The “projected earth system” is put forward as an alternative construction method. The soil from each site is adapted following a specific protocol to make it suitable for spraying. The type of construction and the receiving surface determine the machinery and spraying system used (dry or wet mix). The result will be similar in texture and colour to the original material (in the case of earth walls) or to the surrounding material.
Earth is an integral part of sustainable architecture (
Most research and advances in the use of earth-based construction materials has focussed on strengthening adobe as a material, or on the study of compressed stabilized earth blocks (CSEB) (
The most important advances in rammed earth construction using the “tapial” technique have centred on increasing the density and load-bearing capacity of the earth either by improving the compaction process by replacing earth rammers with small mechanical compactors (in the case of new buildings), or by using hardeners, selecting specific kinds of materials and reinforcing them with natural or artificial fibres (
Other products can also be used to consolidate soils, such as asphalt emulsion, polyvinyl acetate, cellulose, polyvinyl alcohol, plaster, polyacrylamide, rubber emulsions, different types of organic resin and cement-based grout (
Most earthwork restoration projects have involved the use of standard lime or cement mortars. These, however, do not bind with the underlying material for a variety of reasons: chemical and mineral incompatibilities, poor permeability, which results in the formation of vapour barriers, or poor adherence, mainly due to the method of application. Other factors can also be involved, such as shrinkage, formation of surface salts, or the use of bonding agents that are stronger (
At other times, the project requires modern techniques such as micropiling (
We therefore propose the Projected Earth System as an alternative application technique, replacing cement or lime mortar with the same earth used in the original construction, albeit prepared according to a protocol. This would guarantee compatibility with the receiving surface and produce an earth-based mortar with the physical, chemical and mechanical properties needed to ensure stability over time.
Before the “Projected Earth” technique was patented in 2005, the only products available for this technique were gunite, shotcrete, at times with added pigments, and more recently earthen plaster.
The main objective of this project is to adapt any kind of soil for use in projection techniques by means of the protocol described here, which has so far given excellent results. We also recommend the best spraying systems (wet or dry mix) and machinery based on the type of project and construction.
Base Material (BM). The material used in this project was Alhambra Formation (
Typical physic-chemical (1a) and physic-mechanical parameters (1b) of the BM
Physic-chemical parameters (1a) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Maximum size (mm) | Sand equivalent test | Sieve module | Classification on Casagrande chart | USCS classification | Lambe test | Shrinkage limit test | Carbonate content (%) | Soluble sulphates content (%) | Organic matter content (%) | Water content (%) |
20–40 | 22–34 | 4.61–2.79 | CL |
GC |
Low expansivity |
>15% | 13.5–17.1 | 0 | 0 | 0.57–0.62 |
2.6 | 1.7–1.75 | 1.8–2.0 | 9.5–11.0 | 2.0–2.06 | 0.28–0.31 | 30–33 | 10 | 2.06 | 3.7 |
BM and Houben and Doat (37) grading envelope.
The BM is very heterogeneous in texture, a factor that affects not only the coarse but also the fine fraction, silt and clay, which comprise between 22% to 38% of the soil, although this does not affect the suitability of the material, as shown in the grading envelope in
Under X-ray diffraction crystallography, at <50 µ the soil is mainly composed of phyllosilicates, calcite, dolomite, portlandite and quartz. In terms of mechanical properties, BM has a Standard Proctor (SP) density of 2.06 g/cm3
(
Hydrated lime. CL90-S content is 8% of soil weight (
White builder's cement (BL22.5X (
Additives (
The water used meets standard EHE-08 (
Aggregate: this material is needed to improve the gradation of mixtures in which the base material contains over 30% of fines (clay and clayey-silt), to bring this proportion down to 20–25%. The aggregate is white in order to blend in with the structure, and must meet all EHE-08 (
When a graph is plotted with aggregate particle size, Fuller's ideal grading curve (
BM grading envelope (tmax 10), MM + dolomite aggregate grading envelope, Fuller's ideal grading curve (tmax 8) and dolomite aggregate grading curve.
The flow chart in
Field and laboratory methodology.
These allowed us to select a suitable BM collection site, which in turn involved examination of the earth structure to be restored, determining its most important properties in order to subsequently compare them to the BM soil chosen to ensure that it was the most suitable.
In the case of roadside embankments, however, the material removed during construction will be analysed for suitability.
In all cases, the soil used for the mixture should be the same as that used in the original structure, which calls for preliminary field tests together with laboratory tests to characterise the soil (
Since the soil will ultimately be sprayed onto the structure, particles greater that 10 mm were removed from the BM to prevent damage to the pumps and rebound, consistent with the requirements of standard UNE 83-607-94 (
BM, UNE 83-607-94 (
It should be noted that the larger the maximum particle size, the greater the rebound rate, while fines can lead to shrinkage. The BM does not match cement or mortar projection standards due to: (i) a high percentage of fines (between 4% and 9% of fines are needed for sprayed concrete (
It is sometimes advisable to add white aggregate 4–8. This would help reduce shrinkage of sprayed, hardened earth and also reduce the percentage of fines, if needed (>35%) (
Several different mixtures were used for physical, mineralogy, mechanical, durability and chemical testing, including a natural and forced carbonation study using different percentages of lime. On the basis of these tests the two most suitable blends (D1 and D2) were chosen, differentiated by the substitution of 25% of BM for aggregate.
