What about building consumables? Types of building materials The use of various building materials in construction.
For the construction and decoration of buildings and structures, various artificial and natural building materials are used. The use of a particular material depends on the purpose of the structure, its properties and operating conditions.
The most widely used natural building materials are rubble stone, gravel, crushed stone, sand, clay, granite, marble.
rubble stone represents large pieces of sandstones, limestone, irregularly shaped granite from 150 to 500 mm in size. It is used for laying foundations, constructing walls of non-residential premises, paving slopes of canals and slopes of embankments of subgrade roads, etc.
Gravel- a loose accumulation of rocks, consisting of fragments of granite or basalt of a rounded form, ranging in size from 1 to 20 mm. By size, gravel is divided into fine, medium and large; by origin - on the river, lake, sea and glacial. Gravel is widely used in the preparation of concrete, the construction of the top coating of roads, the ballast layer of railways, as well as in hydraulic engineering construction.
Rubble - stone building material, which is obtained by crushing various rocks up to 5-70 mm. The strength of the rock corresponds to the strength of the original rock. Crushed stone is used as a filler in the preparation of concrete, for the construction of crushed stone pavement and pavement layers of highways, as well as for the construction of drainage layers of hydraulic structures.
Sand- fine-clastic loose mass, consisting of grains of various minerals and rocks. Sand contains quartz particles, feldspar crystalline grains, and a number of other minerals. Sand consists of fractions ranging in size from 0.1 to 2 mm. It is widely used in construction as an artificial underlay under foundations, for the preparation of concrete, various mortars and artificial stone materials.
Clay- rock, which includes kaolinite, montmorillonite and a number of other minerals, the size of which does not exceed 0.01 mm. Clay has the properties of plasticity, swelling, and when moisture enters, it can increase its volume several times.
Granite- igneous rock, which includes quartz, feldspar, mica and other minerals. Granite has a very high density, an average of 2600 kg / m 3. It can be machined and is used for facing floors, stairs, columns, walls, as well as for the preparation of high-strength crushed granite.
Marble- a rock of metamorphic origin, formed as a result of recrystallization of limestone. Marble is mined in quarries with the help of stone-cutting, percussion-cutting machines, wire saws. Marble, along with granite, is widely used as a finishing material, and both white marble and its colored varieties with different patterns, which appear after polishing, are used in construction.
In the construction of buildings and structures, shell rock, volcanic tuff, basalt, diabase, syenite, labradorite, and some other materials of igneous and sedimentary rocks are also widely used.
artificial stone materials are used in the manufacture of building structures at factories of reinforced concrete structures and reinforced concrete products.
Brick- one of the most widespread materials in construction. It is obtained by molding and firing a mixture of natural clay and additives in the form of sand and other materials. Bricks as a whole have the properties of water absorption (at least 8%), frost resistance, strength, thermal insulation; properties of specific types of bricks depend on their composition, production technology and purpose. Brick dimensions are $ 250x120x65 mm. Depending on the strength, the brick is divided into eight grades: 50, 70, 100, 125, 150, 200, 250 and 300. The higher the brick grade, the greater its compressive strength.
Cement- one of the most common mineral substances belonging to the group of hydraulic binders. The composition of cement includes calcium silicates, which are formed during high-temperature processing of limestone, clay, bauxite and a number of other minerals. As a result of the firing of natural cement raw materials, sintered clinker is formed, which is ground into powder and mixed with various active additives. The quality of cement depends on the fineness of the clinker, and consumers determine it by brand. Cement is produced in different grades, for example:
- 0 portland slag cement grades: 200, 300, 400 and 500;
- 0 plasticized Portland cement grades: 300, 400 and 500;
- 0 pozzolanic cement grades: 200, 300 and 400;
- 0 alumina cement grades: 400, 500 and 600.
Depending on the purpose, several types of cements are produced with different properties: fast-hardening, acid-resistant, expanding, sulfate-resistant, etc.
building lime belongs to the group of air binders. It is obtained by roasting and subsequent processing of carbonate rocks (limestone, chalk). Lime is slaked and quicklime. It is used for the preparation of mortars, silicate bricks and a number of other autoclaved silicate concrete products.
Building plaster obtained by firing natural gypsum - a fast-hardening binder. It is used in the production of gypsum concrete, plaster mortars and other gypsum products, and also as additives to cements.
