Influence of the Concrete Mixture Compaction Time on Density and Compressive Strength of Hardened Concrete Samples

Journal of Sustainable Architecture and Civil Engineering Vol. 2 / No. 11 / 2015 pp. 79-87 DOI 10.5755/j01.sace.11.2.12660 © Kaunas University of Technology Received 2015/03/02 Accepted after revision 2015/06/01 Influence of the Concrete Mixture Compaction Time on Density and Compressive Strength of Hardened Concrete Samples Influence of the Concrete Mixture Compaction Time on Density and Compressive Strength of Hardened Concrete Samples


Introduction
Journal of Sustainable Architecture and Civil Engineering 2015/2/11 80 ropy usually becomes apparent during vibration when shear stress and viscosity go down (Khayat et al. 2002, Roussel 2006, Mewis and Wagner 2009. Because concrete itself is a porous composite material, the pores in concrete structure are classified as compaction pores, air pores and capillary pores (which affect durability) and gel pores. The change in the compaction pores may significantly affect the carbonation rate and the sorptivity coefficient, and compacting of the fresh concrete is the more important in terms of the durability (Gonen and Yazicioglu 2007).
To assess adequate level of vibration, consideration should be given to match the amplitude of vibration and concrete viscosity. High amplitude means greater vibratory forces. The viscosity of the mix should be strong enough to bear these forces while keeping the suspensions of coarse aggregates. Thus, the amplitude of vibration should match the viscosity of mix when applying vibration (Safawi et al. 2005).
According to authors (Arslan et al. 2005) in normal construction practice, concrete is placed in 1 m layer and compacted by using poker type vibrators, which are immersed into only the top 1 m of the concrete. If concrete is placed to a depth of 1 m, the vibrator is immersed 1 m into the concrete. The concrete is completely fluidised and the lateral pressure is hydrostatic. If concrete is placed to a depth of 2 m, the effect of the vibrator will extend below the vibrator and the 2 m depth of concrete will be fluidised, giving hydrostatic pressure. If concrete is placed to a depth of 3 m, the lower concretes develop significant shear strength, settling vertically under load and developing friction between the concrete and the formwork surface.
According to authors (Banfill et al. 2011) in the inner liquefied zone around the vibrator the flow is due to shear whereas in the outer unsheared zone propagation is due to compressive waves. The research results gives a method of predicting the radial position at which the flow changes, which coincides with the radius of action of the vibrator. Experiment agree that the peak velocity of the vibration governs its efficacy, with radius of action increasing with increasing velocity. The work offers the potential to optimise the design and use of vibrators. Teranishi et al. (1993) investigated theoretically the mean deformation resistance of fresh concrete under vibration by simplifying the vibration as a repeated static shear stress and using the Bingham model to express the deformation behaviors of fresh concrete under static stress. As a result, when the vibration acceleration is greater than a certain value, the shear rate -shear stress relationship in the lower shear rate -range becomes a curve without a yield value, rather than a straight line as in the Bingham model. Gong et al. (2015) developed a UWB-based tracking method for real-time 3D tracking and visualization of concrete vibration effort during concrete placement. The research results indicate that the developed occupancy grid-based vibration effort visualization method can identify defective vibration practices. The research outcome will contribute to the improvement of concrete consolidation quality assurance methods.
Concrete-filled steel tubular columns have better structural performance than that of bare steel or reinforced concrete. For reinforced concrete columns, concrete compaction only affects the mechanical properties of concrete. But for concrete-filled steel tubes, it is well known that the interaction between steel tube and concrete is the key issue to understand the behaviour of this kind of column. The concrete compaction not only affects the properties of the core concrete itself, but also may influence the interaction between the steel tube and its concrete core, and thus influences the behaviour of the composite columns (Han and Yao 2003).
Scientists Grabiec and Piasata (2004) say that the big influence on the concrete's resistance to freezing -thawing cycles is being made by the density and the compressive strength of the concrete.
The outcome of this article was to find how different compaction time influences the density and compressive strength of hardened concrete.
Portland cement CEM II/A-LL 42.5 R (MA) (A) (JSC Akmenės cementas production) was used for the test. Physical, mechanical properties and chemical composition of Portland cement are given at the table 1.
Sand with the fraction of 0/4, bulk density of 1680 kg/m 3 and fineness module of 3.2 was used as fine aggregate. Crushed gravel with the fraction of 4/16 and bulk density of 1450 kg/m 3 was used as the coarse aggregate. Granulometric composition of aggregates is conducted according to LST EN 12620:2013 and presented in Table 2.
Plasticizing admixture based on policarboxile polymers Glenium SKY 628 (BASF Construction Chemicals Italia Spa) was used with density of solution 1.06 g/ml. The total dosage of admixture was 0.5 % of cement.
The concrete mixtures were prepared in 3 minutes by two stages in laboratory forced type con-  crete mixer Zyklos Rotating Pan Mixer ZZ 75 HE. During the first stage cement, aggregates and 2/3 of water were poured into the moistened mixer and mixed for 2 minutes. While during the second phase, the remaining amount of water was poured with concrete admixture and concrete mixture was mixed for 1 minute. During the research, dry aggregates were used for concrete mixtures. Cement and aggregates were dosed by weight while water and chemical admixture were dosed by volume. When preparing the concrete mixture, 90 % of water was instantly poured to the mix. Super-plasticizing admixture was mixed with 10 % of water and poured into the mixer.

