Assessment of Watertight Concrete and Role of Chemical Admixtures

Transcription

1 Assessment of Watertight Concrete and Role of Chemical Admixtures Mohammadreza Hassani 1, Kirk Vessalas 2, Daksh Baweja 3 and Zoe Schmidt 4 1 PhD student in Civil Engineering, The University of Technology, Sydney 2 Lecturer of Civil Engineering, The University of Technology, Sydney 3 Associate Professor of Civil Engineering, The University of Technology, Sydney 4 National Technical Manager, Sika Australia Pty. Ltd. Abstract: The term watertight concrete is often used in design to specify a concrete for structures to achieve watertightness. The durability, or specifically the watertightness of concrete, is largely determined by its resistance to permeation of water or any other dissolved chemical ions. Typically reducing the permeation capacity and developing watertight concrete involves the use of water reducing chemical admixtures. These admixtures are effective in reducing pore volumes in concrete by decreasing water-to-binder ratio (w/b) of concrete, producing a cementing matrix with higher density and higher compressive strength. Permeability-reducing admixtures are also available which are beneficial in terms of watertightness. This paper presents an experimental study into the effectiveness of using chemical admixtures in concrete including water reducers, hydrophobic pore blockers, crystalline crack healers and liquid colloidal silica. The functionality of these admixtures is assessed by monitoring the change in mass transport mechanisms such as absorption, adsorption and permeability. Concretes were evaluated by increasing binder content and assessed for durability including depth of penetration under hydrostatic pressure, initial surface absorption and apparent volume of permeable voids. The admixture systems demonstrate beneficial effects on concrete as they enabled a reduction in w/b. It was concluded that careful consideration is required when specifying concrete for watertightness to achieve a watertight concrete structure. Keywords: watertight concrete, chemical admixtures, apparent volume of permeable voids, initial surface absorption, water permeability. 1. Introduction Long-term performance of concrete structures (typically considered as 50 years) to maintain their serviceability in various exposure conditions is vital considering technical and economic matters. Users of concrete require durable, sustainable, cost and energy effective structures that are able to meet all the requirements within a specified design life. Concrete structures are susceptible to failure earlier than their intended design life due to various internal and external factors (1). Internal factors encompass the deficiencies in the concrete mix design or the quality of materials used in concrete. Deterioration, caused by physical or chemical attack from the environment such as abrasion or ingress of water or chemicals, are listed as external factors. Most of these external factors classified as potentially harmful agents penetrate into the concrete through the capillary pores and voids in the binder or binder-aggregate interfacial zones; hence, the porosity and permeation capacity of a concrete matrix is the governing parameter in controlling the effects of these external factors. Permeable concrete is vulnerable to deterioration by entry of aggressive agents. To achieve a durable concrete the permeation capacity and penetration rate of water, other aggressive ions and air, should be minimised. Traditional materials used in the past to achieve this purpose include waterproof membranes to reduce permeation capacity. Alternatively, integral waterproofing of concrete or watertight concrete is another option available to engineers and contractors, although care is needed in specifying and using such products (2). Definitions of watertight concrete are provided in the Concrete Institute of Australia Current Practice Note 28 (2). To make the concrete watertight, the permeability of concrete needs to be reduced. The level of reduction of permeability required to achieve this is widely debated (2). Previous research carried out forming the basis of durability exposure classifications in Australian Standards AS 3600 and other design standards has been based on carbonation effects in concrete and water sorptivity (3). The role of supplementary cementitious materials has been also extensively studied with respect to durability and water penetration (4, 5, 6). The Australian Standard (i. e., AS Part 1 Appendix F) (7) covering chemical admixtures for concrete defines specialist admixtures normally associated with watertight concrete as permeability reducing admixtures. These admixtures are classified as: Inert pore fillers Chemically reactive pore fillers

