Rami H. Haddad

The use of oil shale ash in Portland cement concrete

Abstract An experimental investigation was undertaken to study the potential use of Jordanian oil shale ash (OSA) as a raw material or an additive to Portland cement mortar and concrete. Different series of mortar and concrete mixtures were prepared at different water to binder ratios, and different OSA replacements of cement and/or sand. The compressive strength of mortar and concrete specimens, cured in water at 23 °C, was determined over different curing periods which ranged from 3 to 90 days. The results of these tests were subjected to a statistical analysis. Equations were developed by regression analysis techniques to relate the effect of batch constituents on the strength developments of OSA mortars and concretes. The models were checked for accuracy by comparing their predictions with actual test results. The obtained results indicated that OSA replacement of cement, sand or both by about 10% (by wt) would yield the optimum compressive strength, and that its replacement of cement by up to 30% would not reduce its compressive strength, significantly. It was found that OSA on its own possesses a limited cementitious value and that its contribution to mortar or concrete comes through its involvement in the pozzolanic reactions. The statistical model developed showed an excellent predictability of the compressive strength for mortar and concrete mixes.

Potential use of phosphogypsum in concrete

Abstract An experimental study was conducted to investigate the potential use of phosphogypsum (PG) in concrete. This was accomplished by preparing mortar mixes at a water/cement ratio of 0.6 using two types of cement, ordinary Portland cement (OPC) and pozzolanic Portland cement (PPC), and two types of fine aggregate, natural river sand and limestone. These mixes were prepared at different replacements (by weight) of PG and purified PG ranging from 10–100%. The purified PG was obtained by calcining PG (washed in water and not washed) at temperatures of 170, 600, 750, 850, and 950°C. The compressive, tensile, and flexural strengths of different hardened mortars were obtained following 3, 7, and 28 days of curing in air and in a moist room. In addition, the setting time and the soundness of cement pastes, prepared using OPC and PPC at a water/cement ratio of 0.6 and at different replacements of PG, were evaluated. At a particular PG percentage of replacement, the results indicated an increasing trend in strength gain over curing time. The strengths of mortars, cured for a specific time, showed a decrease in the strength with further replacement with PG. The purification process, by heating PG up to 900°C, has resulted in improving the strengths of the mortar mixes. The highest percentage increase in strength was found to be for PG calcined at temperatures of 850 and 900°C. The incorporation of PG in the cement paste has dramatically increased its initial and final setting times and has increased the percentage elongation of paste prisms used to evaluate the soundness. The improvement in strength upon calcination is primarily due to the formation of anhydrate at higher temperatures, which subsequently hydrates to gypsum; the latter has some cementing value.

Characterization of Portland cement concrete using electromagnetic waves over the microwave frequencies

A study was conducted to evaluate the effect of the basic properties of Portland cement concrete (PCC) and chloride presence (in PCC) on its dielectric properties over the microwave frequency range of 0.1 to 1 GHz. PCC specimens were prepared at three w/c ratios, 0.35, 0.45, and 0.55, using three aggregate types, limestone, quartzite, and active-silica, and two types of cement, type I and high alkali. Other PCC mixes were prepared and contaminated with two methods, addition of sodium chloride (NaCl) to the mixing water and immersion of hardened PCC specimens (prepared at a 0.45 w/c ratio) in 6% (by wt) NaCl solution after 28 days of curing. The dielectric properties were evaluated using a coaxial transmission line designed to allow measurements over the frequency range of 0.1 to 1 GHz. The dielectric properties of PCC specimens were evaluated over curing time. Powdered PCC samples were obtained from chloride contaminated specimens and chloride contents were determined using Virginia Tech Procedure. The results showed significant effects of curing time on the dielectric properties of PCC specimens and insignificant effects of w/c ratio, air entrainment, and cement type. The dielectric properties showed a significant difference between PCC specimens with different aggregate types. The chloride presence (in PCC) did not influence significantly the PCC dielectric properties.

