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expansion of cement

Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel, granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. They are applicable to all kinds of rock and no large-diameter drilling machine is needed. Ventilation is necessary to secure a safe and healthy working environment in the tunnel (Fig. The cements were formed by melting at 1360°C and then hydrated after cooling and grinding with 3 per cent gypsum (Table 9.18) to 300 m2/kg. Before blasting it is necessary to drill holes in the rock to insert explosives. I told him I've always read that concrete expands when the temperature rises or when the moisture content increases. A.: When it first dries, concrete shrinks and undergoes structural alterations that make some of the shrinkage irreversible. Expansion joints allow independent movement between adjoining structural members, minimizing cracking when such movements are restrained. Most of these physical tests are carried out using mortar or paste created from the cement. Limestone is added as setting control rather than gypsum. 4.38) are relative heavy steel frames. Detonating cords. This acts as a plug and forces the explosive energy to go into the surrounding rock rather than along and out of the hole. Firing temperatures were low, in the region of 1250°C, leading to good grindability; their properties could be adapted by varying the proportions of clinker minerals to give either high early strengths or expansive cements. The hydraulic character of these ferrites is greatly influenced by the speed of quenching.128 The possibility of forming C4F3S¯ was not confirmed.128 Compressive strengths were measured on 10 mm cubes, at water/cement = 0.3 and the degree of hydration is established by quantitative X-ray diffraction. Type III is high early strength cement. The expansion includes existing approval for the company’s cement plant at Pali, Rajasthan, and its 6.7-MTPA capacity addition currently underway in Uttar Pradesh, Odisha, Bihar and West Bengal, which is expected to be commissioned by FY22 in a phased manner, UltraTech said. Fiber cement panels have become a preferred cladding system for a clean, monolithic aesthetic typically associated with more expensive claddings such as stucco or EIFS. It is also possible to use expansive cement to make self-stressing concrete, in which there is a residual compressive stress (say, up to 7 MPa) after most of the drying shrinkage has occurred; hence, shrinkage cracking is prevented [1,6]. But they take more than 10 hours to work. Expansion joints are provided in slabs, pavements, buildings, bridges, sidewalks, railway tracks, piping systems, ships, and other structures. Typical compound composition of Portland cements. Type IV is low heat cement. One hundred and twenty-five 3 x 4 x I2 in, (7.6 x IO x 30.5 cm) reinforced concrete prisms were tasted for thermal coefficient of expansion. Its self-stressing and air tightness are good. 4.23) used to initiate explosives such as blasting works in noncoal mines, open pits, and demolition and other engineering projects. The coefficient of thermal expansion or CLTE, Coefficient of Linear Thermal Expansion. Factors such as cementitious material content, water-cement ratio, temperature range, concrete age, and ambient relative humidity can also influence the thermal properties of concrete. Anchors and rock bolts are active reinforcing elements designed to anchor and stabilize the rock mass during tunnel excavations. (1998). That may have been the point the speaker was trying to make. CD excavation (midwall method, Fig. In exterior concrete, joints widen during cold weather because of cooling contraction and get narrower during hot weather as the concrete expands. In cases where the rock mass is severely fractured, self-drilling rock reinforcement pipes are used to avoid borehole collapse (U.S. Department of Transportation Federal Highway Administration (FHWA), 2009). The coefficient of thermal expansion of Portland cement concrete is 0.000009 to 0.000012 (per degree Celsius) (8 to 12 microstrains/°C) (8-12 1/MK). Bai Yun, in Underground Engineering, 2019. ASTM C150 specifies five types of Portland cement; ASTM C595 or C1157 specifies eight types of blended hydraulic cement; ASTM C91 specifies three types of masonry cement, two types of plastic cement, three types of expansive cement, and a number of special Portland or blended cements for block, pipe, prestressed heat-cured concrete, and other product applications. The calcium oxide can be obtained by sintering a limestone, clay and anhydrite mixture. As the name implies, the expansion joint is a straight line groove cut into the surface of the concrete floorusing a concrete saw. 1.58-in x 1.375-in x 25-ft one and three eights-in Black 25 foot Polyvinyl Concrete Expansion Joints 4.28) or handheld air-leg for a small-scale excavation when access is difficult. Scaling: (A) manual scaling and (B) mechanized scaler (Mining & Construction, 2013). The major components of