Multi-sized fillers to improve strength and flowability of concrete

Concrete with a low carbon dioxide footprint (LCDF) contains less cementitious materials (CM) than ordinary concrete and hence less cementitious paste. Besides the reduced carbon dioxide footprint, LCDF concrete offers other advantages such as reduced cost as well as improved dimensional stability due to reduced hydration heat, creep and shrinkage. Nevertheless, decreasing the quantity of CM has a negative impact on concrete workability. To restore workability, multi-sized fillers are advocated to replace aggregates and CM. Generally, fillers can improve the packing density of concrete due to the filling effect and thus more excess water or paste is available to facilitate spread and flow rate. Two types of fillers were examined in this study – limestone with particles smaller than 75 μm and foundry sand with particles of 75–400 μm. The particles of these two fillers are respectively similar and larger in size when compared with cement. Concrete mixes with no fillers and with mono- or multi-sized fillers were prepared and tested for flowability and strength. The results indicate that more superplasticiser is needed to achieve the same flowability when fillers are added. It was also found that, at the same water/CM ratio, fillers can improve concrete strength, and the use of multi-sized fillers can simultaneously improve the flowability and segregation resistance of concrete

Fillers to improve passing ability of concrete

Concrete possessing high-passing ability needs to be flowable and cohesive. Hence, passing ability cannot be improved by solely adding superplasticizer, which increases both flowability and segregation of concrete simultaneously. Decreasing the maximum size of aggregates so that concrete segregates at lower cohesiveness is a possible but undesirable way as it narrows the aggregates' grading and decrease dimensional stability of concrete. With the same maximum size of aggregates, passing ability can be improved by raising the concurrent flowability-segregation envelope of concrete. In this paper, fly ash and silica fume (cementitious fillers) and limestone (inert filler) were selected to replace cement partially and subsequently the passing ability of concrete was studied. From the results, it was evident that when either type of fillers were used, the passing ability and maximum limits of flowability and segregation achieved simultaneously increase. It is because these fillers are finer than cement that provides better filling effect to increase packing density and excess water leading to better flowability. Concurrently, the cohesiveness of concrete also increases as the content of fine particles increases. These allow concrete to hold the coarse aggregates more firmly when passing through narrow gaps, after which the concrete will keep flowing rapidly

The rapid chemically induced corrosion of concrete sewers at high H2S concentration

Concrete corrosion in sewers is primarily caused by H2S in sewer atmosphere. H2S concentration can vary from several ppm to hundreds of ppm in real sewers. Our understanding of sewer corrosion has increased dramatically in recent years, however, there is limited knowledge of the concrete corrosion at high H2S levels. This study examined the corrosion development in sewers with high H2S concentrations. Fresh concrete coupons, manufactured according to sewer pipe standards, were exposed to corrosive conditions in a pilot-scale gravity sewer system with gaseous H2S at 1100 ± 100 ppm. The corrosion process was continuously monitored by measuring the surface pH, corrosion product composition, corrosion loss and the microbial community. The surface pH of concrete was reduced from 10.5 ± 0.3 to 3.1 ± 0.5 within 20 days and this coincided with a rapid corrosion rate of 3.5 ± 0.3 mm year −1. Microbial community analysis based on 16S rRNA gene sequencing indicated the absence of sulfide-oxidizing microorganisms in the corrosion layer. The chemical analysis of corrosion products supported the reaction of cement with sulfuric acid formed by the chemical oxidation of H2S. The rapid corrosion of concrete in the gravity pipe was confirmed to be caused by the chemical oxidation of hydrogen sulfide at high concentrations. This is in contrast to the conventional knowledge that is focused on microbially induced corrosion. This first-ever systematic investigation shows that chemically induced oxidation of H2S leads to the rapid corrosion of new concrete sewers within a few weeks. These findings contribute novel understanding of in-sewer corrosion processes and hold profound implications for sewer operation and corrosion management

Limestone and silica fume to improve concurrent flowability-segregation limits of concrete

The demanding performance criteria of high-performance concrete (HPC) such as high flowability and low segregation make superplasticiser (SP) indispensable in concrete production. SP is commonly added to concrete to increase flowability without undermining its strength. However, over-dosage of SP will cause segregation. This not only decreases the strength but also implies that there is a maximum limit of flowability that can be reached before segregation occurs. Increasing this maximum limit, or in general the overall flowability-segregation performance, is essential for HPC. In this study, limestone (LS) and silica fume (SF) were used as partial replacements of cement and the overall flowability-segregation performance of concrete was studied. The results showed that, with the addition of LS or SF, more SP was needed to achieve similar flowability but the maximum limit of flowability before reaching unacceptable segregation (>20% sieve segregation ratio) was improved, implying that replacement of cement by LS or SF with additional SP actually enhances the overall flowability-segregation performance of concrete. Accordingly, the design limits of flowability and segregation can be improved when LS or SF is added to the concrete, which is a very useful result for the design of selfcompacting concrete