Concrete- durable artificial stone material, which includes cement, gravel or crushed stone, sand and water. A mixture of these materials until hardening is called a concrete mix. Concrete is characterized by properties such as strength, density, impermeability, frost resistance, shrinkage and expansion, creep, and fire resistance. Concrete mix is produced by mechanical mixing of its components in special concrete concrete blocks with a capacity of 65 to 1600 liters or at special plants and delivered to the construction site in finished form or mixed directly on the construction site.
The best concrete mix is produced in factories where it is obtained in the most balanced and rationally selected composition. Depending on the design solutions, the concrete mortar is placed directly on the construction site into the structure being constructed or poured into a formwork specially designed for this purpose, which gives the mortar the required shape. By density, heavy and light concrete grades are distinguished from 25 to 600. Heavy concretes are mainly used in the construction of load-bearing structures of buildings and structures, and light ones - as a wall material, in such cases porous materials - expanded clay, pumice, vermiculite can be used as fillers .
In cases where the concrete mixture is placed in a formwork with a frame of steel reinforcement, after hardening, a structure is formed, called a monolithic reinforced concrete structure.
In our country, reinforced concrete structures are very widespread. The technological process of their creation consists of the preparation of a concrete mixture, the preparation of reinforcing cages, the formation, laying and compaction of the concrete mixture in an inventory metal formwork, as well as a special heat and moisture treatment of the structure in a steaming chamber to give the concrete the necessary strength by accelerating the hardening process. .
Building mixture is a mixture of water, sand and binder. Depending on the density, solutions are divided into heavy and light. For their preparation, mortar mixers with a capacity of 30 to 1800 liters are used. Mortars are used for filling joints in masonry and brick masonry, plastering surfaces, and for sealing joints in concrete and reinforced concrete structures.
asbestos cement It is formed by mixing water, cement and asbestos and has high mechanical strength in bending, low density, low thermal conductivity, resistance to leaching by mineralized waters, low water permeability and high frost resistance. Asbestos cement is used to produce fibrous or smooth sheets of roofing, facing slabs, pressure or non-pressure pipe products. In agricultural construction, asbestos-cement structures made of sheet asbestos-cement, heat-insulating materials and a wooden frame are widely used.
Astringents organic or inorganic composition form a separate group of building materials.
Mineral binders when mixed with water, they form a pasty mass, which hardens under the action of physicochemical processes.
From organic binders materials most widely used in construction and repair is bitumen- a material consisting of hydrocarbons and their derivatives and obtained as a result of the distillation of oil, from waste acid cleaning of lubricating oils, as well as from coal and peat. It is widely used in road construction for the production of asphalt concrete, for the manufacture of roofing material and glassine, for waterproofing walls and foundations.
Thermal insulation materials in construction, they are necessary to ensure the specified thermal regime of buildings, coops, pipelines, etc. The effectiveness of the selected insulation depends on the bulk density of these materials, which is expressed in kilograms per cubic meter of volume (kg / m 3).
Organic thermal insulation materials include wood fiber boards, reeds, foam plastics, fiber boards, and wood shavings and sawdust. Their volumetric weight is from 10 to 100 kg/m 3 . Inorganic heaters include lightweight concrete, aerated concrete, foam concrete, foam glass, glass wool, from which felt, mats, slabs, and a number of other insulating materials are produced. The volumetric mass of inorganic heat-insulating materials can reach 300 kg/m 3 .
Timber products are processed and unprocessed.
Round raw Timber products are widely used in construction as supports and logs for felling wooden buildings and structures, as well as as a raw material for sawing and processing.
To processed materialsinclude beams, edged and unedged boards, parquet riveting, wood veneer. Processed timber is obtained from the wood of coniferous and deciduous trees. Timber products have low density, strength, ease of processing, etc.
Wood is used to produce load-bearing and enclosing structures: beams, trusses, frames, arches, panels, window and door blocks. Details of various wooden structures are connected using nails, dowels, dowels, various metal fasteners, and glue. Wooden structures connected with glue have increased strength, lightness, durability, as well as fire resistance and low cost. It is these qualities that determine the widespread use of these materials in construction.
From waste wood and woodworking, fibrous and chipboards are obtained, which are widely used in housing, civil and industrial construction as a heat-insulating and finishing material. The use of such boards, finished with fine wood veneer of valuable species, makes it possible to effectively use scarce timber, as well as improve their decorative properties.
