FROM IDEAS TO THEIR REALIZATION
EFFECT
OF CERTAIN
OF READY-MIXED
FACTORS
ON THE STRENGTH
CONCRETE
V. M. Belonozhko and M. Z. Kagan
The Ochakov Reinforced-Concrete Members Plant of the State Construction Administration of the Moscow Subway was acutely faced with the problem of charging ready-mixed concrete in truck mixers (TMs). It was not possible to load the TMs with ready-mixed concrete prepared in a concrete mixer installed at the hatching plant (BP) in view of the shortcomings which were designed in the construction of the BP. As a result of this, for several years the TMs were charged with the proportioned initial components, which, after weighing, separately passed along a steel pipe, bypassing the concrete mixer, into the mixing drum of the TM. However, as the experience of our and certain other Moscow plants showed, the TMs mounted on the chassis of KamAZ and KrAZ trucks cannot always prepare concrete of high grades with such a technological scheme. It was decided to increase the content of cement in order to compensate the gaps in the technology of preparing concrete and to obtain the required strength. Other shortcomings were found in the technological scheme of charging the materials: the sand and rubble entered the mixing drum of the TM through a steel pipe (chute), which was the cause of its rapid wear (abrasion); 3-5% of the cement was spilled due to the existing gap between the pipe, through which it passed after weighing, and the compartment of the drum of the TM under the hopper. The drivers of the TM often preferred mixing the aggregates on arrival at the objects. A comparison of the strength of concrete in TMs and clump trucks was made (Table 1). Concrete grade 300 was compared, the water/cement ratio was the same in all cases (the specimens were subjected to heat and moisture treatment for 13 h). It was found that the average strength (1~) of concrete at age 1 day from the dump trucksexceeds that from the TMs by 40%. It must be assumed that the main cause of this is poor mixing of concrete in the TMs. As for the greater increase of strength of concrete from the TMs than from clump trucks at age 28 days, this apparently occurs due to the fact that the quantity of unreacted initial cement particles during the first day in one case was far greater than in the second. The coefficient of variation of the strength at age 1 day for spccmens of concrete from the TMs is 1.4 times greater than that of the specimens from the dump trucks. The main cause is insufficient mixing of concrete in the first case. The decrease of the coefficient of variation at age 28 days from the TMs and clump trucks is about the same and corresponds to 58% in the first and to 50% in the second case. The results of the strength of concrete [1] taken from the TMs and dump trucks were also compared (Table 2). In the comparison by the sign test the first numeral in the brackets signifies the number of cases with a minus sign, i.e., the strength of the concrete from the clump trucks in six cases was higher than that of concrete prepared in the TMs, and the second with a plus sign, i.e., the strength of the concrete prepared in the TMs in two cases was higher than that of the concrete from the dump trucks. Then follows the tabular significant number [0] and conclusion: [0] signifies confirmation of the null hypothesis and therefore it cannot be stated that the strength of the concrete specimens from the dump trucks is higher than that of the concrete from the TMs. In the comparison by the Wilcoxon test: at first are indicated the limits of inversion, then the number actually obtained U a = 27, and, finally, the conclusion: the number of inversions U a does not lie in the critical region, and therefore the null hypothesis is not rejected and there are no grounds to consider the results in the forms, with the exception of several cases, to be significantly different. According to the data of mathematical statistics given above, the results of the strength in most cases were mainly the same. This is explained by the small number of collected results. Nevertheless, we can observe a tendency toward an excess of the strength of specimens of concrete charged into the dump trucks over the strength of concrete from the TMs.