Blends D1 and D2 were then tested on different spraying guns in order to chose the best equipment ( Piston pump gun for dry mix (DM). Used for sprayed cement and granite. Piston pump gun for dry mix (WM). Used for sprayed cement and granite. Worm drive gun for wet mix (WMW). Used for sprayed mortar, particularly in building.
In all case similar formwork was used: 1.40 m2 sheets of chipboard coated on the inside with a three-centimetre layer of BM, attached using a suitable adhesive.
A predominantly sandy soil with a certain amount of gravel, grit and fines with low to medium plasticity (
Authors such as Rodríguez Ortíz (
The density and compressive strength parameters were based on corresponding values obtained in existing walls built in Granada using this type of material and rammed earth techniques (dry density between 1.7–1.75 g/cm3 and wet density between 1.8–2.0 g/cm3; the real density of the soil should be around 2.6 g/cm3). These parameters have been shown to be particularly durable even after several hundred years.
Using SP energy, density increases significantly with respect to the tamping foot compaction technique, with a dry bulk density [
These values are associated with an “e” void [
To define the best blend using dolomite aggregate, different lime and cement dosages were studied in detail, and forced carbonation was tested in a CO2 chamber. All tests were conducted using the same protocol and samples manufactured with SP energy (
Determination of dry density and compressive strength in samples with different lime percentage and time (days) of forced carbonation in a CO2 chamber
Simple compressive strength (MPa) | Water content (%) | Dry density (g/cm3) | Phenolphthalein carbonation (diameter of non-carbonated region from the core of the sample) | |
---|---|---|---|---|
Lime-free BM | 3.7 | 10.4 | 2.06 | – |
4% lime, no CO2 chamber | 4.6 | 10.9 | 1.98 | Not carbonated |
4% lime, 7d.CO2 | 5.8 | 10.9 | 1.99 | 2.0 cm |
4% lime, 14d.CO2 | 4.5 | 10.9 | 1.98 | total |
4% lime, 28d.CO2 | 4.1 | 10.9 | 1.98 | total |
Lime-free BM | 3.7 | 10.4 | 2.06 | – |
8% lime | 3.3 | 12.7 | 1.89 | Not carbonated |
8% lime, 7d.CO2 | 4.2 | 12.7 | 1.9 | 3 cm |
8% lime, 14d.CO2 | 4.1 | 12.7 | 1.9 | total |
8% lime, 28d.CO2 | 4.5 | 12.7 | 1.9 | total |
Lime-free BM | 3.7 | 10.4 | 2.06 | – |
15% lime | 2.4 | 14.3 | 1.83 | Not carbonated |
15% lime, 7d.CO2 | 2.3 | 14.3 | 1.7 | 5 cm |
15% lime, 14d.CO2 | 3.4 | 14.3 | 1.8 | 4 cm |
15% lime, 28d.CO2 | 4.1 | 14.3 | 1.80 | 1.5 cm |
Lime-free BM | 3.7 | 10.4 | 2.06 | – |
20% lime | 1.9 | 15.1 | 1.81 | Not carbonated |
20% lime, 7d.CO2 | 2.4 | 15.1 | 1.7 | 5 cm |
20% lime, 14d.CO2 | 3.6 | 15.1 | 1.7 | 3.5 cm |
20% lime, 28d.CO2 | 4.6 | 15.1 | 1.80 | 1.5 cm |
In summary,
Densities and compressive strength
Sample characteristics | Density (g/cm3) | Compressive strength (MPa) |
---|---|---|
BM (SP) | 2.06 | 3.7 |
BM + 8% lime (no forced carbonation) (manufactured using SP energy) | 1.89 | 3.2 |
BM + 8% lime (almost wholly carbonated after 7 days in carbonation chamber) (manufactured using SP energy) | 1.89 | 4.2 |
BM + 8% lime + 2% cement (manufactured using SP energy) (breakage at 28 days) | 1.9 | 7.0 |
BM + 8% lime + 2% cement + 25% dolomite aggregate (manufactured using SP energy) (breakage at 28 days) | 2.02 | 7.1 |
Sample taken one year after spraying with BM + 8% cal | 1.48 | 2.7 |
Sample taken two months after spraying with BM + 8% lime + 2% cement 22.5 | 1.64 |
4.9 |
Sample taken one year after spraying with BM + 25% dolomite aggregate + 8% lime + 2% cement 22.5 | 1.98 |
4.8 |
Restoration of a rammed earth structure using projected earth with the formula BM + 25% dolomite aggregate + 8% lime + 2% cement 22.5 (sample taken two years after spraying). | 1.83 |
3.4 |
Several spray tests were needed to determine the best system (dry or wet mix) and spray gun according to the type and scope of the restoration or construction project.