2 Water-repelling substances Water-reducing and air-entraining admixtures Special purpose chemical admixtures Others These permeability-reducing admixtures have many attributes; some are capable of reducing the effective water content of a concrete mix resulting in a less permeable matrix while others containing colloidal silica and hydrophobic agents block the capillary pores of the hydrated cement paste, enhancing the watertightness of the concrete sections. Whilst the specific definition of watertight concrete is broad and open to conjecture (2, 8), it is used in conjunction with reduced permeation in the research described in this paper. A postgraduate research program focusing on the area of watertight (i. e., low permeability) concrete has commenced with the aim of quantifying how selected permeability-reducing admixtures can influence the permeability of concrete and how such concretes could be better specified and used in the design of selected structures. 2. Mass transport mechanisms Water movement in concrete is governed by three main factors: boundary conditions, moisture content and pore structure (1). Boundary conditions, including ambient temperature, liquid viscosity, hydrostatic pressure and chemical reactions, are variables in different applications. Moisture content has the main role in determining the mechanism by which water will transfer through the pores. Porosity of concrete formed from pores and voids in binder, cracks and aggregate interfacial zones controls the permeation capacity. Aggregates used in conventional concrete are assumed to be impervious compared to cement paste. Therefore, the fluid transport in concrete is mainly controlled by the pore structure in the binder (9). Based on the moisture content of the concrete, water moves through the pores following one or a combination of mechanisms (10): 1. Adsorption: as the relative humidity (RH) of concrete approaches zero, any water penetrating into the pores will be adsorbed by the hydrophilic surfaces of cement hydration products. At 11% RH, a single monolayer of water molecules will form on the walls of the capillary pores. With increasing RH, the surface adsorption energy will decrease and water will start to transfer by diffusion. 2. Vapour diffusion: when the RH of concrete is less than 40%, the transport mechanism will be in the form of diffusion. Diffusion is the movement of molecules or suspended substances from an area of high concentration to an area of low concentration. In this case, water molecules located in an area of high concentration will evaporate and then move to another area where there is a low concentration of water molecules. 3. Absorption (i. e., liquid-assisted vapour transfer): for RH between 45% and 100%, menisci can form in the neck of non-uniform capillary pores. In this situation, the transport mechanism will be in the form of absorption by capillary action. Water molecules will condensate at the higher pressure side and evaporate at the lower pressure side of the capillary pore; hence, this type of transfer utilises a combination of connected channels without any coherent flow. 4. Permeability (i. e., saturated liquid flow under hydrostatic pressure): while concrete is fully saturated (RH = 100%) a coherent flow through pores will occur, which is governed by viscosity as well as the pressure gradient (i. e., Darcian flow). There is a direct relationship between applied hydrostatic pressure and final flow. 5. Ionic diffusion under saturated conditions: in saturated concrete, ions in solution (e. g., Cl - ) can transfer from an area of high concentration area to an area of low concentration (Fick s law). This type of fluid transport is the governing mechanism in chloride diffusion, which is the most important cause and concern of deterioration in marine structures. Chemical admixtures are an integral component in developing a concrete with low permeability (11) that can be incorporated in the design of concrete structures to minimise water ingress. The integration of these admixtures into mix design can play a significant role in reducing the permeability of concrete by enabling reductions in w/b in conjunction with imposed design and constructional requirements. Permeability-reducing admixtures can be classified into two types: permeabilityreducing admixtures for hydrostatic conditions (PRAH) and permeability-reducing admixtures for nonhydrostatic conditions (PRAN). Whilst there has been a significant amount of work carried out on such transport mechanisms (12, 13) the data generated in this paper will enable a better understanding of the specific role of chemical admixtures on key permeability measures.

