PFA Concrete Research Articles

Mineral Additives in Concrete Durability: A Comprehensive Review

The body of literature on concrete using mineral admixtures covers a number of review studies on the durability characteristics of various materials used in concrete, such as fly ash (FA), rice husk ash (RHA), ground granular blast furnace slag (GGBS), fly ash (SF), and met kaolin (MK) are reviewed in this work. The features that are related to durability have been evaluated, and they include permeability, resistance to chloride ion penetrations, abrasion, fire resistance efflorescence. Incorporating mineral admixtures in concrete affects various properties. Concrete's permeability decreases and its ability to resist chloride ion penetration rises when admixtures containing a greater alumina content are used. Use of mineral admixtures enhances compressive strength and enhancing abrasion resistance. Moreover, highly reactive mineral admixtures mitigate efflorescence. Notably, while heating PFA concrete improves fire resistance, it reduces overall durability. SF concrete, on the other hand, behaves similarly to standard concrete but can be more brittle. MK concrete exhibits increased strength at 200°C, but its durability and strength decline at higher temperatures compared to other concrete types.RHA pozzolans can replace OPC up to 15% by weight after curing for up to 200 days without lowering the concrete's compressive strength.

Enhancement of Autogenous Healing on Pre-Cracked PFA Concrete Using Response Surface Methodology (RSM)

Concrete is used as a construction material because of its superior mechanical and durability performance and comparably inexpensive cost when compared to other building materials. However, because of its innate heterogeneity, low tensile strength, and unfavourable service environment, concrete is vulnerable to progression and coalescence in micro-cracks production. This study looked into the self-healing assessment of pre-cracked PFA concrete. PFA cement was utilized to replace 10, 20, and 30% of the weight of cement used. To investigate the strength development of PFA concrete, compressive strength was determined. An ultrasonic pulse velocity (UPV) test was performed to monitor the self-healing progress of the concrete to assess the impact of PFA on autogenous healing concrete. The tests were conducted at 7, 28, 60, and 90 days after being cured in water. The result shows that concrete integrated with 10% of PFA as a cement replacement recorded the highest compressive strength compared to those mixes. It is also revealed that UPV readings increased significantly with the increased curing ages for concrete mixes. The results also revealed that the autogenous healing ability of pre-cracked and PFA concrete progressively improved. Based on RSM analysis, the inclusion of PFA in concrete has a strong relationship with strength and autogenous healing progression. Considering all these test results, it was attained that 10% of PFA in concrete exhibits excellent compressive strength and autogenous healing concrete.

Properties of PFA Concrete at Different Curing Conditions

This paper investigates the effect of various curing conditions on the strength development and durability characteristics of PFA concrete. The concrete samples are placed in 3 different curing conditions: (i) saturated wet curing, (ii) air dried at room temperature, and (iii) sea water curing. The systems comprise of concrete mixes at decreasing water-cement ratio from 0.60 to 0.50 to 0.40 and constant PFA replacement at 30%. Neat concrete (100% OPC) and concrete incorporating both PFA and silica fume (SF) (10% SF. 20% PFA), further supports the study. Compressive strength was determined at concrete ages of 3, 7, 28 and 56 days. Water absorption test is done for samples at age of 28 and 56 days to measure the durability properties at the later ages. The study concluded that both seawater cured OPC and PFA concrete samples result in high early age strength up to 7 days. At later ages, there was no significant deterioration in strength for seawater cured concrete. Low water binder (w/b) ratio and silica fume enhance the performance of PFA concrete at all ages for all curing conditions.

Time‐variant reliability analysis of RC bridge girders subjected to corrosion – shear limit state

AbstractChloride‐induced corrosion of reinforced concrete (RC) bridge girders has led to a huge loss of national resources. One of the important concerns affecting RC bridge girders is corrosion of the stirrups, which can even cause the failure mechanism to change from a ductile flexural mode to a brittle shear mode. Hence, analysis of the reduction in shear capacity overtime is essential in the reliability assessment of bridge girders, which is the topic of the paper. This paper proposes a stochastic modelling approach for estimating the time‐variant shear capacity and reliability within the framework of a Monte Carlo simulation, which assists in the sustainability‐based service life design of bridge girders. Such modern design concepts require methodologies for estimating whole life cost at the design stage itself. The development of such methodologies would provide the designer with various options for arriving at an optimum design having the desired performance level during the service life. The proposed approach takes into account: 1) the randomness in basic variables, 2) the effect of micro‐environments and the spatial variation of corrosion, 3) the number of stirrups resisting web shear failure, and 4) the ductile to brittle transition of stirrup steel as corrosion propagates. The incorporation of this transition is found to have a significant influence on the time‐variant reliability of the girder. Although PFA concrete is known to have better durability characteristics than OPC concrete, this paper gives a framework for its quantification in terms of time‐variant reliability.

