Cover Image
close this bookFCR: Fibre Concrete Roofing (SKAT, 1987, 185 p.)
close this folder3. Technical rationale
View the document(introduction...)
View the document3.1 Conclusions
View the document3.2 Evaluation of experience
View the document3.3 Comments

(introduction...)

In chapter 3 Hans Erik Gram is treating the aspects

- Composition of matrix and type of fibres used
- Rejection rates
- The matrix
- The role of the fibre
- The composite
- The tile
- The sheet
- The load on roofing structures
- Quality control
- References

3.1 Conclusions

Roofing elements of natural fibre concrete have been in use since 1977. The experiences so far include both success and failures. To secure a good quality roofing material made of concrete or mortar with natural fibres one has to keep the following in mind:

The cement matrix

- The properties of the hardened concrete are affected by the mix proportions, the mixing procedure, the compaction and the curing during the first 7 days.

The fibres

- The chopped fibre plays its main role in giving the fresh mix a better cohesion and it enables the moulding of corrugated products.

- Fibre length less than 25 mm is recommended.

· All types of natural fibres are possible to use in FC as long as they are clean so that they do not have a negative influence on the setting and hardening of the concrete.

The composite

- Concrete with long natural fibres may be improved in flexural strength but this effect is lost when the fibres are decomposed.

- Concrete with chopped natural fibres is a material with properties more like burned clay than asbestos cement and it has to be looked upon as an unreinforced material. The addition of 1 percent by volume of chopped fibres does not improve the properties of the hardened concrete with exception of a slight increase in toughness.

- The natural fibre is decomposed in the alkaline environment of the concrete . This has the effect of a reduction in toughness but not of the other strength properties.

- The durability of natural fibre concrete is good if the batching, mixing, compaction and curing are done properly and if the material is not subjected to stresses exceeding its capacity. There is need for a good quality control program for the production of natural fibre concrete elements.

- The stresses within the roofing element are dependent on its dimensions and how it is fixed. In this respect the roofing tile seems to have an advantage compared with the roofing sheet.

3.2 Evaluation of experience

16 answered questionnaires and discussions with practitioners as well as earlier evaluations made, give the base for this evaluation. In table 1 basic data from the questionnaires are given


Table 1: Basic data on FC roofing.


Table 2: Composition of matrix and type of fibre used. Compaction method used.


Table 3: Rejection rate before shipment, damage after shipment, mode of fixing, type of observed cracking and expected durability.

From data given in table 3 one can conclude that the rejection rate before shipment and damage rate after shipment is lower for tiles than for sheets. Cracks have been observed in most cases. The sheets seem to have both longitudinal and transverse cracks. Two out of four tile producers report no cracking. The tile has been in use for only 2 years, but the experience so far shows better performance than it is the case for the sheet.

Not too wide cracks can be mended according to the answers in the questionnaire: In reference /69, 72, 75, 78, 93 / the use of mortar is recommended. In reference /72, 75-79, 88 / white PVC glue, asphalt, cement paste or painting with oil are suggested.

The reason for observed cracks are according to reference /71, 78, 93/ poor curing. Other reasons such as not homogenenous mortar mix and not evenly distributed fibres /72, 74, 93 / ,bad properties of constituents (cement, sand or fibres) /72, 74, 78, 93 /, to high water /cement ratio /72, 93/ or bad compaction /72, 76, 78, 93/ have also been mentioned.

Deteriorated fibres in roofing structures have been reported in reference /69/ (Sisal). No sign of fibre deterioration has been reported in references /67, 68, 70, 72-74, 76-79, 87, 88, 93/.

Investigations in the field /45, 85/ give evidence of both successes and failures. In these examples up to 45 % of installed roofing sheets are cracked after a few years. The roofing tile has only been in use for two years and has less tendency to cracking compared with the sheet.

A control of cement, sand and fibre quality on reception and in stock is reported in 14 questionnaires. A control of the correctness of weighing and batching equipment is reported in questionnaires. The product shape is controlled according to 13 questionnaires.

The product thick-ness is controlled according to 10 questionnaires. The surface finish is controlled according to 13 questionnaires. The Impact resistance is controlled by 7 producers. The stiffness and /or strength of the products are controlled by 4 producers. The pull-out strength of fixings is controlled by 5 producers. The porosity and/or water tightness is controlled by 9 producers. The condition of the production equipment is controlled according to 8 questionnaires. “Good housekeeping” of the production area is kept according to 9 questionnaires. The frequency with which controls are undertaken, varies between dally and monthly, depending on type of control ( 8 questionnaires). The proportion of tested samples depends also on the type of test. Non destructive tests may include 100%of products and the destructive test includes normally 1 %or less of the production (data from 10 questionnaires).

