Characterization Of Mortars Research Articles

Feasibility of Ecofriendly Mortars with Different Hemp Additions for Use in Building Sector

Cement mortars are commonly used in building works for the execution of enclosures and exterior rehabilitation of facades. The incorporation of plant-based additives enables the development of ecofriendly construction materials. In this work, a physical and mechanical characterisation of cement mortars with the incorporation of hemp in three morphologies (fibre, powder and pellet) was conducted. The results show how the additions of hemp powder and pellets with a partial replacement of natural aggregate reduce the final density of cement mortars and their thermal conductivity by more than 16% and 19%, respectively. On the other hand, the addition of hemp fibres reduces shrinkage during the setting of cement mortars, improving their flexural strength and increasing their durability. For this reason, it is possible to recommend the use of these mortars with the addition of natural hemp fibres without prior surface treatment to improve the physical-mechanical properties of these construction materials and extend their application field as ecofriendly materials for masonry work.

The durability of basalt-fiber-reinforced cement mortar under exposure to unilateral salt freezing cycles

Basalt fiber and cement-based materials have been widely applied in engineering structures. In this context, the durability of basalt-fiber-reinforced ordinary silicate mortar was systematically studied under exposure to unilateral salt freezing. The mechanical durability, chloride ion diffusion characteristics, and microscopic pore characteristics of cement mortar with basalt fiber content levels in the range of 0 kg/m3–1.5 kg/m3 were tested under exposure to 0–40 freeze–thaw cycles. The relationships of changes in the internal pore structure with mass loss, mechanical damage, and the physical properties of the material were also analyzed under exposure to salt freezing cycles. The results demonstrated that even a small amount of basalt fiber could significantly improve the mechanical properties of cement mortar under unilateral salt freezing and its resistance to salt freezing erosion. In particular, cement mortar with 1.2 kg/m3 basalt fiber content exhibited good durability of compressive and flexural strength, while the specimens with no basalt fibers exhibited a relatively large degree of internal porosity under exposure to unilateral salt freezing. Our work provides concrete evidence for changes in the porosity of mortar under exposure to unilateral salt freezing, with these changes showing an exponential relationship with mortar mass loss and a strong linear correlation with changes in the compressive strength, flexural strength, and chloride ion diffusion coefficient of the material.

Comprehensive assessment of geopolymer concrete mechanical and environmental performance with glass cullet fine aggregates

Despite its established reputation and extensive testing as an alternative to conventional cement, geopolymer continues to heavily rely on natural aggregates. The disposal of waste glass presents a significant global challenge as it constitutes a substantial portion of solid waste. This study investigates the application of geopolymer mortar, wherein waste glass cullet serves as the principal component for fine aggregate. Two distinct curing methods, namely ambient and elevated temperatures, were employed to evaluate the mechanical properties of the material in accordance with standards. An optimal aggregate-to-binder ratio of 1:2.5 was determined to achieve suitable workability for in-situ casting applications. Following a 28-day curing period, the geopolymer mortar underwent testing for compressive and tensile strength, bringing acceptable results at 46 MPa and 2.8 MPa, respectively. Additionally, the study examined the relationship between compression and tension characteristics of geopolymer mortar and, through regression analysis, established correlations that accurately predicted data from other published works. Furthermore, the carbon footprint of geopolymer was compared to that of ordinary Portland cement (OPC) concrete, with geopolymer demonstrating the potential to significantly reduce the environmental impact of cement manufacturing, saving 27.76% of the carbon footprint per m3 in comparison to OPC. This study emphasizes the strong mechanical qualities of geopolymer products, which give them a great probability of being employed in industry and creating a new market for waste materials through the usage of by-products.