3 3. Experimental procedures 3.1 Chemical admixtures investigated In this paper, a range of chemical admixtures were investigated with respect to their permeability reducing capacity and, thus, their applicability to achieve concrete with a low permeability. These include a standard water reducer, a high range water reducer (superplasticiser), two permeabilityreducing admixtures for hydrostatic conditions and three permeability-reducing admixtures for nonhydrostatic conditions. Table 1 presents the classes of these admixtures according to the classification used in the American Concrete Institute Education Bulletin E4-12, Chemical Admixtures for Concrete (13) and AS (7): Water reducer (WR): this is an aqueous solution of highly purified polycarboxylate and carbohydrates. WR is a dispersing agent, which provides uniform, predictable performance and conforms to AS Type WR. It is used to lower the w/b ratio. This in turn decreases the volume of capillary pores within the concrete matrix. High-range water reducer (HWR): this is a high range water reducer, which is used to lower the w/b. It will result in denser concrete with lower capillary pore volume and enhanced durability characteristics. PRAN-1: this is a liquid hydrophobic pore blocker, which consists of active components. These active components will form non-soluble materials throughout the pore and capillary structure of the concrete and seal the concrete against penetration of water and other liquids. PRAN-2: this is a liquid permeability-reducing admixture, which enhances the self-healing properties of concrete and will improve the ability to heal cracks in concrete. PRAN-3: this is a liquid colloidal silica that increases the mix cohesiveness and the viscosity characteristics of concrete. The mix will revert to a dense and highly viscous consistency. This in turn decreases the volume of capillary pores within the concrete matrix, while lending the concrete high workability and low absorption. PRAH-1: this is a powdered water resisting and crystalline waterproofing admixture, which improves water impermeability in addition to enhancing the self-healing properties of the concrete. This in turn decreases the volume of cracks and capillary pores within the concrete matrix, while decreasing its permeation capacity (i. e., permeability and absorption). PRAH-2: this is a powdered additive consisting of Portland cement and various active chemicals. These active chemicals react with the moisture present in concrete in its fresh state and the by-products of cement hydration to cause a catalytic reaction. To examine the performance of chemical admixtures in watertight concrete, 24 concrete mixes were used in this study with the target slump set to mm for reproducibility in workability. These mixes utilised three different binder contents (i. e., 350 kg/m 3, 400 kg/m 3 and 450 kg/m 3 ). Each set of mixes (i. e., mixes with identical binder content) were proportioned to have the same amount of coarse and fine aggregates. Fly ash was used to partially replace General Purpose cement at 25% in all mixes mainly to provide benefits to concrete, which have been previously described in literature (4, 6, 14). For each set of mixes, a control mix without any admixture was designed in addition to a secondary control mix with a fixed amount of standard water reducer. Permeability-reducing admixtures and a superplasticiser were added with the hypothesis being to diminish the water permeation capacity of the concrete. The performance of concrete specimens with the permeabilityreducing admixtures was compared with that of the control concrete mixes. 3.2 Materials and proportioning The raw materials for use in concrete mix design included: General purpose Portland cement conforming to AS Fly ash (Tarong) conforming to AS Coarse aggregate with relative density of 2700 kg/m 3 Fine aggregate with relative density of 2650 kg/m 3 Chemical admixtures as described in Table 1 Potable water Mix designs are summarised in Table 2.

