INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 4, 2011

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1 Assessment of cement- sand mortar under direct compressive load Shaswata Mukherjee 1, Saroj Mandal 2, Adhikari.U.B 3 1- Research Scholar, Department of Civil Engineering, Jadavpur University, Kolkata, 2- Professor, Department of Civil Engineering, Jadavpur University, Kolkata 3- Retired Technical Officer, Central Glass and Ceramic Research Institute, Kolkata shaswataa@yahoo.co.in doi: /ijcser ABSTRACT An experimental investigation has been carried out to study the effect of variation of compressive load on mortar specimens. Ordinary portland cement and river sand as fine aggregate was mixed with water/cement ratio of Loads of 0, 30, 50 and 70% of compressive strength at 7 days were applied to the samples of age 7 days separately and released the load immediately. At higher loading range, cracks of up to 2 mm have been noticed on the surface of the samples. The stressed samples were tested for percentage apparent porosity and water absorption which showed increase in their values with the higher load application. Compressive strength and ultrasonic pulse velocity of the stressed samples confirm that the quality of concrete deteriorate with load application. Keywords: Compressive Load, Mortar, Strength, Apparent Porosity, UPV 1. Introduction Cracking is usually a result of various physical and chemical interactions between concrete and its environment, and can occur in concrete structure at any age. Sometimes these cracks are generated by the nature or sometimes these are manmade inclusions. Natural causes includes earthquake, tsunami, extreme cold, warm, rain, snow etc. and human made causes includes poor cement or concrete quality, volume instability, extreme loading, bad environmental exposure, poor construction procedure or design error (Yingzi Yang et al., 2009). The presence of cracks, due to mechanical stresses or time dependent effects is one of the major factors which can influence durability and serviceability of concrete structures (S. Granger et al., 2005). These micro cracks initially form at the paste-aggregate interface and eventually propagate into the mortar matrix. Cracking of concrete affects both the mechanical and the durability response of the maternal. The load carrying capacity of concrete and it's effectiveness in protecting the reinforcement from corroding will be affected by the extent and type of load induced micro cracking within the matrix (Gordon tat et al. 1997). With this background in views an experimental investigation has been made to study the behaviour of mortar samples loaded with different level of initial compressive load. Parameters like percentage apparent porosity, percentage water absorption, ultrasonic pulse velocity and compressive strength was measured for the stressed samples. 2 Experimental methods 2.1 Raw Materials Ordinary portland cement (OPC) in the preparation of mortar specimens having 28 day compressive strength of 54 MPa was used. Chemical composition of cement is shown in table 782

2 Percentage finer, percent INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING 1. Fine aggregate used was local river sand. The specific gravity and fineness modulus of the sand was 2.50 and 2.65 respectively. Figure 1 shows the results of sieve analysis report of the river sand. The particle size distribution of the sand indicates that it falls in Zone-II, as per IS; classifications. The sand was washed to remove any dirt associated with it and was used in saturated surface dry condition. Water to cement ratio (W/C) used was 0.45 for making cementitious mortar specimen. Chemical compositio n Table 1: Chemical properties of ordinary portland cement SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO Na 2 O K 2 O SO 4 Loss on Ignitio n OPC 18.62% 4.75% 3.02% 61.42% 3.21% 1.51% 1.42% 2.29% 3.55% Paricle size, millimeters Figure 1: Particle size distribution of fine aggregate (sand) 2.2 Specimen preparation and test procedure Cement to fine aggregate ratio of 1: 3 (by weight) and W/C ratio 0.45 was used for making the mortar specimens. Specimens were prepared in 75mm X75mm X 75 mm cube moulds and properly compacted in high frequency vibrating table. Specimens were then demoulded from the mould after 24 hours and kept in water for curing for 7 days. At first apparent porosity, water absorption, ultrasonic pulse velocity and compressive strength was measured for the samples cured for 7 days. Further, compressive load of 0, 30, 50 and 70% of 7 days mortar strength was 0.5 KN/Sec to the specimens and subsequently released. 783