ROLE OF FIBERS IN CONTROLLING UNRESTRAINED EXPANSION AND ARRESTING CRACKING IN PORTLAND CEMENT CONCRETE UNDERGOING ALKALI-SILICA REACTION

Abstract An experimental study was undertaken to investigate the role of polypropylene or brass-coated steel fibers in controlling unrestrained expansions and delaying and arresting cracking in Portland cement concrete due to alkali–silica reaction. Portland cement concrete and fiber-reinforced concrete (FRC) mixtures were prepared at a w/c ratio of 0.40 using modified Type I cement, reactive fine particles, and coarse limestone aggregates. Prism (5×5×30 cm) and plate (13.5×13.5×3 cm) specimens were prepared and cured for 7 or 28 days before exposure to a special treatment to accelerate ASR. Expansion, time of cracking, and ultrasonic pulse velocity were determined over a treatment period of 65 days using prism specimens. Ultimate cracking pattern and extent were determined after a treatment period of 85 days using plate specimens. The results showed that while fibers did not contribute significantly to controlling pre-cracking and post-cracking expansions, they played a significant role in delaying cracks formation and limiting their extent. Considering its lower cost and content, the performance of polypropylene fibers was superior to that of brass-coated steel ones. The potential of brass-coated fibers in arresting ASR cracking was significantly affected by age of concrete when subjected to treatment.

Effect of early overloading of concrete on strength at later ages

Abstract An experimental investigation to study the effect of early over-loading of concrete on its strength development is reported. Concrete and mortar samples were subjected to different loading levels at ages of 8, 16, 24 and 72 hours. The samples were then retested, along with control specimens, at ages of 7, 28, and 90 days. More than 900 specimens were used in this investigation. Ultrasonic pulse velocity measurements were taken to evaluate the healing capacity of damaged specimens. The results indicate that loading concrete, beyond 8 hours after casting, up to 90% of its compressive strenght has no effect on later strength development. However, loading concrete up to failure resulted in strength loss between 10% to 50%, depending on age at time of loading, age at time of retesting and curing conditions.

Prediction of mechanical properties of post-heated self-compacting concrete using non-destructive tests

Statistical models to estimate residual mechanical properties of post-heated self-compacting concrete (SCC) were developed and validated. The data, compiled from a series of non-destructive and destructive tests on various SCC specimens, damaged by exposure to high temperatures in the range of 300–600 °C, were employed in models’ development. Multivariate linear regression analysis was implemented to estimate the residual elasticity modulus and residual compressive strength of SCC as a function of damage indices obtained from non-destructive testing, as well as concrete mix parameters. Results indicated that the selected statistical models exhibited excellent fit of the data and represented well the physical behaviour as stipulated in literature. Hence, could be used as a simple powerful prediction tool to estimate residual mechanical properties of SCC at elevated temperatures reliably and efficiently, instead of cumbersome laboratory measurements.

Analytical Solution of Buckling of Beams with Two Delaminations

A stability problem of composite beams with multiple delamination was tackled. A closed-form solution was found and buckling loads of composite beams with two delaminations were determined in order to obtain their compressive load-carrying capacity. Crack-opening mode was assumed for each detached delaminated region. Seven different regions having different transverse deformations resulted from assumed positions of delaminations. Developing the derived continuity condition equations reduced the number of algebraic equations required to solve the problem analytically. The results of the work were validated by comparing them to those reported in the literature. The effects of length, location, and distribution of multiple delaminations were considered in the comparison, and the results showed very good agreement. Buckling load decreases as delamination size increases. Buckling load for a beam with two delaminations is lower than that for the same beam with a single delamination.A stability problem of composite beams with multiple delamination was tackled. A closed-form solution was found and buckling loads of composite beams with two delaminations were determined in order to obtain their compressive load-carrying capacity. Crack-opening mode was assumed for each detached delaminated region. Seven different regions having different transverse deformations resulted from assumed positions of delaminations. Developing the derived continuity condition equations reduced the number of algebraic equations required to solve the problem analytically. The results of the work were validated by comparing them to those reported in the literature. The effects of length, location, and distribution of multiple delaminations were considered in the comparison, and the results showed very good agreement. Buckling load decreases as delamination size increases. Buckling load for a beam with two delaminations is lower than that for the same beam with a single delamination.

