Accelerated Carbonation Assessment of High-Volume Fly Ash Concrete

The issue of concrete carbonation has gained importance in recent years due to the increase use in supplementary cementing materials (SCMs) in concrete mixtures. While there is general agreement that concrete carbonation progresses at maximum at a relative humidity of about 60%, the rate may differ in the case of cements blended with SCMs, especially with high-volume fly ash replacements. In this study, the effect of high-volume fly ash concrete exposed to low ambient relative humidity (RH) conditions (57%) and accelerated carbonation (4% CO2) is investigated. Twenty-three concrete mixtures were produced varying in cementitious contents (310, 340, 370, and 400 kg/m3), water-to-cementitious materials ratio (0.45 and 0.50), and fly ash content (0%, 15%, 30%, and 50%) using a low and high-calcium fly ash. The specimens were allowed 1 and 7 days of moist curing and monitored for their carbonation rate and depth through phenolphthalein measurements up to 105 days of exposure. The accelerated carbonation test results indicated that increasing the addition of fly ash also led to increasing the depth of carbonation. Mixtures incorporating high-calcium fly ash were also observed to be more resistant against carbonation than low-calcium fly ash due to the higher calcium oxide (CaO) content. However, mixtures incorporating high-volume additions (50%) specimens were fully carbonated regardless of the type of fly ash used. It was evident that the increase in the duration of moist curing from 1 day to 7 days had a positive effect, reducing the carbonation depth for both plain and blended fly ash concrete mixes, however, this effect was minimal in high-volume fly ash mixtures. The results demonstrated that the water-to-cementitious ratio (W/CM) had a more dramatic impact on carbonation resistance than the curing age for mixtures incorporating 30% or less fly ash replacement, whereas those mixtures incorporating 50% showed minor differences regardless of curing age or W/CM. Based on the compressive strength results, carbonation depth appeared to decrease with increase in compressive strength, but this correlation was not significant.

Developing Sustainable Ultra High Strength Concrete Mixtures Using Spent Foundry Sand

This study presents the development of ultra high strength concrete(UHSC)that has been made more sustainable by using both local materials from central Texas and spent foundry sand(FS)from the metal casting industry,which has also been obtained locally.This study first describes various trial mixtures tested as well as the specimen preparation techniques investigated that led to the final UHSC-FS mixtures.The developed mixtures were proportioned with local constituents to increase the sustainable impact of the material by reducing emissions due to shipping as well as making UHSC more affordable to a wider variety of applications.The final mixture design constituents were:river sand,locally available type I/II cement,silica fume,and spent FS,which was obtained from a local steel casting company.Multiple variables were investigated,such as the aggregate type and size,concrete age(7,14,and 28-days),the curing regimen,and the water-to-cement ratio(w/cm)to optimize a UHSC mixture that used local materials and FS.This systematic development revealed that heat curing the specimens in a water bath at 50 oC(122 oF)after demolding and then dry curing at 200 oC(392 oF)two days before testing with a w/cm of 0.20 at 28-days produced the highest compressive strengths.Once an optimum UHSC mixture was identified a partial replacement of the fine aggregate with FS was completed at 10%,20%,and 30%.The results showed an increase of compressive strength performance at 10%replacement,followed by no change at 20%,and finally a slight decrease at 30%.Developing this innovative material with local materials and FS ultimately produces a novel sustainable construction material,reduces the costs,and produces mechanical performance similar to prepackaged,commercially,available construction building materials.

Developing High Strength Pervious Concrete Mixtures with Local Materials

This study focuses on developing pervious concrete mixtures that have higher compressive strengths than conventional pervious concrete. This study also focuses on producing high strength pervious concrete that is also made with locally available materials. The study focused on four aspects of pervious concrete to produce high compressive strengths. These parameters were the effect of the coarse aggregate (type and size), the compaction of the test specimens, the effect of the w/c along with superplasticizers, and lastly the effect of silica fume. This study was completed parametrically in order to isolate each variable in order to see its individual affect. Once an optimum performance was obtained from one variable the best performing mixture was used for the next variable testing. This method allowed for the highest performing mixture to be obtained from each of the investigated variables. The results showed that high strength pervious concrete made with local aggregates, without polymers, and without fibers can be produced in the range of 15.44 MPa - 21.63 MPa. A porosity range 19.1% - 32.9% with a percolation rate range of 5.8 mm/s - 1.9 mm/s was also achieved, with a porosity of 19.4% and percolation rate of 2.6 mm/s for the highest performing mixture.

The Effect of Various Polynaphthalene Sulfonate Based Superplasticizers on the Workability of Reactive Powder Concrete

A superplasticizer is a type of chemical admixture used to alter the workability(viscosity)of fresh concrete.The workability of fresh concrete is often of particular importance when the water-to-cement(w/c)ratio is low and a particular workability is desired.Reactive Powder Concrete(RPC)is a high-strength concrete formulated to provide compressive strengths exceeding 130MPa and made of primarily powders.RPC materials typically have a very low w/c,which requires the use of a chemical admixture in order to create a material that is easier to place,handle and consolidate.Superplasticizer are commonly used for this purpose.Superplasticizers are developed from different formulations,the most common being Polycarboxylate Ether(PCE),Polymelamine Sulfonate(PMS),and Polynaphthalene Sulfonate(PNS).This study investigates the effect of various PNS based superplasticizers on the rheological performance and mechanical(compressive strength)performance of a RPC mixture.Six distinctive types of PNS based superplasticizers were used;three of various compositional strengths(high,medium,low range)from a local provider,and three of the same compositional strengths(high,medium,low)from a leading manufacturer.The properties investigated were the individual superplasticizers’viscosity,the concrete workability,determined through a mortar spread test,the concrete viscosity,and the compressive strength of the hardened RPC mixtures measured at 7,14,and 28 days.Two separate RPC mixtures were prepared,which contained two different water-to-cementitious ratios,which consequently increases the dosage of superplasticizer needed,from 34.8L/m3 to 44.7L/m3.The results show that the name brand high range composition produced the overall highest spread,lowest viscosity,and a highest compressive performance.However,the local provider outperformed the name brand in the mid and low range compositions.Lastly,the rheology assessment also confirmed that the name brand high range,and RPC fabricated with the name brand high range,developed the lowest viscosities.