Metals and metal structures are widely used in construction, as they are relatively light, high strength and combined with materials of any kind. Steel structures are manufactured from structural steel using the industrial method and joined together by welding or rivets. In construction, aluminum alloys are also widely used, which are distinguished by high specific strength, decorative effect, and good anticorrosion properties. Wall panels, suspended ceilings, window frames, finishing and profile sheets are made from them.
Roofing materials used for roofing. These include asbestos-cement sheets and tiles, roofing material, roofing felt, tiles of various types, galvanized sheet steel, but the latter is used quite rarely, as it has a high cost. Roofing materials are characterized by water resistance, durability, frost resistance, fire resistance.
Decoration Materials give buildings and structures high aesthetic qualities, as well as protect structures from external influences. This group of materials includes: finishing plasters, natural and artificial stone materials, ceramic products, paints, varnishes, glass, wallpaper, linoleum, veneer, chipboard and metals.
plastics widely used in construction. They are lightweight and have high specific strength. Plastics are used to manufacture floor coverings, plumbing equipment and pipes for various purposes, including for water management construction, skirting boards, handrails, and facing material.
Film materials have become widespread in land reclamation construction as an impervious material in the construction of canals and reservoirs for various purposes.
asphalt concrete, used in road construction, is obtained from a compacted and rationally selected mixture of crushed stone, sand, mineral powder and bitumen in asphalt mixing plants.
Building materials and products used in the construction, reconstruction and repair of various buildings and structures are divided into natural and artificial, which, in turn, are divided into two main categories. The first category includes general-purpose building materials: brick, concrete, cement, timber, roofing material, etc. They are used in the construction of various elements of buildings (walls, ceilings, coatings, roofs, floors). To the second category - special purpose: waterproofing, heat-insulating, refractory, acoustic, etc.
The main types of building materials and products are: natural stone building materials and products made from them; binders, inorganic and organic; artificial stone materials and products and prefabricated structures; forest materials and products from them; metal products, synthetic resins and plastics. Depending on the purpose, conditions of construction and operation of buildings and structures, appropriate building materials, products and structures are selected that have certain qualities and protective properties from exposure to various external environments. Given these features, any building material must have certain construction and technical properties. So, for example, the material for the exterior walls of buildings (bricks, concrete and ceramic blocks) should have the lowest thermal conductivity with sufficient strength to protect the premises from the outside cold and withstand the loads transferred to the walls from other structures (floors, roofs); material of structures for irrigation and drainage purposes (lining of canals, trays, pipes, etc.) - water impermeability and resistance to alternate moistening (during the field season) and drying out (during breaks between irrigation); pavement material (asphalt, concrete) must have sufficient strength and low abrasion to withstand the loads from passing traffic and not collapse from the systematic effects of water, temperature changes and frost.
Starting to study the section "Building materials and products", it is necessary to understand that all building materials and products can be classified into groups according to various classification criteria: types of products (piece, roll, mastic, etc.); the main raw materials used (ceramic, based on mineral binders, polymeric); production methods (pressed, roller-calender, extrusion, etc.); purpose (structural, structural and finishing, decorative and finishing); specific areas of application (wall, roof, heat-insulating); origin (natural or natural, artificial, mineral and organic origin).
Building materials are divided into raw materials (lime, cement, gypsum, raw wood), semi-finished materials (fibreboard and chipboard, plywood, beams, metal profiles, two-component mastics) and ready-to-use materials (bricks, ceramic facing tiles, tiles for floors and suspended acoustic ceilings).
The products include carpentry (window and door blocks, panel parquet, etc.), hardware (locks, handles, other carpentry fittings, etc.), electrical (lighting fittings, sockets, switches, etc.). etc.), sanitary products (baths, sinks, sinks and fittings for them, etc.). Products include parts of building structures - concrete and reinforced concrete wall and foundation blocks, beams, columns, floor slabs and other products of reinforced concrete products factories and construction industry enterprises.
When classifying materials and products, it must be remembered that they must have good properties and quality. Property - a characteristic of a material (product) that manifests itself in the process of its processing, application or operation. Quality - a set of properties of a material (product) that determine its ability to meet certain requirements in accordance with its purpose.
