Obada Kayali

SOME CHARACTERISTICS OF HIGH STRENGTH FIBER REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE

Abstract The effect of polypropylene and steel fibers on high strength lightweight aggregate concrete is investigated. Sintered fly ash aggregates were used in the lightweight concrete; the fines were partially replaced by fly ash. The effects on compressive strength, indirect tensile strength, modulus of rupture, modulus of elasticity, stress–strain relationship and compression toughness are reported. Compared to plain sintered fly ash lightweight aggregate concrete, polypropylene fiber addition at 0.56% by volume of the concrete, caused a 90% increase in the indirect tensile strength and a 20% increase in the modulus of rupture. Polypropylene fiber addition did not significantly affect the other mechanical properties that were investigated. Steel fibers at 1.7% by volume of the concrete caused an increase in the indirect tensile strength by about 118% and an increase in the modulus of rupture by about 80%. Steel fiber reinforcement also caused a small decrease in the modulus of elasticity and changed the shape of the stress–strain relationship to become more curvilinear. A large increase in the compression toughness was recorded. This indicated a significant gain in ductility when steel fiber reinforcement is used.

Strength and durability of lightweight concrete

Two lightweight aggregate concretes, SLWC35 and SLWC50, of 35 and 50 MPa 28 day cube compressive strength were cast. The concrete specimens made with lightweight coarse aggregates and a dune sand were continuously cured in water for one or 7 days and then exposed to predominantly hot and humid seaside ambient conditions containing air-borne salts. After 7 days of initial curing and on subsequent exposure to hot and humid air both SLWCs attained an almost similar strength to those continuously water cured cubes at an age of 12 months. In contrast, the water penetrability of SLWC35 and SLWC50 after 7 days of initial curing and subsequent exposure to the sea side was about 2 and 1.8 times the water penetration of those slabs which were water cured for the entire duration of 12 months. However, the depth of carbonation of the two sand lightweight concretes up to an age of 12 months were negligibly small. The results suggest that compressive strength is comparatively less sensitive to the curing regimes investigated. Both the chloride and sulphate penetration after 12 months exposure were found to be within tolerable limits. Also replacement of lightweight fine aggregate with normal weight sand produces a concrete that is somewhat more durable as indicated by their water penetrability and depth of carbonation when concretes are of equal strength.

Drying shrinkage of fibre-reinforced lightweight aggregate concrete containing fly ash

Lightweight aggregate concretes containing fly ash with a compressive strength between 61 to 67 MPa were produced. The lightweight aggregate used was sintered fly ash. The concretes were reinforced with either polypropylene or steel fibres. The fibres did not affect the compressive strength, but did increase the tensile strength of these concretes. The modulus of elasticity of all the lightweight concretes tested was about 21 GPa, compared to 35 GPa for the normal-weight concrete. Fibre reinforcement did not affect the value of the elastic modulus. This type of lightweight concrete, containing fly ash as 23% of the total cementitious content, resulted in long-term shrinkage that is nearly twice as large as normal-weight concrete of somewhat similar strength. Polypropylene fibre reinforcement did not reduce drying shrinkage, while steel fibres did. Early shrinkage behaviour of this type of lightweight concrete was similar to normal-weight concrete. However, the rate of shrinkage of the lightweight concrete remained constant until nearly 100 days of drying. This is different from normal-weight concrete that slowed appreciably after 56 days. Shrinkage of normal-weight concrete stabilised after 400 days, while shrinkage of lightweight concrete did not appear to stabilise after a similar period of continuous drying.

PROPERTIES OF HIGH STRENGTH CONCRETE USING A FINE FLY ASH

A Class F fine fly ash (FFA) with a fineness of 99% passing a 45 μm sieve was used to produce workable high-strength concrete. Six mixtures were cast with total cementitious contents of 400 and 500 kg/m3. The replacement of cement by FFA, on equal mass basis, was 0, 10, and 15%. The mixtures were tested for workability and strength. Drying shrinkage and water absorption characteristics were determined as indicative of durability. The slump varied between 45 to 110 mm and fluid/cementitious ratio varied between 0.25 to 0.38. The optimum cement replacement for both 400 and 500 kg total cementitious material mixtures was 10%. The 28-day maximum strength for the two optimum mixtures was 94 and 111 MPa with a slump of 45 and 85 mm, respectively. The indirect tensile strength of the two concretes was only 5 and 6% of their compressive strength, respectively. The 2 h water penetration of the two concretes was comparatively low, 11 and 13mm, respectively. The drying shrinkage of all the six concretes were very similar with a maximum value of 470 microstrain after 56 days of standard exposure. The 28-day modulus of elasticity of all the concretes varied between 40–46 GPa.

Bond of ribbed galvanized reinforcing steel in concrete

Abstract The ASTM beam end test (ASTM A944) has been used to compare the bond and slip behaviour of deformed (i.e. ribbed) galvanized, epoxy-coated and black steel bars in concrete. The objective was to determine whether galvanizing adversely affects bond strength. From a series of thirty specimens, the average bond strength of black steel and galvanized steel reinforcement used in these tests has been determined and bond stress has been shown to act uniformly over the embedded bar area. A slip value of approximately 0.4 mm has been confirmed to be associated with bond failure by concrete splitting. The results indicated that while epoxy coating resulted in a significant loss in bond strength of the order of 20% compared to black steel, there is no adverse effect on bond with the use of galvanized steel. Chromate treatment of galvanized bars is deemed unnecessary since there was no evidence of long term reduction in bond due to the possible effects of hydrogen gas evolution resulting from the reaction between zinc and wet concrete.