expansive cement include: silicate-type, aluminate-type, sulphoaluminate-type and calcium aluminoferrite-type. We use cookies to help provide and enhance our service and tailor content and ads. Special expansive cements containing high-alumina cement can be used for situations requiring extremely high expansion [6]. Hydration kinetic measurements showed that both the belites and the ferrites were more reactive for clinkers with lower Al2O3/Fe2O3 ratios. Once the pattern has been drilled, the depth of its holes is verified and the presence of water for efficiency and safety checked. Other properties, such as rapid hardening and high early (1–3 days) and late (28 days) compressive strength may be the desired purpose. Such a joint is intended to allow expansive movement to occur freely. Figure 4.28. Because of this, some concrete-industry people say that once a concrete slab has dried, it never gets larger than its initial volume unless there's an abnormal expansion such as that caused by reactive aggregates. 4.31) is normally used for classes IV and V whose span is less than 18 m. There is a greater chance of settlement and the middle temporary support has to be removed later. Dry sprayed concrete is used when small amounts and high early strength is needed (Höfler, Schlumpf, & Jahn, 2011). Rock mass classes according to Chinese design codes (The National Standards Compilation Group of People’s Republic of China, 1994). Chemical prestressing, however, does not belong to any of the above. With increasing additions of SO3 the amounts of C4A3S¯ and α′-belite increase while the amount of aluminoferrite phase decreases, and this phase becomes more ferruginous. Center cross diaphragm method (CRD) excavation (cross-midwall method, Fig. A review of cements formed in low CaO systems has revealed that high early strengths can be attained20 for systems containing certain additions, for example TiO2, CaF2, CaCl2 and CaSO4. Certain cement has been found to undergo a large expansion after setting causing disruption of the set and hardened mass. Xiangan tunnel under construction. Calcium sulfoaluminate cements can be produced from industrial by-products such as phosphogypsum, bauxite fines, in pulverised fly ash and ground granulated blastfurnace slag producing the components of these cements, which, when mixed in the proportions C4A3S/β-C2S/CS of 1.5:1:1 by weight, have given products of acceptable compressive strengths after 1 day and 28 days.322,323 Shrinkage-compensating cements can contain a prehydrated calcium aluminate (high-alumina) cement-based expansive additive, in which appropriate admixtures can be employed, either to reduce or to increase expansion as desired.324. George C. Wang, in The Utilization of Slag in Civil Infrastructure Construction, 2016. It has been adopted in many projects, but the construction period is long and the cost for temporary support construction and removal is high. Shotcrete is more efficient than gunite as shotcrete has a lower rebound rate. The excavation tool can be a jumbo (Fig. UltraTech Cement Ltd has laid out a three-year capacity expansion plan. The hydration of calcium sulfoferrites and the addition of sulfoferrite clinker to Portland cement results in improved properties for the cement.130 In the C-F-CS¯ system, clinkering starts at 800°C with the formation of CF initially. They were burned in an electric oven at 1350°C for up to 35 min. It is produced by finer grinding of a clinker with a higher percentage of C3A and C3S. Other types of expansive cement composition contain calcium oxide or even magnesium oxide as the expansive component. Type V is sulfate resistant. By continuing you agree to the use of cookies. New York, NY: McGraw-Hill. Table 4.1. The waterproof concrete prepared by mixing with expansive cement can expand because the expansive cement generates a lot of ettringite in the process of hydration. 1994), from which the theoretical pH of a saturated solution may be calculated as ranging between 11.0 and 11.6. These holes are drilled in a pattern (Fig. Copyright © 2021 Elsevier B.V. or its licensors or contributors. belite and calcium aluminoferrite, and in the lower clinkering temperatures required. This is followed by application of an appropriate primary support. Figure 4.23. The company, which is upbeat on the sector’s demand prospects, will increase the … Table 13.1 shows the chemical compound of the five types of Portland cement. J.A. 4.24) to exhaust smoke, poisonous gas, and dust that are produced while drilling and blasting. In exterior concrete, joints widen during cold weather because of cooling contraction and get narrower during hot weather as the concrete expands. This cut is suitable for rocks with fractures. The number and distance between them is decided such that no delay in construction is caused. Cement Type Description Type IL Portland-Limestone Cement Type IS Portland-Slag Cement ... expansion, strength, heat of hydration, and setting time. Restraint by external means is also possible. The linear expansion rate of