The dry mix system is not recommended, since this causes significant segregation as the mixture is projected from the nozzle. When the mixture was sprayed, a large quantity of aggregate was separated from the soil by the pressure of the compressed air, and subsequently lost; it was impossible to determine the exact aggregate content of the sprayed mixture. We also noted a slight delay between projection of the dry mass and the water, making it impossible to control the volume of water added and to add a water reducing agent (superplasticiser).
The wet mix spray gun, meanwhile, had several advantages, including control of (i) added water, (ii) superplasticiser, (iii) and binders. This gave an evenly blended, dense and strong spray mixture while reducing rebound and poor adhesion.
Samples were taken of the sprayed mixture from different guns: DM, WM and WMW and the density and compressive strength were compared with the same results from the SP compaction tests using the same dosages (
Density/Strength of projected earth samples
Soil samples manufactured using SP energy | Samples taken after the sprayed | ||||
---|---|---|---|---|---|
Maximum density (g/cm3) | Optimal humidity (%) | 28 days Compressive strength (MPa) | Maximum density (g/cm3) | 28 days Compressive strength (MPa) | |
D1VH | 1.9 | 12.4 | 7.02 | 1.93 | 4.36 |
D2VH | 2.02 | 10.1 | 7.07 | 1.98 | 4.87 |
D1VHTS | 1.9 | 12.4 | 7.02 | 1.85 | 3.86 |
D2VHTS | 2.02 | 10.1 | 7.07 | 1.87 | 4.23 |
The mechanical strength of the mixture sprayed from the smallest gun (WMW), once totally dry, ranges from 3.8 to 4.2 MPa. This will increase over time due to the effect of natural carbonation. In the case of spray guns typically used in civil engineering works (WM), strength ranges from 4.3 to 4.8 MPa.
Spray tests were also crucial in establishing the order in which the ingredients are added to the mixture, since Projected Earth does not behave in the same way as other mortars mixed before spraying. Based on the test results, the following was determined to be the best order: first, all the water content, then all the cement + lime, half the soil, half the superplasticiser, half the soil and finally half the superplasticiser. If anti-shrinkage agents are used, these should be added at the end of the foregoing stages.
Projected earth in Houben's graph of other earth-based construction systems.
Restoration of a rammed earth wall with significant loss of mass at the base.
This projected earth technique has three possible applications: 1. Restoration of earthen structures. In this case it can be used to coat the structure or to compensate for lost material. 2. As an alternative building material in the context of sustainable architecture. 3. As a means of protecting previously stabilised roadway embankments and adapting them to the surroundings (restoration). This would prevent damage from years of erosion and would greatly benefit the environment by reducing the visual impact of such constructions.
The same earth as the original structure is always used to give the sprayed earth the same physical, chemical and mechanical properties as the receiving structure and ensuring it is consistent in terms of colour and texture. When spraying projected earth on to a damaged rammed earth structure, earth from the surroundings should be used to ensure it is similar to that used in the original construction. In the a new earth building, earth should be taken from a nearby site to ensure the colour and texture of the finished building is compatible with the surroundings.
Projected earth is an effective method of repairing damage using material with the same texture and colour while meeting acceptable standards of density and strength and ensuring excellent adherence to the receiving surface.
In the absence of official standards for this technique, we hope this test protocol will help adapt other soils for use with sprayed mortar techniques.
We propose this BM soil as a standard. Grading envelopes should be adjusted to the particle sizes put forward here, and SP-type samples should be tested in the laboratory to ensure they have 40% more compressive strength than that required by the finished, sprayed earth mortar.
It is important to bear in mind that the mixing order outlined here is unique to projected earth, and is not similar to that used in any other material. The volume of water added should be adjusted to account for the humidity of the source soil (
For reasons of performance and projection pressure, piston pump guns (mainly used in civil engineering projects) should not be used in the restoration of rammed earth structures, although they are suitable for restoration of roadway embankments or other surfaces that can withstand such pressure.
During the restoration or building process, anti-shrinkage materials should be used, such a glass or organic fibre or dramix-type steel fibre, although the latter are not suitable for worm drive guns since they cause obstructions in the nozzle. Mesh made of plastic or other materials can also be sandwiched between layers or projected earth.
Durability was tested by means of accelerated ageing, water resistance, rain resistance, ice and salt spray tests; based on the results obtained, we recommend applying a water repellent to the finished surface.
We would like to thank the staff of SITE S.A. (Granada, Spain) for their help in this study, and also the Department of Building Construction of Granada University.