Performance of pfa concrete in a marine environment––10-year results

Abstract Steel reinforced 100×100×300 mm concrete prisms, with nominal strength grades C25, C35 and C45 and different pfa levels (0–50%), were exposed to various curing treatments during the first 28 days prior to exposure in the tidal zone of the BRE marine exposure site. Chloride concentration profiles and rebar weight losses had previously been measured on specimens exposed for 1, 2 and 4 years and this paper reports the results of similar measurements after 10 years exposure. Chloride profiles were also measured for specimens after 1 and 28 days immersion in seawater under laboratory conditions. Pfa concretes showed substantially increased resistance to the penetration of chlorides compared with control Portland cement (PC) concrete specimens. The improved resistance of the pfa concrete to the penetration of chlorides resulted in reduced corrosion of steel bars imbedded in the concrete. Threshold chloride levels for corrosion, estimated from relationships between steel weight loss and chloride content at the location of the steel, were found to decrease with increasing pfa content. Chloride concentration profiles after 28 days of immersion in seawater showed that considerable chloride penetration occurred during this period due to sorption (capillary suction) of the seawater into the unsaturated specimens. This results in a significant error in diffusion coefficients calculated from the concentration profile using the standard solution to Fick’s second law. The error may be substantial for pfa concretes where chlorides penetrating due to sorption immediately after exposure may outweigh subsequent diffusion during continued seawater exposure. The performance of the concretes, particularly the PC concretes, in this programme is considered in the light of current and new British Standard recommendations for concrete exposed to marine tidal conditions. The adequacy of these recommendations is, however, difficult to assess because the highest concrete quality tested fell a little short of the minimum quality required in the recommendations and cover to reinforcement was also less than required. Nevertheless, the superior performance of concretes containing 30% or more pfa was clearly demonstrated.

Characterization of conditioned pulverized fuel ash for use as a cement component in concrete

This paper considers the influence of moisture addition (conditioning) to PFA of up to 40% by mass on its physical and chemical properties and performance as a cement component in concrete. The work examined conditioned PFA, covering a range of fineness levels and lime contents, stored for up to 18 months, either in the laboratory or in stockpiles established at four UK power stations. Data for laboratory-conditioned PFA indicated progressive particle agglomeration (mainly in the finer fractions) with storage period, which was greatest at moisture levels of 10–20% and for material with total and free lime contents in excess of 3·0% and 0·1%, respectively. Corresponding increases in water demand and reductions in strength factor were also noted with conditioned material. Other changes, including increased LOI, reduced free lime content and formation of hydration products such as bassanite and gypsum, indicated that the effects were due to chemical activity. However, no significant changes to bulk oxide, modal composition and glass contents of PFA were obtained, suggesting that conditioning effects are mainly localized at particle surfaces. In general, similar behaviour to that in the laboratory was noted for PFA stored in power station stockpiles. The workability and strength development to 28 days of conditioned PFA concrete were comparable to that containing dry PFA, with only slight reductions in both properties with long-term conditioned PFA storage.

Effects of pulverized fuel ash on alkali-silica reaction in concrete

The partial replacement of Portland cement 〈 pc〉 by pulverized fuel ash 〈 pfa〉 in a series of potentially reactive concretes significantly reduced expansion due to alkali-silica reaction 〈 asr〉 in high alkali mixes, whereas, in marginally reactive concretes with lower alkali levels, 25% addition of a high alkali pfa slightly increased the expansion compared with that developed by the equivalent pc mix. This extra expansion, however, was insignificant in terms of the likelihood of causing physical damage. Pore solution analysis indicated a continuing slow release of potassium ions from the pfa but the observed relationships between asr expansion and pore solution composition of the various concretes studied were somewhat unclear. Petrographic analysis suggested that although no significant expansion-induced microcracking was observed in the cement matrix of pfa concretes, the degree of internal microcracking and gel formation in the reactive aggregate particles was very similar to that seen in the equivalent mixes containing no pfa. This suggests that physical changes to the cement matrix as well as variations in the pore solution alkalinity affect the degree of asr expansion developed by concretes containing pfa.

PFA concrete: chloride-induced reinforcement corrosion

The Paper describes a study undertaken to determine the effect of PFA on reinforcement corrosion due to chlorides in concrete. The results indicate that the time to corrosion initiation when PFA is included is greatly increased, but that the quantity of chlorides causing this does not differ significantly between PFA and OPC concretes. After a given period of exposure, the concentration of chlorides at the surface of the reinforcement and the resulting corrosion intensity were lower in PFA concrete. A relationship between corrosion rate, water-soluble chloride content and coeficient of chloride diffusion has been developed, suggesting that the ability of the binder system to limit chloride migration is an important factor influencing the corrosion process. From this a means is proposed of evaluating corrosion risk in concrete under chloride attack.

