Chloride and sulphate concentration in concrete exposed to marine conditions

Transcription

1 Chloride and sulphate concentration in concrete exposed to marine conditions H AI-Khaiat*, Kuwait University, Kuwait M N Haque, Kuwait University, Kuwait 27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, 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 27 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore Chloride and sulphate concentration in concrete exposed to marine conditions H AI-Khaiat*, Kuwait University, Kuwait M N Haque, Kuwait University, Kuwait Abstract A normal weight concrete of 50 MPa (NWC50) and a total lightweight concrete of 50 MPa (LWC50) compressive strength were cast and tested. The mix proportions, strength and modulus of rupture development, and water, carbon dioxide, chloride and sulphate penetration after 3 years of exposure in hot humid ambient conditions after different initial curing periods have already been reported. This paper presents both the chloride and sulphate profiles of the NWC50 and LWC50 concretes after 1 and 7 days of initial curing and subsequent exposure to hot humid (seaside) conditions for a period of 3 years. The chloride and sulphate concentrations in the two concretes, at 5-mm depth intervals are reported. It is concluded that the extent and amount of penetration of both chloride and sulphates in concrete can be considerably reduced by providing 7 -day moist curing, thus enhancing the performance and design life of concrete structures exposed to severe conditions. The chloride concentrations both in NWC50 and LWC50 were practically not much different on seaside exposure. Also, sulphate concentration in LWC50 was only marginally higher than that in NWC50. Overall, it is concluded that properly designed lightweight concretes of adequate strength are durable against chloride and sulphate penetration in severe hot humid exposure conditions. Keywords: Lightweight concrete, chloride profile, sulphate concentration, seasidel 1. Introduction Concrete, in reality, is a highly versatile and a flexible family of construction material, which has performed extremely well in some of the most demanding and aggressive service and exposure conditions. As a material of construction it embodies high strength, high performance, heavy weight, lightweight, fiber-reinforced, polymer concrete and the list goes on. If designed and constructed with proper care, concrete construction has often out-performed its design life. Some of the more challenging concrete construction is in marine, off-shore and gravity platforms, etc. In marine and gravity structures durability against chlorides and sulphate rich waters and soils, and the self-weight of the concrete are often the overriding constraints. Accordingly high strength structural lightweight concrete has emerged as a highly suitable material of construction in gravity and marine structures. Structural lightweight concrete (LWC), of course, has well established advantages in comparison with normal weight concrete (1,2). It is now about six years (initiated in early 1996) that the authors started a research project on the performance evaluation of the similar grades of NWC and LWC exposed to marine and inland 131

3 conditions in Kuwait (3). The mix proportions, strengths and modulus of elasticity development, and water, carbon dioxide, chloride and sulphate penetration up to 3 years of exposure in both hot humid and hot dry ambient conditions after different initial curing periods have already been reported (4-6). This paper presents both the chloride and sulphate profiles of a normal weight concrete of 50 MPa (NWC50) and a total lightweight concrete of 50 MPa (LWC50) strength concretes after 1 and 7 days of initial curing and subsequent exposure to hot humid (seaside) conditions for a period of 3 years. The chloride and sulphate concentrations in the two concretes, at 5-mm depth intervals, are presented and discussed. 2. Experimental Program The details of the experimental program have been reported elsewhere (4-6). A brief summary is given below: Two concretes, one normal weight and the other lightweight of 50 MPa 28 day compressive strength were designed. They are referred to NWC50 and LWC50, respectively. The mix quantities used and some characteristics of the two concretes are included in Table 1. Table [1]: Mix Proportions (kg/m 3 ) and characteristics of fresh concrete Material/Mix LWC50 NWC50 Cement CSF Water Lightweight-coarse (normal weight) Lightweight-fine (washed sand) Superplasticizer Slump (mm) Density (kg/m 3 ) The specimens were initially cured as given below: Full curing Curing in water tank maintained at 23 ± 20 C till the age of testing; 1 day curing No water curing after demoulding; 3 day curing Curing in water tank for 2 days after demoulding; 7 day curing Curing in water for 6 days after demoulding. In addition to the continuous water curing, specimens were placed at an exposure site near sea (but beyond the reach of seawater) after 1 and 7 days of initial curing (as described above) and are referred to as 1 SS and 7SS exposure regimes, respectively. The relevant British Standards (7) were used to evaluate durability characteristics of the concrete. Permeability of concrete to water was determined by 01 N 1048 Part 2 [8]. This test was performed on plate shape specimens of 200 x 200 x 120 mm. To determine sulphate and chloride profiles for the beam specimens exposed on the seaside, 5 mm slices were cut, crushed and ground to extract the powdery specimen for the chemical analysis according to the British Standard (7). 3. Results An abridged version of the strength, depth of water penetration, extent of carbonation, sulphate and chloride concentration in LWC50 and NWC50 are included in Tables 2-4 for information and completeness (5). These results, however, are not discussed in this paper. Table [2]: Strength after 3 years exposure on seaside (MPa) Curing Compressive strength Modulus of rupture regime LWC50 NWC50 LWC50 NWC50 Age (days) Full SS SS

