control
At this stage several quality controls may result in the reject of defective tiles:
- the surface of the tile must be smooth, ripple and wrinkle-free. Minor surface defects may be corrected. If the defects are too important (holes, wrinkles, etc.) the tile should be discarded;
- the long edges and sides of the tiles should be parallel two by two and form a 90º angle;
- tile parallelism is checked with the control bar of the demoulding jig. If the tile is warped, it will not overlap correctly. It should be discarded (figure 52);
- the curve of the tile is correct if the long side of the tile lines up with the edge of the control bar. If the gap between tile and bar is too wide (several millimetres) the tile should be rejected.
Figure 52. Checking parallelism
Figure 53. Trimming the edges
STEP 16: Trimming the edges
During vibration, the cement milk runs off through the cracks between the frame and the vibrating table. The cement milk produces an untidy fringe. At this stage of demoulding, the mortar is still fresh enough for this to be corrected: with a blade or rigid piece of metal the four sides may be trimmed and smoothed (figure 53).
STEP 17: Transfer to curing tanks
The tiles should then be carried to the tanks or containers for curing. Each tile should be carried with both hands in a vertical position and placed carefully in the tank. During transfer the tile should be kept level. The mortar being still fresh, the tiles will break if these precautions are not taken.
The tiles are up-ended on their small side and stacked vertically against each other. Space can also be saved by stacking them in threes with the nibs overlapping. In this case more caution is required since the tiles are handled in threes (figure 54).
Figure 54. Cross-section of two stacking methods
STEP 18: Immersion or airtight packing
Curing of the tiles in water tanks or airtight plastic sheets prevents the water used in the mix from evaporating too quickly. Curing is necessary for the cement to set properly and maximize tile strength. The consequences of insufficient curing are:
- excessive shrinkage;
- development of cracks;
- reduced tensile strength.
Curing may be carried out by either of two methods:
The first method consist in packing the freshly demoulded tiles hermetically (figure 55). The tiles are placed vertically in wooden racks. A large plastic sheet, preferably black (to capture radiating heat) is used to wrap the crates. A small amount of water should be poured in the plastic sheet before wrapping so as to maintain a high degree of humidity in the pack through the action of heat on the black plastic material.
Figure 55. Airtight packing of tiles
The second method consist in immersing the tiles in large water tanks (figure 56). Curing tanks are often built above ground level but they can also be buried. In the first instance, drainage will be easier. In both cases the curing water should be drained twice a month since it would otherwise become caustic.
The tank should be filled with water until all the tiles are fully immersed. No part of the tiles should be left uncovered: this would produce differential strengths in the tile and induce micro-cracking. The tiles should be placed in the tank day after day and each batch clearly marked in order to check immersion time.
Figure 56. Immersion of tiles
Curing time in immersion tanks or plastic wrappings depends on the quality of the cement, temperature and air humidity. In theory, the tiles should be sampled for various curing times and tested to define the ideal duration of curing (see chapter V). In practice, however, a curing time of five to six days is considered appropriate.
STEP 19: Drying and air curing
A certain time is required for the mortar to achieve maximum strength. Four weeks after hydration (mixing the mortar), the increase in strength begins to slow down. As a rule, final strength will be achieved after 24 months (figure 57).
The production cycle, from mortar mixing to the end of immersion or airtight stocking, lasts six days. It is usually considered that the tile should be stored for another three weeks before handling. During this period of air curing, handling is very delicate and requires a high degree of caution.
Figure 57. Strength of a high quality mortar over two years (350 kg cement per cubic metre) (ref. 9)
The tiles are stacked vertically resting on their small edge. The rows are stacked against a solid support. They may be stacked up to three layers thick provided each layer is covered with a protective material such as straw. However, if there is enough space available around the production unit, it is preferable to stack them in single layers. As in wet curing the tiles may be stacked in groups of three with the top tiles resting on the lower nibs. This method gives a storage capacity of 200 tiles per square metre.
