Rood no. 10 section Rc

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PERIODiCA POLYTECHSICA SER. CIVIL ENC. FOL. 37, XO 4. PP. 313-.320 (199.3)

Gyorgy J6zsef BORJJ'\N*, Albert HORV SCHWERTECZKyh*

Department of Building Materials

Technical of Budapest

H-1521 Budapest, Hungary

Hungarian Consumer Protection Superintendence Szende Pal u. 3.

H-I051 Budapest, Hll:nO'~rv

and Ferenc

,·'"rpT-nr;,,,, Residential and Public t)uliGlIlF;S Ltd_

T'he article discusses the expe;:irrlentaI results of hov/ the in!lO!llogeneⁱity of concrete innu- ences the next factors:

- preparation of road concrete

- moulding the concrete prism in a lying position and testing in standing position - the effect of gauge length

- the effect of the type, shape and size of the aggregate used.

Introduction

Material characteristics (strength, deformation) are understood as mean values depending on shape, size, test method, material composition and homogeneity. Among the factors, the effect of homogeneity will only be pointed out on hand of some examples.

Inhomogeneity of Concrete of .H.ig.hvva:v Construction Strength of a specimen under central compression and tension load is understood as quotient of the ultimate force and the surface area. It can easily be demonstrated, however, by ultrasonic concrete tests how the ultrasonic velocity varies along the specimen height. Core specimens were drilled out from the highway concrete at corners and midpoints of a square with 1 m sides (Fig. 1).

Cores No. 2, 5 and 8 were tested for strength as well as for ultrasonic velocity along the specimen height during compression and tension. Among several test results only one example will be demonstrated.

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314 Gy. BALAZS .t al.

a)

b)

I I

30 40 Rc oMPa

I

Rood no. 10 section Rc

0

2= 27,8 MPa

, J

RcoS=Z5,1 MPo

_, ₁

Rc

0

x = 33,1 MPa

I I ' "

at 48+000 km Rood no 10 section at 64+ 000 km R e ,o/2 :::: 45,9 MPa

Rc

0

5 ::: 40,5 MPa

I J

Rc

⁰

8 :::: 48,'1 MPa

J J

Fig. 1. Ultrasonic testing of highway concrete inhomogeneity

According to Fig. la., ultrasonic velocity varies considerably along the specimen height. Both the mean velocity and the respective strength are the lowest for specimen No. 5. Velocities along specimen height are more uniform in Fig. 1 b than in Fig. 1 a.

It is, however, a contradiction that according to velocities the strength

of No. 5 is to be the while to the

mechanical compressive test, it resulted the lowest values. Also these examples point to the fact that, on one hand, cores drilled out of a 1 sq. m area of pavement concrete do not show the same strength (concrete is also inhomogeneous in the plo,ne) , on the other hand, strength is an intricate outcome of inhomogeneity along the specimen height, moreover, although the ultrasonic propagation velocity points out concrete inhomogeneity, it is insufficient to measure it along a single diagonal.

Casting Horizontally, Testing Vertically

Inhomogeneity may also arise from the specimen being cast horizontally, and loaded vertically during testing. Checking was made by means of 20

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EFFECT OF INHOMOGENEITY 315

mm long strain gauges stuck on the surface of the rectangular specimens made with aggregates of 20 mm maximum size according to Fig. 2a. Along a specified length, the cross section of the specimen was constant: 12 X 12 sq.cm.

a) b)

C

2 10 17 12 3

19 - 22 8

15

1 - ₀

13

23 - 26 6

4 11 18 9 A

Fig. 2. Spread shell of a tensile specimen with the strain gauges (al, and the sequence of strain gauge readings Cb)

For the sake of checking, in each load step, deformations were plotted in the order of measurements (Fig. 3).

1:%0

+0,1 Tension: 30 kN

Switching number

o

^I ^I ₅ ₁₀

20

25

-0,1 Compression: 50 kN

Fig. 3. Deformations in the sequence of measurements according to Fig. 2b

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316 Gy. BALAZS et 01.

It seems that in the order of measurements, there were only random fluc- tuations. Locally differing deformations could be well observed with the strain gauge measurements. The order of extensometries appears in Fig. 2.

Thereafter, specimens were first exposed to compression load, then, after unloading, were put into a tensile testing machine with a special head, and tested for tension. The results were registered in the sequence according to Fig. 2b. Deformations are seen in Fig.

4.

Maximum deformations appeared at the A-B edge, attributable neither to the mentioned error pos- sibilities, nor to the excentric load (the specimen had a geometrically central position both during tensile and compressive loading), but to the de- viation between material and geometrical centroidal axes.