3 These samples were further tested to compare the physical and mechanical properties on effect of previous loading. Ultrasonic diagnostics is among the most widely used techniques for the detection of cracks in concrete structures and the determination of the pulse velocity is a standard in concrete testing (K. Komlos et al., 1996). An optical microscope was used to study the surface deterioration of the specimens by predetermined compressive loading. To determine the apparent porosity and water absorption of the preloaded mortar specimens, three cubes from each series were dried in hot air oven at 110 O C for 24 hours and its weight was taken as dry weight (DW). The specimens were then boiled in water for 2 hours and kept for another 24 hours in the same warm water for water to penetrate the pores. Specimens were then suspended in water with copper wire of 0.5 mm thickness to take the suspended weight (S1 W) as well as soaked weight (S2 W) was also recorded by carefully removing the surface water and the copper wire. The following equations were used to find out the apparent porosity and water absorption of the specimens. 3 Results and Discussion 3.1 Surface deterioration and Ultrasonic Pulse Velocity Figure 2 shows the optical micrographs of the surface of mortar samples with different initial compressive load application at the age of 7days. Though cracks can be seen in naked eye but to find out the extent of damage optical microscopy was done. On 30% loading the microcracks was very small in size which could not be measured. But with 50% loading up to 1mm wide cracks was generated and on 70% loading up to 2.5 mm cracks has been observed on the surface. Table 2 represents the ultrasonic pulse velocity (UPV) result of mortar specimens after different load application. The test was conducted as per IS: (Part 1) Results indicate that the mortar specimens fall in GOOD category before load applications. It is evident from UPV results that on increasing the compressive load on the specimens the quality of the concrete begins to deteriorate. Table 2: Ultrasonic Pulse Velocity Test Results of Mortar Specimens (Km/Sec) Load Applications 0% 30% 50% 70% Before Loading After Loading % change Apparent Porosity and Water Absorption Results of change in percentage apparent porosity and water absorption before and after 0, 30, 50 and 70% compressive load are presented in figure 3, figure 4 and figure 5 respectively. The change in percentage apparent porosity and water absorption confirms the extent of damage due to the application of higher load. Apparent porosity and water absorption 784

4 increase with load application and maximum in 70% of compressive load. Increase of porosity confirms that the quality of the concrete begins to deteriorate. The more porous the concrete, the weaker it will be. 0% Loading 30% Loading 50% Loading 70% Loading Figure 2: Images of sample surfaces through optical microscope after different load applications 785

Figure 4: Change in % apparent porosity on application of compressive load Figure 5: Change in % water absorption on application of compressive load 3.

5 Figure 4: Change in % apparent porosity on application of compressive load Figure 5: Change in % water absorption on application of compressive load 3.3 Compressive Strength Results of deterioration in compressive strength after different compressive load application are presented in Figure 6. The compressive strength was determined using a digital compression testing machine. Almost linear drop in compressive strength was found with increasing load. About 8 MPa strength was lost with 70% compressive loading. It indicated that the up to about 30% loading, no such deterioration was noticed. This study will help to select the different repair schemes. 786

6 Figure 6: Compressive strength of specimens that were previously loaded 4. Conclusion On the basis of the present study it may be conclude that the quality of mortar specimen generally decreases with application of higher load. Optical micrograph and UPV results confirm the quality of the concrete deteriorates with higher loading. Percentage apparent porosity and water absorption of the concrete begin to increase with load application. The compressive strength of different load applied specimens decrease almost linearly. Thus compressive loading will definitely cause the concrete to deteriorate. Acknowledgement The authors thank the Department of Science and Technology, (PURSE Scheme) Government of India, India, for funding this research. 5. References 1. Yingzi Yang, Michael D. Lepech, En-Hua Yang and Victor C. Li (2009), Autogenous healing of engineered cementitious composites under wet dry cycles, Cement and Concrete Research, 39, pp S. Granger, A. Loukili, G. Pijaudier-Cabot and G. Chanvillard (2005), Mechanical characterization of the self-healing effect of cracks in Ultra High Performance Concrete (UHPC), Third International conference on construction materials, Innovation and structural applications, CONMAT 05, Vancouver, Canada, Aug Gordon Tat, Kwong Lok, (1997), Load induced microcracking and its effects on the air permeability of mortar, MS THESIS, Department of Civil Engineering, University of Toronto. 787

7 4. IS 383:1970, Specification for coarse and fine aggregates from natural sources for Concrete, Bureau of Indian Standards, New Delhi. 5. K. Komlos, S. Popovics, T. Nurnbergerova, B. Babal, J.S. Popovics (1996), Ultrasonic pulse velocity test of concrete properties as specified in various standards, Cement & Concrete Composites, 18, pp IS (Part 1): 1992, Non-destructive testing of concrete: Part 1 Ultrasonic pulse velocity, Bureau of Indian Standards, New Delhi. 788

Chapter 8 Design of Concrete Mixes

Chapter 8 Design of Concrete Mixes 1 The basic procedure for mix design is applicable to concrete for most purposes including pavements. Concrete mixes should meet; Workability (slump/vebe) Compressive