Models for strength prediction of foam concrete

There are several strength prediction relations developed for plain cement paste, mortar and concrete. In concrete where air voids contribute significantly to volume of voids (like aerated and foam concrete), more general expressions including the volume of air voids is to be developed as the better alternative. The objective of this paper is to propose prediction relations for the compressive strength of foam concrete by extending two of the well-known relations available for cement paste, mortar and normal concrete, viz., Balshin’s strength-porosity model and Power’s gel-space ratio equation. For this, theoretical equations were derived for porosity and gel-space ratio relating it to the density, proportion of ingredients in the mix and material characteristics like specific gravity. Foam concrete with fly ash showed lesser dependency on pore parameters than cement-sand mixes. As both the prediction relations developed in this study consider the effect of composition on the strength, it can serve as a simple tool for predicting the strength of foam concrete. But strength-porosity model stands out compared to gel-space model as it correlates well with the measured strength and also because of its ease in application since it employs the composition of constituents and easily measurable parameters.     

粉煤灰对泡沫混凝土物理力学性能的影响

研究了粉煤灰掺量和水灰比对粉煤灰掺量较大的泡沫混凝土的干密度和抗压强度的影响,利用经验公式计算泡沫混凝土的孔隙率,重点研究泡沫混凝土的孔隙率和干密度、抗压强度的关系。研究结果表明随着粉煤灰掺量的增加泡沫混凝土的干密度和抗压强度均呈下降的趋势。当粉煤灰掺量由25%上升到30%时,不同水胶比的泡沫混凝土干密度下降均超过了60 kg/m3;粉煤灰掺量由30%提高到40%时,不同水胶比的泡沫混凝土抗压强度下降的趋势都明显减小。以粉煤灰取代水泥后,孔隙率和干密度的拟合公式为 Y =27126.8-64295.9X +38334.4X 2,相关系数为0.9097;孔隙率和抗压强度的拟合公式为Y =58.7-142.2X +86.3X 2,相关系数为0.9802。     

On the magnitude and variability of the fatigue strength of acrylic bone cement

When used for the fixation of orthopaedic implants poly(methyl methacrylate) bone cement is prepared during surgery, and polymerises in situ. The technique for preparation of the bone cement involves mixing the liquid monomer and powder: two common mixing methods are hand mixing and vacuum mixing. Previous studies have shown that porosity depends on mixing technique. In this study, the fatigue strength of hand-mixed and vacuum-mixed cements is measured and correlated with the pore distribution resulting from each mixing technique. S–N curves show that vacuum mixing improves the fatigue strength by an order of magnitude. However, there is greater variability of fatigue strength associated with vacuum-mixed cement. This is correlated with the appearance of an occasional large pore in the vacuum-mixed cement. If the cross-sectional area is corrected to take account of porosity in vacuum-mixed cement, an 8% increase in the association of the data is found. Using a two-parameter Weibull model, it can be shown that the vacuum-mixed cement has a greater Weibull life at the 50% probability-of-survival level. However, if a probability-of-survival close to 100% is required (i.e. high reliability), the hand-mixed cement is found to have superior fatigue behaviour. The S–N curves can be explained by examination of the fracture surface features. The initiation stage of fatigue cracking is notably different for the two different mixing techniques. The lower fatigue strength of the hand-mixed cement can be explained by the interactions of pores on the fracture surface causing stress concentrations, whereas no such pore interactions occur in the vacuum-mixed cement.     

Microwave near-field reflection property analysis of concrete formaterial content determination

One of the most important parameters associated with concrete is its compressive strength. Currently, there is no reliable nondestructive testing technique that is capable of robust determination of this parameter. Concrete is a heterogeneous mixture composed of water, cement powder, sand (fine aggregate), rocks of various size or grade (coarse aggregate), and air (porosity). Water and cement powder chemically combine into a cement paste binder which, in due curing time, produces concrete with its specified compressive strength. Compressive strength of concrete is strongly influenced by its water-to-cement (w/c) ratio as well as its coarse aggregate-to-cement (ca/c) ratio. Therefore, if these two parameters are determined using a nondestructive testing technique, then they may be correlated to the compressive strength. Near-field microwave nondestructive testing techniques, employing open-ended rectangular waveguide probes, have shown tremendous potential for evaluating concrete constituent make-up. In this paper, the results of an extensive set of measurements, using these probes, are presented. The results demonstrate that the statistical distribution of the multiple measurements of the magnitude of reflection coefficient of concrete specimens with various constituent make-ups follows two well-known distributions as a function of frequency. It is shown that for the specimens investigated this distribution is Gaussian at 10 GHz and uniform at 3 GHz. Furthermore, the standard deviation of the measured magnitude of reflection coefficient at 10 GHz is shown to correlate well with ca/c ratio, whereas, the mean of this parameter at 3 GHz is correlated well with w/c ratio. Subsequently, these parameters may be used in conjunction with well established formulae or a look-up table to determine the compressive strength of a given concrete specimen     