Nitrite admixed concrete for wastewater structures: Mechanical properties, leaching behavior and biofilm development

This study systematically investigated the impacts of calcium nitrite addition on the mechanical properties and biofilm communities of concrete-based wastewater infrastructures using sulfate resistant cement through standard tests and DNA sequencing, respectively. The results revealed that setting time and water demand for normal consistency were reduced, but slump, drying shrinkage, and apparent volume of permeable voids increased with calcium nitrite dosage up to 4% weight of cement. The cumulative leached fraction of nitrite, 28-day compressive strength and biofilm communities were not significantly affected by calcium nitrite dosages. The addition of calcium nitrite into concrete is environmentally friendly to wastewater infrastructures

Filler to improve concurrent flowability and segregation performance of concrete

Post-blending limestone (LS) filler to replace cement partially in concrete can lower the cost of production and embodied carbon. In order not to jeopardise the compressive strength, the water-to-cementitious materials (W/CM) ratio shall be kept. Because of the reduction in water-to-powder ratio, more superplasticiser (SP) is required to restore workability. Nonetheless, too much SP causes segregation. Hence, it is not clear if post-blending limestone will increase the segregation at similar strength and workability. In this study, various concrete mixes with post-blended limestone are tested for strength, flowability and most importantly, concurrent flowability-segregation performance. It is evident that at the same W/CM ratio, post-blending limestone increases the compressive strength and needs more SP to achieve similar flowability. Nevertheless, at the same water-to-powder ratio, the SP dosage required to maintain similar flowability does not change by too much. More importantly, the overall flowability-segregation performance of concrete containing LS is generally improved. It is because the cohesiveness of concrete improves when part of the cement is replaced by finer limestone. The results suggest that post-blending limestone can increase workability at the same segregation stability or vice versa

Cause and mitigation of dilatancy in cement powder paste

Superplasticiser (SP) becomes essential in contemporary concrete manufacturing because it decreases the water demand for a prescribed set of concurrent strength and workability requirement. On the other hand, SP also creates unfavourable dilatancy (i.e. shear thickening) that decreases the uniformity of mixing and pumpable distance of concrete, as well as creates difficulty in in-situ handling. Although it is known that the dilatancy is created by clustering of free and adsorbed polymers of SP, the correlation of them is not fully understood. Herein, the dilatancy of cement powder paste was studied using a co-axial rheometer. It is evident that with the addition of SP, dilatancy of paste increases initially from zero up to certain dosage. Further addition of SP decreases dilatancy as it effectively disperses the cement particles. At a given SP dosage, replacing cement partially with pozzolanic filler such as fly ash or silica fume can effectively decrease dilatancy depending on SP dosage. It is because the fillers can fill up the interstitial void of the cement particles and free up more water for cement hydration to produce Ca via ettringite formation. Hence, it attracts more negatively charged SP to adsorb to the cement surface, and decreases the free SP polymers that cause dilatancy

Dilatancy mitigation of cement powder paste by pozzolanic and inert fillers

Third generation polycarboxylate-based superplasticiser (SP) can easily extend the maximum concurrent design limits of strength and flowability of concrete or cement paste, but simultaneously creates unfavorable dilatancy (or shear thickening) that decreases mixing efficiency and pumping range of concrete. The dilatancy is created initially by clustering of free and adsorbed polymers of SP, but further addition of SP can conversely mitigate the dilatancy by improved wet packing density. Because of the negative correlation of packing density and dilatancy, it is believed that partial replacement of cement by finer pozzolanic or inert filler can decrease the dilatancy via improved filling effect. Herein, the dilatancy of cement powder paste with or without pozzolanic fly ash or inert limestone powder replacing partial cement was studied using a coaxial rheometer. The results revealed that there exists a threshold limit of SP, beyond which the effect on dilatancy reverses. At a given SP dosage, partial cement replacement by either filler decreases dilatancy via improved packing density up to an optimal ratio, after which the dilatancy increases because the packing density reverses

Failure Mode of Concrete Under Polyaxial Stresses

Advances in instrumentation, methods for data collection and techniques in manufacturing precision testing machines have paved the way for very promising recent researches related to the failure characteristics of construction materials under true field loading conditions. Nevertheless, the majority of available biaxial and true triaxial failure envelopes for rock-like materials shows that the induced octahedral shear stresses on a stressed specimen at failure is a monotonically increasing function of the effective mean applied pressures. This function could be linear, exponential, or polynomial of any reasonable power depending on the material type. Curve fitting procedures are therefore needed to obtain the function’s constants. To determine the constitutive response of concrete under conventional triaxial, biaxial, and true triaxial stress states, a series of tests has been performed in this study using carefully designed concrete specimens. The validity of some of the available true triaxial failure models in the literature was then examined against the collected data, the results which are summarised in this paper