The properties of building materials and products are classified into three main groups - physical, mechanical, chemical. Important properties that affect the choice of a method for the production of building materials are manufacturability, i.e., the simplicity and ease of processing or processing them to obtain products of the desired shape and size, and energy intensity - the amount of energy required to extract raw materials and obtain building material and products from it.
When evaluating the economic efficiency of building materials, in addition to these properties, the durability of the material is very important, which is characterized by its service life in the structure without repair and restoration or replacement.
If the materials are mined near the construction site, they are called local building materials. The cost of such materials is significantly reduced due to savings in transportation costs.
Light steel thin-walled structures have good thermal performance, low cost, ease of construction. The LSTK technology allows you to build prefabricated houses, cottages, apartment buildings, etc.
All building materials by type are divided into natural and artificial. At the same time, artificial ones include those that are subjected to thermal, chemical or other processing during the manufacturing process, which changes their structure, chemical composition, etc.
In construction, the following types of building materials are mainly used:
- natural timber and artificial materials made from wood;
- metals;
- stone materials - natural and artificial;
- binders or simply binders - mineral and organic (lime, cement, asphalt, etc.);
- mortars and concretes;
- special building materials - heat-insulating, waterproofing, roofing, finishing, etc.
The above classification is conditional, since brick, concrete, and even window glass are essentially varieties of stone materials. Therefore, unlike machines and equipment, which are made mainly of metals, buildings and structures in many cases are built almost entirely of stone!
The need for a separate consideration of concretes and mortars is dictated by their special significance in modern construction.
Widely introduced synthetic materials (plastics), which are a kind of artificial materials, are still used in construction on a limited scale - for floors, wall decoration, thermal insulation (porous plastics), etc.
One of the most important properties of building materials used for load-bearing structures is strength.
In construction, two indicators of strength are mainly used:
- for brittle materials (stone, concrete) - compressive strength (tensile strength);
- for ductile (mild steel) - yield strength.
In both cases, strength is measured in kg/cm2 (sometimes in kg/mm2).
Materials for enclosing structures must first of all have a sufficiently low coefficient of thermal conductivity.
Coefficient of thermal conductivity k is measured in kcal / m - deg - hour. Its direct determination is possible only in laboratory conditions.
A very convenient and easier to determine indicator that characterizes the heat-shielding properties of materials quite well is volumetric weight - the weight of a unit volume of a material in its natural state (i.e., in the presence of pores and voids in it).
Besides, volume weight directly affects the dead weight of individual structures, as well as buildings and structures in general, and, therefore, determines the tonnage of transportation of large quantities of materials used by the construction industry.
For dense materials such as steel, the bulk density is the same as the specific weight; for porous materials, the bulk density is less than the specific weight.
The volumetric weight of building materials is usually determined in kg / m3 or in T / m3.
moisture permeability(or rather, impermeability) is the main property of roofing, waterproofing and other materials.
Frost resistance is an important indicator for exterior wall materials subject to alternate freezing and thawing (in the outer layers). It is verified by repeated freezing and thawing of samples in a water-saturated state and is estimated by the number of test cycles that the samples can withstand without a significant decrease in strength and weight loss. Frost resistance is indicated by the Mrz symbol with the addition of a number indicating the number of cycles, for example, Mrz 15, Mrz50. Frost resistance significantly depends on the water absorption of the material, since the destruction during freezing is due to the expansion of water when it freezes in the pores of the material.
fire resistance. In relation to the action of fire (in case of fire), building materials are characterized by combustibility, and building elements by fire resistance.
On the basis of combustibility, materials are divided into 3 categories:
- combustible (wood),
- fireproof (stones, metals)
- and slow-burning, which ignite and continue to burn or smolder only in the presence of a source of fire.
The fire resistance of structures is characterized by the fire resistance limit (hour), showing the duration of the structure's resistance to fire in case of fire, which depends both on the type of material used and on the thickness of the structure, its massiveness, etc. For various elements of buildings, the fire resistance limit is set by norms from 0.25 up to 5 o'clock.
The concepts of incombustibility and fire resistance do not always coincide. For example, such a fireproof material as steel has a relatively low fire resistance, since at temperatures above 500-600 ° the elastic modulus and strength characteristics of steel are sharply reduced and the structures undergo catastrophic deformations.
