Материал: Scientific Herald of the Voronezh State University of Architecture and Civil Engineering

Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture

Table 2

Distribution of temperature during cooling of the hot mix in time at different air temperatures

The above data suggest that regardless of the air temperature, the distribution of the temperature in time conforms to general laws and can be described with an exponential dependence. A correlation coefficient for all the equations ranges from 0,98 to 0,99. In order to identify the general laws of the influence of the air temperature on the temperature of the hot mix we will represent the data from Table 2 in relative units. It was found experimentally that the temperature of the hot mix changes most considerably following the distribution along the base depending on the thickness of a layer within 5 to 10 minutes after it has been laid (Table 3).

Table 3

Distribution of the temperature during cooling of the hot mix in time at different air temperatures

66

of the hot mix Кв. Fig. 2 sho s the dependence of the coeffici nt of the effect of the air temperatur on the tem perature of the hot mix with its numerical va lues given by

The ab ove Figure suggests t at as the a ir tempera ure increas es, Кв has more effect, which causes the cooling rate of the hot mix to reduce. It was noted that depending on a type of a mix an bitumen l abel, the te mperature o f making and laying i specified in guideline s. However, de pending o how far a roadway i s, the temperature of an asphalt concrete mi x as it is delivere d on site can be minimum as sp ecified by the guidelines [6, 7]. umerical v alues of the temperature of a mix as it is being c ooled in time depending on the temperatur e as it is delivere d on site is in Table 4.

Coefficient Km

67

Scientific Herald of t he Voronezh State University o f Architecture and Civil Engineering. Construction and Architecture

vered on site and p resent num erical values of the temperature in relative u nits. Let us assume

The ab ve data su ggests that regardless of the temperature of the mix deli vered on site, there is a gen eral law of cooling of the mix in time whic h can be accounted for by introdu cing the coefficient Ксм which is in Fig. 3. The numerical v lue of the coefficient Ксм is given by the followi ng

where tсм is the tem perature o f the mix d ring laying; the correlation coefficient is 0,99.

KmCoefficient

Temperature of the mix, ºС

Fig. 3. Dependence o f Ксм on the temperature o the mix as it is being distributed along a roadway ba e course

68

the air temperature and the wind speed [8]. As the air temperature drops, current guidelines certify that the hot asphalt mix is paved onto the base course provided that the wind speed is not over than that as specified in Table 6 [6]. These restrictions depending on the wind speed are due to intensive heat transfer into the environment as the hot mix is distributed in a small layer on a large surfacing area which causes the cooling rate to increase and construction times to increase. The heat transfer coefficient is found to depend on the wind speed [9]. As a result of modeling of thermal processes in asphalt concrete mixes, the wind speed was found to influence the cooling rate of the mix considerably. Fig. 4 shows the cooling rate at different wind speeds and Table 6 –– the temperatures depending on time.

Time, min

Fig. 4. Effect of the wind speed on the cooling rate of an asphalt concrete mix

Table 6

Change in the temperature of the hot mix in time at different wind speeds

In order to specify the effect of the wind speed on cooling of the hot asphalt concrete mix, let us introduce Table 6 in relative units (Table 7).

69

Scientific Herald of t he Voronezh State University o f Architecture and Civil Engineering. Construction and Architecture

The ab ve data su ggests that the wind sp eed influen ces the co ling rate of the hot mix and it increase s as does t he air mass displacement. Cooling complies ith an exp onential dep endence and is g eneral. Ta ing into account the temperature of the mix at the zero speed per unit, we obtain the depende nce of the w ind speed o n the cooli ng of the asphalt concr te mix (Fig. 5).

The nu merical val e of the coefficient is given by the equation

where v is the win speed, m/ seс; the correlation coe fficient is 0 ,99.

Consid ring the specified stru cture of a oadway surfacing, the thickness of a layer can vary. According to the guidelines, a minimum thickness for the upper layer of a surfacing is 0,03 m and for the lower layer 0,08 [13]. Thus the cooli ng rate of t he hot mix and thereb y paving times a re different. Fig. 6 shows the dist ribution of the temper ture of the hot mix de pending on the thickness of a paved la yer at the mix tempera ure during laying 160 0С.

Coefficient Кv

Wind speed, /sec

Fig. 5. Co fficient Кv allowing for th effect of the wind speed

70