Reuse of crushed concrete

A number of materials in foreign journals are devoted to the reuse of crushed concrete.

The service life of concrete structures, as well as of other materials, is unfortunately limited and the annual cost of their repair and restoration exceeds half the cost of new construction. In accordance with European statistics, on average throughout the Community countries, annual waste during construction, reconstruction and demolition of buildings that have served their time also amounts to about 1 ton per inhabitant. Construction waste can be disposed of in two directions: it is the reuse of certain parts of the building (foundations, walls) or its separate structures (beams, slabs, columns) for its intended purpose in new construction or recycling of this waste (recycling) for their use as secondary (recycled) raw materials. Those wastes that cannot be recycled for various reasons go to dumps. In Great Britain, in particular, in order to preserve natural resources and stimulate recycling (1) a tax was imposed on the use of each ton of natural aggregate (“primary raw materials”) in the amount of 1.6 pounds sterling. Annually in the EEC countries over the last decade according to (2 ) traditionally took place:

This amount of waste causes a certain concern of society and the corresponding pressure of the governments of many countries on the construction industry for their utmost utilization. As can be seen from the table in some countries, industrial processing of waste is much better developed than in others, which can be partially explained by the lack of sources of natural raw materials in these countries and free places for dumping dumps.

Due to its remarkable properties, concrete in many cases serves both for the protection of nature in the so-called environmental protection facilities, and for the protection of man himself from radiation and other harmful wastes, including absorbing this waste as coarse or fine aggregates. No wonder the famous American specialist P. Metha compared concrete with the god Shiva (in Indian mythology, this god drank poison to save humanity). However, concrete, on the other hand, cannot serve as a waste bin for all construction waste, since They may contain substances harmful to the concrete itself, especially for its durability. Therefore, construction waste can only be partially used in new construction, and almost 40% of them, including small fractions of up to 2 mm in size, can only serve as backfilling in road construction. The remainder, after appropriate recycling, can be used as aggregate for concrete. In accordance with the recommendations of RILEM (3), recycled or secondary aggregate of ≥4mm size is classified by origin into 3 types:

1) waste masonry; 2) crushed concrete; and 3) a mixture of crushed concrete (max 20%) and primary aggregate (min 80%). The recommendations contain mandatory requirements for all three classes of secondary aggregate: by volume, water absorption, content of foreign bodies, etc. The Recommendations also contain requirements for the properties of recycled aggregates, including particle size distribution, static strength, abrasion coefficient, chloride content, frost resistance, etc., which are not binding unless they are specified by national standards.

The use of recycled secondary aggregates has a significant effect on many properties of concrete. For example, at 100% content of crushed concrete as coarse aggregate, the strength of concrete does not exceed 10 MPa, when 50% of primary aggregate is introduced into the mixture, the concrete strength increases to 15 MPa, and even when 80% of primary aggregate is introduced, the concrete produced rarely meets all the requirements shown for standard concretes. Therefore, manufacturers of concrete with recycled aggregate are required to inform their customers of this in order to avoid possible serious consequences.

Concrete manufacturers with recycled coarse aggregate often face the need for increased cement consumption and the use of superplasticizers. The cost of such aggregate is also quite high due to the significant energy costs for its crushing, cleaning, sieving and washing. Because the economic feasibility of using recycled aggregates is not always obvious, then their successful application requires the support of civil society and the political will of governments.

The relevant committee of RILEM (4) supports the widespread use of recycled materials. Based on the ambiguity of the term “sustainable development” in relation to construction, the committee proposed several theoretical-experimental models for solving this problem, including:

1. Calculation of the life cycle (life cycle) of the structure, according to which during the whole period of operation, including repair and possible reuse and recycling, it turns out to have minimal impact on the environment. A Dutch approach was considered to examine 10 stages of the product life cycle with possible environmental consequences (the “Delft Staircase”), including a return system whereby construction products that have served their time return to the original supplier for repair, restoration and reuse in the same role. The committee has proposed a two-level assessment of the use of waste: “low” - in which the waste goes only for use in road construction and “high” - in which the waste goes to the manufacture of concrete or other products.

2. Design for recycling. Primary raw materials are usually homogeneous (sand, gravel, clay, wood, iron ore, etc.), which makes it quite easy to use them in the process of making any products. This cannot be said of secondary raw materials, since recycling requires a significant amount of processing: recognition, grinding, separation, etc. But if earlier most of these operations were performed manually, now there are mechanized methods that significantly speed up and facilitate this work. Therefore, the government of the Netherlands and a number of other countries have recognized the need and desirability of introducing SDI, the basic principles of which are: reducing the quantitative composition of materials in the product; easy disassembly of products; marking all parts of plastic so that they can be easily distinguished and separated for further separate disposal.

3. Determination of cost-effectiveness / price ratio (CEC). This coefficient has a numerical value and can be used as a comparative evaluation of various design solutions. Cost is a virtual or hidden cost of manufacturing and operating products in an environmentally friendly manner that provides the well-known principles of sustainable development. The price of the product includes the cost of materials, energy, labor, depreciation, taxes and profits. This coefficient serves as an indicator of the environmental friendliness of the solution, which includes all the parameters of manufacture and operation, expressed in the same dimension. The committee’s calculations of the CEC values, for example, confirmed the roughly equal feasibility of using crushed concrete both as underlying layers in road construction (low level) and for use as coarse aggregate in concrete (high level).

Re-use of crushed concrete in many cases is very appropriate and meets the principles of the concept of "sustainable development", the main provisions of which include saving materials and energy, increasing the durability of structures and reducing the negative impact on the environment, including the preservation of irreplaceable sources of natural resources. The most advanced manufacturers of building materials are now accompanying their materials with a voluntary “Ecological Declaration”.

Literature:

1. A. Leshchinsky, M. Lesinskij. Concrete aggregate from construction and demolition waste, BFT, N8, 2003, pp. 14-22.

2. Recycling of offshore concrete structures, fib Bulletin N18, 2002, pp.29

3. Specification for concrete with the recycled aggregate, RILEM TC 121-DRG, Materials and Structures, V. 27, 1994, pp. 557-559

4. Ch. Hendrics and G. Jansen. Use of recycled materials in construction. Materials and Structures, V. 263, November 2003, pp. 604-608