The curing area should be protected from the wind and direct exposure to the sun in order to prevent the tiles from drying too quickly.
There are several types of ridge tiles. They differ in their mode of fixing or thickness. Hip tiles follow the same principle as ridge tiles. If their dimension is not different for architectural reasons, they will be identical with ridge tiles.
The process for ridge tile production is similar to that of Roman tiles, although the frame and mould are different. The screeding frame is thicker for ridge tiles (ridge tiles should be 10 mm thick) and is rectangular in shape (without the blanked-off corners which are necessary for pantiles). Two marks are etched half-way along the long side of the screeding frame. Once the vibration is over, a line is drawn on the screed with the trowel, joining the two marks (figure 58).
This line marks the exact folding line for correct placing on the mould (figure 59).
Figure 58. Fold mark
Figure 59. Shaping ridge tile on the mould
When the tile is placed in the mould the wire loops should be cast in the angle of the tile. For secure fixing a small lump of mortar should be added (figure 60).
Figure 60. Placing wire loops
Two ridge tiles may be formed side by side in a mould. For demoulding, a "negative" jig is placed in the mould. The mould and tile are given a 180º flip. The mould is then in a topmost position and may be removed. The ridge tile is now resting on the demoulding jig. The interface sheet is removed.
Curing and storage for ridge tiles are the same as for pantiles.
Ridge tiles with single overlap ensure improved watertightness. They are made of two layers of screed of 6 mm to 10 mm thickness (depth of frame to be adjusted). The two screeds are joined together, leaving a small space at either end for the overlap. They are made in two stages:
- The first screed is made as for a normal ridge tile, including casting of the wire loops and initial 24-hour curing (figure 61);
- Twenty-four hours later, a second 10 mm screed is vibrated and positioned on the mould. The tile moulded the day before is immediately demoulded and placed on the fresh screed of the second tile (figure 62). The first tile has already set and is pressed gently flat against the second tile (figure 63), leaving a few centimetres on the side of the fresh tile before bonding the hardened tile. This will create an overlap ensuring a tighter fit of the ridge tiles (figure 64).
After allowing to dry for another 24 hours, curing and storage are carried out as for pantiles.
Figure 61. First standard ridge tile
Figure 62. Placing the first tile on fresh tile
Figure 64. Overlap
Figure 63. Joining first tile on second tile
These connecting tiles are shaped specifically to form the joint between the ridge and the hip. They cannot be made beforehand and should be formed on the roof by joining the ridge and edges of the roofing with identical mortar.
Another solution consists in cutting out the hip tile and ridge tiles in such a way as to ensure the tightest fit possible (figure 65).
Figure 65. Angle joint tile for hip-roof
The first row of tiles is placed at the top of the roof under the ridge tile. The lower edge of the ridge tile rests on the concave part of the pantile. The channel part of the pantile or roman tile thus leaves a gaping hole connecting with the roof structure. In order to avoid the wind pushing under the roof through these openings, and/or wind suction on the roof, a small bar should be moulded on the uppermost side of the pantile. The lower part of the bar follows and adheres to the hollow part of the tile, whilst the upper part is level. Together with ridge tiles, this first row of tiles provides a tight fit across the whole length of the ridge (figure 66).
The bar is moulded on a pantile after the initial 24-hour curing period. Once the bar is moulded, curing and storage of the tiles are the same as for pantiles.
Valley tiles, are used to dispose of run-off water in the roof angle of L-shaped structure. Ridge tiles placed upside down may be used as valley tiles. In such a case, no wire loops should be placed in the angle of the tile.
Valley tiles may also be moulded on special moulds (figure 67 b). The same screeding frame as for ridge tiles may be used. The mould (figure 67 a) may be produced locally, e.g. from wood.
Figure 66. First row of tiles
Figure 67 a. Valley tile mould
Figure 67 b. Valley tile