Influence of compaction on chloride ingress

Proc. of the 9th _{fib International PhD Symposium in Civil Engineering, July 22 to 25, 2012,}

Karlsruhe Institute of Technology (KIT), Germany, H. S. Müller, M. Haist, F. Acosta (Eds.),

Influence of compaction on chloride ingress

Jure Zlopasa

Microlab, Faculty of Civil Engineering and Geosciences, Technical University of Delft,

Stevinweg 1, 2628 CN Delft, the Netherlands

Supervisors: Eduard Koenders and Aad van der Horst

Abstract

Experiences from practice show the need for more of an understanding and optimization of the com-paction process in order to design a more durable concrete structure. Local variations in comcom-paction are very often the reason for initiation of local damage and initiation of chloride induced corrosion. Poor compaction is often manifested locally in concrete structures in the form of large voids, profuse honeycombing and heterogeneity. Controlling compaction in practice today depends largely on the operator and his experience. The reliability of such a practice goes with many uncertainties. Depend-ing on the exposure conditions, cement based materials maybe attacked by aggressive substances which can influence the performance of a concrete structure. Therefore the change in chloride ingress was investigated as a function of compaction time and sample depth. In this paper the Rapid Chloride Migration method, (RCM), was used and test results show a change of the diffusion coefficient with time of compaction. Along with the RCM results, the compressive strength was measured and the results are presented as well.

1 Introduction

Durability of concrete depends on the long-term quality of a microstructure, which depends, on its turn, on the used materials, quality of construction, quality of design, the exposure conditions, and the on-site execution. When placed in a form, concretes may contain from 5 up to 20 % of entrapped air by volume. Presence of air voids in hardened concrete turned out to have a significant effect on me-chanical properties and can be controlled by compaction. Compaction has a very good correlation with several important properties such as strength, permeability (and hence durability) and shrinkage [1]. It has been found that for every 1 % of air voids there is a 5 % of decrease in compressive strength [2]. Air voids are expelled by compaction. Compaction can be achieved by vibration, centrif-ugation, rodding, tamping, or combination of these actions [3].The compaction consists of two pro-cesses; in the first stage the particles are set in motion and concrete is “liquefied” and slumps to fill the forms, while during the second stage air bubbles escape from the concrete element. There is a distinction between the two stages because they don’t start and finish at the same time. The conse-quence of inadequate compaction is appearance of sand streaks, honeycomb and as mentioned exces-sive entrapped air voids. If the workability of the concrete is constant, the skill and consistency of workers in operating the vibrators (i.e., compaction effort) will significantly influence the degree of compaction attained and with this the long-term durability [4]. Because concrete contains 65-75 %vol. of aggregate and all of the air voids reside in the cement paste, a small amount of air entrain-ment causes a significant change of the microstructure of the paste, and to its pore structure in particu-lar[5]. The focus of this research was on the relationship between time of compaction, transport and mechanical properties of concrete a specimen. Transport properties are measured by the Rapid Chlo-ride Migration, RCM, test and the mechanical properties are presented in terms of compressive strength after 7 and 28 days. Because the chloride transport is of great significance, but in real life this process lasts for many years, the RCM test is used which has shown that it can adequately represent chloride transport in cement-based materials [6].

2 Materials and experimental procedure

Concrete samples with the same mix design, table 1, were casted. In order to view the influence of compaction of the properties of concrete mix design, mixing, vibration frequency (65 Hz), vibration amplitude (0,087 mm), curing conditions and temperature were kept constant. Samples were cast in cube moulds 150mm×150mm×150mm. The compaction was done on a vibration table. After the

9 fib International PhD Symposium in Civil Engineering

compaction was done the samples were stored in the fog room at 20 °C. The specimens were tested for compressive strength after 7 and 28 days and chloride migration coefficient, which was performed by the RCM test. The whole experimental procedure is outlined in figure 1.

Table 1 Mix proportions

CEM III/B 42.5N [Kgm-3_{] 370}

Water [Kgm-3_{] 185}

Aggregate [Kgm-3_{] 1794}

Fig. 1 Experimental scheme

In order to evaluate the difference in transport properties of the samples determination of the chloride coefficient in concrete using the RCM is used. The method is prescribed in NT Build 492 [7]. The principle behind this method is the application of external electrical potential axially across the con-crete specimen which forces the chloride ions to migrate into the specimen. A sketch of the RCM test is shown in figure 2. Sampling was done by drilling the cubes and slicing the cylinders in order to obtain 100 mm diameter and 50 mm height sample. The samples were then preconditioned by putting them in vacuum for 3 hours and then, with the vacuum pump still running, the samples were covered

with saturated Ca(OH)2 solution. After 1 hour the vacuum pump was turned off and air was allowed

to enter the container and the specimens were kept in the Ca(OH)2 solution for 18 ± 2 hours. After

the preconditioning the samples were mounted in the RCM setup which consists of a rubber sleeve in which the sample is placed and anolyte solution (0.3 M NaOH) was poured in the sleeve. The speci-men was then put into a catholyte solution (2 M NaOH). The electrodes were immersed in anolyte and catholyte solutions and connected with the power supply.

Fig. 2 Schematic representation of the RCM test

The initial current through the specimen at 30 V is recorded and the voltage was adjusted according to the standard which then states the duration of the test as well. Initial and final temperatures were recorded. After the stated duration of the test the specimens were removed from the RCM setup and

sliced axially into two pieces. On the freshly split surface 0.1 M AgNO3 was sprayed and after a few

minutes white silver chloride started to precipitate. The white precipitated silver chloride represents the penetration depth of the chlorides which was measured by a calliper. For each measurement three samples were made.