Heat resistance requirements are imposed on materials intended for operation at high temperatures, and fire resistance at very high temperatures.
Materials operating in conditions where their corrosion is possible must have sufficient corrosion resistance. Under the influence of various chemical agents, most building materials (steel, concrete, masonry, etc.) are susceptible to corrosion.
The resistance of organic building materials to decay is referred to as bioresistance. By using various antiseptic agents, the biocompatibility of materials can be increased, but usually only for a limited period of time.
The wide scope of construction in the Soviet Union is accompanied by an expansion in the production of local materials and the introduction of new types of materials into construction practice, as well as an increase in building parts and semi-finished factory-made products. The main building materials include: forest materials, natural stone, ceramic, mineral binders, concrete and products made from them, artificial stone materials, bituminous and heat-insulating materials, metal products, etc.
Forest materials- pine, spruce, fir, cedar and larch are widely used in construction. These materials are divided into roundwood (logs, bollards and poles) and sawn timber (plates, quarters, boards, slabs, beams and bars). In construction, wood with a moisture content of not more than 20% is used. To protect the wooden structures of buildings from moisture and decay, they are coated or sprayed with antiseptics (tar, creosote, etc.)
natural stone materials used in construction both without processing and after preliminary processing (splits, hewing and sawing). The volumetric weight of natural stones ranges from 1100 to 2300 kg / m3, and their thermal conductivity coefficient ranges from 0.5 to 2. Therefore, rubble and cobblestones are used mainly for laying foundations, paving roads and for processing into crushed stone. Rocks are also used to make lime, gypsum, cement and bricks. Materials such as sand, gravel and crushed stone are used as aggregates for the preparation of concrete.
Ceramic materials and products- These are artificial stone products that are obtained by molding and subsequent firing of the clay mass. These include porous ceramic products (ordinary clay brick, porous brick, hollow brick, facing tiles, roof tiles, etc.) and dense ceramic products (clinker and floor tiles). Recently, a new material, expanded clay, has been widely used in construction. This is a light material in the form of gravel and crushed stone with accelerated firing of fusible clays. During firing, the clay swells and a porous material with a bulk density of 300-900 kg/m3 is obtained. Expanded clay is used for the manufacture of concrete and reinforced concrete.
Mineral binders- these are powdery materials, when mixed with water, form a pasty mass, which gradually hardens and turns into a stone-like state. There are air binders that can harden only in air (building gypsum, air lime, etc.), and hydraulic ones that harden not only in air, but also in water (hydraulic lime and cements).
concretes and products from them - artificial stones obtained as a result of hardening of a mixture of binder, water and aggregates (fine sand and coarse gravel or crushed stone). Concretes are heavy (volumetric weight above 1800 kg/m3), light (volume weight from 600 to 1800 kg/m3) and heat-insulating or cellular (volume weight less than 600 kg/m3). Cellular concrete includes foam concrete and aerated concrete.
foam concrete obtained by mixing cement paste or mortar with a special, stable foam. To obtain aerated concrete, gas-forming substances are introduced into the cement paste containing sand, slag and other aggregates. Concrete structures and parts into which a steel frame is introduced - reinforcement consisting of steel rods interconnected by welding or connected by wire, are called reinforced concrete.
Artificial stone non-fired materials- these are gypsum and gypsum-like products (slabs and panels for partitions and sheets of dry plaster, magnesite) used for flooring and making fiberboard, silicate products (silicate brick, etc.) and asbestos-cement products, smooth roofing slabs and corrugated sheets (slate) .
Bituminous materials contain natural bitumen or tar oils, pitches, raw tars in their composition. A mixture of bitumen and sand is called asphalt mortar, used as a base for laying tile floors, asphalt floors and for waterproofing. Bituminous materials include roofing material, glassine, hydroisol, borulin, roofing felt. These materials are used for roofing, waterproofing and vapor barrier.
Thermal insulation materials used to protect rooms or individual structures from heat loss or from heating. These materials have high porosity, low bulk density and low thermal conductivity up to 0.25. There are thermal insulation materials of organic and mineral origin. Organic include: fibreboard (hardboard) from crushed wood fiber; straw and reeds - slabs pressed from straw or reeds and stitched with wire; fibrolite - plates pressed from wood shavings bound with a magnesian binder solution. Of the mineral heat-insulating materials, foam concrete and aerated concrete, mineral wool, foam silicate, etc. have become widespread. Recently, products based on plastics have been introduced into construction practice. This is a large group of materials, which is based on natural artificial high-molecular compounds. For sheathing the interior surfaces of the room, you can use aluminum sheets that reflect thermal radiation from animals and heaters.