""'0"-",,,,"""" on Deviations Effect of the Gauge

of the 5 - c:

Extension measurements under load are applied to conclude on deforma- tional properties of concrete (modulus of elasticity, fj - c: diagram). Strain gauge length is of extreme importal"lCe, strain gauge is applied for measur- ing the mean elongation over the gauge length. Elongation of the mentioned rectangular specimen has been tested by means of 180 mm and 40 mm induction strain transmitters and amplifiers. Loaded up to 1.4 MPa tensile stress, after three repetitions (following each repetition, the instru- ment was removed and then mounted back again), the elongation resulted as 0.043 to 0.044 %0. Vvhile over a gauge length of 40 mm it varied from 0.038 to 0.053 0/00, averaging again 0.044 %v (Fig. 5).

Thus, a short-gauge length measurement indicates rather the local elongations, while a long-gauge shows the mean elongation typical of the given material. Local deformations depend on shape and size of the spec-

on richness in mortar of COHCTe~e. on on concrete mOlE,tllre concrete

as 'well as on the kind of extensometer and gauge length.

Effect of Agjl;:r~=gate on In.h('nJlo~~e11eJlty

Deformation characteristics are different between coarse aggregate and cement mortar and indicate an essential inhomogeneity. Inhomogeneity is mainly affected also by the shape and size of the aggregate. Theoretical and experimental models were applied to test the effects of grain shape (circle, ellipse), grain size (in case of a circle, 10 to 80 mm), aggregate material (steel and glass) and of mortar-matrix quality on the failure pattern.

(The mortar quality was described by its modulus of elasticity: Eh 20

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EFFECT OF INHOAfOGEl,tEiTY

o~mm$#Effii

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I P=i

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f

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I

ⁱ

^! I! i

^{1 1}¹

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-oA

¹¹

^A

Î Î1ⁱ Î^B

^I

^C ¹¹ ^D

111

°fM=e~:p"~'~=fJRti=Ffitt=tS:~

Fig. 4. Deformations on the surface of the rectangular specimen

317

30 or 40 G Pa.) Centroids of the aggregate mortar disc and ones of the disc coincided.

Research results have been recapitulated in Fig. 6.

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318 ^Gy.^BALAZS^{et al.}

2,0 9

1,8 1,6

1,4 7

0.. Cl 1,0 _{4,-_}

::L

0,8 ^5~

, 11

10' 8

1-3 9

0,6 6

7

E %0

Fig. 5. Tensile b c diagrams over 40 and 180 mm strain gauge lengths

Conclusions

a) Stress peaks and failures occurred at four points of the aggregate grain indicated in Fig. 6a. Tensile cracks appeared at somewhat lower forces than did compression cracks. The force causing tensile crack was more dependent on diameter and modulus of elasticity of

the thai1 that crack.

b) In case of a poor mortar, the ultimate force much depended on grain size and kind. For medium or good quality material (Eh

=

^{40 GPa)}

the dependence was negligible (Fig. 6 and Fig. 6b).

c) For compression the two end points of the smaller axis of the ellipse were critical with respect to the bond between aggregate and mortar.

It was always the bond - rather than the mortar - that failed first.

The ultimate force was little affected by the ratio of greater to smaller axis of the ellipse (n), but it was significantly affected by the mortar quality (Fig. 6c).

d) In case of a tension force parallel to the greater axis of the ellipse, cir- cular aggregate grains (n

=

¹⁾behaved in the best way. By increas- ing the value of 'n', the ultimate force decreased for n

=

1, failure

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EFFECT OF INHOMOGENEITY

17,6 17,4

01

~ Failure (gloss)

Failure (steel)

Stress peak in [ tension and-

~ress ~~~-- I

I

17,2 compression -t-tl--,F compression

- 10

17,0 16,8 16,

16,0 15.8 15,6 '6,4

First crack in cOl11!:lression

~ glass) . Medium grade mortar

Eh = 30 GPo Fir s t cf'1JCk in tension

~

^.^steetrFlt:~L.c:@.0_lnt§~siof1

^---

. (gloss)

80 40 2010 Diameter, mm

c)

10, 9,8

b)

Poor mortar Eh= I 20 GPo

~"t~LI

~-.

'~,Foilure ^(gloss)

00 40 20 10

Diameter, mm

d)

Legend:

319

- - critical for bonds

- - - - -11 - - , - mortar

Fig. 6. Effect of grain shape, size, aggregate type, mortar grade on load type on the bond between aggregate and mortar

a.) Effect of grain size and aggregate kind on the stress causing cracking or failure of the bond between aggregate and cement mortar

b.) Effect of grain diameter and aggregate type on stress causing failure of the bond between aggregate and cement mortar

c.) Effect of form factor and mortar type on the ultimate force d.) Effect of form factor and mortar type on the ultimate force

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320 Cy. BALAzs et al.

started at the bond, while for n

=

5, in the mortar. Failure depended less on mortar quality than on compression (Fig. 6d).

Summary

The effect of concrete inhomogeneity on its strength and deformation characteristics was illustrated by examples in the following fields:

inhomogeneity of highway concrete structures;

effect of the mode of production;

effect of the gauge length;

effect of the grain size, shape and material, as well as of the embedding mortar (matrix) quality.