Conventional Vis–a–Vis Mineral Admixed Concrete for Cement Concrete Pavements Construction

Dr Rakesh Kumar, Scientist and Dr Renu Mathur, Scientist & HoD, Rigid Pavements Division, Central Road Research Institute (CRRI) New Delhi.

This article presents a comparison between conventional concrete and mineral admixed concrete to be used in the construction of cement concrete pavements. Mineral admixtures such as flyash, rice-husk ash and silica fume were used in this study. These admixtures were selected to arrive at an economical way for enhancing flexural strength of concrete without significantly affecting compressive strength. Ten concrete mixes were used in this investigation. The performance of concrete mixes was evaluated at fresh state as well as hardened state. Hardened state concrete properties such as compressive strength and flexural strength were evaluated. The study suggests that mineral admixed concrete shows higher flexural strength in comparison with conventional concrete. It also reveals that flexural strength can be drastically enhanced by using 5 to 10 percent of flyash, rice husk ash and silica fume as partial replacement of cement. However, from economical point of view replacement of 10 per– cent cement by mass with flyash is a better option for enhancing flexural strength of control concrete to be used in the construction of cement concrete pavements.

Introduction

The design of concrete pavements (also known as rigid pavements) is based on the flexural tensile strength of concrete as these pavements fail due to bending stresses. The flexural strength (sometimes called the modulus of rupture) of concrete is determined by use of beam specimens under either three-point or center-point loading following any standard test procedure.

The third-point loading is preferred because in the middle third of the span, the specimen is subjected to pure moment which is not possible in the center-point loading device. Hence, the center-point loading test gives results about 15 percent higher in comparison to three-point loading test. Depending on axle load, present traffic, design life, fatigue consumption etc., the average 28-day flexural strength of at least 3.5 MPa is commonly encountered in practice. This range of flexural strength can be achieved by using concrete of M 35 and above grade depending on its constituent ingredients. In recent past, a lot of research was carried out throughout globe for improving the performance of concrete in terms of its rheology, strength and durability qualities^1,2. Consequently, several new constituents such as mineral and chemical admixture of concrete were used to meet the specific need of strength and durability of concrete. The use of mineral admixture in combination with chemical admixture has allowed the concrete technologists to tailor the concrete for many specific requirements. Among the several types of mineral admixture available, the most common are industrial by-products, such as silica fume, fly– ash, blast furnace slag, limestone powder and rice-husk ash etc. Use of these mineral admixtures improve the particle packing density, rheological properties in fresh state and mechanical properties including flexural strength and durability of cementitious system^3-9. Amongst the mineral admixtures, silica fume seems to be the most useful for the development of very high strength concretes and/or high performance concrete. Various advantages of using silica fume, flyash and rice husk ash in concrete need not to be overemphasized. Therefore, in this study the relative performance with respect to compressive and flexural strength development of conventional vis-à-vis mineral admixed concretes containing ASTM Class F flyash, Silica fume, and rice-husk ash, for the construction of cement concrete pavements has been reported.

Experimental Study Materials

Ordinary Portland cement with properties as given in Table 1 was used throughout this experimental investigation. The cement met the requirements of the Indian Standard specification IS: 8112-1989¹⁰.

The flyash (FA) used in this study was obtained from a thermal power plant near Delhi and was evaluated as per ASTM C 618 requirements.¹¹ Table2 shows some of the important physical and chemical properties of the flyash.

This flyash met the ASTM C 618 requirements. It is therefore, concluded that the flyash may perform satisfactorily in cement concrete.

Silica fume (SF) used in the experimental work had specific gravity of 2.18 and Blaine specific surface was around 450 m²/kg. Some important properties of silica fume used are presented in Table 3.

The rice husk ash (RHA) used was obtained from a commercial plant located at Tharsguda, Orissa, India. Its specific gravity and bulk density were 2.06 and 718 kg/m³respectively. The particles of rice husk ash were finer than 45mm. The various properties of rice husk ash are given in Table 4.