Questions:
1) The main types of building materials;
2) Advantages and disadvantages of structures made of reinforced concrete, stone, steel, wood;
The main types of building materials are: reinforced concrete, steel, stone (artificial and natural), wood. Artificial stones include ceramic and silicate bricks, as well as concrete, slag concrete, foam concrete, aerated concrete, polystyrene concrete, ceramic and other blocks. Natural stones include blocks of tuff, shell rock, limestone, buta, etc. Aluminum, duralumin, polymers, bitumen and tar are also used for the manufacture of building structures.
The variety of materials and structures used in construction is determined by the large number of requirements imposed on them (strength, deformation, heat engineering, fire protection, acoustic, economic, aesthetic, etc.). There is no ideal building material that meets all these requirements.
Structures made of different materials have their own advantages and disadvantages.
Concrete structures were known before our era. However, a real breakthrough in construction was the invention of reinforced concrete in the middle of the century before last. Although reinforced concrete structures began to be widely used in the 1950s. Concrete is a composite material made using aggregates (gravel, crushed stone, sand) and a binder (adhesive composition). Reinforced concrete is a material consisting of concrete and reinforcement. The term reinforced concrete is traditional, but not entirely correct. The fact is that iron used to be called steel, which is now used for reinforcement. Concrete structures are not widely used due to its serious drawback. Concrete performs well in compression but poorly in tension. Steel, on the contrary, works well in tension, and loses stability at high compressive stresses. Therefore, the main principle of designing reinforced concrete structures is the installation of reinforcement in zones stretched during operation, manufacture, transportation and installation. The essence of obtaining such a highly effective material lies in a number of factors:
1) steel and concrete have approximately the same coefficients of thermal expansion;
2) concrete is resistant to many aggressive influences and perfectly protects steel from them;
3) concrete has a high heat capacity, which protects the reinforcement during emergency temperature effects (fires);
4) concrete and reinforcement mutually compensate for each other's shortcomings under force effects (tensile and compression).
Reinforced concrete structures have the following advantages:
1) strength, especially in compression and bending;
2) rigidity;
3) durability;
4) fire resistance and fire resistance;
5) resistance to aggressive influences;
6) the ability to be made in any shape;
7) industrialism.
Despite all the advantages, reinforced concrete has a number of disadvantages. Concrete has a high thermal conductivity. It is problematic to make enclosing structures from reinforced concrete. There are ways to increase the heat-insulating ability of concrete: the manufacture of air voids (hollow blocks), increasing porosity (foam and aerated concrete), the introduction of heat-insulating materials (polystyrene, slag, expanded clay concrete, etc.). All these methods lead to a change for the worse in the strength and deformation properties of manufactured products and structures.
Reinforced concrete structures are heavy. In this regard, their use in high-rise and large-span structures is difficult.
Reinforced concrete is a porous material with open and closed pores. This contributes to its water and breathability. It is possible to make tanks and pipelines for some liquids from reinforced concrete, but it is impossible to make gas holders.
Prefabricated reinforced concrete structures require additional steel consumption for embedded parts for their connection. In addition, they often require additional reinforcement due to the peculiarities of transportation and installation. However, prefabricated structures are highly industrial and require less time for manufacture and installation, which reduces construction time.
stone structures by the nature of work under load and by properties similar to concrete. Stone is one of the ancient building materials. Stone materials work well in compression and poorly in tension. They are resistant to aggressive influences, fire-resistant, fire-resistant, durable. However, such designs have several disadvantages:
1) it is difficult to make bendable structures from stone and it is almost impossible to make stretched ones;
2) they cannot take a variety of forms;
3) they have low industrialization, which leads to an increase in construction time;
4) they have a high thermal conductivity, which leads to an overrun of the material;
5) they are heavy.
3) high operating costs.
Wooden structures without special measures have low durability. In addition, one should be aware of the weak reproducibility of this resource.
In the oil and gas industry, wooden structures are used for temporary buildings, as well as for the production of temporary retaining walls in