3.3 Comments

The matrix

The matrix is formed by the binder and the aggregate. The binder, cement, consists mainly of lime, silica, alumina and iron oxide. During hydration heat develops during the acceleration stage, which lasts about 24 hours.

During the retardation stage, which lasts about 28 days, the hydration sequence is retarded. About 1/3 of the hydration products consist of calcium hydroxide which does not take part in forming the cement gel, the binder.

Normally about three-quarters of the volume of the matrix is occupied by aggregate. The properties of the matrix are of course influenced by the properties of the aggregate. The properties depend on the petrology, mineralogy, particle shape and texture, mechanical properties, specific gravity and porosity. The aggregate may contain deleterious substances like organic impurities, clay and other fine material, salt, and unsound particles.

The aggregate shall be graded so that the matrix gets maximum density. The voids between the coarse aggregate shall be filled with smaller particles. The grading shall also result In a good workability of the fresh mix.

The properties of the hardened concrete are affected by the properties of the mix in the fresh stage. The fresh mix shall be workable, be possible to compact, surround the reinforcement and fill out the mould without segregation of the constituents and have as little bleeding as possible. The fresh mix gradually converts to the hardened stage, the workability decreases and the strength of the matrix is built up. Cracks can develop very easily during this phase (see the figure). The cracks can be initiated by outer or inner forces. One example of an inner force is plastic shrinkage caused by rapid evaporation.


Relation between mouldability and age and relation between strength and age.

The setting depends on the evaporation and the hydration of cement. Factors as cement content, water content, the temperature of the mix and the temperature and relative humidity of the ambient air, admixtures and porosity of the aggregate are all affecting the setting.

For a 1: 3 mortar with a water/coment ratio of 0.5, the shriakage Is 0.4 % if the temperature is 520 ° C and the relative humidity 50 percent and the wind velocity 1.0 m/s.

During this setting time, the modulus of elasticity increases with increasing strength.

During curia´’, keeping the concrete saturated or as nearly saturated as possible during the setting time, is very essential for the properties of the hardened product.

The cement paste is not homogeneous. It contains pores of different sizes and weak zones can develop for instance in the phase boundaries between cement paste and aggregate particles or reinforcement material, inter alia, due to water segregation in the fresh concrete state.


Structure of the phase boundary between cement paste and an aggregate.

When concrete is fully compacted, its strength is taken to be inversely proportional to the water/cement ratio.


The relation between strength and water/cement ratio.

The tensile strength of concrete is low, less than 10 % of the compressive strength. The type of aggregate and the moisture content have influence on the tensile strength. The tensile strength of concrete is about 2 MPa. The flexural strength of concrete is affected by the dimension of the tested specimen, the surface of the aggregate maximum aggregate size, temperature and moisture gradients. The flexural strength of concrete is about 5 MPa.

The impact strength is related to the tensile strength of concrete and the brittle behaviour of the material. The impact strength can be increased by lower water/cement ratio, high volume of aggregate, small maximum aggregate size and crushed aggregates. The impact strength of concrete is about 2 kJ/m².


Typical stress-strain curve for concrete.

A characteristic value of the modulus of elasticity for concrete is 30 GPa. The strain at failure is 0,01 - 0,02 %.

The drying shrinkage is caused by withdrawal of water. The main factor affecting shrinkage Is the amount of water available for evaporation from the concrete, i.e. how much water has been added to the mix.


Relation between the water content of fresh concrete and drying shrinkage.

The dimension of the concrete structure, the relative humidity of the ambient air, and the time affect the shrinkage.

Concrete also undergoes shrinkage due to carbonation and this shrinkage is in the same order of magnitude as the drying shrinkage.

The thermal coefficient of expansion of concrete is in the magnitude of 13 x 10(-6)/°C.

The fibre

Fibres can be classified as shown below.


Fibre classification chart.


Table 4. Properties of some fibres.

There are three main factors which can cause partial or complete deterioration of the natural fibre. They include:

- long exposure to moisture
- stress corrosion and
- exposure to aggressive environment.

