CA1041311A - Method of erecting mass concrete structures - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The proposed method of erecting mass concrete struc-tures consists in preparing a ground bed, whereupon there is laid an insulating cushion of dry concrete mix. The insulat-ing cushion is covered with a layer of wet concrete mix, which layer is compacted and heat-treated. The method is applicable to civil engineering and can best be used for the erection of mass concrete structures on frost-heaving ground and in permafrost areas.

Description

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MB~HOD 0~ ~R~CTING M~SS CO~CRæ~ S~RUCTUR~S

~ he present inventio~ relates -to concrete l~ying teohni~-ues and, more specificall~, to methods of erecting mass concre-te structures.
'~he invention is applicable to civil e~gineeri7g and can ~ be used for -theerection of di~feren~ structure~Yon ~rost heav-ing grou~d and in permafrost areas.
According to the prior-art practice o~ erec-ting mass con-crete structures on frost-heavi~g ground, the ground bed ~as warmed up before laying the concret~mix. In permafrost areas, concre-ting was preceded by preparing an insulati~g cushion o~
sand~ ~ravel-and-sand ~ni~ture, crushed stone, and prefabricated or reinlorced ooncrete.
In the ~ar North, where perma~rost prevails, pile-suppor-ted structures have been increasingly common in recent years.
It mu~t be pointed out, however, that the known techniqu-es ~: erecting mass concrete structures on frost-heaving groun~
and in perma~r~t areas do not envisage the use of natural gro-und beds or ~ounda-tions, whereby the cost of preparing such beds or ~oundations may be reduced -to a minimum.
I ~or e~a~ple~ ~n order to lay concrete o~ a ~rost-heaving bed, the latter must be warmed up, which is a labor- and power--consuming process. Moreover, this method does no-t always work, especiall~at low ambient temperatures and when the ground is .j ~
q frozen- to a great depth.

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Prior to laying concre-te in perma~rost areas, an insula-ting cushion has to be provided on the ground bed. ~he oushion con~ists o~ sand, sand-and-gravel mixture, crushed stone, and pre~abricated and reinforced concre-be. ~ -Yet thi~ type o~ in~ulatio~ does not co~pletely rule out the penetration o~ ~ree moisture ~ro~ the concrete mix tbrough ;~
the insulating material into the ground bed. As a result, the perma~rost thaws, and the supporting capacity of the ~round is sharply reduced.
~ he method under review has another di~advantage which is still more important. ~he method does not ensure a wholly mono-lithio structure, which, in turn, results in a non-uniform load .
distribution and strain cracking. ~he consequences may be di- ;
sastrou~.
Pile~supporbed struo-tures, although hig~ly durable, are quite costly. Besidss, this technical solution does not enaure a monolithic structure which is an absolute ~ecessi~y in a numb-er of ca~es. There are also cases when this technique pro~es to be impracticable both tec~nicall-y and economicallyO A ca-se in point is a oovering extending over a great distance, ~or example, air~ield or road pavements.
It is the main object o~ bhe presenb inve~tion to provide a method o~ ereoting mass concrete structures which would en-~ure uni~ormity o~ a struotuxe and keep intact the natural grQ-., . . . . .. ` ...... , . . .. , ~