4 Table [3]: Water penetration and carbonation depth after 3 years exposure on seaside (mm) Curing Water penetration Carbonation depth regime LWC50 NWC50 LWC50 NWC50 Age (days) Full 1SS 7SS Table [4]: Sulphate and chloride contents after 2 years exposure on seaside (% by mass of cement) Curing Sulphate content Chloride content regime LWC50 NWC50 LWC50 NWC50 Age (days) SS SS The chloride and sulphate concentrations in the two concretes at 5-mm depth intervals, exposed to hot humid (seaside) conditions for a period of 3 years are included in Table 5. The chloride and sulphate profiles are also plotted in Figs Discussion 4.1. Chloride profile As can be seen in Table 5, the chloride concentration is maximum in the outer 0-5 mm thickness of both the lightweight and normal weight concrete for both one and 7-day initially cured specimens. As expected the chloride concentration decreases as the path length increases as shown in Figs. 1 and 2. Also, it can be seen in Figs. 1 and 2, both for 1 and 7 day initially water cured specimens, the chloride penetration profiles are very similar both for the lightweight and normal weight concretes. Table [5]: Chloride and sulphate content after 3 years exposure on seaside (% by weight of cement) Concrete LWC50 NWC50 depth [15 [15 [0-5] [5-10] 10-15] [0-5] [5-10] 10-15] (mm) 20] 20] 1SS - C SS - C SS - S SS - S Average chloride concentration After three years of seaside exposure and initial curing of 1 day only, chloride concentration in both the concretes is more than 0.2% by mass of cement (see Table 4). It is to be noted that the salt concentration shown in the table are average value in the whole of specimen, i.e., average over the depth. As shown in the table, this chloride buildup has been the result of the soaking process. Chloride that has soaked in are known to be able to initiate corrosion at levels of 0.2% by mass of cement (9). These results suggest that even an inadequately cured 50 MPa normal weight concrete is prone to the undesirable buildup of chlorides in hot-marine exposure conditions. It is known that chloride-induced reinforcement corrosion, due to the ingress of sea water, can occur with dramatic rapidity, especially in hot climates (10). 133

5 ~ 0.40 I'll c: IV c: ~ E0.30 IV IV Q. IJ 0.25 c: '0 020.Sl (I) S ~ 0.15 IJ E ~ > '.c LWCSO ---A- NWCSO 1 ~ , , , [0-5] [5-1a [1G-15j [15-2a Fig. [1]: Chloride profile of concrete after 3 years exposure on seaside after one-day curing 1-.- LWCSO ---A- NWCSO 1 IV Cl 0.40 I'll - c: 0.35 IV c:... E 0.30 IJ IV -- IV IV Q.IJ 0.25 ~ c: (I) (I) o I'll 0.15 IJ E 0.10 ~ > '':::..c '= 0.00 IJ -- [0-5] [5-1a [1G-15j [15-2a Fig. [2]: Chloride profile of concrete after 3 years exposure on seaside after seven-day curing Whilst the chloride penetration in LWC50 is somewhat higher than that in NWC50, the difference is only nominal. Again extent of curing has improved the resistance of both the concretes to the ingress of chloride. Osborne (11) also observed that chloride levels in lightweight concretes exposed to seawater were somewhat higher than those in the NWC. However, the diffusivity of chloride is known to decrease with increased exposure time (12-13). The salt content at the surface of the LWC has also been reported to be higher than in the NWC, although the depth of penetration was small (14). The results reported earlier (5) indicate that concretes with higher water penetrability also yielded higher chloride concentration. Since the chloride and sulphate penetration into concrete is a result of ionic transport, the higher the water penetrability of a concrete, the higher seems to be the depth of carbonation and concentration of the damaging salts in a concrete. 134