BOND OF STEEL IN CONCRETE AND THE EFFECT OF GALVANIZING

Publisher Summary This chapter presents the significance of bond of steel in concrete and the effect of galvanizing. It also presents that the main sources of bond strength depend upon whether the bars are ribbed. These are the bearing capacity of the concrete between the lugs and the shear strength of the concrete cylindrical surface located between the lugs. Bond stresses are presented in the three common situations—anchorage, changing moment along the beam, and constant moment along certain beam sections. The first situation is represented by a cantilever beam example. The chapter explains the principle of the method, the pullout test, which evaluates bond strength. It also discusses the difficulty in devising a test that can give a reliable estimate of bond with minimum influence from those parameters that govern changes in the concrete and the reinforcement. It also describes that the bond strength is not a single value for a certain combination of concrete and bar; rather, it is a variable that depends upon many factors, among which is the mode of failure, whether by splitting or pullout. The mode of failure itself is also dependent upon several factors, which include the cover depth, the concrete strength, the reinforcement size, the presence of coatings on the steel, the size of the concrete member, and the confinement of the main reinforcement. The chapter explains the issue of the role of galvanization in improving the mechanical performance of conventional and fibre reinforcement from the perspective of bond strength.

Chloride binding ability and the onset corrosion threat on alkali-activated GGBFS and binary blend pastes

This paper presents an experimental study to investigate the chloride binding ability of alkali-activated ground granulated blast furnace slag (GGBFS) and binary blend pastes. Free chloride and total chloride were measured to assess the chloride binding ability. pH measurement was performed to obtain [Cl−/OH−] ratio to assess the onset corrosion threat. The results showed that bound chloride significantly increased in GGBFS pastes and it gradually increased with GGBFS content in the binary blends. GGBFS paste mixed with deionised water and binary blend pastes showed corrosion risks although their free chloride contents were significantly low. The results further demonstrated that Ordinary Portland cement may be a more appropriate option than NaOH for activating GGBFS in terms of corrosion resistance.

Using Soft Systems Thinking to Confront the Politics of Innovation in Engineering Education

Engineering curriculum innovators face a range of formidable barriers which, singly or in combination, have thwarted countless attempts at sustainable curricular quality improvement initiatives regardless, of their educational efficacy. The often ignored elephant in the room of programmatic quality improvement is the politics of change. The essential point of this chapter is this: a whole-of-programme curriculum innovation demands an intervention strategy capable of effectively responding to multiple stakeholder perspectives and therefore to the politics of change. It is argued that Soft Systems Methodology embedded within a Systemic Action Research approach will give engineering educators that capability.

Autogenous Shrinkage in Plain Cement Mixtures

Determination of a realistic model for the estimation of autogenous shrinkage in plain cement mixtures has been an ongoing research among researchers in high performance concrete. While no standard test method exists for the determination of autogenous shrinkage, various researchers have designed different test methods for measurement of autogenous shrinkage. Current study involved the experimental determination of autogenous shrinkage using the test method developed by O.M.Jensen and co-workers, complimented with non-contact eddy current sensors. Measurements were conducted from as early as 1.5 hours from the time of casting. The samples were placed in a constant temperature chamber and the temperature of the sample was also monitored using a thermocouple. The study was carried out on plain cement mixtures at three water cement ratios of 0.25, 0.32 and 0.38. Measurements were also conducted on simple sealed prismatic samples but these measurements could only be collected after 24 hours of casting. The work is supplemented with CEMHYD3D simulations of the samples at similar water-cement ratios under sealed conditions so as to understand the development of the microstructure of the cement responsible for autogenous shrinkage. While experimental determination of internal relative humidity is quite difficult, data regarding chemical shrinkage, amount of water left and the development of the discontinuous capillary network from the simulations help to understand the determined experimental values of autogenous shrinkage. A detailed explanation on the causes of autogenous shrinkage and the basic mechanism responsible for it has been presented.

Study on the Disposition of Water in Fly Ash-Based Geopolymers Using ATR–IR

This paper addresses the question of whether the main product of low calcium fly ash-based geopolymer is a hydrate, namely, sodium aluminosilicate hydrate (N-A-S-H). The answer to this question is important for understanding geopolymer characteristics. One of these is its fire resistance. In this study, fly ash-based geopolymers were synthesized using the combination of Na2CO3 and Ca(OH)2. Samples were cured at ambient temperature for 7 days, then placed in the oven at 105°C for 24 h, and then calcined at 1050°C for 48 h. IR was used to examine the produced geopolymers at each stage so as to further knowledge on the following issues: the possibility of co-existence of calcium aluminosilicate hydrate (C-A-S-H) and N-A-S-H, the roles of cations as compensator and network modifier as well as the role of water. The results obtained suggest that the primary geopolymerization products, which are potentially good fire-resistant, are unlikely to contain hydrates. Even if there were hydrates, their amount must be very small such that the escaping of water does not compromise the structure’s integrity.