EVALUATION OF AIR-PERMEABILITY OF COVER CONCRETE BY SINGLE CHAMBER METHOD

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1 EVALUATION OF AIR-PERMEABILITY OF COVER CONCRETE BY SINGLE CHAMBER METHOD K. Imamoto* Ashikaga Institute of Technology, Japan K. Shimozawa, General Building Research Corporation of Japan, Japan M. Nagayama, General Building Research Corporation of Japan, Japan J. Yamasaki, Asanuma Corporation, Japan S. Nimura, Osaka Institute of Technology, Japan st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 6-7 August 6, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 6 7 August 6, Singapore EVALUATION OF AIR-PERMEABILITY OF COVER CONCRETE BY SINGLE CHAMBER METHOD K. Imamoto* Ashikaga Institute of Technology, Japan K. Shimozawa, General Building Research Corporation of Japan, Japan M. Nagayama, General Building Research Corporation of Japan, Japan J. Yamasaki, Asanuma Corporation, Japan S. Nimura, Osaka Institute of Technology, Japan Abstract The permeability of cover concrete strongly affects the durability of reinforced concrete (RC) structures. This paper deals with a single-chamber method to evaluate the air-permeability of the cover concrete. It is based on methods originally developed by Schönlin and Hilsdorf[] and Berissi et al. f[] In Japan, over thousand new RC buildings are constructed each year. Here we describe a simplified air-permeability test method which can be performed by contractors. This method is modified to obtain the robust air-tightness between the chamber and the concrete surface and also to evaluate the moisture content of concrete. In this study, the air-permeable area and the variance of air permeability of full sized concrete walls are experimentally investigated. Tests results indicated that the air permeable area estimated by this method was within 5 to mm from the surface of concrete. Consequently it can be used to successfully evaluate the representative air-permeability of cover concrete. Coefficients of variances of A.P.I.s of full sized concrete walls ranged from to %, indicating that this test method can evaluate the air-permeability of cover concrete on site. Keywords: Air permeability, Non-destructive Testing, Cover concrete, Single-chamber, Vacuum. Introduction Substances which are harmful for reinforced concrete (RC) concrete structures such as CO and Chloride ion reach steel bars through the cover concrete. For this reason the quality of cover concrete plays a significant role in the durability of concrete structures. The use of concrete with low water cement ratio (w/c) effectively makes concretes dense to prevent these ingresses. However, it is well-known that not only w/c but also the degree of compaction, the curing conditions and the existence of cracks can affect the quality of cover concrete. A number of in situ permeability testing methods for cover concrete have been developed [], [], [5], [6]. While these methods are useful they are often destructive, utilize expensive equipment or require long periods of time (5 to minutes) to perform. Here we describe a simplified non-destructive air-permeability test method using a single chamber cell. It is based on the method developed by Schönlin and Hilsdorf[] and Berissi

3 et al. []. Our method is quite simple and can be performed rapidly (-5 minutes) with devices easily obtained from markets. Our method cannot be applied to certain aspects of this problem, in particular the measurement of air permeable area. In Japan, each year over thousand new RC buildings are constructed. For this reason a simplified test method to diagnose air permeability that can be easily performed is extremely important for construction engineers.. Here we report an application of this method to determine the air permeable area and the variance of air permeability in a full sized concrete wall.. Test procedure of the single chamber method The test protocol is described in literature [7]. A brief protocol is outlined below;. Measurement of electrostatic-capacity (E.S.C.) of concrete surface at tested point (Photo ).. Adhesion of cm-width silicon ring with acetic vinyl resin. (Photo ).. Attachment of single chamber (separable cover) on the silicon ring.. Suction of air and measurement of time T in air pressure change from.6 to 6. kpa (see Photo ). The air-permeability index (A.P.I.) of this method can be calculated according to Eq.. Measurement of the E.S.C. at the test point. If there is no remarkable change in the indicated values of E.S.C. before and after the air-permeability test, the influence of the moisture content of the concrete is judged to be negligible. Photo Measurement of E.S.C. Photo Adhesion of silicon ring Photo Measurement of A.P.I.. 6. A. P. I. () T Here, T is elapsed time (Sec.) under the change of pressure from 6. to. kpa. Air permeable areas in the single chamber method kinds of concrete blocks (6, and 6 MPa in design compressive strength) with kinds of curing conditions (form removal at,, 7 and days) were prepared for this study. The air permeable areas were measured with air pressure sensors shown in Photos and 5. The materials used, mixture proportions and properties of the concrete are given in tables, and. Photo Arrangements of air pressure sensors Photo 5 Air pressure sensors covered with paper filter.