Throughout the study, the same land-quarried local sand was used. The specific gravity of the sand was 2.62 and its water absorption was 0.73%. The bulk density of the fine aggregate was 1615 kg/m³. The fineness modulus of the sand was 2.4. The sand was evaluated for the water absorption, specific gravityand grading as per the procedure given in IS: 383-1970¹². The data on grading of sand is reported in Table5. 20 mm (maximum nominal size) graded crushed stone was used as the coarse aggregate for this study. The specific gravity of he coarse aggregate was 2.62. Its water absorption was 0.27%.

The bulk density of the coarse aggregate was 1610 kg/m³. The sieve analysis data on the coarse aggregate both for d" 20 mm and d" 12.5 mm are given in Table 6.

A new generation carboxylic ether polymer based super plasticizer was used as a high-range water reducing agent for this study.

Mix Proportions

Ten concrete mixes were used for this study whose mix proportions are summarized in Table 7. This includes one controlled conventional concrete i.e. a mix without mineral admixture and nine minerals admixed concrete mixes. The mineral admixtures used were Class F flyash, silica fume and rice husk ash. Three concrete mixes using each of the mineral admixtures were manufactured. Admixed concrete mixes were made with 5%, 8%, and 10% replacement of cement in conventional concrete with these mineral admixtures.

The replacement level was limited to 10% with a notion to achieve 28-day compressive strength similar to conventional concrete and flexural strength at least about 4.0 MPa. The water-to-cementitious material ratio was kept around 0.42 and dosages of superplasticizer were varied in range of 0.2% to 0.4% of binder's mass to achieve a slump value of 75±5 mm. It is worth to mention hat the slump specifications are different for the cement concrete road construction for fixed form paving and slipform paving. Hence, the range of slump value is so selected to fit the requirements of both forms of the paving. The mixing procedure adopted was as described below. All the ingredients except superplasticizer were mixed in dry state for few seconds in a tilted drum type concrete mixer, then ¾ of total required water was added and the mix was further mixed for a couple of minutes. The superplasticer was mixed in remaining 1/4^th water and added to the mix in the final stage of mixing. The mixture was mixed for another two to three minutes before evaluating its properties at fresh state.

Sample Preparation

For compression testing, 150 × 150 × 150-mm³ cubes and for flexural tensile strength prism specimens with dimensions 100 × 100 × 500-mm³ were prepared from all concrete mixes. All the specimens were demoulded after 24 hours of casting and curing in the steel mould. Thereafter, the demoulded specimens were marked for identifications and kept submerged in a curing tank till the age of testing.

Results and Discussion Properties of Fresh Concrete

The results obtained on the fresh state properties i.e. slump, concrete temperature and concrete unit weight are presented in Table 8. The targeted slump was 75 ± 5 mm for all the concrete mixes. Additions of silica fume as well as rice husk ash slightly decreased the slump value in comparison of control concrete. Therefore, the same was brought within the range required by increasing the amount of more water reducing agent (Table 7). This indicates that addition of silica fume and rice husk ash decreases the slump of the concrete. However, in the case of FA concrete the slump value increased marginally. Based, on the results given on the slump values in Table 8 and the amount of super plasticizer required to maintain those value Table 7, it is obvious that replacement of cement by SF and RHA reduces the slump of the fresh concrete which is mainly due to increased fineness of the SF and RHA particles in comparison of cement particles.

The data on concrete temperature presented in Table 8 indicates that the replacement of cement by RH & silica fume is similar. In general, an insignificant effect on concrete temperature can be noticed. A close look, of the Table 8 indicates that the mineral admixed concrete shows higher unit weight than the controlled one, which is due to micro filling effect of the mineral admixture. However, it can be noticed that the increase in replacement levels have insignificant effect on unit weight of concrete containing mineral admixture, which may be due to the fact of close range of replacement, levels.

Mechanical Properties

The most important mechanical properties of concrete for the use in pavement construction is flexural strength. In addition to it, the compressive strength of concretes was determined for their comparison. The results have been discussed in the following sections.

