16-02-2013, 09:11 AM
PERFORMANCE AND COMPATABILITY OF PERMEABILITY REDUCING AND OTHER CHEMICAL ADMIXTURES IN AUSTRALIAN CONCRETES
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ABSTRACT
A substantial research program has been undertaken at the Australian Centre for Construction Innovation of the University of New South Wales to determine the benefits resulting from the use of permeability reducing admixtures as integral components of concrete required to demonstrate superior durability in aggressive environments. This program used commercial concretes which contained conventional water reducing admixture, different types of supplementary cementitious materials and permeability reducing admixtures at various dose rates.
The program included testing of both plastic state and hardened state properties of these concretes to assess the compatibility and impact on performance properties. Both concrete and mortar testing have been undertaken in order to determine a range of properties including setting time, strength, drying shrinkage, chloride resistance, and sulphate resistance. Assessment of these tests indicates that these permeability reducing admixtures can positively influence the key performance properties indicative of improved durability.
INTRODUCTION
Durability is one of the major design criteria for concrete structures exposed to aggressive environments including water pressure, chloride and sulphate attacks. In appropriate selections of materials or mixture proportions can allow corrosion of reinforcement and eventually lead to reduction of structural capacity and service life. Durability factors important for underground and off shore structures may include 1) early strength gain; 2) drying shrinkage; 3) sulphate resistance; and 4) chloride resistance. Concrete is normally considered to be porous due to existence of capillary pores, gel pores and potentially porous cement-aggregate interface zones. Important traditional means to improve concrete durability are through reduction of water to cement ratio and/or increase of the moist curing period. Recently, many new materials and techniques have been developed to control corrosion by inclusion of inhibitors (1) or by reduction of penetration of water, chloride, and sulphate ions into concrete, eg. membrane coatings or concretes with high proportions of supplementary cementitious materials. For vary aggressive environments, such as tidal-zone and walls of underground tunnels, serial protection to restrict corrosive ion diffusion and leaking need to be taken into consideration, to minimise maintenance and repair cost. Partial replacement of Portland cement with supplementary cementitious materials (SCM) has been used widely in aggressive environments applications and the effects have been discussed and published widely in the literature (2,3).
MIXTURE PROPORTIONS AND TEST PROGRAM
Materials and Sample Preparation
To minimise the difference in performance between “lab concrete” and “site concrete” commercial concrete batches of two cubic metres each were used in this research for investigation of real concrete batches for construction applications. Three types of cement used in the concrete mixtures were AS3972 Type-SL Portland cement only, or AS3972 Type-GB fly ash blend with 20% fine fly ash or AS3972 Type-GB slag blend with 38% slag. For each type of cement, a control batch without permeability reducing admixtures was produced and at least one batch of concrete was produced with the addition of a permeability reducing admixture at various dose rates recommended by the manufacturer.
All concrete batches were supplied by a premixed concrete plant based on 32 MPa strength grade commercial concretes. AS1478 Type-WR admixture was added at a rate of 300ml/100kg cement to achieve a target slump of 80mm. Two types of permeability reducing admixture (PRA-1 and PRA-2) were added into selected concrete batches at the premixed plant according to manufacturer’s specification and recommendations. The crystal growth types of permeability reducing admixtures react with by-products of cement hydration and generate insoluble crystals within the pores and capillary (5).
EXPERIMENTAL RESULTS AND DISCUSSION
Setting Time
The influence of permeability reducing admixtures on initial and final setting time of concretes is summarised in Table 2 and Fig. 1. An addition of 0.8% PRA-1 or 0.8% PRA-2 has increased initial setting time of Type-SL cement concretes by 18~30% (Mixture-B and Mixture-D) whereas, at dosage of 1.2% of PRA-1 (Mixture-C), by approximately 60% compared with Mixture-A. One concrete mixture using PRA-2 (Mixture-D) also demonstrated a longer initial setting time compared to PRA-1 mixture (Mixture-B).
For the fly ash concretes, an addition of 0.8% PRA-1 in Mixture G-1 increased the initial setting time by 48% whereas for Mixture-G-2, the increase was 80% compared with Mixture-F. Concrete Mixture-E (with 1.2% PRA-2) also had the initial setting time increased by 50% compared with control Mixture-F. For the slag concretes, Mixture-J had the initial setting time increased by 40% compared with Mixture-I.