Innovative use of copper mine tailing as an additive in cement mortar

This research assesses the feasibility of recycling copper mine tailings (CMT) by analyzing the durability and mechanical characteristics of cement mortar using these tailings as filler additives. CMT are mineral wastes generated during the process of mining. In this work, specimens of cement mortar were incorporated with up to 30 wt.% of a CMT. Bulk density, dynamic modulus of elasticity, apparent density, ultrasonic pulse velocity, flexural and compressive strengths tests were evaluated. Total amount of voids, sorptivity, water absorption and chemical resistance tests were also obtained to evaluate the mortar durability. When 10 wt.% CMT was incorporated, overall amount of voids in the mortar was reduced by 20% and mechanical performance was improved by 16% after 28 days. The flexural strength of the mortar was also found to increase, with the 20% wt.% CMT mortar incorporation reaching a flexural strength of 5.89 MPa. Thus represents 16% increase compared to the control 0% CMT strength. The results indicated that there was not a perfect correlation between these results and the mechanical strength results for the 15 and 20 wt.% CMT mortars. In addition, the CMT acts as a protective barrier against harmful chemicals. The results of this research indicate that reusing CMT by incorporating into cement mortar is a feasible method for their recycling. Mortar made with as much as 15 wt.% CMT presented the same strength and durability as mortar with traditional sand and cement.

Evaluation of non-destructive techniques for mechanical characterisation of earth-based mortars in masonry joints

In this paper, the use of non-destructive tests for the mechanical characterisation of earth-based mortars in masonry joints is discussed. Four testing methods, namely the penetrometer, Schmidt hammer, pendulum hammer and scratch test, originally developed for other types of mortar, are reviewed. The methods are applied to the earth-based mortars at the Wupatki Pueblo archaeological site, in Arizona, US. The outcomes of the experimental programme allowed to assess the reliability of the methods and to identify their limitations. Finally, the methods are compared in terms of six qualitative indicators, namely easy-of-use, consistency of results, range and granularity of results, respect towards cultural value, depth of investigation under the visible surface and versatility in application. Overall, the penetrometer test is recommended as the preferable method to characterise the mechanical performance of earth-based mortars.

Investigation on the multimodal failure characteristics of cement mortar under uniaxial compression loading

The physical condition and durability of cement-based structures can easily decrease after years of operation due to exposure to severe scenarios associated with internal defects, aggressive usage, and continuous load changes. Therefore, accurately assessing such infrastructures is essential for ensuring safety and serviceability and preventing hazards. In this study, an experimental investigation of the multimodal failure characteristics of cement mortar with various sand contents at different uniaxial compressive loading rates was conducted. Specifically, the multimodal failure characteristics include three main categories: mechanical properties (e.g., compressive strength), one-dimensional signals (e.g., micro-vibration and electromagnetic radiation), and two-dimensional videos/images (e.g., crack propagation trajectory). A geophysical acquisition and a high frame rate camera were employed to simultaneously record the micro-vibration, electromagnetic radiation and crack propagation videos of the cement specimens in a uniaxial compression test. Based on the computer vision technique and the feature pyramid network (FPN), an automatic pixel-level crack segmentation method was proposed to extract cracks of each critical moment in the fracture video. Then, the fractal dimension and crack area were calculated to quantitatively depict the crack propagation trajectory. Finally, the stress drop, micro-vibration, electromagnetic radiation, and crack propagation trajectory characteristics were analysed and discussed. The experimental results indicate the following. (1) The FPN offers a practical way to identify cracks in structures of cement-based materials at the pixel-level. (2) The micro-vibration, electromagnetic radiation, and crack propagation trajectory exhibit good consistency. In particular, the area and fractal dimension of cracks positively correlate with the stress drop of cement under compression loading. (3) The peak and cumulative energy values of micro-vibration and electromagnetic radiation tend to increase as the sand content increases when subjected to the same loading rate.

Reuse of mixed masonry wastes in plastering mortars

According to Eurostat statistics, the volume of waste collected is constantly increasing, the amount of waste produced in Europe in 2020 reaching 2151 million tons. In the European Union, the most waste-producing activity is construction and demolition, accounting for 37.1% of the total waste produced in 2020. This type of waste has a high occupancy rate. This article proposes the reuse of mixed masonry waste in the production of plaster mortars. The maximum amount of mixed masonry waste used in the experiment was 35%, partially replacing the aggregate component. The physical and mechanical characteristics that have been measured in the experimental program may be divided into two categories: characteristics of mortars in fresh state (apparent density, consistency) and in hardened state (bending strength, compressive strength and adhesion to the support layer). In order to highlight the influence of the addition of waste on the composition and on the evolution of the mechanical properties of the studied mortars, the experimental program was distributed over 3, 7 and 28 days. The results showed an increase in the mechanical strength of mortars containing mixed masonry waste compared to standard mortar. Mixed masonry waste can be a substitute for raw materials in the production of new types of plaster mortars and contributes to the reduction of the resulting waste by recycling and reuse. The experimental program will be continued with microscopic investigations using modern investigation methods, to highlight the behaviour of mortars with masonry waste, and to identify the correct proportion of waste that can be used in mortar networks so as not to affect the characteristics required by the acting norms.