Compressive Strength

The compressive strength of concrete mixes was determined as per the standard procedure described IS: 516¹³ at 7 and 28 days. Average result obtained on triplicate specimens was used for the reporting. Figures 2-4 illustrate the development of compressive strength of concrete mixes containing 5%, 8% and 10% of mineral admixtures i.e. silica fume (SF), rice husk ash (RHA) and Class F flyash in comparison of controlled one, respectively. It is obvious from the Figure 2 that the concrete containing silica fume, out performs other concrete mixes including control one in term of compressive strength development. Similar trends for strength development can be observed in the cases of replacement levels of cement with mineral admixtures under nvestigation i.e. 8% and 10% (Figures 3-4). It can further be seen that concrete containing fly– ash (FA) develops least compressive strength. However, in the replacement levels used in the study, it shows strength nearly at par with control concrete. The insignificant reduction in strength of concrete containing flyash in comparison with control one may be due to the lower level of replacement of cement with flyash.

Figure 2: Strength development of concrete mixes containing 5% of mineral admixture

Further, the effect of levels of replacement of cement with the mineral admixture on strength development is shown in Figures 5-7. Figure 5, 6, and 7 show the development of strength of concrete containing silica fume, rice husk ash and flyash, respectively along with control concrete. From Figure 5, it can be observed that the increase in the level of replacement of cement by silica fume increases the compressive strength of concrete; however, the increase in replacement level from 8% to 10% is not much significant. Therefore, optimum level of silica fume seems to be 8% in concrete. In the case of concrete containing rice husk ash (Figure 6) the replacement level has not much significant effect on the strength gain. But, in the case of flyash concrete (Figure 7) no significant reduction in strength in comparison with control concrete can be seen with increase of replacement levels from 5% to 10%.

Figure 3: Strength development of concrete mixes containing 8% of mineral admixture

Figure 4: Strength development of concrete mixes containing 10% of mineral admixture

Figure 5: Effect of replacement levels of SF on strength of concrete mixes

Flexural Strength

The determination of flexural tensile strength or modulus of rupture is essential to estimate the stress at which the concrete member may crack. Its knowledge is useful in the design of pavement slabs and airfield runway as flexural tension is critical in these cases. For this, specimens of standard dimension of 100 mm × 100 mm × 500 mm were used. The specimen was placed in the Universal testing machine such that the load was applied to the upper most surface as cast in the mould. The test was conducted at the age of 28 days as per the standard procedure described in IS: 516¹³. The average result obtained on three specimens was taken as the representative flexural strength of the concrete. Table 9 presents the flexural strength of concrete. It is obvious from the Table 9 that concrete mix containing SF outperform all other concretes including controlled concrete in flexural strengths development at all the levels of replacement. Concrete mix containing RHA performs next to concrete mix containing SF. Concrete mix containing FA also shows improvement over flexural strength in comparison of control concrete mix. The increase in flexural strength of concrete containing admixtures is mainly due to the densification of transition zone of concrete and overall improvement in the homogeneity of concrete in comparison with controlled concrete. Therefore, the easiest economical way for enhancing flexural strength of conventional concrete up to 20 percent of 28-day value is incorporation of good quality of flyash in it.

Figure 6: Effect of replacement levels of RHA on strength of concrete mixes

Conclusion

The following important conclusions can be drawn from this experimental study:

• Replacement of cement by SF and RHA reduces the slump of the fresh concrete in comparison of control concrete.
• The increase of replacement levels of silica fume from 8% to 10% has not much significant effect on the development of compressive strength.
• The replacement levels of rice husk ash have no significant effect on strength development and perform at par with control concrete.
• The replacement levels of flyash have no significant effect on strength development and it performs nearly at par with control concrete.
• Concrete mix containing mineral admixture such as flyash, silica fume and rice husk ash develops higher flexural strength than the conventional concrete without compromising compressive strength.
• The most economicalconcrete mix with improved flexural strength for the construction of rigid pavements be produced by using 370 kg/m³ of cement and 40 kg/m³of flyash.

Figure 7: Effect of replacement levels of FA on strength of concrete mixes

Acknowledgments

The kind permission of the Director, Central Road Research Institute, Mathura Road, New Delhi – India to publish this research work is highly acknowledged.

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