4 Table 1. Chemical Admixture Classification Admixture ACI E4-12 Classification AS :2000 Classification WR HWR PRAN-1 PRAN-2 PRAN-3 PRAH-1 PRAH Test program Type A, water-reducing admixtures High-range, water-reducing admixtures Permeability-reducing admixture for nonhydrostatic conditions (PRAN) Permeability-reducing admixture for nonhydrostatic conditions (PRAN) Permeability-reducing admixture for nonhydrostatic conditions (PRAN) Permeability-reducing admixture for hydrostatic conditions (PRAH) Permeability-reducing admixture for hydrostatic conditions (PRAH) Water-reducing admixture (Type WR) High range water-reducing admixture (Type HWR) Special purpose set-retarding admixture (Type SRe) Special purpose set-retarding admixture (Type SRe) Special purpose normal-setting admixture (Type SN) Special purpose set-retarding admixture (Type SRe) Special purpose set-retarding admixture (Type SRe) Several authors consider permeation test methods a measure of two or more transport mechanisms in saturated or partially saturated conditions and generally, a few of the permeation test methods measure absolute values of fundamental transport mechanisms (15). AS is a standard test method used in Australia to measure the permeation capacity of concrete as represented by the apparent volume of permeable voids (16).This test method along with other test methods employed included the depth of water penetration test and the initial surface absorption test. These tests were selected as they are commonly used to evaluate the degree of watertightness of concrete achievable in practice (2, 11). The depth of water penetration test is a test method that has been developed for non-steady-state penetration of water under hydrostatic pressure (17). This test method was conducted in accordance with BS EN :2009 (17). Cube moulds (150 mm) were used to cast specimens following the procedures given in AS (18). After 24 hours, specimens were removed from the moulds and cured in lime-saturated water for 28 days prior to testing. During the testing phase, water under pressure was applied to the top surface of the specimen. The test was terminated after 72 hours in accordance to the standard test method requirements. In order to determine the depth of water penetration, the specimen was split perpendicular to the penetration surface and the maximum depth of penetration was determined by means of the application of methylene blue. Methylene blue was used as the tracer to specify the exact penetration depth as it yields a blue solution when dissolved in water. For reproducibility of experimental data, the average of 3 readings (obtained from 3 specimens for each mix) was reported as the depth of water penetration. With the water absorption and apparent volume of permeable voids tests, a standard cylinder (100 mm x 200 mm) was tested conforming to AS test method requirements (16). The cylinder was cut into 4 discs. According to AS , immersed and boiled absorption in addition to the apparent volume of permeable voids (AVPV) were determined for each disc. The average of 4 readings was reported. The initial surface absorption test (ISAT) was used for establishing correlations between porosity and initial surface absorption in concrete. For ISAT, 3 cube (150 mm) specimens were cast and then cured for 28 days in lime-saturated water. Following curing, the specimens were oven-dried for 4 days and then placed under air storage, at 23 C +/- 2 C at 50% RH for a further 2 days. Initial surface absorption was determined on each cube in accordance to BS :1996 test method requirements (19). For reproducibility of experimental data, the average of 3 specimens after 10, 30 and 60 minutes were reported as the initial surface absorption parameters for each mix. In addition to the aforementioned test methods, fresh and hardened properties were determined. Slump compliant to AS test method requirements, air content of freshly mixed concrete as per AS test method requirements and setting time conforming to AS test method requirements were also carried out. Compressive strength was evaluated after 7 and 28 days curing in lime-saturated water in accordance to AS test method requirements.