Microstructure, chloride diffusion and reinforcement corrosion in blended cement paste and concrete

This paper presents some results from a wide-ranging investigation of porosity and pore-structure, chloride-diffusion, and reinforcement-corrosion studies in blended cement paste and concrete. The cement-replacement materials used were pfa, slag, and microsilica. Mercury-intrusion-porosimetry experiments were carried out on pastes with w/c of 0·45. Chloride-diffusion studies and corrosion investigation on rebars were carried out by using concrete mixes of proportions by weight 1:2·5:1·2 and w/c of 0·58. In all these experiments, initial curing for at least 14 days was under relatively low-humidity environments instead of idealised wet curing. The results show that microsilica cement pastes have a larger pore volume than other pastes used in the investigation. The chloride penetration, however, is the minimum in silica-fume concrete. The chloride-binding capacity of pfa concrete is less than that of normal concrete in the surface zone, but, at a greater depth (> 20 mm), the binding capacity of pfa concretes exceeds that of plain concrete. The results generally indicate a higher corrosion activity of reinforcement with increasing chloride concentration in the pore fluid. Microsilica was by far the most effective cement-replacement material in increasing the corrosion resistance of reinforcement in concrete.

PFA concrete: influence of simulated in situ curing on elasto-plastic load response

A study comparing the elasto-plastic load response of OPC and PFA concretes was undertaken using temperaturematched curing (TMC) to simulate in situ conditions. For a given strength, the concrete type and curing regime had no sign$cant effect on the modulus of elasticity. However, PFA concrete showed a more ductile failure than the corresponding OPC concrete. In addition, higher plastic strain energies for the PFA concretes indicated better postfailure toughness than that of the OPC concretes.

PFA concrete: Permeation properties of cover to steel reinforcement

The effect of PFA, at up to 30% replacement, on the water absorption and air permeability properties of cover concrete to reinforcement is described. PFA concrete designed for 25 to 50N/mm 2 and cured under different conditions of relative humidity and temperature and tested at the age of 28 and 180 days is shown to give better resistance to fluid permeation properties than the corresponding series of OPC concrete.

Discussion: Marine performance of PFA concrete

Discussion: Marine performance of PFA concrete

A comparison between the carbonation of OPC and PFA concrete in a mass concrete structure

The carbonation of an OPC and equivalent PFA concrete (with the same cementitious content) taken from a 30 year old mass concrete buttress dam have been investigated. The results presented compare the determination of the carbonation front made by spraying a freshly broken surface with phenolphthalein, with those determined from calcium hydroxide determination by thermogravimetry (TG), and from the analysis of silicate anions in calcium silicate hydrate (CSH) using trimethylsilylation (TMS). TG and TMS analyses gave similar results for OPC and PFA concrete and indicated in both cases the presence of a carbonating zone of finite width, although the results of using these methods for the OPC concrete showed carbonation to a greater depth than is indicated by phenolphthalein. For the PFA concrete, however, the different methods gave comparable results. Carbonation affected calcium hydroxide and CSH simultaneously within the carbonating zone. A residue of partially carbonated CSH remained in the carbonated zone, indicated by phenolphthalein, although the calcium hydroxide had completely carbonated. It is concluded that the chemical processes of carbonation are very similar for OPC and PFA concrete.

Development of pfa use in precast concrete manhole units

A series of full-scale factory trials of precast concrete manhole units using pfa are discussed. The trials demonstrated that the use of pfa was succesful in this process. However, the link between rheology of zero slump concrete and surface finish was unclear. Unit production cost was important and the advantage of pfa concrete is shown.

Marine performance of PFA concrete

Results are presented from a study of the performance of reinforced concrete in a marine environment. Reinforced concrete specimens from three series of concrete mixes of nominal strength grades C25, C35 and C45 and with a range of PFA levels (0–50%) were placed in the tidal zone of the BRE marine exposure site. Measurements of reinforcement weight loss and chloride ion penetration were carried out on the specimens after two years' exposure. The results indicate that chloride ion penetration during early exposure is controlled by sorptivity rather than diffusion, and predictions of diffusion coefficients derived from chloride concentration profiles are not valid. No significant increase in the chloride content at depths below 10 mm were observed between one and two years for concretes containing 30% or more PFA, whereas substantial increases were observed for the OPC control concretes. The PFA concretes were far more resistant to the penetration of chloride ions than the OPC concretes and consequently corrosion of steel reinforcement was reduced in the PFA concrete. It is concluded from results to two years that in marine conditions PFA concrete provides much improved protection to steel reinforcement, compared with OPC concrete of equal strength grade, cementitious content (OPC + PFA) or water–cementitious ratio. Further assessments will be carried out after 5 and 10 years exposure. These results are discussed in the context of current specifications for concrete exposed to marine conditions.