Mix design of concrete with high content of mineral additions: Optimisation to improve early age strength

The concrete industry is an important source of CO₂ gas emissions. The cement used in the design of concrete is the result of a chemical process linked to the decarbonation of limestone conducted at high temperature and results in a significant release of carbon dioxide. Under the project EcoBéton (Green concrete) funded by the French National Research Agency (ANR), concrete mixtures have been designed with a low cement quantity, by replacing cement by mineral additions i.e., blast-furnace slag, fly ash or limestone fillers. Replacement of cement by other materials at high percentages generally lowers the early age strength of the resulting concrete. To cope with this problem, an optimisation method for mix design of concrete using Bolomey’s law has been used. Following the encouraging results obtained from mortar, a series of tests on concretes with various substitution percentages were carried out to validate the optimisation method.     

Static shear strength between polished stem and seven commercial acrylic bone cements

The stem–cement interface is one of the most significant sites in cemented total hip replacement and has long been implicated in failure of the whole joint system. However, shear strength at this interface has rarely been compared across a range of commercially available bone cements. The present study seeks to address this issue by carrying out a comparative study. The results indicated that the static shear strength was more dependent on cement type than cement viscosity and volume. However, both cement type and viscosity were contributory factors on porosity and micropore size in the cement surface. There was no significant difference between Simplex P and Simplex P with Tobramycin. Although the bone cements were all hand mixed in this study, the static shear strength was significantly larger than the values recorded by other researchers, and the porosity and micropore size showed much lower values. Bone cement transfer films were detected on the stem surface, typically about 4–10 μm thick. They were considered to be an important factor contributing to high friction at the stem–cement interface after initial debonding.     

Acid attack on pore-reduced cements

Because the durability of high-performance cements is as important as their strength, the performance of pore-reduced cement (PRC) in aggressive media such as sulfuric, hydrochloric and ethanoic acids, was studied and compared with that of ordinary Portland cement (OPC). The effects of exposure to these media on these cements were monitored by periodic visual inspection and sample weighing. Specific interactions with regard to interconnected porosity were addressed and the corrosion products characterized. PRC is less susceptible than OPC against hydrochloric and ethanoic acids. However, sulfuric acid damages PRC and OPC to almost the same extent. It is shown by electron microprobe analysis that the hydrochloric and ethanoic acids quickly penetrate the interior of normal cement pastes by acid leaching of the interconnected porosity. The reduced porosity of PRC reduces the susceptibility to attack by this mechanism. Sulfuric acid exposure causes extensive formation of gypsum in the cement surface regions, which results in mechanical stress and ultimately leads to spalling. Thus fresh surfaces are exposed regularly and therefore the relatively closed microstructure of PRC is no hindrance to this kind of attack. This revised version was published online in November 2006 with corrections to the Cover Date.     

Damage assessment in cellulose–cement composites using dynamic mechanical characteristics

This paper reports on an experimental investigation of test methods that can detect damage in cement composites, incorporating cellulose macro-nodule fibers, subjected to freezing/thawing and immersion in hot water. Conventional methods of ascertaining damage, such as porosity and flexural strength testing, were carried out, along with dynamic mechanical testing for determination of elastic modulus and damping capacity. Increase in porosity and reduction in flexural strength (as compared to normally cured specimens) were observed for low fiber volume composites subjected to freezing and thawing, whereas no significant changes were observed for specimens with higher fiber volumes. Determination of dynamic elastic modulus and specific damping capacity at regular intervals of exposure showed that the high volume of porous macro-nodules in the mixture prevented damage due to freezing and thawing by acting as stress release sites. No appreciable change in porosity and flexural strength was observed for specimens continuously exposed to hot water; however, reduction in relative modulus and increase in damping capacity was observed. The changes in these system properties suggest that a certain degree of damage might be occurring under sustained hot water exposure, which the determination of porosity and flexural strength could not capture. Using a stiffness-loss map, it is shown in this paper that the damping characteristic is more sensitive than the stiffness in detecting damage as a result of continuous exposure to hot and wet conditions.     