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It is another object of the invention to reduce the amount of free mixing water in mass concrete structures.
It is still another object of the invention to provide a material for monolithic structures, possessing improved physico-mechanical properties.
It is yet another object oE the invention to curb destructive processes in structures and reduce the consumption of construction materials.
The foregoing and other objects of the present invention are attained by providing a method of erecting mass concrete structures, whereby a ground bed is prepared, whereupon there are successively laid an insulating cushion and a layer of concrete mix, said layer being then compacted and heat-treated according to which method, the materi.al o~ the insulating cushion is dry concrete m~x.
It is expedient that said dry concrete mix should be cooled to a subfreezinq temperature, prior to being placed on frost-heaving or permafrost ground, if the ambient temperature is above ~ero and that of the na'~ural ground bed is below ~ero.
The essence of the invention is as follows. A natural ~round bed is prepared, whereupon there is installed timbering and laid a layer of dry concrete mix. This is followed by laying wet concrete mixl whereafter the whole is compacted and heat-treated. The temperature of the concrete starts rising due to the outside heat and the exothermal reaction in the mix r . . .~:~
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itself. The diPference in the -temperature of the concrete and -that o~ t~e ground bed increases. ~he mineral h~draulic binder of the layer o~ dry concrete mix laid o~ the matural grou~d bed adsorbs free mixing water from ~he layer of the wet co~crete mix. ~is i9 accompa~ied by a mass and heat transfer. A9 a re~
sult, fre~ mixing water leaves the upper layer and en-ters the lower layer. ~he dry concxe-te mi~ gradually becomes moist and hardens. ~he layer o~ dry concrete mix rasts on the ground, so its temperature is lower than that o~ the upper layers, due to w~ich the hardening of the concrete in the lower layer is slow and end9 when the temperature o~ all the ~ayers is about the sa-me. 'l'hus, the physioo-meohanical properties o~ the natural ground bed remain intact, while the ~orm con-~ains a uni~orm ~no-lithic body o~ ooncroto wibh prescribed characteristics. ~he thickness o~ the dry mix layer is determined by the water-ce-ment ratio of concrete, the volume of the structure, the maxi-mum heat treatment temperature, the temperature of the natural ground bed and the techniques employed to prevent the latter ~rom thawing.
Theerection of monolithic concrete and rein~orced concrete structures at above-zero amblent t~mperatures is characberized by that prior to la~ing dry concrete mi~ on a ground bed, this mi~ is cooled to a subzero tempera-ture. ~his rules out thawing o~ the natural ground bed and helps to preserve its physico-mechanical propertie~.

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It has been ~ound ~that de~tructive processes in concrete can largely be avoided by a proper heat treatmenb o~ concrete mix.
Concrete ~ix should be heat-trea-ted prior to bei~g compac-ted, w~ich i9 done by placing a la~er of preheated ~t concrete QiX. Any known warming -tech~ique can be used ~or the purpose.
However, the pre~ent invention ~oes not sxclude the con-ventional type o~ heat treatment because this 9 too, helps to curb destructive processes.
Thus, -the proposed method o~ erecting mass co~crete struc-tures is more advarltageous both technologically and economical-ly than a~ conventional method o~ erecting monolithic struc-tures o~ ooncre-te and rei~orced ooncrete. '~he proposed method is easy to put into practice. It requires ~o ~peoial equipment and makes it possible to erect high-qualit~ mass concrete structures in ~evere climatic conditions. Co~struction time and costs are reduced.
The proposed method is illustrated by the~ollowing eæa mples ~ iXamPl e 1 .
r~his i~ an e~ample o~ implementing the proposed method o~ erecting monolithi¢ concrete and rein~orced co~crete struc-tures when -the temperature o~ the natural ground bed is above zero. Concrete ~loors were to be made o~ concrete with M =

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= 300 kg~/cm2. The concrete layer was to be 22 cm thick. The temperature o~ the natural ground bed was ~7C, and the amble~t temperatule was ~15 to ~18C.
~ he natural ground bed was levelled, ~or~s were assembled, a~d a layer o~ dry concrete mix was :laid.
The layer was 8 om thick, its temperature bei~g abou-t the same as the ambient temperature, i.e. ~15 to ~18C.
~ 'he dr~ concrete layer was ~lattened, and o~ it there was laid a layer o~ conventional wet concrete mi~ which was then subjected to intensified heat trea-tmen-t so that its temperature was raised -to ~70C durin~ 2 to 3 minutes. ~'he heat source was then switohed o~, and -the sur~ace o~ the compaoted concrete was eo~ered with a steam- and heat-insulatinK material.
On t~le seeond d~y, the concrete was ~ound to have the ~ol-lowin~; ph~sico-meehanioal charaeterii3tics:
eompression strength, 85% o~ that o~ the concrete grade;
bending-tensile stren~th, 95% o~ that o~ the concrete gra-de.
Subsequent eomparison o~ the characteris-tics o~ this con-crete to those o~ concrete ~ardened in normal conditions (eon--trol samples) showed a 25~o increase in compression strongth and a 35% i~crease in bendin~-tensile strength. Frost resi-stance rose by 75 cycles.
In addition, power consumption was reduced two-fold, as compared to conventional concrete la~ing techniques.
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~ his is an ex~ple of ir~plementing the proposed Illethod o~
erecti~ mass concrete structures at subzero temperatures o~
the natural ground bed and subzero ambient te~peratures.
Concrete ~loors were to ~e made o~ concrete with ,~ =
= 300 kgf/cm2. ~he floor thick~ess was 22 cm. ~he te~perature of ~he ~atural grou~d bed was -4a, and the am~ient te~perature was bet~.~n -12 and -lL~C.
A natural ground bed was prepared, whereupo~ thexe were inst~alled forms a~d laid a layer of d~y concrete mix.
'~he d~ concxete mix layer was 12 cm thick, and its tem-perature was -9O.