3 Results and discussion

In figure 3 the change in compressive strength after 7 and 28 days is shown. There is a difference in compressive strength with time of compaction. This is due to the presence of the air voids that are entrapped in the specimen. Air voids are reduced by the time of compaction which is shown as an increase of the compressive strength. There is a big rise in the compressive strength from no compac-tion to 10 seconds of compaccompac-tion, after that increase in compressive strength is shown. So it could be said that most of the air inside of the concrete has escaped.

9 fib International PhD Symposium in Civil Engineering 0 5 10 15 20 25 30 5 10 15 20 25 30 35 40 45 50 55

x 7 day compressive strength - 28 day compressive strength

Co m p re ss ive s tre ng th [MPa ] Compaction [s]

Fig. 3 7 and 28 day compressive strength as a function of compaction duration.

Also shown in the figure 3 is less scatter in the attained compressive strength results for compaction from10 to 30 seconds. That shows a more homogenous and consistent microstructure and results.

0

5

10

15

20

25

30

0

1x10

2x10

3x10

4x10

5x10

6x10

Fig. 4 Chloride migration coefficient measured from the chloride penetration depth after RCM

test as a function of compaction time

From the penetration depths, measured after silver chloride, AgCl, precipitation, chloride non-steady state migration coefficient was calculated. It can be seen in figure 4 and table 2 that the migration coefficient decreases with time of compaction. The reason for the lowering in the migration

coeffi-cient can be found in change in aggregate to cement ratio which changes during compaction, and decrease of volume fraction of air voids. Wong et al. [5] found that the microstructure of the air void-paste interface is similar to that of the aggregate-void-paste interfacial transition zone. Kreijger [8] showed that the “skin” of a concrete consists of three layers, and during compaction the biggest change in composition, aggregate to cement ratio, is within few cm. This skin concrete layer that is rich in cement paste has different transport properties than the bulk concrete. That difference could also be enhanced due to chloride binding by cement hydration products. That is one of the downfalls of RCM method in which up to 2 cm of “concrete skin” is cut off.

Table 2 Summary of the results measured for compressive strength after 7 and 28 days, and RCM

test after 28 days.

Sample Compressive strength [MPa] DRCM [m2s-1]

7 day 28 day CC0 10.13 ± 1.92 14.76 ± 1.60 4.69×10-12 _{± 1.05×10}-12 CC5 25.46 ± 3.28 45.79 ± 1.12 3.34×10-12 ± 0.75×10-12 CC10 29.05 ± 1.94 44.44 ± 0.68 3.10×10-12 ± 0.74×10-12 CC15 29.12 ± 0.87 45.76 ± 0.13 3.06×10-12 ± 0.51×10-12 CC25 30.45 ± 1.82 46.62 ± 0.65 3.05×10-12 ± 0.73×10-12 CC30 29.50 ± 0.71 49.74 ± 0.28 2.94×10-12 ± 0.43×10-12 4 Conclusions

This study was undertaken to investigate the importance and the influence of compaction on the chloride migration coefficient and compressive strength. Specimens with constant mix design were characterized and the findings are outlined below.

RCM test shows sensitivity towards time of compaction, this sensitivity is greater while there is a higher volume fraction of air inside the concrete specimens. For compaction longer than 10 seconds the part that changes most is the outermost part where the segregation happens with the change in aggregate to cement ratio which in the standard RCM test is cut off.

Compressive strength showed an increase with time of compaction which is a consequence of en-trapped air leaving the concrete specimen due to vibration. The biggest difference was until 10 se-conds of vibration, later little difference was observed. The difference was more pronounced on the standard deviation rather than the compressive strength.

Up to thirty seconds of compaction leads to a more uniform and homogenous concrete element, in which the properties differentiate less than compared with no compaction.

5 Acknowledgements

The authors like to acknowledge the Dutch National Science foundation STW. The research conduct-ed within this project is financconduct-ed by STW as part of the IS2C program (www.is2c.nl), number 10962. References

[1] Ayton, G., A Recipe for Compaction of Concrete, In: 7th International Conference on

Con-crete Pavements (2001)

[2] Heaton, B. S., Relationship in Concrete between Strength, Compaction and Slump, In:

Con-structional Review 39 (1966) No.2, pp.16-22

[3] ACI 309R-96: Guide for Consolidation of Concrete. In: ACI Manual of Concrete Practice

(2005)

[4] Stewart, M.G.: Concreting Workmanship and Its Influence on Serviceability Reliability. In:

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[5] Wong, H.S., Pappas A.M., Zimmerman, R.W., Buenfeld, N.R.: Effect of entrained air voids

on the microstructure and mass transport properties of concrete. In: Cement and Concrete Re-search 41 (2011), pp. 1067-1077

[6] Bouwmeester - van den Bos, W.J., Schlangen, E.: Influence of curing on the pore structure of

concrete, In: Tailor Made Concrete Structures (2008), pp.65-70

[7] NT Build 492, 1999, Concrete, Mortar and Cement-based repair materials: Chloride

Migra-tion Coefficient from non-steady-state migraMigra-tion experiments. Nordtest Finland.

[8] Kreijger, P.C.: The skin of concrete: Composition and properties. In: Materials and Strucutres