14.4. Ways of saving material resources during the production of reinforced concrete products
In the production of precast reinforced concrete, product cost for materials mainly include the expenses for cement, aggregates and reinforcement. On the average, for 1 m3 of precast reinforced concrete more than 90 kg of equivalent fuel (conditional) are outlaid. Of these, thermal energy constitutes near to 90%, whilst electric energy constitutes about 10% of the energy resources used.
Saving of cement. The saving of cement in the production of concrete and reinforced concrete is achieved by the removal of its production losses, by improving project decisions, technological processes, forms and methods of production organization, wide utilization of industrial wastes and by-products.
Part of cement’s losses take place through the inefficient use of cements of different kinds and strength, their use for the wrong purpose, mixing of different grades of cement, shipping of hot cements with activity lower their quality. Sometimes, a 40 MPa cement is used for making concretes with ultimate compressive strength of 10 to 15 MPa and low strength cement mortars. Part of cement’s losses is due to the use of low-grade aggregates in concrete mixtures. Every percent of muddiness of the crushed stone, for example, equals additional amount approximately 1% of cement.
A significant part of cement losses are conditioned by imperfections in the technology of manufacturing of the reinforced concrete products; the use of inappropriate equipment for transporting, unloading and dosage of cement; losses due to preparation and transporting of concrete mixture, the increase of slump and strength of concrete in comparison to specific project indexes; wear of equipment, issue of defective products and others like that. It is has been established that, on average, losses are 10 times less if transportation is in cement carriers, than in simple carriages, and about 40 times less in open transport vehicles.
Reduction in cement content is promoted by use of high-strength concretes and precast reinforced structures which are proven to have reduced cross-section of the elements and their mass. Examples of economic reinforced concrete products include- large size slabs completely factory assembled, which allow reductions of up to 2-3 times the number of elements which are assembled, and centrifugated concrete columns with ultimate compressive strength 50-60 MPa, etc.
Considerable saving on the cost of cement is related to wide use of light-weight concrete. The use of light-weight concrete instead of normal-weight one normally increases the cement content per m3. However, taking into account the reduction in cross-sectional dimensions, the overall expenses on cement is reduced. With respect to construction mass, the use of light weight concretes for external walls, slabs and ceilings result in up to 30% reduction in the mass of buildings and about 10% on the cost of cement.
Chemical admixtures are a most powerful means for cutting the cement content in concrete. The use of superplasticizers allows the reduction of cement content by 15-20% without worsening the concrete properties. In mortars containing fine sands, the content of cement may be reduced by 5-20% by introducing enlarger admixtures into the concrete mixture. The admixtures may be classified stone sifting out, wastes from ore mining and processing enterprises, or slag produced as waste from power plants.
The use of fly ash in the optimum amount as an admixture in concrete mixture allows cement saving of up to 50-70 kg/m3, and more. Re-grinding even low-active ash to the specific surface 5000 sm2/kg allows saving 20-30% of cement without the decline of concrete’s class.
A considerable saving of cement can be attained by optimization of composition of concrete mixtures, taking into account growth of concrete strength after thermal treatment, expansion of volume of products’ manufacturing with negative tolerances to the sizes and due to other technological means.
Saving of metal. Reinforcement for reinforced concrete constructions accounts for approximately one-third of metal that is used in the whole of construction. The basic ways of reducing metal consumption of reinforced concrete products are by improvement of the strength indices of reinforcement steel, development of production and use of wire reinforcement, enhancement of reinforcement production and use of effective embedded parts.
The amount of reinforcement steel of any class (T)
can be approximately shown to be conditionally equivalent to the
strength amount of steel of class (
):
, (14.1)
where Ktr - transitional coefficient.
It is possible to save about 40% of metal due to pretensioning of reinforcement. For prestressed structures it is effective to use high-strength reinforcement steel. High-strength thermally fixed reinforcement of enhanced corrosive resistance is widely used in recent years.
A considerable saving of steel can be achieved through production of welding fabrics and cages from reinforcing wires from die-rolled sections in place of the smooth ones. An important reserve of cutback of steel’s spending is through production of reinforcement elements on the automated no-waste lines. On average 6-35 kg of steel of metallic forms are outlaid for 1 m3 of reinforced concretes. The cutting of metal's cost is achieved by improvement in the construction of forms, as well as their rational planning and use.
Saving of fuel. The principal reason of overrun of fuel at a steaming-out is the insufficient state of steam-curing chambers, and lack of controls for steam flow consumption. The non-productive losses of fuel reduce by the increase of thermal resistance of steam-curing chambers, diminishing of thermal conductivity of chambers walls with the help of different heat-insulation materials and light-weight concrete.
In comparison with pit more profit-proof are vertical, tunnel and low downstream chambers. In the last one, for example, charges of steam are about 30-40 % lower than in pit ones.
For the saving of fuel and energy resources in the production of the precast reinforced concrete the use of energy-efficient technologies is of great importance. For example, application of hot mixtures with a heating temperature of 65-70°C results in the acceleration of thermal treatment for 3.5-4 hours during the production of massive elements. Possible reduction of the treatment duration in the chambers or cassettes does take into account the fact that subsequent curing of products reaches 3 hours and more. Considerable reduction of temperature and duration of thermal treatment is possible through combination of intensive mechanical and thermal influences on concrete, creation of surplus pressure in the chambers, use of more rigid mixtures, use of chemical admixtures to accelerate the hardening of concrete.
The decline of power-consumption of precast reinforced concrete is possible also due to low temperature modes of thermal treatment and reduced heat of hydration of the cement in the process of products warming up, application of non-steam methods of thermal treatment (use of combustion products of natural gas, high temperature organic heat-transfers). In warmer regions energy consumptions for the acceleration of concrete hardening can be reduced considerably by application of sun energy.