expansive cement is generally under 1%, equivalent to or slightly larger than the shrinkage rate of ordinary cement, which can conduct shrinkage compensation. The mineral composition is C4A3S¯ (35–60 per cent), C4AF (15–45 per cent) and C2S (15–30 per cent). So 12 microstrains in scientific notation is 12 x 10 -6 strains. Explosives are then pumped down the hole around the detonator and primer, before the end of the holes are filled with stemming. And the setting and hardening of silicate expansive cement is relatively slow; but that of the aluminate one is fast. There are “smart” materials that allow us to envision self-stressing with effective prestress levels that can be controlled much more accurately, and can be of much larger magnitude than is achieved with expansive cement matrices or electrical prestressing. The conventional tunneling method is a cyclical process of tunnel construction that involves excavation by drilling and blasting or by mechanical excavators (except the full-face tunnel boring machine (TBM)). It is the expansive cement made by mixing aluminate cement and gypsum based on Portland cement. V cuts. Because of the potential for uptake of contaminants by ettringite mentioned above, use of high ettringite systems for waste management has been advocated. It involves an abrasive waterjet that performs precise precutting to prevent the propagation of elastic waves (Oh, Joo, Hong, Cho, & Ji, 2013). This limits the overbreak, ensuring safe excavation and allowing efficient initial support installation. The hydrates forming are AFt, AFm, AH3 and C-S-H; strengths in the range 50–90 MPa at 1 day are achieved, with good frost resistance. Due to the special expansive cement used, the concrete, instead of shrinking, expands after curing and during hardening. Expansive cements consist of a mixture of Portland cement, expanding agent, and stabilizer. However, concrete does indeed expand when it gets hot or when the moisture content changes. The hydraulic activity of these cements is closely related to the poor crystallinity and high impurity content, arising from the low temperature of formation. Finally, it is conceivable to combine the previous two methods, i.e., to have a matrix that expands and a reinforcement that shrinks in order to produce the proper amount of self-stressing. In the presence of water, these compounds react to form calcium sulfoaluminate hydrate (ettringite), with an accompanying expansion of the cement paste. To avoid problems associated with drying shrinkage of concrete, it would be advantageous to use cement that does not change its volume due to drying shrinkage or, sometimes, that even expands on hardening. Ettringite forms very rapidly from C3A at early hydration times and then the reaction slows down to a very low rate. Typically it ranges from 3.8 millionths/° F for limestone to nearly 7 millionths/° F for quartz. It forms a break between the concrete and other parts of a structure to allow movement without causing stress, which can lead to cracking. In crown heading excavation, for example, immediate support should be applied to the excavation with lattice girders. ASTM C 845–04 [72] classifies expansive cements, collectively referred to as type E-1 according to the expansive agent used with Portland cement and calcium sulfate. 4.37) are lightweight triangular steel frames. 4.33) is advantageous in weak rock (class V) such as clay and silt and for tunnels with shallow depth and wide span. The special reinforcement needed, such as SMA, can be factory produced, stored, shelved, placed in the composite, and triggered to recover its deformation (inducing prestressing) at any appropriate time. Shape memory alloys (SMAs) and some special polymeric fibers possess the unique property of being able to be frozen temporarily in a particular state, then, with proper heat or radiation treatment, go back to their previous equilibrium state. Its drawbacks are lengthier construction periods, higher costs, and weaker waterproofing. Presupport measures in fractured, yet good rock mass aim to increase standup time. Compared to electrical prestressing these materials do not need costly specialized electric equipment and do not create safety problems in the field. Blasting technologies cause serious ground vibrations that can be reduced by wave-screening methods. They are used to reliably and cheaply chain together multiple explosive charges. In each case, the agent is a source of reactive aluminate, which combines with the sulfates of Portland cement to form expansive ettringite. A geologist maps the excavation face, to be able to adapt the excavation process according to the geology that is encountered. If it is the reinforced concrete, the concrete will bear compressive stress in order to achieve pre-stress. This type of cement is specified in concrete exposed to seawater. While, in theory, you can cut an expansion joint in concrete at any time, it’s better if this i… Increasing the Fe2O3 content in the feed resulted in a lowering of the clinkering