Factors affecting the static shear strength of the prosthetic stem–bone cement interface

Debonding of the stem–cement interface has been implicated in the initiation of failure of cemented femoral stems. The objective of this work was to examine some of the parameters which influence the interface static shear strength, including surface finish, cement type, pre-treatments and porosity. Surface finish was found to have the greatest effect on the interface strength. Increasing the surface roughness by a factor of 100 increases the interface shear strength by a factor of 20. However, increasing the surface roughness above a certain value was found to have no additional affect. This was due to failure in the cement itself rather than at the cement–stem interface. There were significant differences between some of the different cement types regarding the interface strength. Pre-heating the stem produced a six fold reduction in cement porosity at the stem–cement interface, however, resulting in only a minor influence on the static interface strength. Generally, no significant correlation was found between the cement porosity and the static interfacial shear strength.     

Influence of different types of water on strength, porosity and hydric parameters of metakaolin admixtured cement

The present research deals with strength, porosity and hydric behavior of metakaolin cement admixtured with different types of water. The hydration of ordinary Portland cement in the presence of 0%, 10%, 20% and 30% metakaolin treated with distilled, ground and sea water with the water to cement ratio of 0.4 was studied. The experimental results on setting time, strength, porosity and hydric parameters are reported. The results show that, metakaolin percentage increases in strength with a decrease in porosity. The observed results are discussed with SEM micrographs. Further, sea water accelerates the cement hydration at the early stages but retards it in the latter stages of hydration.     

Modelling the loss of strength and porosity increase due to the leaching of cement pastes

Pure water leads to a progressive decalcification of hardened cement pastes. The physical and chemical processes of this degradation are relatively well defined today, but the consequences of the leaching on the porosity increase and the loss of strength have been only partially studied. Our work consists of correlating the alteration of these properties with the rate of degradation of the material. The leaching is achieved by an accelerated process. Compressive strength and water porosity are measured after different processing times in an aggressive solution. A physical phenomenon analysis leads us to propose a stress distribution model at rupture, and a calculation model of the porosity increase, taking into account the nature of the degraded hydrates (CSH).     

Comparison of the fatigue behaviour of two different forms of PMMA

This study investigates the fatigue behaviour of two forms of polymethylmethacrylate (PMMA): pure PMMA in the form of acrylic glass and PMMA-based bone cement. Acrylic glass demonstrated superior fatigue strength compared to hand-mixed porous bone cement but was significantly weaker than pore-free bone cement. The greater fatigue strength of bone cement in comparison to acrylic glass was attributed to its composite-like microstructure, containing pre-polymerized beads which are absent in the amorphous structure of acrylic glass. In tests conducted on notched specimens, the porous bone cement demonstrated superior fatigue strength to acrylic glass. The results could be predicted using the theory of critical distances, with errors no larger than 14%. This approach allowed us to show that, although porosity had a negative influence on the fatigue strength of the plain specimens in comparison to acrylic glass, the presence of porosity had no effect when the samples contained a notch or a hole. This finding is interesting when considering the effort put into removing porosity from bone cement during surgical operations, where it is used in situations where there are sharp stress concentrations in the form of protruding bone and the design features of artificial implants.     

The role of aggregates on the structure and properties of lime mortars

Lime mortars have been used for centuries in civil engineering construction. Considering ancient monuments and historical buildings it seems that these mortars have proved to be durable and reliable materials although they are of low strength in comparison with cement mortars. Nowadays, they are used for the repair of monuments and for the manufacture of renderings and plasters.

In the present paper the role of aggregates on the structure and behaviour of lime mortars is examined by studying the influence of the aggregate content and the grain size on strength, porosity and volume stability of the mortars. Capillary porosity by suction was also measured as an indicator of resistance to weathering. It is shown that coarse aggregates contribute to the volume stability of lime mortars independent of strength enhancement when adequate compaction reduces the capillary pores. The highest strength values, and consequently, the low porosity, were attained by lime mortars of low binder/aggregate ratio which contained aggregates of maximum size 0–4 mm.     