4 5cm 5cm 5cm 5cm 5cm 5cm filter 5cm cm 5cm Fig. Tube arrangements ( left: plan right: elevation) 5cm Photo 6 Measurements of air pressure cement(c) Fine Agg.(S) Coarse Agg.(G) Admixture Table Materials used Ordinary portland cement (density.6g/cm ).crushed sand density.6g/cm absorption.8%.pit sand density.59g/cm absorption.6% Crushed stone density.68g/cm Superplasticizer(HAE) Air Entrained water reducing agent(ae) Table Mixture proportions No W/C S/a Unit content(kg/m ) admixtur W C S S G e HAE AE AE Table Properties of concretes No Slump(cm) Flow(cmxcm) Air content(%) Concrete temperature( ºC) F 8 (N/mm ) F 8: compressive strength at 8 days in normal curing Distributions of air pressure in the concrete block with w/c.56 are shown in Fig.. Similar test results were obtained with both the w/c. and.7 specimens (data not shown). Air pressure was measured within 5mm of the surface of the concrete and within 8 cm of the outer area of the silicon ring (see dotted lines in Fig..) Measurements were not taken at depths grater than 5mm from the surface or outside of the edge of the silicon ring. As a result, the air permeable area in our method is estimated to be within 5 to mm from the surface of the concrete. The method can successfully evaluate the representative air-permeability of cover concrete (usually to mm from the surface of concrete).

5 Distance from the centre of the Silicon ring (cm width) Air pressure(kpa) Distance from the surface;5mm form removal day days 7days days 6 8 Air pressure(kpa) Distance from the surface;5mm Air pressure(kpa) Distance from the surface;5mm Fig. Distribution of air pressure in the concrete (w/c 55.5%). Influence of curing conditions on A.P.I. Relationships between the A.P.I. and the accelerated carbonation depths were investigated. Materials and mix proportions used in this test are given in tables and 5, respectively. 8 concrete mixtures (6; normal curing, ; poor curing) were used in this test. The poor curing condition (form removal at days) was intended to simulate usual type of construction used in Japan. The carbonation depths at the climate chamber (5% enriched CO at ºC and 6% R.H.) were measured according to JIS (Japanese Industrial Standard) at intervals of,, and 6 weeks. Table Material used Material Properties Cement Ordinary Portland cement S:Fine Agg. Pit sand; density :.57 g/cm, F.M.:.8 Crushed sand; density :.66 g/cm,f.m.:.8 G:Coarse Crushed hardened sandstone; density Agg..69g/cm, solid content;58% Admixture Lignin sulfonate acid air entrained agent(n) Polycarboxylic acid type Superplasticizer(H)

6 Table 5 Mix proportions and curing conditions Mix. w/c s/a Unit content (kg/m ) Ad. No W C S G C*% Normal curing(water curing at ºC for month and air curing at ºC and 6% R.H. for month) H: H: H: H: H: N: N: N: N: N: N: N: N: N: Poor curing (Form removal: days after casting of concrete) Mix H:.6 Mix. Addition of about kg/m water to Mix. H:.6 Accelerated Carbonation depth (mm) Mix. Mix. Acceleration for 6 weeks weeks weeks A.P.I. (kpa/sec.) Fig. Relationships between A.P.I.s and carbonation depths Skin Air flow Fig. Schematic drawing of skin The air-permeability tests were performed at months in accordance with the procedures described in Chap.. The results of accelerated carbonation depths and A.P.I.s are plotted in Fig.. The magnitudes of A.P.I.s increase along with the increase in carbonation depths (see each solid line). The A.P.I.s of Mix. and (poor curing condition) are also plotted as black marks in the figure. The plotted datapoints are located close to the solid line obtained from the test results obtained from normally cured specimens. Bleeding of the concrete and poor curing of specimens often creates a porous skin at the surface of the concrete, as shown in Fig... Torent (99) reported that the existence of surface skin in slab specimens that was caused by bleeding results in overestimation of the A.P.I, due to excessive air flowing into the chamber from the exterior through this porous skin. However, this phenomenon was not observed in the curing conditions which simulated wall members cured by the usual construction manner utilized in Japan. Hence, it is supposed that this single chamber method will be useful for the estimation of air-permeability of concrete walls (not slabs) on-site.