The influence of calcium oxide doses as an activator on the compressive strength and mechanical characteristics of cement-free mortar containing ground granulated blast furnace slag

In the construction industry, cement-based materials as aggregate binding agent are still widely used. Due to the environmental concern, researchers are working to create eco-friendly cement-free binding materials. Calcium oxide (CaO) is a reactive earth metal oxide that could serve as a potential activator for ground granulated blast furnace slag (GGBFS). In this experimental study, the cement-free mortar was produced by using GGBFS as the base material and CaO as an activator with varied doses of 5%, 15%, and 25% with a fixed water-to-binder ratio (w/b) of 0.4. The results showed that the 28-day compressive strength values of the mixture with an amount of CaO of 5%, 15%, and 25% were 21.45, 32.03, and 25.80 MPa. Moreover, the values of bulk density and porosity of free-cement mortar at the age of 28 days ranged between 1.932 - 1.939 g/cm3 and 22.66% - 24.95%, respectively. According to the research findings, the optimal additional CaO amount of 15% exhibited the series’ highest compressive strength and lowest porosity.

Study on Usage of Fabric Fibre Slime Waste as Fractional Substitution on the Characteristics of Cement Mortar

Study on Usage of Fabric Fibre Slime Waste as Fractional Substitution on the Characteristics of Cement Mortar

A Neural Network-Based Prediction of Superplasticizers Effect on the Workability and Compressive Characteristics of Portland Pozzolana Cement-Based Mortars

Portland Pozzolana Cement (PPC) mortars are predominantly employed in plastering works to achieve better workability, superior surface finish, and higher fineness to offer better cohesion with fine aggregates than the ordinary Portland cement (OPC) mortars. To achieve high performance in the cement mortar similar to cement concrete, the addition of a superplasticizer is recommended. The present study investigates the impact of addition of sulphonated naphthalene formaldehyde- (SNF)-based (0.5%, 0.6%, 0.7%, and 0.8%) and lignosulphate- (LS)-based (0.2%, 0.3%, 0.4%, and 0.5%) superplasticizers on the workability and compressive strength characteristics of PPC mortars. Plastering mortars of ratio 1 : 4 were prepared with natural sand and manufacturing sand (M sand) as fine aggregates. A flow table test was conducted on all the mortar mix proportions, and the effects of the inclusion of superplasticizers on flow properties were recorded at different time intervals (0, 30, 60, 90, and 120 minutes). PPC mortar cubes were prepared, cured, and examined to assess the inclusion of chemical admixtures on compressive strength at different ages (1, 3, 7, 14, and 28 days). The experimental findings from the workability and compressive strength of PPC mortars were analyzed, and the corresponding results were predicted using artificial intelligence. Experimental investigations demonstrated that the desired flow characteristics and higher compressive strength results were achieved from a 0.7% dosage of ligno-based superplasticizer. The predicted workability and compressive strength results at various ages acquired by implementing an Artificial Neural Network (ANN) were found to be in close agreement with the experimental results.

Effect of internal curing and shrinkage-mitigating materials on microstructural characteristics of fiber-reinforced mortar

Internal curing (IC) and other shrinkage mitigating materials are employed to reduce shrinkage and risk of cracking. This study investigates the efficiency of individual versus combined use of IC and shrinkage mitigating materials on key characteristics of fiber-reinforced mortar (FRM). The investigated mixtures include a 25% pre-saturated lightweight sand (LWS) that is added individually and combined with 10% CaO-based expansive agent (EA) and 2% shrinkage reducing admixture (SRA). This elucidates the synergistic effect of high content IC and EA/SRA on macro- and micro-mechanical characteristics of FRM, especially at the interface of the matrix with fibers. Mechanical properties, microstructural characteristics, and fiber–matrix bonding of FRM made with 0.5% steel fibers are investigated. The results show that the use of IC with EA and SRA completely compensates for shrinkage at 56 days. The highest compressive and fiber pull out strengths are observed for FRM with IC and without EA/SRA due to the densification of the interfacial transition zone (ITZ) confirmed by microstructural analysis. Such improvement is associated with the lower porosity of the cement paste and longer silicate chain—higher Si/Ca—obtained by FRM made with LWS. Although the combination of both EA and SRA with IC leads to 180 μstrain expansion after 56 days, the corresponding mixture presents the weakest ITZ and inferior mechanical properties.