5 WR + PRAH-1 (2%) H H Table 2. Summary of Mix Designs Mix Description Mix Design Type Cement: Fly Ash 75:25 75:25 75:25 Slump (mm) Binder Content (kg/m 3 ) mm Aggregate (kg/m 3 ) mm Aggregate (kg/m 3 ) Man Sand (kg/m 3 ) Fine Sand (kg/m 3 ) Cement (kg/m 3 ) Fly Ash (kg/m 3 ) Admixture - WR (ml/100 kg) * Admixture PRAN-1 (%)* Admixture PRAN-3 (%)* Admixture PRAN-2 (%)* Admixture PRAH-1 (%)* Admixture PRAH-2 (%)* Admixture - HWR (ml/100 kg) * * of binder content (kg) 4. Results and discussion The fresh, hardened and durability properties of the 24 mixes studied in this investigation were analysed and the results are presented in Table 3. In Figure 1, the compressive strength versus waterto-binder (w/b) ratio is shown for all mixes investigated. As expected (20), the strength was found to decrease with an increase in w/b for all concretes. This relationship was established for all mixes trialled in order to draw further information from the data analysed. Initial surface absorption was measured after 10 minutes, 30 minutes and 60 minutes following exposure to water. In Figure 2, data taken from ISAT results after 10 minutes of water exposure are compared to results taken after 30 minutes and 60 minutes. Direct correlations were found in the data with derived correlation coefficients shown to be significant at the 95% confidence level. This indicates that ISAT values taken after 10 minutes could be used to represent data taken after 30 minutes and 60 minutes. In Figure 3, relationships between compressive strength of concrete after 28 days and the apparent volume of permeable voids following AS testing methodology are shown. There are direct relationships between the parameters for each of the three sets of binder contents considered, 350 kg/m 3, 400 kg/m 3 and 450 kg/m 3 and, an increase in compressive strength shows a decrease in AVPV value. Furthermore, it can be seen that some concrete mixes with higher compressive strength have higher porosity, which suggests that compressive strength should not by itself be used as appropriate reliable indicator of porosity and by extension watertightness. The relationship between initial surface absorption (ISAT) and AVPV tests is shown in Figure 4. ISAT is highly sensitive to the surface moisture condition, while increasing the surface moisture decreases the initial surface absorption. Therefore, specimens that have been prepared exactly under the same conditions are compared. Direct relationships observed between ISAT and AVPV test results suggest that these two test methods measure the particular transport mechanism in concrete (i.e. absorption). To determine the effect of permeability-reducing admixtures for all binder contents, the average changes (average of results corresponding to 3 binder contents) of each parameter (ISAT, depth of water penetration and AVPV) were deduced. In Figure 5, the influence of chemical admixture systems on compressive strength behaviour is shown. The percent increase in compressive strength was calculated based on comparing average changes in strength for the three binder contents (350 kg/m 3, 400 kg/m 3 and 450 kg/m 3 ) considered in this study. It can be seen that all chemical admixture systems

6 Compressive Strength (MPa) WR + PRAH-1 (2%) H H (including WR and permeability reducing admixtures) contributed in reduction of w/b ratio and had the effect of increasing compressive strength. Table 3. Test results - fresh, hardened and durability properties of concrete mixes. Mix Description Binder Content Slump (mm) W/B Ratio Water Reduction (%) Compressive Strength - 7 days (MPa) Compressive Strength - 28 days (MPa) Water Penetration - Maximum (mm) ISAT - 10 min (ml/(m 2.s)) ISAT - 30 min (ml/(m 2.s)) ISAT - 60 min (ml/(m 2.s) AVPV (%) R² = Water:Binder Ratio Figure 1. Compressive strength versus w/b for all concretes studied

7 Initial surface absorption after 10 min (ml/m 2 /s) AVPV (%) Initial surface absorption (ml/m 2 /s) R² = R² = ISAT after 30 min ISAT after 60 min Initial surface absorption after 10 min (ml/m 2 /s) Figure 2. Relationships between ISAT results recorded after 10 minutes to those recorded after 30 minutes and 60 minutes R² = R² = R² = Binder content = 350 kg/m3 11 Binder content = 400 kg/m3 Binder content = 450 kg/m Compressive Strength (MPa) Figure 3. Compressive strength at 28 days versus apparent volume of permeable voids (AVPV) R² = R² = R² = Mixes Mixes Mixes AVPV (%) Figure 4. Apparent volume of permeable voids (AVPV) versus initial surface absorption test results (ISAT)

