·Table of Contents
Evaluation of Actual Compressive Strength of High Strength Concrete by NDTGiovanni Pascale, Antonio Di Leo, Roberto Carli
DISTART - University of Bologna
Viale Risorgimento, 2 - 40136 BOLOGNA - ITALY
DICASM - University of Bologna
Viale Risorgimento, 2 - 40136 BOLOGNA - ITALY
E-mail : email@example.com
Keywords : Concrete, HSC, Non destructive methods, Combined methods.
The materials used in this investigation and their characteristics are here summarized.
High early-strength Portland cement CEM I 52.5R (Italcementi, Bergamo, Italy) conforming to italian standard UNI EN 197/1.
Supplementary cementitious materials
Silica fume, Starkzement 1 (ADDIMENT Italia S.p.A., Bergamo, Italy), in the form of powder with the characteristics showed in Table 1.
Commercial, locally available sand was used (Cave AGES, Bologna Italy). Based on petrographic analysis it was composed of 94% of limestone. The fineness modulus, density, on a saturated and surface-dried basis, and water absorption were 2.67, 2.70 and 0.23 respectively.
Commercial, locally available crushed coarse aggregate (Cave AGES, Bologna Italy) with a nominal maximum aggregate size of 15.0 mm was used. Based on petrographic analysis it was composed of 95% of limestone. The fineness modulus, density, on a saturated and surface-dried basis, and water absorption were 6.78, 2.74 and 0.71 respectively.
A new superplastifier based on carboxylic ether polymer with long side chains (Glenium 51, MAC S.p.A., Treviso, Italy) was used.
|Chemical composition||Physical characteristics|
|SiO2||min. 85%||Fineness||grading 100% < 100 mm|
|CaO||ca. 1%||BET spec. surf. area 22 mq/g|
|MgO||max. 1%||Density||2.2 kg/dm3|
|Al2O3||max. 1%||Apparent density||0.2 kg/dm3|
|Loss of ignition||max. 5%|
|Moisture content||max. 1%|
|Table 1: Silica fume characteristics|
Concrete and mix proportions
Ten concrete mixes were prepared with five different water/(cement + silica fume) ratios. 11% of silica fume by wt. of cement was added. The curing regime of 21 ± 2 °C and 72 ± 5% of relative humidity (R.U.) was adopted. The concrete mix proportions are summarised in Table 2.
The mix 1 was chosen as reference mixing  and the others were obtained changing the w/(cement + silica fume) ratio and, as a consequence, the nominal strength.
Three batches of 38 dm3 of concrete were cast for each mix to provide the necessary specimens for the investigation.
|Mix||1/1 - 1/2||2/1 - 2/2||3/1 - 3/2||4/1 - 4/2||5/1 - 5/2|
|Water (w), kg/m3||126||126||126||126||126|
|Silica fume, kg/m3 (powder)||60||48||42||36||32|
|Fine aggregate, kg/m3||467||673||775||904||940|
|Coarse aggregate, kg/m3||1367||1273||1232||1190||1166|
|Table 2: Concrete mix proportions|
Test specimens and schedule
For each mix different specimens were done, as a function of the specific characterisation as follows:
Batching, casting and curing
A horizontal rotary drum mixer with 50 dm3 capacity was used. The surface of the mixer was wetted before mixing to avoid water absorption and to insure the same mixing conditions for all mixes. First, coarse aggregate, fine aggregate, cement and silica fume were added and then, starting the mixing, water and admixture, at the end. Mixing was continued for about 5 min. Slump test was then done according with Italian standard UNI 9418. The cubic and beam specimens were cast in steel moulds and cylinders specimens in plastic moulds. All the samples were consolidated on an standard vibrating table for 2 min. All the moulds were covered with polyethylene sheets for 24 h, in a laboratory environment, until demoulding. After this time, the specimens were removed from the moulds and transferred to the controlled environment room for the curing. The tests were carried out at 1, 3, 7, 28, 90 days. Two 150 mm cubes were cured in water for standard compressive testing at 28 days.
Non destructive tests and equipment
The following equipment were used for non-destructive testing.
Pulse velocity tests
A microseismic analyzer was utilized, with the following characteristics:
A standard Schmidt rebound hammer type N was used, and the tests were done according to the italian standard UNI 9189.
The commercial apparatus FISCHER TCP was used, with post-inserted, forced expansion inserts, according to the italian standard UNI 10157.
Probe penetration tests
The Windsor Probe System was used, according to ASTM C803M-97.
A Hilti DCM-1 drilling machine was used to core the cylinders. A REMET MT70/LS3 apparatus was used to cut and smooth the specimens. A standard laboratory testing machine was used to do the compressive tests (UNI 10766).
The regression curves were established according to the law:
y = a.xb
where the coefficient a and the exponent b were determined by the least square method. The correlation coefficient r and the 95% confidence limits were determined for each method.
The values of the rebound index and the pulse velocity were correlated to the compressive strength separately, as well as in combination, according to the "SonReb" method. In the second case, the following form was adopted for the correlation curve:
z = a . x b . yg
All the experimental results are plotted in Figures 1 to 6.
The correlation laws determined are reported in Table 3, where:
|Rs||=||estimated cubic compressive strength (MPa);|
|Vl||=||Pulse velocity (m/s);|
|P||=||Pull-out pressure (bar);|
|L||=||Exposed probe length (in);|
|Rc,28||=||Compressive strength of microcores Æ 28 mm;|
|Rc,50||=||Compressive strength of microcores Æ 50 mm;|
|Rs = 10-28V18.1272||0.84|
|Rs = 0.000135 Ir 3.4424||0.81|
|Rs = 0.01714 P 1.6383||0.78|
|Rs = 4.95654 L 3.4383||0.83|
|Rs = 29.326 + 0.6144 Rc,28||0.86|
|Rs = 19.8938 + 0.7633 Rc,50||0.91|
|Table 3: correlation laws|
Comparing the correlation laws found to the ones usually employed for normal concrete, we can observe that the exponents are generally higher. This is due to a lower sensitivity of non-destructive parameters to strength variations, when the strength is very high .
Correlations of Figures 1 and 2 can be favourably judged, with reference to the whole range of experimental results, according to the value of r calculated.
With reference to the other curves, it was not possible to use the results relative to the mixes with the highest strength.
The pull-out method (see Fig. 3) showed good performance for the entire strength range, but it was necessary to discard some results, because of anomalous cracking around the testing area.
|Fig 1: Compressive strength vs pulse velocity.|
|Fig 2: Compressive strength vs rebound index.|
|Fig 3: Compressive strength vs pull-out pressure.|
|Fig 4: Compressive strength vs exposed probe length.|
|Fig 5: Compressive strength vs microcore compressive strength (Æ 28 mm).|
|Fig 6: Compressive strength vs microcore compressive strength (Æ 50 mm).|
|Fig 7: Compressive strength esitimated by SonReb method|
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