Water permeability and chloride penetrability of lightweight aggregate concrete. Chen Chung Kho

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1 ABSTRACT Water permeability and chloride penetrability of lightweight aggregate concrete Chen Chung Kho Department of Civil Engineering, National University of Singapore, Engineering Drive 2, Singapore Experimental studies have been carried out to compare the water permeability and chloride penetrability between lightweight aggregate concrete (LWAC) of different densities and normal weight concrete (NWC). Also, the influence of presoaking of lightweight aggregate prior to casting is presented. The lightweight and normal weight concrete had 28-day compressive strength of ~40-50 MPa and had the same proportion of constituents by volume. The results show that the water permeability of the NWC is higher than that of LWAC, regardless of the density of the LWA used. For accelerated chloride penetration test, the chloride content in the first 20 mm from the surface varied slightly for the concrete with different aggregates. No definite trend was observed comparing different concretes. However, in the depth of more than 20 mm the chloride content for the concrete with different aggregates was similar. Due to the limited data available, no definite conclusion can be drawn in terms of the effect of the density and surface texture of lightweight aggregate on the water permeability and chloride penetration, and further study is needed. INTRODUCTION Concrete is a composite material with fine and coarse aggregates embedded in a cement paste matrix. It is well known that the interfacial zone between the coarse aggregate and cement paste will affect the durability of the concrete. In general, the interfacial zone between the lightweight aggregate and cement paste is better than that when normal weight aggregate is involved. However, for lightweight aggregate concrete (LWAC) with relatively denser outer shell, the microstructure of the interfacial zone is of concern since a more porous interfacial zone was formed sometimes with characteristics layers of massive calcium hydroxide and some ettringite []. As a result, the permeability of LWAC made with LWA with dense outer layer may be similar to that of NWC. The main objective of this study is to evaluate the water permeability and chloride penetration in LWAC and NWC at strength level of 40 to 50 MPa. Lightweight aggregates of two different densities were used. The influence of pre-soaking for one of the lightweight aggregates was also investigated.

2 2 EXPERIMENTAL 2. Materials used 2.. Cement ASTM Type I ordinary Portland cement was used for all the concrete mixtures. The specific gravity of the cement is Lightweight coarse aggregate. The lightweight aggregates used in this study were Liapor 6.5 (F6.5) and Liapor 9.5 (F9.5). These are manufactured expanded clay aggregates from Germany. The grain size for both F6.5 & F9.5 was between 4 to 8 mm. The bulk density and particle density of F6.5 are 650 ± 25 kg/m 3,.2 g/cm 3 and that of F9.5 are 950 ± 25 kg/m 3,.7 g/cm 3. The water absorption rate of the LWA is determined according to the procedure given in ASTM C The -hour absorption rate of F6.5 and F9.5 are 7% and 9.2%, respectively Normal-weight coarse aggregate. Crushed granite with a nominal size of 0 mm and specific gravity of 2.65 was used for NWC Sand. The fine aggregate used was natural sand with specific gravity of Concrete mixtures The constituent proportions of the concrete studies are given in Table. All the concrete mixtures had water-cement ratio (w/c) of 0.5 and volumes of each constituents were kept constant in each concrete mixtures. Table. Mixture details. Mix No. Aggregate type Crushed granite Cement kg/m 3 w/c Mix proportion w : c : FA : CA Moisture condition of aggregate Density of Day concrete, kg/m 3 Slump mm 28-day Compressive strength, MPa : :.28 : Liapor F : :.28 :.56 Presoaked Liapor F : :.28 :.56 Unsoaked Liapor F : :.28 :.0 Presoaked Preparation and curing of concrete specimens The LWA used in Mix 2 & 4 were presoaked for hour before casting. In Mix 3, the additional water required for the LWA water absorption was predetermined from the - hour absorption values and added during the mixing of the concrete. A pan-mixer was used to prepare the concrete and specimens.