DRMS for the mechanical characterization of lime mortars: Influence of user-controlled variables on the correlations between UCS and drilling results

The paper investigates the influence of the user-controlled variables on the results of the drilling test (DRMS) performed on lime mortars used for ancient buildings restoration. In particular, the ratio of the penetration and rotational speed (δ), namely the depth of cut per revolution, is considered. The experimental results show a relevant variation in the shapes of the drilling profiles with δ, especially in mortars with lower strengths. The efficacy of the regression models between the uniaxial compressive strength (UCS) and the drilling results changes with the depth of cut. An optimal range of δ to perform the DRMS test is identified for the analyzed mortars.The paper also investigates the possibility of performing the DRMS test according to another procedure poorly studied in the literature to evaluate the UCS of materials. It consists of making more holes in the same material at different values of δ. This method is based on the analytical and experimental results obtained with another scratching technique, the scratch test. The statistical analysis of results shows that the correlation between the mean of the drilling forces and δ can be described by linear regression in a limited range of δ. The slope of the linear regression has a high correlation with UCS. The obtained positive results move to deepen the experimental research on this topic.

Mortar Characterization of Historical Masonry Damaged by Riverbank Failure: The Case of Lungarno Torrigiani (Florence)

The research of structural masonry associated with geo-hydrological hazards in Cultural Heritage is a multidisciplinary issue, requiring consideration of several aspects including the characterization of used materials. On 25 May 2016, loss of water from the subterranean pipes and of the aqueduct caused an Arno riverbank failure damaging a 100 m long portion of the historical embankment wall of Lungarno Torrigiani in Florence. The historical masonry was built from 1854–1855 by Giuseppe Poggi and represents a historical example of an engineering approach to riverbank construction, composed of a scarp massive wall on foundation piles, with a rubble masonry internal core. The failure event caused only a cusp-shaped deformation to the wall without any shattering or toppling. A complete characterization of the mortars was performed to identify the technologies, raw materials and state of conservation in order to understand why the wall has not collapsed. Indeed, the mortars utilized influenced the structural behavior of masonry, and their characterization was fundamental to improve the knowledge of mechanical properties of civil architectural heritage walls. Therefore, the aim of this research was to analyze the mortars from mineralogical–petrographic, physical and mechanical points of view, to evaluate the contribution of the materials to damage events. Moreover, the results of this study helped to identify compatible project solutions for the installation of hydraulically and statically functional structures to contain the riverbank.

Alkali-activated slag & fly ash as sustainable alternatives to OPC: Sorptivity and strength development characteristics of mortar

One approach to decreasing the environmental footprint of the construction industry is to replace ordinary Portland cement (OPC) with recycled slag and fly ash activated using alkaline materials. This article presents the outcomes of an experimental study that evaluated water absorption characteristics and strength development of mortar that uses blends of alkali-activated ASTM class F slag and fly ash binders. Mortar mixes were developed using three binder combinations: 100% slag and no fly ash (S1F0), 75% slag + 25% fly ash (S3F1), and 50% slag + 50% fly ash (S1F1). Slag and fly ash binders were activated using NaOH solution with molarity ranging from 10 mol/L to 16 mol/L mixed with sodium silicate solution. Two sets of samples were created, one set was immersed in an acidic medium after casting, which represented a closed environment, and the second was left in the lab exposed to air until the test day. For mortar cured in a closed system, the highest strength development during the first seven days after casting occurred in S1F0 mortar followed by S3F1, then S1F1. The observation indicates strength development is dominated by the fast reactivity of slag during the first 7 days. The pattern is similar for all NaOH activator concentrations evaluated in this study. Mortar samples with high slag content (S1F0 and S3F1) that were cured in air experienced a decrease in strength during the 28–90 day curing phase as the strength at the age of 90 days decreased relative to the strength at 28 days. However, S1F1 gained strength during the 28–90 day phase when NaOH molarity was the highest (16 mol/L). This is due to the dissolution and activation of fly ash at high solution alkalinity which contributed to the continued increase in strength. A strong linear correlation between the square root of time (t0.5) and water absorption existed in all activated mortars evaluated in the study, except for mortar samples prepared using NaOH with high concentration and the highest ratio of Na2SiO3/NaOH. Therefore, alkali-activated slag-based mortars share similar water capillary absorption characteristics with conventional cement-based mortar. It was found that increasing the NaOH concentration increases the global warming potential (gwp) and that the mix with the least environmental impact was the S1F1 developed using the lowest NaOH concentration of 10 mol/L and 50% fly ash.