Methods
The concrete specimens, columns of diameter about 106 mm and height about 1000 mm dimensions size, were formed in plastic angular tube and compacted by using poker vibrator ENAR DINGO (frequency: 50÷60 Hz, vibrations: 18.000 r.p.m.). Concrete mixture was compacted by poker vibrator, when vibrating element was placed to the top part of the form. Different time was used for compacting mix-ture in forms: 0, 3, 6, 9 and 12 s. The concrete mixture was compacted immediately after placing in the forms in such a way as to produce full compaction of the concrete with neither excessive segregation nor laitance. The specimens were kept for 28 days in forms at the temperature about 16° C. When specimens were removed from the forms, they were cut by100 mm. After cutting the specimens, we got 10 samples, cylinders of diameter about 106 mm and height about 100 mm dimensions size. The cut surfaces of samples were polished by abrasive stone. If surface was extremely rough, it was smoothed by cement mortar.  Technological properties of the concrete mixture were determined. Mixture fulfilled class S3 (S3=(100-150)±30 mm) according to cone abatement. Determined density of the mixture was 2230 kg/m 3 . From the prepared concrete mixture 5 specimens were formed in the shape of column with dimensions: diameter 106 mm, the height 1000 mm (Fig. 1).

Fig. 1
The The dependence of the concrete samples' (106 mm diameter and 100 mm height -H/ D=0.94) density on the duration of compacting is presented in Fig. 2. From the presented graphic it could be observed, that the average values of the concrete samples' density is in the range of 2130÷2190 kg/ m 3 . The highest average value of the density of the concrete samples (2190 kg/m 3 ) was obtained when mixture was compacted for 3 seconds during the formation (H/D=9.4). As it was expected, the lowest average value of concrete sample's density (2130 kg/m 3 ) was obtained when mixture was not compacted during the formation (H/D=9.4).
The dependence of concrete sample's (H/D=0.94) compressive strength on the duration of the compacting is presented in Fig. 3. The average values of compressive strength of the concrete samples were in a small range 30.3÷32.6 MPa.

Fig. 2
The change of density of concrete samples depending on the compacting time

Fig. 3
The change of compressive strength of concrete samples depending on the compacting time The dependence of the concrete samples' (106 mm diameter and 100 mm heighton the duration of compacting is presented in Fig. 2. From the presented graphic it that the average values of the concrete samples' density is in the range of 213 highest average value of the density of the concrete samples (2190 kg/m 3 ) was obt was compacted for 3 seconds during the formation (H/D=9.4). As it was expected value of concrete sample's density (2130 kg/m 3 ) was obtained when mixture was no the formation (H/D=9.4).   Fig. 4. Equations of dispersion of the results, empirical and correlation coefficients values are presented in Table 4. Correlation coefficient (Pirson), which is evaluating the strength of linear relationship, and presented in the Table 4, was calculated according to coefficient of empirical equation. The closer correlation coefficient is to 0, the better representation of the dispersion of values in the curve. According to the obtained correlation coefficient, it was determined which equation describes the best distribution of statistical data. From Table 4 we can see, that values of dispersion of dependence of concrete samples density on the duration of the compaction time can be described by gradual, linear and parabolic dependencies. The values of correlation coefficient were in the range from 0.88 till 0.98. The relationship between the variables will be stronger a) Compaction time, 0 s

Fig. 4
Distribution of density and compressive strength of concrete depending on location of sample in the specimen's height, when the duration of compaction time is different similar. At higher sample's location, density and compressive strength were increasing. Also it noticed, that regardless the duration of the compacting of the specimens (H/D=9.4), values of den and compressive strength at 500-600 mm height were close to values of specimens taken at 100 height. That could be influenced by filling the forms with concrete mixture during the formation of specimens: forms of 1000 mm height were filled with mixture in two steps and only after the mix was compacted in forms. Filing forms with several layers and instantly compacting separate la could have more significant influence on density and compressive strength of hardened concrete.  Table 4. Correlation coefficient (Pirson), which is evaluating the linear relationship, and presented in the Table 4, was calculated according to coefficient of equation. The closer correlation coefficient is to 0, the better representation of the dispersion in the curve. According to the obtained correlation coefficient, it was determined which escribes the best distribution of statistical data. From Table 4 we can see, that values of of dependence of concrete samples density on the duration of the compaction time can be by gradual, linear and parabolic dependencies. The values of correlation coefficient were in from 0.88 till 0.98. The relationship between the variables will be stronger when the coefficient values is close to 1. it means, that variables are statistically dependent and based es of the correlation coefficient, there is a strong connection between variables.
he dependence of dispersion of data