Quality criteria

Impact comminution of concretes with defined properties

Concrete rubble extracted by the demolition of the test centre of the Department of Building Materials Processing and Recycling at the Bauhaus University of Weimar

Measured variables and parameters of the image analysis

Cumulative size distribution curves of screen underflow of the finest comminution products of the concrete B 25 and the coarsest comminution product of the lightweight concrete compared to the impact crusher comminution products of the concretes B 15 and B 35

Fraction portions of the finest comminution products of concrete the B 25 and of the coarsest comminution product of the lightweight concrete

Fraction portions of the concretes of the strength class B 25 (gravel concrete b_D, 28 d = 36,1 N/mm², chippings concrete b_D, 28 d = 39,4 N/mm², RC concrete b_D, 28 d = 37,7 N/mm²)

Fraction portions of the concretes of the strength class B 45 (gravel concrete b_D, 28 d = 50,0 N/mm², chippings concrete b_D, 28 d = 58,3 N/mm², RC concrete b_D, 28 d = 53,0 N/mm²)

Fraction portions of the high-strength concrete HPC and the lightweight concrete LB (HPC b_D, 28 d = 91,4 N/mm², LB b_D, 28 d = 52,9 N/mm²)

Comparison of the comminution products of lightweight concrete (a) and high-strength concrete (b)

$Sphericity of the particles of the fraction 8/31.5 mm as a function of the impact speed$

Sphericity of the particles of the fraction 8/31.5 mm as a function of the impact speed

$Length-width ratio of the particles of the fraction 8/31.5 mm as a function of the impact speed$

Length-width ratio of the particles of the fraction 8/31.5 mm
as a function of the impact speed

Comparison of the apparent densities of the hardened concrete and the comminution products (impact speed 50 – 60 m/s)

Apparent densities of the recyclates produced in the impact crusher, compared to the apparent density of the original concretes

Increase of the specific surface as a function of the mass-related kinetic energy

Fictitious comminution resistance as a function of the compressive strength of the concretes

Demolition of roofs in Gera/Germany

Summary: Building waste that can be recycled as building material is processed in various steps and comminution has a central importance here. It is required for particle size reduction. As regards composite building materials, comminution furthermore represents the possibility to disintegrate connected components. The impact crusher has proved a success for the comminution of concrete or masonry building waste. On the one hand, the comminution result depends on the stress conditions. On the other hand, it can also be expected that the concrete quality will influence the particle size reduction achieved. This question is investigated in the present article. Various types of concrete were crushed under defined conditions in a impact compressed-air cannon in order to discover the influence of the concrete on the particle size reduction and the disintegration achieved. Statements were made as regards the interdependence between the crushability and the compressive strength of the concrete to be comminuted. Conclusions are drawn for the processing of concretes with higher strength classes as one of the development trends in the range of concretes.

1 Types of concrete investigated

Normal concretes consisting of gravel, chippings and recycled aggregates with graded strengths – according to the designation valid at the time of manufacturing as B 25, B 35 and B 45 – were made for the investigations in the compressed-air unit. The recyclates were produced from pure concrete specimens in the laboratory. In addition, a high-strength and a lightweight concrete were also included in the investigations (Table 1). The particle size distribution of the aggregates used corresponded to the grading curve A/B with a maximum particle size of 16 mm for...