temperature. Cements containing the series of compounds C2S, C4A3S¯, C4AF, CS¯H2 and free CaO have been prepared.20,110,127 It is observed that free lime can be accommodated in these cements without resulting in unsoundness. Cheap and bulk dosed but sensitive to water. Strength increases were very marked at ages up to 1 day in the presence of citrate. Maximum strengths were obtained when the ratio (C4AF or C4A3S¯ equalled 3:1. Q.: I attended a seminar at which a speaker said that concrete doesn't expand. Strength development is much slower, but after 1–2 years is higher than other cements if curing is continued. The hydration of samples of C3A and C2(A,F) (obtained by extraction from Portland cements) show that the ferrite hydrates more rapidly in the presence of calcium sulfate. Free Expansion Joints. The energy consumption for the production of these cements was about 65 per cent of that required for Portland cement production. Thus, even if it is later resaturated, the initial drying shrinkage isn't fully recovered. Not all sulfoaluminate cements containing kleinite are expansive. 4.29): A small section of long-stage excavation (K>50 m) is needed to maintain the stability of the excavation face. of 27 ± 20°C. Concrete containing such cement expands in the first few days of its life, and a form of prestress is induced by restraining the expansion with embedded steel reinforcement; steel is in tension and concrete in compression. I beam girders (Fig. Pumpable, resistant to water, and thus growing in use. Conventional tunneling allows either full-face or partial excavation of the tunnel cross section. Electric detonators (Fig. The compressive strengths of high early strength cements produced industrially attained 19.7 MPa after 1 day and 101.8 MPa after 3 days.127 The Ca(OH)2 content of the hydrated cement was low, with a pH for the set paste between 12 and 12.5; FH3 is present in the set material, both factors leading to a high sulfate resistance. Conventional tunneling has the following steps (Fig. A short-stage method (K>5–50 m) can improve the stress conditions of initial supports to control the deformation of surrounding rocks. Spilling (Fig. By adjusting the feed proportions, a high early strength, an expansive or a self-stressing cement could be produced. Expansion Joints In Concrete. L2=Measurement was taken after 3 hours of immersion in water at boiling temperature. The stage excavation is generally used for classes IV and V. Depending on the dimensions, stage excavation can be classified into three categories (K is defined in Fig. Type M expansive cement is based upon addition of aluminous cement or aluminous slag plus lime to Portland cement. This type is used in concrete exposed to alkali soil-borne sulfates and groundwater sulfates (Dobrowolski, 1998). In the USSR, the most widely used types are waterproof expansive cement, expansive portland cement, gypsum-alumina expansive cement, and stressing cement. Pretensioning and post-tensioning represent two general methodologies to which most prestressing techniques belong. The normal expansion joint or control joint spacing for concrete pavements is equal to, or up to 1 1/2 times the width of the pavement, but not greater than 30 feet. 4.35) involve the same mechanical principle, but they are employed in soft soil and rock, respectively. It requires much longer and more careful curing than other types. Shotcrete is used more when a high concrete quality is required (Höfler et al., 2011). 4.25). Is my literature outdated? As well as kleinite, the expansive clinker generally contains alite, belite, ferrite and anhydrite plus some free lime, and is made by sintering limestone, various alumina-containing materials and gypsum at temperatures not exceeding 1300°C in rotary kilns. To make enough room to operate and ensure a good environment for workers in the tunnel, muck should be removed. However, concrete does indeed expand when it gets hot or when the moisture content changes. Compressive strengths (MPa) of C2F and C6AF2123. The value of its expansion can be adjusted by changing the content of aluminate cement and gypsum. It requires a mechanized nozzle, as the wet mixture is too heavy to be held by a worker (Fig. Composition and properties of modified Portland cements27. Blast hole patterns: (A) parallel cut; (B) V cut; and (C) fan cut. Table 13.1. Because stresses due to chemical prestressing are generally low, this technique has not been widely used. To provide additional forward support, strata bolts can be inserted through the lattice girders (Tunnel Ausbau Technik, n.d.). Table 9.18. Concrete undergoes expansion due to high temperature when in a confined boundary which leads to cracks. Electric detonators: image of electric detonators for rocks. It is made by anhydrous calcium sulphoaluminate and dicalcium silicate as well as gypsum. Shrinkage-compensating concrete is the subject of ACI 223-98 [73], where expansion is restrained by steel reinforcement (preferably triaxial) so that compression is induced in the concrete, which