6 4.3 Sulphate penetration The sulphate penetration profile is similar to that of chloride penetration and is shown in Figs. 3 and 4. The average value of the sulphate concentration in the two concretes, after differing periods of exposure, is included in Table 4. Sulphate concentration in LWC50 is higher than that in NWC50. Likewise seven day initial curing has improved the resistance of both the concretes to the ingress of sulphate ions. The sulphate concentration in the two concretes is much lower than the limits of 5% as prescribed by the concrete structures codes (15,16). Q) CI It! C ~ ; 200 ; ~ 1.75 Q. u ; : ~ E0.75.! > ~.Q 0.25 I-+- LWC50 -Ir-NWC50 I Q. O.OO ~ ~ :::l en [0-5] [5-ia [UH5I [15-2a Fig. [3]: Sulfate profile of concrete after 3 years exposure on seaside after one-day curing I-+- LWC50 -Ir-NWC50 I Q) CI It! -c- Q) C 225j U Q) 200 ~ E Q) Q) Q. u 1.50 ; : ~ o It! 0.75 U E Q) > ~.Q Q. :::l en -----, , , , [0-5] [5-ia [UH5I [15-2a Fig. [4]: Sulfate profile of concrete after 3 years exposure on seaside after seven-day curing 5. Conclusions i) The extent and amount of penetration of both chloride and sulphate in concrete can be considerably reduced by providing 7-days moist curing, thus enhancing the performance and design life of concrete structures exposed to severe conditions. ii) The chloride concentration both in NWC50 and LWC50 was practically not much different on seaside exposure. 135

7 iii) Sulphate concentration in LWC50 is only marginally higher than that in NWC50. Overall, it is concluded that properly designed lightweight concretes of adequate strength are durable against chloride and sulphate penetration in severe hot humid exposure conditions. 6. References [1] Owens, P. L., "Structural Lightweight Aggregate Concrete - the Future?". Concrete, Vol. 33, No. 10, pp ,1999. [2] Hughes, D.W.; Hodgkinson, L.; Cullen, D. and Seaton, H., "Adjusted Density Concrete". Concrete Engineering International, Vol. 3, No. 7, pp , [3] AI-Khaiat, H. and Haque, M.N., "Effect of Initial Curing on Early Strength and Physical Properties of Lightweight Concrete". Cement and Concrete Research, Vol. 28, No.6, pp ,1998. [4] AI-Khaiat, H. and Haque, M.N., "Strength and Durability of Lightweight and Normal Weight Concrete". ASCE Materials Journal, Vol. 11, No.3, pp , [5] Haque, M.N. and AI-Khaiat, H., "Strength and Durability of Lightweight Concrete in Hot Marine Exposure Conditions". Mateirals and Structures, Vol. 32, pp , August-Sept [6] Haque, M.N., and AI-Khaiat, H. "Long Term Durability Indicator of Total Lightweight Concrete Exposed to Hot Marine Conditions". Proceedings, 2 nd Int. Symp. on Structural Lightweight Agg. Concrete. Edited by S. Helland and I. Holland. Kristiansand, Norway, pp , June, [7] British Standard Institution, "Part Testing Concrete: Methods for Analysis of Hardened Concrete", BS 8110 BSI, London, [8] German Standard "DIN Test Methods of Concrete Impermeability to Water: Part 2", Deutscher Institute Fur Normung, Germany, [9] Parker, J., "Carbonation and Chloride in Concrete". Concrete, Vol. 31, No.8, pp , [10] Parrot, L.J., "Some Effects of Cement and Curing upon Carbonation and Reinforcement Corrosion in Concrete". Materials and Structures, Vol. 29, No. 187, pp , [11] Osborne, G.J., "The Durability of Lightweight Aggregate Concrete after 10 years in Marine and Acid Water Environments". Int. Symp. on Structural Lightweight Aggregate Concrete, Norway, pp , [12] Dhir, R.K.; Jones, M.R. and Ahmed, E.H., "Concrete Durability: Estimation of Chloride Concentration during Design Life". Magazine of Concrete Research, Vol. 43, No. 154, pp ,1991. [13] GjoN, O.E.; Tan, K. and Zhang, M.H., "Diffusivity of Chlorides from Sea Water into High Strength Lightweight Concrete". ACI Materials Journal, Vol. 91, No. 5, pp , [14] Mays, G.C. and Barne, RA, "The Performance of Lightweight Aggregate Concrete Structures in SeNice". The Structural Engineer, Vol. 69, No. 2, pp , [15] American Concrete Institute, "Building Code/Commentary". 318/318R, [16] Standard Australia, "AS Concrete Structures", 2001, p