8 PRAH-1 (1%) PRAH-2 PRAN-2 PRAN-1 PRAN-3 PRAH-1 (2%) H AVPV reduction (%) PRAH-1 (1%) PRAH-2 PRAN-2 PRAN-1 PRAN-3 PRAH-1 (2%) H Increase in compressive strength (%) Figure 5. Influence of chemical admixture systems on compressive strength deduced by comparing average changes in strength for the three binder contents In Figure 6, similar data to what is depicted in Figure 5 is presented, but in this case AVPV data was used as the criterion to assess the efficacy of the chemical admixture systems. For the most part, admixture system inclusions had the effect of reducing the AVPV percent in concrete except for the PRAH-2 admixture system, which was a powder permeability-reducing admixture capable of enhancing the self-healing properties of concrete for improving the ability to heal cracks in concrete. This admixture demanded higher free water content compared to other mixes to achieve the target slump suggesting a higher porosity resulting in the matrix. This suggests that although a chemical admixture may have permeability reducing effects by pore blocking or crack healing, its water demand requirement should be considered as well. In contrast, the combination of a HWR and PRAN-1 demonstrated improved performance in terms of reducing the apparent volume of permeable voids by an average reduction of 16.75%. This admixture system was noted to reduce the free water content in the mix, which lead to a lower volume of capillary pores in the matrix in addition to the blocking of the capillary pores by its permeability reducing effects under non-hydrostatic condition Figure 6. Influence of chemical admixtures on AVPV - derived by comparing average changes in AVPV for the three binder contents considered in this study Admixture systems are a necessary part of the concrete matrix used to manage early age properties of concrete as well as influencing the hardened properties. The selection of an admixture system needs to be matched to the imposed design criteria that would apply on a structure or structural element. Similar data to that shown in Figure 7 was found when admixture systems were compared with other water ingress measurement criteria investigated in this study. Some of the investigated chemical admixture systems also had retarding effects on the concrete as shown in Figure 7, where the PRAH-1, PRAH-2, PRAN-1 and PRAN-2 mixes retarded the initial set time between minutes while PRAN-3 and HWR and PRAN-1 were found to have an accelerating effect.

9 PRAH-1 (1%) PRAH-2 PRAN-2 PRAN-1 PRAN-3 HWR+PRAN-1 Increase in initial set time (min) Figure 7. Influence of chemical admixtures on initial set time - derived by comparing average changes in initial set time for the three binder contents considered in this study It is necessary to incorporate admixtures into concrete to meet the constructional (early age) properties and design criteria of a concrete structure. The results of this study suggest that the main method of achieving this criterion was to use water reducing and high range water reducing based admixtures in concrete. This had the effect of reducing w/b and increasing compressive strength and reducing permeability of water into concrete. The results of this study suggest that an individual test method is not sufficient to quantitatively define the benefits of the admixtures over control concretes. Instead, chemical admixtures should be examined by different test methods considering the various transport mechanisms available under hydrostatic and non-hydrostatic conditions. 5. Conclusions Based on the experimental investigation into the effect of chemical admixtures on the watertightness of concrete the following conclusions can be drawn: ISAT results are highly sensitive to the surface moisture and surface finish conditions. However, ISAT is an indicator of absorption in the concrete surface and correlates well with the AVPV test method. In comparing control concrete made with a high range water reducing admixture and hydrophobic poreblocker, to concretes made with only a liquid hydrophobic pore blocker, liquid permeability-reducing admixtures, liquid colloidal silica, liquid water resisting and crystalline waterproofing admixtures, improvements were noted in compressive strength, AVPV and water penetration Concrete will show different performance characteristics depending on the water transport mechanism considered. Therefore, appropriate test methods should be adopted taking into account the application of the concrete element, external factors and cracking potential. The combination of HWR and PRAN-1 demonstrated the best performance in reducing the measured water penetration in the concretes studied. This was due to the water reduction effect as well as the permeability reducing effect. Four out of six admixture systems retarded the initial set of concrete; however, PRAN-3 and H had accelerating effects The results demonstrate that there are a number of considerations that need to be taken into account when specifying concrete for watertightness to achieve a watertight concrete structure. Using specifications based solely on values of water permeability as determined by test methods such as water penetration into concrete, ISAT and AVPV can lead to confusion in terms of data interpretation, resulting in the rejection of applicable concrete systems and the acceptance of inappropriate concrete systems. The issue of water penetration and cracking potential also needs to be further considered.

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