Effects of retempering on consistency and compressive strength of concrete subjected to prolonged mixing

In this study, effects of prolonged mixing and four different retempering processes on some proper-ties of fresh and hardened concrete, such as temperature, slump loss, and strength, were investigated. Two types of concrete mixtures with different compression strength having 15 cm initial slump were produced in a laboratory mixer. After mixing for 5 min at 20 rpm speed to ensure homogeneity, the mixing was continued at 4 rpm for a period of up to 4 h to simulate the prolonged agitation of ready-mixed concrete in truck mixers. Concrete samples were taken out of the mixer at the end of first, second, third, and fourth hour for estimating the effects of prolonged mixing on propel-ties of fresh concrete. For restoring the initial workability, four different retempering methods were used and their effects oil properties of concrete were investigated. Results show that compared to the untempered concrete mixtures, those tempered with solutions prepared by 3% or 4.5% solid superplasticizer by mass of retempering water had significantly less loss of 28-day compressive strength

Setting time: An important criterion to determine the length of the delay period before steam curing of concrete

Some precast concrete plants expose freshly made concrete elements to steam curing immediately after casting. This is detrimental to properties of the product; therefore, some delay prior to the steam curing process is beneficial. This paper contains the results of an investigation on the effects of various delay intervals selected based on initial setting time of concrete. Four different delay periods and two different steam curing periods at 80 degreesC (5 and 10 h) were used with two concrete types, namely C25 and C40. Compressive strength tests were performed at 1, 3, 7, 28, and 90 days. Setting time of the concrete was found to be an important criterion to determine the length of the delay periods

Effect of ground perlite incorporation on the performance of blended cements

Perlite is a volcanic rock that contains relatively high amounts of SiO2 and Al2O3. Due to its proper chemical composition and glassy structure, it can be used as a pozzolanic addition in blended cements. In this study, ground perlite was used as a cement replacement material in blended cements. Several mortar mixes were prepared to investigate the performance of those cements. The results showed that perlite incorporation caused early age strength losses when compared to the control mortars containing only portland cement; however, the difference between them decreased in time due to the pozzolanic reactions. The strengths of the blended cements were still within the limits of the EN standards. Moreover, it was observed that use of ground perlite increased the durability of portland cement mixes

Spherical harmonic-based random fields based on real particle 3D data: Improved numerical algorithm and quantitative comparison to real particles

The shape of particles often plays an important role in how they are used and in the properties of composite systems in which they are incorporated. When building models of systems that include real particles, it is often of interest to generate new, virtual particles whose 3D shape statistics are based on the 3D shape statistics of a collection of real particles. A previous paper showed mathematically how this can be carried out, but only had a small set of real particle shape data to use and only made a limited amount of qualitative comparisons to the real particle data. The present paper shows how the numerical method used to create virtual particles has been improved and immensely accelerated, allowing the use of large particle datasets. Making use of several large particle shape datasets, the paper confirms that the algorithm creates particles whose statistical shape properties closely match the real particles from which they were generated. Another question that can now be addressed with these larger particle datasets is: how many real particles are enough to be representative of the particle class from which they were drawn? The types of particles analyzed include two size ranges of crushed granite-hornblende rocks, silica sand, calcium carbonate powder, and ground granulated blast furnace slag. Published by Elsevier B.V