Translated from Gidroteklanicheskoe Stroitel'stvo, No. 11, pp. 21-22, November, 1991. 0018-8220/91/251 t-0695512.50 9
Plenum Publishing Corporation
695
TABLE 1 Age, days
Strengthof concrete, MPa,,taken from dump truck 16,6 29,8
I
28
23,6 31,7
TABLE 2
Comparison with respect to sign test l Wilcoxontest I
[6,2]
>0;
[0]
[
9 < U < 54
Ua=27 Tlae null hypothesis is confirmed
I
TABLE 3
Age, days 1
28
IAverage strengthof 9 MPa/coefficient of variation (v, %) for a concrete yield, volume, m 3 3,5 [ 4,0 17,6/7
30, l/ll
]
I
15,3/12
29,6/6
Experiments of a somewhat different character were conducted later. It was decided to reduce the total volume of charging individual concrete components into the TMs and to compare the concrete strength obtained in this case with the strength obtained with the usual charging volume. The works were conduced with preparation of concrete mix grade 300 in TMs with a yield of ready-mixed concrete in one case of 3.5 m 3 and in another 4 m 3 (Table 3). The concrete specimens were subjected to heat and moisture treatment for 13 h. The average strength at age 1 day for the smaller concrete yield was 15% higher, by 28 days the strengths equalized. It is of interest to predict the results of the gain of strength of concrete (Tables 1 and 3) not on the basis of laboratory data but directly in structures during operation of tunnels constructed by open method. As is known, conditions under which the ambient temperature is 20 ~ +__ 2 ~ and the humidity is within 90% are the optimal conditions for sampling the strength of concrete. The most active hydration of cement particles with water and, as a consequence of this, the attainment of the required concrete strength at the prescribed times occur precisely under the given conditions. Deviation from the optimal temperature and hydrophysical parameters leads to redardation of the activity of cement hydration, and this means that the final strength of concrete of the required grade should be expected not at 28 days but at later times. The average temperature in the winter months varies from - 1 4 ~ in January to -15.5~ in December. The ambient humidity is 51.5% in January and 58.5% in December. December is apparently the most favorable winter month, when the rate of increase of concrete strength is highest. At the same time, the ambient temperature and humidity are insufficient to guarantee obtaining both the wholesale and grade strength of concrete at the required age. The strength of concrete will increase, but slowly. The difference between the ambient temperature and humidity in winter months is small. In the spring the ambient temperature and hydrophysical quantities differ a little from these same quantities in the winter, and hence the strength of concrete at these times will be the same (Fig. 1). The ambient temperature in the summer is relatively high, from 24~ in June to 27~ in August. However, the ambient humidity, reaching a maximum value during the year of 62% in July, reaches a minimum of 52% in August. Apparently, in the summer, in June and July, hydration of cement particles will occur more actively than at any other time of the year. At the same time, there are great differences in August between the air temperature and humidity. This means that there will be insufficient moisture for active hydration of cement, the concrete will have shrinkage and the gain of concrete strength may not only be greatly retarded but may stop entirely.
696
6q -27 ~5 - 2# .s 9 25 5'1
-
2q
60 9- 23 5~ - 2 2 58 "21 '57
-
20
56
-
19
55
-
18
5q -17 53
:Ic
Y2
-
15
-
lq
_
13
51 5.n
to I
1
11 11/lV
V
L
i
~
1
1/1 Vll LTIllX .X XI XII months
Fig. 1. Variation of temperature and humidity in the tunnel during the year.
In the fall the minimum air temperature is - 1 5 ~
in November and the maximum - 2 1 ~
in September.
At the same time, the minimum humidity 55% is noted in September, and in November and Octeobr it is respectively 61.5% and 58.5%. CONCLUSION The strength of concrete was investigated. It was found that in the case of charging individual concrete components into TMs mounted on the chassis of K r A Z and K a m A Z trucks, high-grade concrete cannot always be prepared with such a technological scheme. It was established that with a yield of various volumes of ready-mixed concrete from TMs, the strength of the concrete from the smaller volume (3.5 m 3) was higher an age 1 day than I~ of concrete from a volume of 4 m 3. However, at age 28 days the strengths equalize. Furthermore, the concrete taken from 3.5 m 3 was more uniform. In addition to this, an attempt was made to predict how the concrete strength would increase directly in structures during operation of tunnels constructed by the open method during the year. It was established that the maximum gain of strength of concrete will occur in the summer, in June and July. At the same time, in August there are great differences between the ambient temperature and humidity. This means that there will not be enough moisture for active hydration of cement, the concrete will have shrinkage and an increase of concrete strength may not only be markedly retarded but may stop entirely. In the winter, spring, and fall, the strength of concrete will increase, but apparently less intensely than in the summer. LITERATURE CITED N. V. Smirnov and I, V. Dunin-Barkovskii, A Course in Probability Theory and Mathematical Statistics for Engineering Applications [in Russian], Stroiizdat, Moscow (1965).
697