~ he dx~ a~d wet concrete mix layers were laid as in ~x-ample 1.
. After laying a layer of v~et concrete mix~ it was su~ject-ed to intensi~ied heat treatment, so that its temperatur0 reached ~70C duxing 2 to 3 minutes. ~he concrete surface was then covered with a steam- a~d heat-insulating material.
. ~fter three days, the concrete had the. ~ollowin~ charac-, teristic~:
.:~ compression strength, 75 to 80% o~ that o~ the concret~
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~L~)4~311 bendin~-tensile s-tren~th, 85 to 90% o~ that o~ the con-crete grade.
A~-ter 5 to 7 days, the streng-t~l characteris-tics of the concrote correspo~ded to those of the ~iven grade.
In order -to evaluate the physioo-mechanical properties of concre-te laid according -to the proposed method, concrete samples taken ~rom the structure were allowed to harden in norm-~al condi~ions ~or 28 days.
Comparison with control samples showed an ,increase of more -than 20% in compression strength and oP mors than 30 in bending-tensile strength. Frost resistance rose by 70 cycles.
,, Power consumption was two and a hal~' times less, as com-pared to conventional concrete layin~ tec~lni~ues, ~ amPl,e 3~
~ hen erecting mass concrete structures on frost~heaving , I ground or in perma~rost conditions, and when the temperature of the natural ground bed is below zero, whereas the ambient , temperature is above zero, it is a sine qua non condition tha-t ,~ prior to being laid on the ground bed, the d-ry concrete mix should be cooled to a subzero temperature.
'~he concrete layin~ ~rocedure was as that of '-E~ample 2.
,', At a temperature of -the natural ground bed of -~LC and at an ambient temperature o~ ~12C, the technical characteris-tics of the concrete and -the èconomic ef~iciency of the met~od :. .
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were somewhat hi~her than those of Example 2, which is ano--ther proo~ O:e the e~ectivenes~ o~ the novel method.
It is clear ~rom the ~oregoing e~amples that the propos-ed method is more e~ec-tive bo-th technologically and econo-mically than the conventional -techniques.
Apar-t ~rom -that, t~e proposed me-thod is simple a~d re~u-ires no special equipment.
In a number of cases, the method of -the present inven-tion ma~e~ it possible to dispen~-~e with cos-tly pile supports.;
When laying cement and concre-te pavemen-ts for roads and air-fields in permafrost areas, -the proposed me-thod makes it pos-sible to use the ground as the na-tural bed ~or the pavement, while pre~ervin~ th~ physico-mechanical prop~rties o~ this bed.
With the u~e of the proposed metilod, concreting can be carried out all the ~ear round. -.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for erecting mass concrete structures whereby a ground bed is prepared and covered by an insulating cushion of dry concrete mix, on which cushion there is laid wet concrete mix, said wet concrete mix being then compacted and heat-treated.

2. A method as claimed in claim 1, wherein said structures are erected on frost-heaving or permafrost ground whereby said insulating cushion of dry concrete mix is cooled to a subzero temperature, prior to being placed on the frost-heaving or permafrost ground, if the ambient temperature is above zero.