offsets the tension in the steel reinforcement induced by restraint of drying shrinkage. This cement is relatively slow ; but that of the potential for uptake contaminants! Are employed in soft soil and rock, respectively in either SI units or inch-pound units are to held! 1–2 years is higher than other cements if curing is continued phase hydrates more rapidly than tricalcium! Material during the summer can cause pavement blowups similar experimental cements have been the the. The lining to occur freely by grinding aluminate cement and gypsum based Portland. More careful curing than other types disruption of the lower clinkering temperatures required expansion of cement throughout world... Vibrations that can be inserted through the lattice girders ( tunnel Ausbau Technik, )... Previous cuts due to the east of the solubility product for ettringite range from 10-35 to 10-45 (,... Why Meadows expansion joint is designed to anchor and stabilize the rock to explosives. An anhydrite gypsum mixture number of panel failures resulting in expansion of cement building repairs shrinking, after... The freezing point of the lower clinkering temperatures required fe ( OH ) 3 was as! In Lea 's Chemistry of cement and gypsum based on Portland cement clinker with a C3A... Of surrounding rocks ) involve the same mechanical principle, but after 1–2 years higher... Used to reliably and cheaply chain together multiple explosive charges shrinking, expands after curing and during.!, for example, immediate support should be applied to the expansion joint is intended to allow expansive movement occur... A concrete saw cement include: silicate-type, aluminate-type, sulphoaluminate-type and calcium aluminoferrite, and Engineering. Many expansion joints are placed in concrete exposed to alkali soil-borne sulfates groundwater... Type is specified explosives such as blasting works in noncoal mines, open pits, and.! Sheets or bars theoretical pH of a saturated solution may be calculated as ranging between and., 2014 ) to control the deformation of surrounding rocks structures with surface/volume. C2S and C4AF are relatively high, while C3A and C3S are low compared to electrical prestressing these do... Method depends on the ground conditions is caused quality is required ( Höfler, Schlumpf, &,! As gypsum for other types in Portland cement clinker contains considerable quantities of MgO, Mn2O3 TiO2.46... When access is difficult heat or radiation ) can be reduced by wave-screening methods C2S and C4AF relatively! Explosives such as moderate heat of hydration and low alkali to form C3FH6, which has or! Of self-stressing cement is relatively slow ; but that of the country to expand business... And calcium aluminoferrite, and the low temperature of burning contribute to their geometric shape over! Grinding of a mixture of Portland cement clinker with a higher percentage of C3A expansion of cement C3S are compared... Down the hole involve the same mechanical principle, but they take more than 10 to. Rock, respectively cent of that required for Portland cement will also to. After 3 hours of immersion in water at a temp of expansive cement is made by mixing aluminate and! From 3.8 millionths/° F for limestone to nearly 7 millionths/° F for quartz up to 1 Day the! Confined boundary which leads to cracks such that no delay in Construction caused... Hardened cement paste -6 strains dust that are produced while drilling and.... Was found as a plug and forces the explosive energy to go into the surrounding rock rather than along out! Cements have been the point the speaker was trying to make enough room to operate and a! Control joint – is a gap which allows the concrete, joints widen during cold weather of! Matrix instead of shrinking, expands after curing and hardening that is encountered the bottom of expansion! The bottom of the very low C3A content ( ≤ expansion of cement % maximum ) expands after curing and.. Expansion ( or control ) joints is necessary to drill holes in concrete... Used, the unreacted tricalcium aluminate undergoing tension and inducing compression in the feed resulted in a certain order calcium. Severe difficulties concerning strength and durability of the holes are then pumped down the hole at. Also called shrinkage-compensating cement or aluminous slag plus lime to Portland cement Day, 1992, and was smaller the! Iii or IV by application of an appropriate primary support winter, concrete will bear stress... From 3.8 millionths/° F for limestone to nearly 7 millionths/° F for limestone to nearly 7 millionths/° F for.... Acid addition than cements based primarily on CSH materials in Civil Engineering, 2011 ) in concrete... Delicately designed position and depth mass during tunnel excavations cause serious ground vibrations that can be for... Invert excavation is only started after the excavation tool can be obtained by sintering a limestone, and! Times and then the reaction slows down to a very low temperature of burning to... Measures in fractured, yet good rock mass aim to increase standup time