Effects of extended mixing processes on fresh, hardened and durable properties of cement systems incorporating fly ash

Specifications that correspond with system performance may guarantee the addition of value. Most specifications for ready-mixed concrete address limits on discharge time and truck-drum revolution counts. These limits have been developed for conventional concrete. As the uses of supplementary cementing materials (SCMs) become ubiquitous, it is important to determine whether these specifications are applicable to SCMs, that is, systems containing fly ash. This paper presents results of the effects of mixing time and mixer revolution counts on characteristics of lab-made pastes and mortars containing 20% and 50% fly ash. Their characteristics assessed include time-variant ion concentrations, setting time, flow, compressive strength, porosity, and apparent chloride diffusivity coefficient. Results indicate that with increasing mixing time and mixer revolution counts, mixtures with a replacement of fly ash exhibit improved both fresh and hardened characteristics. When mixed for 60 min or 25,505 revolution count, the 28-day compressive strengths of mixtures containing 20% and 50% fly ash are 50% to 100% higher than the neat cement. Fly ash is suggested to adopt in the extended mixing processes of cement systems.

Microstructure tests, flow, and mechanical behavior of polymerized cement mortar

The influence of two types of polymeric admixture on the stress–strain (σ-ε) behavior of cement mortar is discussed in this article. The characteristics of cement mortars modified with two types of polymeric admixtures containing 0.20 percent (% wt.) were investigated. For early age (1 day) and 28-day curing periods, the σ-ε behavior of modified cement mortar with polymeric admixtures was investigated. Depending on the polymeric structure and content, adding polymeric admixture reduced the water/cement ratio (w/c) by 14.5 to 25.5 %. When 0.20 % of polymeric admixture was added to cement mortar, the compression strength improved by 72 % to 153 %. The cement mortar samples became brittle as the polymeric admixtures content increased. The amorphous gel fills the spaces between cement particles and sand particles, decreasing voids and porosity and improving the density of the cement mortar. To forecast the connection of the modified cement mortar with polymer, the nonlinear Vipulanandan p-q equation was used, and it was compared to the Farazdaghi- Harris-YD and Sinusoidal models. The compressive stress–strain models were evaluated using statistical methods, and nonlinear approaches were utilized to predict compressive strength as a function of water/cement ratio, curing age, and polymer quantity. The less complicated model, the Vipulanandan p-q model, already has a smaller sum of squares than the other two models. Therefore, the Vipulanandan p-q model is the best one.

Pressure mapped squeeze flow (PMSF): Extending rheological characterization of mortars beyond traditional rheometry

AbstractThe rheometric techniques available for the evaluation of mortars involve a different set of flow conditions. Squeeze flow is based on the compression of the sample with gap reduction and geometric restrictions, providing important information even when phase separation occurs and an evaluation in conditions similar to those found in several practical situations for many classes of materials, including cement‐based and ceramics. Traditional squeeze flow results are related to bulk normal force and height variation of a sample compressed between parallel plates. Additional information regarding boundary conditions at the interfaces or phenomena related to differential flow can be obtained through further instrumentation of the test. The combination of squeeze flow and a pressure mapping technique has been recently proposed, with great potential for the analysis of cement‐based materials and other granular suspensions. In this work, four mortars were evaluated by the pressure mapped squeeze flow (PMSF) method in two different displacement rates, and new ways to analyze the results were developed to expand the understanding of the flow through the technique, including plotting the pressure along multiple circumferences and an analysis of variation in each radial position. PMSF results were also compared to rotational rheometry and flow table tests for the first time, and concepts of interparticle separation were employed to discuss microstructural aspects of the flow. Due to the variety of mix designs (admixtures, particle size distribution, air content, water content, and other factors), the mortars presented diverse behaviors, ranging from primarily viscous or plastic flows to more granular responses (related to friction between particles with localized formation of jammed structures due to liquid phase migration). This work is part of an effort to establish a foundation for PMSF as a rheometric method that can be used for the analysis of a wide range of materials.

