M. Hulusi Ozkul

THE INFLUENCE OF INITIAL WATER CURING ON THE STRENGTH DEVELOPMENT OF ORDINARY PORTLAND AND POZZOLANIC CEMENT CONCRETES

The effect of initial water-curing period on the strength properties of concretes was investigated. Three types of cement, one ordinary Portland cement (OPC) and two natural pozzolanic cements (blended and trass cements), were used in the concrete mixtures. Six different curing regimes were applied to the specimens, the first of which was continuous water storing, and the second continuous air storing. In the remaining four regimes, the specimens were stored under varying initial water-curing periods of 3, 7, 14, and 28 days, respectively. The compressive strength tests were carried out on the cubic specimens at the ages of 7, 14, 28, 90, and 180 days. The variation of compressive strength with time was evaluated by using a semilogarithmic function and the strength-gaining rates were calculated by using this equation for different curing conditions. It was found that poor curing conditions are more adversely effective on the strength of concretes made by pozzolanic cements than that of OPC, and it is necessary to apply water curing to the former concretes at least for the initial 7 days to expose the pozzolanic activity. However, when the pozzolanic cement concretes have sufficient initial curing, they can reach the strength of OPC concretes in reasonable periods of time.

Utilization of citro- and desulphogypsum as set retarders in Portland cement

The utilization of citro- and desulphogypsum in the cement industry as set retarders was tested and compared with natural gypsum. Chemical gypsums were used in two different forms, powder and agglomerated under pressure. Setting times and volume stability of cement pastes, as well as compressive and flexural strengths of cement mortars were evaluated. A Students t statistic was applied to test the equality of strength results at the ages of 3, 7, and 28 days.

Efficiency of accelerated curing in concrete

The relation between 28-day strength of normal cured concrete and accelerated strength is investigated by using an ordinary portland cement and a trass cement under two different accelerated curing conditions, warm water and boiling water. Linear regression analysis was applied on the test results and evaluated by using the efficiency concept, i.e., the ratio of accelerated strength and 28-day normal cured strength. It is concluded that the ordinary portland cement gives higher efficiency than that of the trass cement. The difference due to the cement type is less in the boiling water method than that in the warm water method.

Size and Wall Effects on Compressive Strength of Concretes

This objective of this article is to eliminate the wall effect experienced in molded specimens by either sawing or drilling them out of concrete blocks. For this purpose, the concrete blocks were prepared in four different thicknesses: 50, 100, 150, and 200 mm (2, 4, 6, and 8 in.) and in two different strength classes: moderate and high. The maximum sizes of the aggregates used in the concretes were 12, 22, and 31.5 mm (0.47, 0.87, and 1.24 in.) for each concrete class. The cubic and core specimens were sawn or drilled out of concrete blocks with an equal length or diameter to the thickness of blocks. The damage that occurred during sawing or drilling was related to the maximum aggregate size of the concrete and, by using the size effect law, the damage was predicted. After eliminating the wall effect and the damage of sawing or drilling, it was observed that a considerable size effect exists between the specimens tested.

Benchmarking Concretes with Pozzolanic Materials in Terms of Rapid Chloride Penetration Test

Silica fume (both in densified and undensified forms), granulated blast-furnace slag, and fly ash are three pozzolanic materials that have been used as additives in concrete production. In this study, there was application of the rapid chloride penetration test over water-cured and air-stored specimens at 90 days of age. A statistical experimental design method was used to determine chosen variable values of water-binder ratio (w/b), pozzolanic material content, and binder content. Twenty concrete mixtures were prepared for each type of pozzolan used, and under each curing condition, 80 mixtures total were tested. That pozzolanic material content had more significance on chloride permeability than w/b, particularly for water-cured concretes, was found through use of the same statistical software used in design. Furthermore, when efficient curing was applied to air-cured concretes, optimal pozzolan amounts increased to higher levels.

Effect of Epoxy Resin Addition on the Moisture Sensitivity of Macro Defect Free Polymer-Cement Composites

Macro-defect-free (MDF) cements are cement-polymer composites and were developed by Birchall et al. three decades ago. The composites are produced by mixing small amounts of polymer and water with cement. However, they have a different production method than that of cement pastes, which was inspired by rubber production. Mixtures of cement, polymer and water are processed by using a two-roll mill. The composites are known with their high flexural strengths. Unfortunately, there are not any known commercial products using MDF cements because of their poor durability under moisture. In this study, MDF cements were prepared by using poly(vinyl alcohol--vinyl acetate) PVA, calcium aluminate cements and two different types of epoxy resins. Epoxy resins were a diglycidyl ether of bisphenol A and a mixture of a diglycidyl ethers of bisphenol A and F. Durability performance was compared with respect to biaxial flexural strengths, contact angle and atomic force microscopy (AFM) for the specimens stored in water.

A Comparative Study on Colloidal Nanosilica Incorporation in Polymer-Modified Cement Mortars

Nanosilica and polymers are both well-known admixtures of cementitious mixtures, changing their properties differently upon employment. However, having them incorporated simultaneously, and their possible interactions in the mixtures, is yet to be studied. In this research, mortar mixtures, modified by one type of polymer latex (Styrene butadiene acetate) and two types of nanosilica colloids (different average particle sizes), were investigated for their flowability and long-term mechanical properties. Interesting observations were made in both fresh and hardened state; in the fresh state, the high water demand of nanosilica incorporating mixtures was moderated at the presence of the polymer, and in the hardened state, improvements were observed, especially in terms of flexural strength. Mortar mixtures were produced with different amounts of polymer and colloidal nanosilica, and, accordingly, numerical analyses and simulations have been conducted to model and gain a better understanding of the investigated properties.

Effect of Initial Water Curing on Sorptivity Properties of Ordinary Portland and Pozzolanic Cement Concretes

AbstractThe water absorption properties of concretes measured by sorptivity test were investigated by using natural pozzolanic cements—CEM II/A and CEM II/B—and compared with those of ordinary port...