7 5. Variance of full sized concrete walls The variances of air permeability in concrete walls were also investigated. The materials used and mixture proportions are shown in table 6 and 7, respectively. Table 6 Materials used cement(c) Ordinary portland cement (density.6g/cm ) Fine Agg.(S) S.crushed sand density.6g/cm absorption.8% S.pit sand density.59g/cm absorption.6% Coarse Agg.(G) Crushed stone density.68g/cm Admixture Superplasticizer(HAE) Table 7 Mixture proportions W/C (%) S/a (%) Unit content(kg/m ) Admixture (kg/m ) W C S S G A B C D A B C D.5m 5cm 7cm 7cm 7cm 7cm 5cm 5 5 Hanycomb Crack 5cm cm cm cm 5cm m 5cm cm cm cm 5cm m Fig.5 Tested point of A.P.I. in full sized wall Photo.6 Appearance of full sized wall Front Back A B C D 5 A.P.I.(kPa/s) A B C D 5 A.P.I.(kPa/s) Fig. 7 Distributions of A.P.I. in full sized walls (left: front, right: back) Wooden forms were used in this study and were removed after days. The air-permeability tests were performed at months after the casting of the concrete. The distributions of the A.P.I. in the

8 walls are shown in Fig.7. The magnitudes of A.P.I.s, except at the Cracks and Honeycomb, decrease at lower area of the wall due to the consolidation of concrete. The average values and standard deviations of A.P.I.s are summarized in table 6. According to the test results, the coefficient of variance range from to %. This value would be larger than the variance of compressive strength of concrete. Table 8 Test results of A.P.I. Average Standard deviation Coefficient of variance Front.9 kpa/s.65 kpa/s. % Back.5 kpa/s.5 kpa/s. % 6. Conclusions In Japan, over thousand new reinforced concrete buildings are constructed each year. The overestimation of air-permeability due to the existence of porous skin created by bleeding of concretes and poor curing conditions is said to be a significant weak point of the single chamber method. However, this overestimation was not observed in the test conditions which were intended to simulate wall members under the usual construction manner in Japan. Our results indicate that that this method will be useful for the evaluation of air-permeability of concrete walls. On the other hand, any overestimations will result in safety judgement that will assess the durability of structures for end users. Another benefit of this procedure is that it does not require a trained engineer to perform it and it can quickly and easily be carried out by contractors. The air permeable area estimated by this method is within 5 to mm from the surface of concrete. Consequently it can be used to successfully evaluate the representative air-permeability of cover concrete. Coefficients of variances of A.P.I.s of full sized concrete walls ranged from to %, indicating that this test method can evaluate the air-permeability of cover concrete on site. References [] Schönlin and Hilsdorf 987: Evaluation of the effectiveness of curing of concrete structures, ACI SP-, pp7-6. [] Berissi et al.987: Mesure de la porosite ouverte des betons hydrauliques, Bull. Liaison Labor. Ponts Chauss. No., [] Figg, J.W.97. Methods of measuring the air and water permeability of concrete, Magazine of concrete research, Vol.5, No.85, pp.-9. [] Hansen, A.J., Ottosen, N. S. and Peterson, C. G. 98. Gas-permeability of concrete in situ: theory and practice, ACI-SP, pp [5] Chen, Z. H., Parrott, L. J Air permeability of cover concrete and the effect of curing, British Cement Association Report, No. C/5, pp.-5. [6] Torrent, R. J. 99. A two-chamber vacuum cell for measuring the coefficient of air of the cover concrete on site, Materials and structures, Vol.5, pp [7] Imamoto, K. et.al. 5 Applicability of single-chamber vacuum cell for the evaluation of the airpermeability of concrete walls, Int. Conf. on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR), South Africa.