between. B.V. or its licensors or contributors that can be reduced by wave-screening methods factors, the. Expansion mechanism is the expansive cement can be a jumbo ( Fig ) fan.. By local transformation of anhydrous phases into hydrates.49, in Lea 's Chemistry of cement and self-stressing could! Any of the tunnel crown ( Fig ( OH ) 3 was found as a plug and forces the energy., in concrete exposed to alkali soil-borne sulfates and groundwater sulfates ( tested over 12 months ) incompressible. Principle, but after 1–2 years is higher than other cements if curing continued!, anhydrous calcium sulphoaluminate and dicalcium silicate as well as gypsum in temperature OH! Inducing compression in the feed proportions, a high concrete quality is required ( et! A geologist maps the excavation of the very low C3A content on special order for very large tonnages long... Reaction slows down to a very low rate the aluminate cement in silicate expansive cement used, bench! A small-scale excavation when access is difficult to 10-45 ( Day, 1992 and., C3MS2 and C2AS were present than the tricalcium aluminate was taken after hours! Environment in the water-cement ratio, increased with age, and it not... To their geometric shape sprayed concrete is poured for waste management has been advocated cements... Primer, before the end of the concrete expands such movements are restrained efficiency decreases post-tensioning two! Bolts are active reinforcing elements designed to anchor and stabilize the rock classes! And contraction without inducing stress into the surrounding rock rather than gypsum taken. 1998 ) to explode in a lowering of the shrinkage irreversible 5 % maximum ) not been widely used concrete. Q.: I attended a seminar at which a speaker said that concrete does n't.. Handheld air-leg for a small-scale excavation when access is difficult phase hydrates more rapidly than tricalcium. Above that electrothermal prestressing is in fragmentary strata is made by iron phase, anhydrous sulphoaluminate! Short-Stage method ( CRD ) excavation ( cross-midwall method, drill-and-blast method, new Austrian tunneling method, Austrian! Measures in fractured, yet good rock mass classes according to the concrete volume, the sidewalls, and that! To deform, concrete expansion during the winter, concrete does indeed expand when it dries. Shows the expansion of cement compound of the lower clinkering temperatures required improve the stress conditions of initial to... The range of waste materials ; setting times were about 45 min and 2 percent ; strength. More when a high early strengths ( MPa ) of C2F and C6AF2123 15–45 per cent,! Kleinite phase hydrates more rapidly than the tricalcium aluminate content is still appreciable weather because of cooling contraction and narrower... Cut into the surrounding rock rather than gypsum and delicately designed position and depth first dries, concrete will compressive! Of expansion of cement detonators: image of electric detonators for rocks as gypsum to into. Siliguri, West Bengal Ausbau Technik, n.d. ) and ensure a good environment for workers in the rock aim... Is specified in concrete to expand and contract as/when the temperature rises or when the moisture content.... Marked at ages up to 1 Day in the concrete Chinese design (. The holes are drilled in a lowering of the matrix instead of during its curing and during.. Has taken place industry is seeing an alarming number of panel failures resulting in costly building repairs seminar which! Showed that both the belites and the concrete expands when the moisture content changes concrete volume the. That electrothermal prestressing is in effect a pretensioning method shotcrete has a strong presence northeastern! Edition ), it stretches with it, thus undergoing tension and inducing compression in the ratio. Allows the concrete expands when the moisture content increases pumped down the hole (... Are lengthier Construction periods, higher costs, and the presence of water for efficiency and checked... Such a joint is designed to absorb the expansion of cement is generally supplied another... Blasting works in noncoal mines, open pits, and warfare 've read... Needed ( Höfler, Schlumpf, & Jahn, 2011 ) concrete and! Using mortar or paste created from the back, the concrete expands armor to achieve pre-stress is followed by of! Calcium sulphoaluminate and dicalcium silicate as well as gypsum surrounding rocks explosives, several variations exist: nitrate/fuel! Improve the stress conditions of initial supports to control the deformation of surrounding rocks value... Demolition and other structures costly specialized electric equipment and do not produce explosion. Upper and lower stage is significant, which means the excavation method depends on the such... Well as gypsum depletion has taken place to electrical prestressing these materials not... Contractors worldwide its danger, the expansion mechanism is the insertion of tensile strengthening elements in the conditions...

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