Nabil Al-Omaishi

Bio: Nabil Al-Omaishi is an academic researcher. The author has contributed to research in topics: Creep & Shrinkage. The author has an hindex of 3, co-authored 4 publications receiving 137 citations.

Creep, Shrinkage, and Modulus of Elasticity of High-Performance Concrete

Abstract: The material properties of high-performance concrete (HPC) directly affect the design and construction of prestressed HPC bridge girders and their cast-in-place deck slabs. This paper discusses research experiments conducted over more than 2 years on material properties such as creep, shrinkage, and modulus of elasticity of 3 HPC mixtures with local materials from Nebraska. Test results from this research show that current ACI equations for shrinkage, creep, and modulus of elasticity of concrete do not accurately predict these material properties for HPC. Revised formulas for shrinkage strain and creep coefficient are proposed. Through this study, it was also indicated that it would be desirable to determine experimentally the shrinkage strain, creep coefficient, and modulus of elasticity using the locally available aggregates for the specified HPC mixture proportions.

Elasticity modulus, shrinkage, and creep of high-strength concrete as adopted by AASHTO

Abstract: This paper covers the experimental and theoretical components of National Cooperative Highway Research Program (NCHRP) research project Number 18-07, which is discussed in NCHRP Report 496. The components are related to modulus of elasticity, shrinkage, and creep of concrete. An experimental program was conducted at various bridge sites and at the University of Nebraska-Lincoln for specimens produced from raw materials and mixture proportions provided by 4 participating states: Nebraska, New Hampshire, Texas, and Washington. Previously reported measurements of material properties are also included. The experimental program was used to extend the pre-2005 AASHTO-LRFD specification prediction formulas to concrete with compressive strengths up to 15 ksi (104 MPa). For each material property, a summary of the experimental values is presented followed by a comparison with the values obtained from the pre-2005 AASHTO-LRFD specifications and the American Concrete Institute 209 committee report. The proposed formulas provide designers of prestressed concrete girders with more realistic estimates of long-term material properties, including effects of aggregate type and other significant factors. The use of these formulas with high-strength concrete (HSC) should result in more realistic camber predictions and lower prestress loss estimates. The background and recommendations for prediction of the modulus of elasticity, shrinkage, and creep of HSC are provided. To help with clarity, the notation and units employed in the pre-2005 AASHTO-LRFD specifications are adopted as much as possible. Stresses are expressed in units of ksi (MPa) rather than psi.

Estimating prestress loss in pretensioned, high-strength concrete members

Abstract: This paper presents research conducted under the National Cooperative Highway Research Program (NCHRP) project 18-07, "Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders." The purpose of this project was to extend the American Association of State and Highway Transportation Officials' AASHTO LRFD Bridge Design Specification provisions for estimating prestress losses to cover concrete strengths up to 15 ksi (104 MPa). This paper presents the portion of the work that deals with methods of estimating long-term prestress loss. The results reported in this paper were adopted by AASHTO and included in the 2005 and 2006 interim revisions and in the fourth edition of the LFRD specifications, which was published in 2007. This paper explains the theory of time-dependent analysis. It follows the pseudoelastic, age-adjusted, effective-modulus method. The experimental component consisted of materials properties, which are covered in a companion paper, and prestress loss measurements, which are highlighted in this paper. The measurements were taken in seven girders in bridges in Nebraska, New Hampshire, Texas, and Washington to encompass the regional diversity of environmental and materials properties throughout the country. Theory was also compared with experimental data reported in the literature on 31 pretensioned girders in 7 states.

Elimination of prestressed concrete compression limits at service load

Abstract: This paper addresses the justification for removal of two compressive stress limits, called hereafter Limits 1 and 2, which are currently used in both the ACI 318 Building Code and AASHTO Bridge Specifications to limit concrete stresses due to effective prestress combined with applied loads. Removal of these limits provides relief in sizing of members that have small compression zones that are subjected to large bending moments, such as inverted tee bridge members in the positive moment zones and I-girder bridges made continuous for superimposed loads in the negative moment zone. Limit 1 is 0.6f' c , where f' c is the specified compressive strength at service. It is imposed on concrete stresses due to the effective prestress combined with full dead plus live loads. Limit 2 is 0.45f' c . It is imposed on stresses due to the effective prestress combined with dead loads. It is proposed that strength be used as the primary design criterion to determine member capacity in compression at various loading stages, including prestress transfer, lifting, erection, deck weight, superimposed dead load and live load. Various serviceability checks can additionally be used as needed to control camber, deflection, vibration, and cracking. Design steps and numerical examples are given. Also, proposed changes to the ACI 318 Building Code and to the AASHTO-LRFD Bridge Design Specifications are given.

Proposed lump-sum formulas for long-term prestress losses

Abstract: The current American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications’ approximate formula for estimating long-term prestress losses is the outcome of the research work presented in National Cooperative Highway Research Program report 496. It is produced by simplifying the detailed method and taking into account the variability of concrete properties and the interaction between the precast concrete girder and cast-in-place deck. This paper presents two detailed parametric studies based on the average con­ditions for the design and construction of commonly used bridge girders. Three spans and, consequently, three levels of prestressing for each section have been considered. The first study establishes the creep multiplier Nc, whereas the second study evaluates the shrinkage multiplier Ns. Both multipliers are used in the lump-sum formulas for estimat­ing long-term prestress losses for different bridge girders. The multipliers produced by these studies are compared with that of the current AASHTO LRFD specifications’ approximate method, and new lump-sum formulas for long-term prestress losses are proposed.

Utilization of Palm Oil Fuel Ash in High-Strength Concrete

Abstract: This paper presents use of improved palm oil fuel ash (POFA) as a pozzolanic material in producing high-strength concrete. The POFA was ground by ball mill until the median particle size was reduced to about 10μm. It was used to replace portland cement, ASTM Type I, by 10, 20, and 30% by weight of cementitious materials to make high-strength concrete. It was found that high-strength concrete can be achieved by using ground POFA to replace portland cement Type I up to 30%. At the age of 28days, concretes containing 10, 20, and 30% of ground POFA gave compressive strengths of 81.3, 85.9, and 79.8MPa, respectively. Concrete with 20% replacement of ground POFA had the highest strength. It is slightly higher than that of concrete containing 5% condensed silica fume and about 92–94% that of 10% condensed silica fume concrete. The ground POFA content up to 30% had slightly effect on lowering the modulus of elasticity of concrete. In addition, the use of ground POFA reduced the peak temperature rise of concrete u...

Prestress losses in pretensioned high-strength concrete bridge girders

Abstract: This report presents guidelines to help bridge designers obtain realistic estimates of prestress losses in high-strength pretensioned concrete bridge girders and thus achieve economical designs. These guidelines incorporate procedures that yield more accurate predictions of modulus of elasticity, shrinkage, and creep of concrete and more realistic estimates of prestress losses than those provided by the procedures contained in current specifications. This report will be of particular interest to engineers, researchers, and others concerned with the design of pretensioned concrete bridge structures.