The concrete is aggressive. Its alkalinity is high.


Schematic sketch of the decomposition of sisal fibres In concrete. The middle lamella Is dissolved by the alkaline pore water In the concrete.

In carbonated concrete, the alkalinity is low and therefore the fibre is not decomposed. Fibres are added to concrete In length up to 25 mm ( in some cases even 75 mm ) in the mix or as long fibres between layers of matrix.

The composite

The properties of fresh fibre concrete are dependent on the properties of the matrix, the type of fibre added, the fibre length and the fibre volume. The workability of the mix is changed, compared to the workability of matrix without fibres. Extra water is needed to maintain constant workability. If the volume of added fibres is high, balling of the fibres may occur. The fibre length and thickness do also influence the balling. The thinner the fibre, the shorter must it be to avoid balling.

The addition of fibres enhances the cohesion of the fresh concrete and makes It possible to mould a product In a simple manner. Depending on the properties of the matrix and of the fibre, the suitable fibre volume and fibre length can be found In pretests. If, for instance, the intended product is a corrugated roofing sheet, it should be established that the thickness does not change during the setting time. Concrete Is extra weak during the first few hours, before the hardening process has given the material a certain strength. Water evaporation can cause cracking, due to plastic shrinkage. It is the same process which occurs when mud dries. It has been known for thousands of years, that this cracking can be limited or completely avoided by reinforcing the material, for instance sun-dried bricks with straw. The incorporation of fibres in concrete can have a similar effect. But it is important to point out that concrete can be protected against plastic shrinkage by stopping the water evaporation by an effective cover or by water spraying on the surface of the concrete.

Constituents in the fibre material or on the fibre surface can cause retardation of the cement hydration, and even affect the resulting strength of the fibre concrete. The harmful substances can be removed by properly cleaning the fibres, but in most cases it appears that the normal fibre processing will give satisfactorily clean material. The main advantage of fibre reinforcement of concrete is the improvement of the fracture toughness, the impact, flexural and tensile strength.


Diagram of composite

The composite Is elastic until the first crack has developed. This point, the limit of proportionality, is not influenced by the fibres or only very limited. The occurrence of the first crack is therefore mainly determined by the properties of the matrix. If we use chopped fibres, the flexural strength or modulus of rupture of the composite will be the same as the value obtained at the limit of proportionality, i.e. the flexural strength of the unreinforced concrete.

As can be seen in the figure, the modulus of rupture and the flexural strength of the composite reinforced with long fibres can exceed the limit of proportionality or flexural strength of the matrix. But this means that the composite is cracked and the design load should therefore not exceed the limit of proportionality. The fracture toughness and impact strength of the composite is considerably higher than in the case of short fibres, compare the area under the stress- strain curves.

The compressive strength of the composite Is not at all affected by the addition of natural fibres. If fibres which are not clean are used, the compressive strength can decrease. The incorporation of fibres may require extra addition of water which results in a higher water/cement ratio and therefore a lower compressive strength.

The water tightness of the composite Is not affected by the incorporation of fibres.

But as is the case with the compressive strength, the water tightness can be Influenced in a negative way If the water/ cement ratio Is Increased to achieve a good workability of the fresh mortar mix.

Drying shrinkage of the hardened composite Is only linked to the water content of the fresh matrix.

The fibres can give a better crack distribution, smaller crack widths, but larger number of cracks. Again, it should be pointed out that the incorporation of short fibres may require more water in the mix to obtain good workability which will increase the shrinkage.

Properties like thermal conductivity, sound transmission, fire resistance, water absorption, linear expansion. moisture movements and temperature movements are not influenced by the incorporation of a few percent by volume of natural fibres.

The natural fibres are protected against microbiological decay in the matrix because of the high alkalinity of the pore water. Constituents of the pore water, for instance calcium hydroxide, penetrate the fibre and “mineralize” it. it is not known what happens to a fibre in a crack, possibly there is a crack width limit within which the fibre is protected against microbiological decay.

The high alkalinity of the pore water results In a chemical attack on the fibre and causes a reduction or even nullisation of Its strength. Fibres In carbonated concrete, where the alkalinity of the pore water is low, are not decomposed chemically and seem to be protected against microbiological decay.