Microstructural and Macroperformance of Recycled Mortar with High-Quality Recycled Aggregate and Powder from High-Performance Concrete Waste

The amount of high-performance concrete (HPC) in construction engineering is growing at a predictable high rate following rapid urban development. However, the utilization of HPC waste as secondary building materials has received little attention until now because previous investigations have focused on the reclamation of ordinary concrete waste. Therefore, this work studied the characterization of mortar containing high-quality recycled aggregate (HRA) and high-quality recycled powder (HRP), both sourced from HPC waste. HRP contains some active components, and the pore structure of paste incorporated with HRP is finer than that of paste containing ordinary recycled powder. Incorporating HRP in paste shortens the setting time and improves the fluidity, and mixing HRA with coarse surface benefits the performance of interfacial transition zone. Incorporating HRA and HRP can both reduce the drying shrinkage, and the maximum shrinkage value of HRA mixed mortar is obviously lower than that of ordinary recycled-aggregate mixed mortar. The mechanical strength first increases and then drops with the HRA incorporation. The incorporated HRP reduces the mechanical strength, but decreasing HRP particle size enhances the mechanical strength of mortar. The water absorption of mortar increases as HRA or HRP is incorporated, but decreasing HRP particle size reduces the water absorption of HRP-incorporated mortar. The mortar with both HRA and HRP has a distinct lower water absorption than the mortar with both ordinary recycled aggregate and powder. High-quality recycled mortar owning good performance can be made by intermixing a suitable amount of HRA and HRP.

Physical and Mechanical Characterization of Lime Pastes and Mortars for Use in Restoration

Slaked lime is traditionally a suitable material for use in construction activities in historic and traditional buildings whose constitutive characteristics demand compatible and appropriate material solutions. Therefore, knowing the physical and mechanical characterization of lime pastes and mortars for use in restoration is considered an important step in the process of scientific development of a material that, in the Mexican case, has lacked scientific rigor in its production, use and commercialization. With this in mind, the present article aims to investigate the characteristics of lime pastes and mortars that have been used in the traditional way and the new limes that exist in the market, offering suitable options for restoration, for which an integrated methodology was used consisting of physical tests such as surface area, electrical resistivity, pulse velocity, colorimetry, and mechanical tests such as compressive strength, giving as a result that “traditional slaked” lime is more likely to deteriorate than powdered lime, which leads us to conclude that powdered hydrated lime has optimal characteristics for restoration activities.

A comprehensive study on the compressive strength, durability-related parameters and microstructure of geopolymer mortars based on mixed construction and demolition waste

As a viable option to upcycle construction and demolition waste (CDW) into value-added materials, geopolymer technology is emerging. Most studies investigate CDWs in a separated form or in combination with mainstream pozzolanic/cementitious materials focusing only on fundamental properties of geopolymer pastes, not considering to scale such materials to the level of their application in the forms of structural mortars/concretes or to characterize long-term performance/durability. This study investigated the development and characterization of ambient-cured mortars with mixed CDW-based geopolymer binders and untreated fine recycled concrete aggregates (FRCA). Mixture of CDW-based roof tile (RT), red clay brick (RCB), hollow brick (HB), concrete (C), and glass (G) was used as the precursor, while ground granulated blast furnace slag (S) was used in some mixtures to partly replace CDW precursors. Compressive strength, durability-related parameters including drying shrinkage, water absorption, and efflorescence, microstructure and materials sustainability were evaluated. Results showed that 28 d compressive strength results above 30 and 50 MPa are achievable with the entirely CDW-based and slag-substituted mortars, which were found improvable to have entirely CDW-based structural concretes. Drying shrinkage of the mortars is slightly higher than that of conventional cementitious/geopolymeric systems although it can be minimized significantly through mixture optimization. Water absorption values remain comparable with the literature. CDW-based geopolymer mortars outperform Portland cement mortars in terms of CO2 emission and energy requirement. Our findings show that via utilizing CDW-based constituents in mixed form as precursor and waste aggregates, it is possible to develop greener construction materials with acceptable strength and long-term performance.