FOUNDATIONS AND FLOOR MATERIALS

A foundation is one of the most important parts of the greenhouse. No matter where the foundation lies, it must be leveled and square.

According to the Construction Specifications and Regulations SNiP II-B.1-62 (Kiselev, 1975), the depth of the foundations on heaving soils depends on the depth of freezing of the soils and is taken to be no less than the calculated value of the latter.

However, for lightweight structures with shallow foundations the construction mea-surements taken with respect to the depth of the foundations do not provide stability for the buildings, since under the effect of the tangential forces of frost heaving con-sidered non uniform, the vertical displacement of the foundation occurs in time as a consequence of the accumulation of residual heaving deformations during annual freezing and thawing of the soils. The concrete foundation of a greenhouse should fulfill the following requirements (Zabeltitz 1990):

1 It should safely sustain and transmit the loads of the greenhouse to the ground.

2 The footing of the foundation should rest on undisturbed soil at a depth of about 500–600 mm below the ground surface.

The materials for greenhouse floors go from bare ground to concrete: some examples are (Schnelle & Dole 1990, Smith, 1992):

Standard Concrete: Regular concrete will endure about 2500 pounds per square inch.

• Advantage:

This mix is appropriate for heavy loads such as soil-mixing areas and locations in the greenhouse where heavy equipment is used.

• Disadvantage:

It will not drain properly.

Porous concrete: Allows for drainage, will help prevent paddling, and still provides a barrier for weed control. Concrete will have a capacity to endure 600 pounds per square inch of surface.

• Advantage:

A four-inch floor of this mixture will adequately endure light vehicle traffic and personnel.

• Disadvantage:

Often not worth the initial savings.

Gravel or dirt floors: Use only with some fine rocks to stabilize the floor. These surfaces drain well and are great on hot days.

• Advantage:

It can be watered down and the heat of the day will cause evaporative cooling.

It is cheap to purchase and easy to install.

• Disadvantage:

Floors will be chronically muddy with frequent irrigations and will generally appear unacceptable, particularly in retail operations.

These muddy, unstable floors will be a liability because of the risk of customers falling and injuring themselves. But all floors have a problem: the cost and pollution they produce had led to look for others solutions such as eco-materials.

4.3.1 Connections and clamps

All steel components of the greenhouse structure should be connected by screws or clamps (Zabeltitz 1990), as it is important for the wind resistance of the greenhouse structure and the stable connection of the steel components.The clamps must not slide on the tubes, but have to be tightened firmly. After the mounting has been finished, it has to be checked whether all clamps, screws and bolts are screwed and fixed tightly.

4.3.2 Leaks

In the greenhouse structure leaks must be avoided, wherever they occur at doors, ventilation openings, plastic-film fastenings, etc., for the following reasons (Zabeltitz 1990):

1 Solar energy will be stored during the daytime and will keep the air temperature in unheated greenhouses some degrees above outside temperature at night.

2 If there are holes in the plastic film caused by the installation of the fastening clips, those holes are the starting point for damage to the plastic film caused by wind forces.

3 Leaks in the structure, vents and insect screens are not permissible when: the integrated production and protection (IPP) system is used, when there are useful insects inside the greenhouse and when pest insects must be kept out.

4 Leaks in gutters and cladding material cause rainwater penetration, crop flooding and disease infestation.

4.3.3 Windbreak

The relationship between windbreak structures and their function has been summa-rized as follows (Heisler and Dewalle 1988):

The horizontal extent of wind protection is generally proportional to windbreak height.

The wind speed reduction is related to the open area of the windbreak.

Very dense barriers are less effective than medium porous barriers for wind speed reduction of 10–30% at larger distances.

Height growth of a natural windbreak may be more important than density when areas as large as possible have to be protected.

Natural barriers with width less than height and a steep side produce a larger wind reduction over a greater distance than very wide windbreaks or streamlined windbreaks in cross-section.

Tree windbreaks lose less effectiveness in oblique winds than thin artificial wind screens.

Turbulent wind flow decreases with the increasing of the open area of the windbreak.

4.3.4 Insect screens

Used in front of the ventilation openings and doors to keep useful insects inside and to prevent pest insects from penetrating the greenhouse. Criteria for the choice of insect screens are (Zabeltitz 1990):

The species of insects to be screened out.

The influence on the greenhouse climate.

The UV stability and the mechanical durability (thickness of threads).

The cost in comparison to the economic value of the crop.

A disadvantage of insect screens is the reduction of the ventilation efficiency with influence on temperature and humidity, as well as the reduction of light transmittance.

The main factors of characterization of insect screens are: the porosity, the ratio of the open area against the total area of the screen, the mesh or hole size, the thread dimension (woven or knitted), The light transmittance, the color and its influence on pest behavior.

On the other hand it is important to mention that in addition to the different materials used in structures, claddings and foundations materials, there are new devel-opments that are moving materials towards the improvement of their mechanical properties and to radiation selectivity, quantity and quality (Theorem Ambient, 2002), which were designed to be sustainable and avoid environmental pollution without decreasing their quality or life span. For example in greenhouses components, in terms of covers you can find the following selection: photo-selective plastics, antivirus films, botrytis films, photodegradable films, multilayers, Drip, Biodegradable Films, Films for solarisation. With these kind of materials it is intended to develop longer-life span materials (with no reduction of their properties), one of the best options is to reduce at source the use of plastics, and use instead bamboo structures and foundations for hydraulic cement, all these alternatives belonging to the eco-materials.