3 All the specimens were demoulded from the steel molds after 24 hours. All the specimens were cured in the fog room for 28 days before testing. 2.4 Testing 2.4. Water permeability test Tapered cylindrical specimens with a height of 80mm height and diameters of 00 mm on the top and 0mm on the bottom were prepared. At 28 days, the specimens were coated with epoxy, and left overnight for the setting of the epoxy. The concrete specimens were then fitted into test rigs and exposed to a water pressure 4 MPa. After 4 days, each specimen was removed from the rig and split into two halves. The water penetration depth was measured and the average depth was taken from 0 equidistant points along each split face of the specimen Accelerated chloride penetration test Specimens with the sectional area of 50X50-mm and thickness of 00mm were prepared. After 28-days of moist curing, the specimens were immersed in Ca(OH) 2 solution (g/litre) for water saturation purpose according to Nordtest Method NT Build 443 [2]. After 6 days, the specimens were taken out from the solution and coated with a water insensitive epoxy on all the surfaces except for the top face which will be exposed to NaCl solution. After the setting of the epoxy the specimens were exposed to the CaOH solution again for day to ensure full water saturation. The specimens were transferred to a NaCl solution (65g/litre) with the exposing surface facing up for 40 days (due to technical problems, all the specimens were immersed in the NaCl solution outside the temperature control room for the first 0 days, and the remaining 30 days inside the temperature control room at 27º C). After the soaking, the specimens were split into 2 halves from the exposed surface. Silver nitrate solution was sprayed on the split faces to observe the solution penetration depth in order to determine the depth interval for drilling and getting powder samples for chloride content analysis Chloride analysis The determination of chloride content was performed according to the BS 88: Part 24: Powder samples weighing 3-4g (results normalized by taking the unit weight of concrete into considerations) were each placed into a beaker with 50ml deionized water. 0ml nitric acid was then added to each beaker followed by 50ml of hot water. The solution was boiled for 4-5 minutes and kept warm for the next 0-5 minutes. The solution was filtered and cooled to room temperature. 0ml of silver nitrate and 2.5ml of 3,5,5-trimethylhexanol (Nananol) were added to the solution. The solution was shaken vigorously in order to coagulate the precipitate. Finally, ml of Iron III indicator was added and the solution was titrated with thiocyanate (KSCN) solution until the first permanent red colour appeared.

4 3 RESULTS AND DISCUSSION 3. Water permeability From the results obtained (Table 2), the water penetration depth of the NWC was higher than that of the LWAC. However, there was no significant difference between the lightweight concretes F9.5 PS, F9.5 US and F6.5 PS in water penetration. This indicates that the water permeability was not affected significantly by the porosity and presoaking of the lightweight aggregate used. It was noticed that the water penetration depth of all LWAC are relatively shallow (~ 20mm). Hence it is suspected that the duration of water pressure applied might be too short (4 days) in this study. To better distinguish the water penetration between LWAC of different densities, it is recommended that a longer period be used for future study. Examination of the microstructure in the interfacial zone between the various aggregates and cement paste would help to determine if the interfacial zone between the lightweight aggregate with denser shell (F9.5) is more porous and similar to that in the normal weight concrete. Table 2. Results of water penetration test. Mix No. Aggregate type Crushed granite Moisture condition of aggregate Designation Water penetration depth on 4 th day (mm) - NWC Liapor F9.5 Presoaked F9.5 PS Liapor F9.5 Unsoaked F9.5 US Liapor F6.5 Presoaked F6.5 PS Resistance to chloride-ion penetration Figure 3. shows the results of the accelerated chloride penetration test done according to Nordtest Method NT Build 443. Powder samples from the concrete F9.5 PS and F9.5 US were collected at 2 mm interval while the samples from the NWC and the concrete F6.5 PS were collected at 5mm interval. The results indicated that in the first 20 mm from the surface, the chloride content varied slightly for the concrete with different aggregates. No definite trend was observed comparing different concretes. However, in depth of more than 20 mm the chloride content for the concrete with different aggregates was similar.

5 Graph 3. Chloride penetration of different concrete.6 Cl (%) NWC F6.5 PS F9.5 PS F9.5 US depth (mm) 4 SUMMARY Based on the limited experimental results, it seems that:. The water permeability of the NWC is higher than that of LWAC, regardless of the densities of LWA used. 2. In the first 20 mm from the surface, the chloride content varied slightly for the concrete with different aggregates. No definite trend was observed comparing different concretes. However, in depth of more than 20 mm the chloride content for the concrete with different aggregates was similar. Due to the limited data available, no definite conclusion can be drawn on the effect of the density and surface texture of lightweight aggregate on the water permeability and chloride penetration, further study is needed. REFERENCES [] Min-Hong Zhang and Odd E. Gjørv (990), Microstructure of the interfacial zone between lightweight aggregate and cement paste. Cement and Concrete Research, Vol. 20, 990, pp [2] Nordtest Method NT Build 443: Concrete, Hardened: Accelerated Chloride Penetration. [3] Chia Kok Seng (200), Permeability on high strength lightweight aggregate concrete, Dissertation for the Degree of Bachelor of Engineering (Civil), National University of Singapore. [4] Kok Seng Chia, Min-Hong Zhang (200), Water permeability and chloride penetrability of high-strength lightweight aggregate concrete. Cement and Concrete Research 32 (2002), pp

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