The carbonation of the matrix is linked with a shrinkage. This shrinkage is of the same size as the drying shrinkage. The carbonation process is slow and this allows the creep of the matrix to play a role. The carbonation shrinkage causes micro - cracking and cracks can sometimes be seen on the surface of the carbonated concrete. The natural fibre does not play any or only a very limited role in this crack formation. With time the carbonation front moves Inwards Into the concrete and if we have a thin concrete product, it can be completely carbonated within a few years. The microcracking can have a negative Influence on the flexural and tensile strength of the composite. The addition of fibres in the fresh mortar may require more water In the mix to obtain good workability and this leads to a higher water/cement ratio, a faster carbonation process and a bigger carbonation shrinkage. Research in this field to clear the effects of the carbonation on the properties of thin natural fibre concrete products is recommended.

The tile

Concrete tiles have been used on buildings for at least 80 years and the experience with them has been good. The concrete tile is not reinforced. Nowadays, they are normally manufactured of a very dry sand- cement mix with very low water / cement ratios. The thickness varies between 10 and 20 mm. Extrusion is used in the production process.

In 1983 Parry introduced a S-shaped pantile with the dimensions 500 x 250 x 6 mm. The tile is light and easy to handle. The production process Includes the use of a vibro-screeding machine which secures a good compaction of the product. Short fibres are included in the fresh mortar mix to give higher cohesion.

The long term behaviour of this thin product is not yet known. Two out of four producers of this tile report no cracking. In Sri Lanka and Brazil thicker tiles, 10 - 30 mm, are produced. The concrete tile seems to be an Interesting rooting element. Further development concerning the shape of the tile and the need of fibres in the tile is recommended.

The sheet

Natural fibre concrete cannot directly substitute for asbestos cement. The flexural strength of natural fibre concrete is only 25 per cent of that of asbestos cement. Many sheets have been produced with dimensions common for asbestos cement products. Several instances of cracking have been reported and the dimension of sheets have changed to smaller sizes and thicker products. Still the cracking seems to cause problems and the need for repair or replacement of roofing streets already placed on a roof. The roofing sheet must be regarded as an unreinforced element.

The use of short or long fibres seems not to have any different Influence on the long term behaviour. In Tanzania production of 70 000 m2 roofing sheets per year will be initiated in 1986, using a concept with long fibres and a manufacturers guarantee of durability. Further development concerning the roofing sheet, suitable dimensions and properties, is recommended.

The load on roofing structures

A roofing element is subjected to many different types of loads. During production and curing we have the plastic shrinkage phenomena due to water evaporation. Later, the drying shrinkage of maybe 0.5 % may cause internal stresses and cracking. During handling, demoulding and transport cracking can occur. Storing and transporting In the wrong way, for instance piled on each other, may also result in too high stresses and cracking. During installation on the roof, the element also has to take static and dynamic loads. On the roof, stresses may be concentrated at different points or lines on the roofing elements.

Deformations in the timber support may also cause stress concentrations in the element. For Instance, a temperature cycle from maybe + 10° C in the night and + 70° C on the day results in a movement of 0.7 mm per meter. The moisture movements caused by rain and sun also initiate movements in the elements which have to be taken into consideration. These movements may cause too high stresses and therefore cracks. The moisture movement of the natural fibres is bigger than that of the matrix, which may result in internal stresses, especially if the volume of fibres is high and the fibres are utilized as reinforcement (long fibres). If the fibres are decomposed due to chemical attack, also new stress situations may occur. Finally, the carbonation process in the matrix also leads to a shrinkage and micro crack development.

The quality control

The control should comprise a check on the cement (no lumps), on the fibres (clean so that they do not contain “sugar”), that an account on the addition of constituents to the mix is made that the compaction is well done, that product shape, thickness and surface finish are acceptable, that the curing Is started immediately after casting in a proper manner and continued for one week, that possibly 10 percent of the products are tested in a non-destructive load bearing test and maybe 1 percent of the products are tested to failure. The fixings should always be checked. If the water/cement ratio is below 0.6, there is no need for a control of water tightness.

For further research

The following items should be the subject of further research:

- What is the optimal shape of a roofing element made of concrete considering the non existence of a reinforcement? The stresses which occur must be identified and taken care of.

- Might the tile be produced without fibres? What consequences would this have on the shape of the tile?

- Is it possible to use locally available pozzolanas as a partial substitute of cement?

- Can the curing procedure be improved in an appropriate way?

- Is it possible to use natural fibre as a reinforcement in the long term and to produce self supporting roof structures?

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