Thermal performance of steel fibrous lightweight aggregate concrete short columns

The thermal performance of fibrous lightweight aggregate concrete short columns was investigated in terms of compressive mechanical response and thermal and load cracking patterns and characteristics. Seventy-two scaled column specimens (120 × 120 × 400 mm) having varying lateral steel confinements were cast without and with hooked steel fiber reinforcement, before cured for 28 days and exposed to high temperatures (from 300 to 700℃) in an electric oven followed by slow cooling in laboratory air. The results indicated a marked reduction in their compressive load capacity and rigidity, and an increase in their strain at peak stress and compressive toughness as exposure temperatures exceeded 400℃. An enhancement in mechanical properties under high temperatures was noticed at moderate confinement levels. The benefit of using hooked-steel fibers in preserving post-heating mechanical properties of short columns was more pronounced in unconfined rather than in confined lightweight aggregate concrete specimens. Thermal cracking was proportional to exposure temperature with its extent reduced upon use of steel fibers or steel confinement. Failure modes of column specimens varied from brittle to semi-ductile fracture with longitudinal steel buckling noticed for those treated at 700℃.

Repair of Reinforced Concrete Beams Damaged by Alkali-Silica Reaction

High-performance fiber-reinforced cementitious (FRC) mixtures offer promise as a repair alternative for structures damaged by alkali-aggregate (silica) reaction (ASR). This paper investigates the effects of externally applied U-shaped high-strength FRC jackets on the flexural behavior of ASR-damaged reinforced concrete beams. A series of 13 under-reinforced high-strength concrete rectangular beams were cast, subjected to exposure conditions chosen to generate a rapid increase of ASR damage, repaired, and then tested under four-point loading to determine the effectiveness of the FRC jackets as a method of increasing the beams’ flexural capacity. The overall response of the beams (loaded up to failure); the onset of cracking; and cracking pattern, serviceability, stiffness, and ductility are described. Three types of fibers were used: high-performance polypropylene, hooked-end steel, and brass-coated steel with fiber content ranging from 1 to 2%. Results showed that all repaired beams exhibited a substantial improvement in their flexural behavior compared with the control beams. They regained a large proportion of the control beams’ ultimate load capacity, and showed a typical flexural failure with an increase in the first cracking and service loads. Beams repaired with a mixture of hooked steel and brass-coated steel fibers showed the highest improvement in flexural load capacity, while those repaired with brass-coated steel fibers exhibited the highest improvement in displacement ductility and toughness. An analytical model was developed and shown to be reliable for predicting the ultimate load capacity of the repaired beams. The findings from this study indicate that the proposed repair technique is an effective method for restoring the flexural capacity of ASR-damaged reinforced concrete members.

Detecting flaws in Portland cement concrete using TEM horn antennae

To understand the dielectric properties of PCC and better correlate them with type and severity of PCC internal defects, a study was conducted to evaluate PCC complex permittivity and magnetic permeability over a wideband of frequencies using both time domain and frequency domain techniques. Three measuring devices were designed and fabricated: a parallel plate capacitor, a coaxial transmission line, and transverse electromagnetic (TEM) horn antennae. The TEM horn antenna covers the microwave frequencies. The measurement technique involves a time domain setup that was verified by a frequency domain measurement. Portland cement concrete slabs, 60 by 75 by 14 cm, were cast; defects include delamination, delamination filled with water, segregation, and chloride contamination. In this paper, measurements using the TEM horn antennae and the feasibility of detecting flaws at microwave frequency are presented.