Compressive Strength
The influence of permeability reducing admixture on compressive strength of concretes made with Type-SL cement is shown in Fig. 2. All concrete strengths increased with time at a similar rate as shown by tests at 3, 7, and 28 days. All concrete mixtures modified with PRA (Mixture-B, C, and D) had higher strength than control Mixture-A at the same age. At the age of 3 days, all PRA modified mixtures had compressive strengths higher than control mixture by 8% to 14%. At ages of 7 and 28 days, PRA modified concrete mixtures recorded 4% to 8% higher strength than the control mixture.
Fig. 3 compares compressive strength results of the concrete mixtures containing 20% fly ash or 38% slag in the cement. Fly ash concretes (Mixtures-F, G-1 and G-2) had similar compressive strengths at each of 3 and 7 days, while PRA modified mixtures (Mixtures-G-1 and G-2) had 6% higher strength at 28 days compared with Mixture-F. PRA-2 modified mixture (Mixture-E) had higher strength by 16% to 26% at early ages whereas shown 12% at 28 days.
Drying Shrinkage
Shrinkage results of concrete mixtures made with Type-SL cement are shown in Fig. 4. Concretes containing PRA-1 (Mixture-B and Mixture-C) had very similar drying shrinkage to control Mixture-A. However, lower drying shrinkage was recorded with the concretes Mixture-D which contained PRA-2. At 56 days, the drying shrinkage of Mixture-D was lower than control Mixture-A by 22%.
Drying shrinkage results of fly ash concretes (Mixtures-F, G-1, G-2 and E) are shown in Fig. 5. PRA-1 modified concretes Mixture-G-1 and Mixture-G-2 both had lower drying shrinkages than control Mixture-F by 12% to 14% at 56 days. PRA-2 modified concrete Mixture-E had similar drying shrinkages compared with control Mixture-F at each age. Fig. 6 shows the drying shrinkage results of slag concrete Mixture-I and Mixture-J. Both control Mixture-I and PRA modified Mixture-J had very similar drying shrinkages at each age tested.
Length Change in Sulphate Solution
Potential expansion of concretes in sulphate environments was assessed in accordance with AS2350.14 by immersion samples in a sulphate solution over 16 weeks. Fig. 7 presents length changes of mortar samples sieved out of Type-SL concretes and measured according to AS2350.14. Except for a similar performance between PRA-1 modified Mixture-B and control Mixture-A, all other PRA modified concretes (Mixtures-C and D) had lower expansion than the control Mixture-A at each age.
Length changes of samples of fly ash and slag concrete mixtures in sulphate solution when tested to AS2350.14 are shown in Fig. 8. Comparing two slag concrete mixtures, PRA modified concrete Mixture-J had 58% lower expansion than control Mixture-I. Each of the 20% fly ash concrete mixtures recorded excellent sulphate resistance, while PRA modified concretes Mixture-G-1 and Mixture-E had 7% and 27% less expansion respectively than control Mixture-F. Concretes with addition of permeability reducing admixtures have shown significant reductions in sulphate expansion especially when portland cement and 38% slag cement are used.
Rapid Electrical Chloride Ion Penetration (CSIRO modified ASTM-C1202)
Rapid electrical chloride ion penetration tests were undertaken according to CSIRO modified ASTM-C1202 method (8). The tests were carried out with samples of concrete mixtures with Type-SL cement and those containing Type-GB cement with 20% fly ash. Concretes Mixture-I and J using Type-GB cement with 38% slag were not assessed using this test method. The rapid electrical chloride ion penetration test results are presented in Fig. 9.
A minor reduction in coulomb values was found for Type-SL PRA modified concretes, Mixtures-B, C, and D. At dosage rate of 0.8% PRA-1 or PRA-2, Mixture-B and Mixture-D recorded an average of 10% lower coulomb values than control Mixture-A. At a higher dosage rate of 1.2%, Mixture-C with PRA-1 recorded 16% lower coulomb value than the control concrete.