Influence of high early temperatures on properties of PFA concrete

The paper presents results of a laboratory investigation using temperature matched curing (TMC) to study the effects of high early temperatures (65–85°C) on properties of concrete made with both ordinary Portland cement (OPC) and blends of OPC and fly ash. TMC is a technique in which test specimens are cured under the temperature regime similar to that which would occur at a chosen point of a pour, particularly for a thick section. Concrete mixes with three different total cementitious contents were studied with PFA replacement of cement at 0%, 15% or 30% by weight. Strength of each concrete mix was monitored for ages from one day to one year. The properties investigated were peak temperature rise, compressive and tensile strength development. The results show that for the case of massive structures in hot climate, partial replacement with PFA lends itself as one of the most effective techniques for reducing the potential loss of strength due to high early temperatures.

The effect of curing on the hydration and pore structure of hardened cement paste containing pulverized fuel ash

Tests were carried out on ordinary Portland cement (OPC) and OPC/pulverized fuel ash (PFA) paste specimens stored at a range of relative humidities in order to study the effects of curing on the hydration and development of pore structure. The results of thermogravimetric analysis show that P FA pastes are more susceptible to inadequate curing with no significant reaction between PFA and Ca(OH)2 observed in early age pastes nor in pastes stored at low relative humidities (below 80%). For pastes cast at equal water/solid ratios, methanol adsorption studies show that PFA pastes have a larger total porosity than OPC pastes, but with adequate curing the PFA paste develops a more refined pore structure (i.e. smaller volume of large pores). However, in early age or poorly cured (storage at low humidity following short curing periods) pastes with equal water/solid ratios, those containing PFA may have a considerably coarser pore structure. Generally, for pastes stored at low relative humidity, PFA paste required an additional curing period of one day in order for the volume of large pores (< 36·8 nm dia.) not to exceed that ofOPC paste. A comparison of OPC and PFA pastes cast at the water/solid ratios used in equal strength grade OPC and PFA concretes, show the PFA paste to have a more refined pore structure, even under conditions of poorest curing (i.e. stored at low humidity following demoulding at 24 h).

Behaviour of foamed slag concrete at high temperatures

Lightweight concrete beams (50 × 75 × 850 mm) using foamed slag aggregate were tested in flexure at steady state temperatures up to 600°C. The properties investigated were thermal movement, ultimate strength, flexural rigidity and weight loss. The results were compared with those of identically dimensioned specimens made from concrete using sintered pulverized fuel-ash (pfa) aggregate and gravel aggregate. Foamed slag concrete exhibited characteristics somewhat similar to those of sintered pfa concrete, and performed better than gravel aggregate concrete.

Contribution of PFA to concrete workability and strength development

An assessment of the nature of the components of pulverized-fuel ash (pfa) has shown a dominance of hollow, glassy spheres with a composite shell. Total of partial coating by magnetite can be present. The form, size and physical nature of the particles are shown to be the main influences affecting the workability and water demand of fresh pfa concretes. The relationship between progressive pfa particle reaction and strength gain with time has been observed for standard cured concretes up to two years old. The early pfa contribution to strength development is largely a beneficial interaction with the normal hydration of Portland cement while the pozzolanic reaction becomes significant after 28 days. The potential performance of pfa can be predicted from a consideration of its fineness, as measured by 45 μm sieve residue.

A STUDY OF TWENTY FIVE YEAR OLD PULVERIZED FUEL ASH CONCRETE USED IN F OUNDATION STRUCTURES.

It is unusual for detailed records to be preserved beyond the construction stages of reinforced concrete structures, but one exception occurs in the case of the concrete foundations of the 275 kv substation at High Marnham Power Station built in 1957. Twenty-Five years later these foundations have been examined in detail and the paper presents findings of this examination. Modern examination techniques have been employed to investigate the concrete, and the results have been compared with construction data recorded in a 1958 report. This is a rare, possibly unique, investigation into the performance of concrete placed with a 20% replacement of the cement with pulverized fuel ash (pfa). It is particularly relevant that the pfa used did not comply with the present British standard specification. Nevertheless, the compressive and tensile strengths continued to improve beyond that achieved by all-cement control samples. Consideration of durability shows that the pfa concrete had a marked advantage. Even at 25 years virtually no carbonation could be found. It is suggested that the lower porosity and much smaller pore size found in the pfa concrete contributed largely to this excellent durability performance. (Author/TRRL)