Effect of grout proportions on strength of two-stage concrete
Hakim S. Abdelgader, Department of Civil Engineering, Al-Fateh University, Tripoli, Libya
Abdurrahman A. Elgalhud, El-Mergib University, Al-Khoms, Libya
The objective of this paper was to study the manufacture of suitable grouts for two-stage (pre-placed aggregate) concrete using different admixtures to improve flowability of grout and hence the strength of two-stage concrete. The hypothesis of two-stage concrete is different from conventional concrete, because the particle-to-particle contact of coarse aggregate shrinkage is lower than that of normal concrete. The most important aspect of two-stage concrete strength still remains unclear. The strength of two-stage (pre-placed aggregate) concrete has been described in different ways, by experiment or theory. As no reliable data are available showing the strength of two-stage (pre-placed aggregate) concrete, it was necessary to investigate its strength at different mixes. This paper deals with the effect of different types of admixture on the concrete strength of two-stage (pre-placed aggregate) concrete. The paper also presents experimental results of pre-placed crushed aggregate concrete using 36 different grout mixture proportions. A total of 360 standard concrete cylinders were tested in unconfined compression and tensile strength at 28 days. On the basis of these results, a relationship between the tensile strength and compressive strength of two-stage (pre-placed aggregate) concrete has been statistically derived.
Mechanical properties of high-volume fly ash self-compacting concrete mixtures
P. Dinakar, Indian Institute of Technology, Chennai, India
K. G. Babu, CBRI, Uttranchal, India
M. Santhanam, Indian Institute of Technology, Chennai, India
Self-compacting concrete (SCC), a recent addition to the concrete scenario, is gaining popularity worldwide owing to the ease of its placement without any need for compaction. This paper describes the results of an investigation aimed at producing and evaluating SCC mixtures made with high volumes of class F fly ash. Eight fly ash SCC mixtures of various strength grades (20 - 100 MPa) were designed at the desired fly ash percentages of 0, 10, 30, 50, 70 and 85%, and were compared with five different mixtures of normal vibrated concrete mixtures (20 - 100 MPa). Tests were carried out on all mixtures to obtain the properties of fresh concretes in terms of viscosity and stability. The mechanical properties of hardened concretes such as compressive strength, splitting tensile strength and elastic modulus were also determined. The different amounts of paste caused differences in the properties of the two types of concrete. Test results indicated that the use of high volumes of class F fly ash in SCC mixtures decreases its 28-day compressive strength. However, the strength results showed continuous and significant improvement at the ages of 90 and 180 days, which was most probably owing to the pozzolanic reaction of fly ash. Self-compacted fly ash concrete mixtures exhibited higher splitting tensile strengths and lower elastic modulus compared with normal vibrated concretes.