Influence of different types of water on strength,porosity and hydric parameters of metakaolin admixtured cement

The present research deals with strength, porosity and hydric behavior of metakaolin cement admixtured with different types of water. The hydration of ordinary Portland cement in the presence of 0%, 10%, 20% and 30% metakaolin treated with distilled, ground and sea water with the water to cement ratio of 0.4 was studied. The experimental results on setting time, strength, porosity and hydric parameters are reported. The results show that, metakaolin percentage increases in strength with a decrease in porosity. The observed results are discussed with SEM micrographs. Further, sea water accelerates the cement hydration at the early stages but retards it in the latter stages of hydration.     

Managing trace elements in Portland cement – Part I: Interactions between cement paste and heavy metals added during mixing as soluble salts

The aim of this work is to investigate the effect of the addition of Cu, Cd, Ni, Pb and Zn nitrate salts on the compressive strength of a CEM I Portland cement. Concentrations of 0.018 or 0.18 mol/kg of cement of each trace element were tested. After 2 days age, the compressive strength was reduced by various extents by addition of heavy metals, with the exception of Ni. This difference is due to a delay in tricalcium silicate hydration (C₃S) as shown by an isothermal calorimetry test. Trace elements also influence the 28-days compressive strength, whereas the measured degree of hydration of these cement pastes is the same. As shown by scanning electron microscopy and X-ray diffraction, Cu and Pb are predominantly absorbed in the calcium silicate hydrate gel (C–S–H) while Cd, Ni and Zn are mainly precipitated as hydroxides within the intergranular porosity. Thus, trace elements precipitated as hydroxides have only a slight effect on the compressive strength. In contrast, Cu and Pb cause an increase in mechanical resistance by changing the C–S–H nanometric assembly and its density.     

Atomic force and lateral force microscopy (AFM and LFM) examinations of cement and cement hydration products

The objective of this work was to better understand how atomic force microscopy (AFM) and lateral force microscopy (LFM) techniques can be used as tools to understand the nanostructure and microstructure of cement and cement hydration products. AFM and LFM techniques were used on mortar samples to distinguish between CSH, CH, and unhydrated cement particles. The LFM technique appears to be more sensitive to topographic changes than conventional AFM and it can more clearly distinguish between the different phases at high magnification (low scan range). AFM could also be used to calculate the roughness of the interfacial transition zone (ITZ) between aggregate and the cement paste at different ages. The rough surface at the interface of the paste and aggregate is generally interpreted as higher porosity. It was found that a reduction in roughness (i.e., porosity) occurred for samples that were cured for a longer time which are consistent with the explanation of porosity.     

The influence of mixing on the microstructure of the cement paste/aggregate interfacial zone and on the strength of mortar

The influence of mixing on the microstructure of the cement paste/aggregate bond has been investigated. Back-scattered electron microscopy was used in conjunction with quantitative image analysis to examine the microstructure of the interface between limestone aggregate and the cement matrix in a series of mortars. The distribution of porosity and anhydrous material along the paste/aggregate interface was shown to be dependent upon the relative abundance of water at the aggregate surface during mixing. Improvements in the interfacial microstructure were shown to correlate with improvements in strength and fracture properties. The interfacial zones seen in the limestone mortars were compared with a model interfacial system. A new classification system for two types of interfacial regions in mortar is proposed.     

碱式硫酸镁水泥耐酸腐蚀性能研究

为了研究碱式硫酸镁水泥耐酸腐蚀性能,将不同配比的水泥试样分别在柠檬酸溶液及水中浸泡不同龄期,再进行质量变化测定及抗折强度和抗压强度试验。采用XRD与SEM技术分析不同配比水泥试样浸泡于两种溶液后的物相组成和显微形貌。结果表明,掺入的矿渣和粉煤灰对碱式硫酸镁水泥具有良好的密实填充作用,降低了水泥的孔隙率,有效阻止了侵蚀介质的进入,其耐酸腐蚀性能与未掺矿渣和粉煤灰的碱式硫酸镁水泥相比有明显提升,其中,掺矿渣的碱式硫酸镁水泥耐酸腐蚀性能更优。     

改性磷石膏的强度与孔结构的研究

本工作研究了矿物掺合料(矿渣、粉煤灰)和激发剂(熟石灰和水泥)对磷石膏强度的影响,并且探索了水泥对磷石膏耐水系数的影响。此外对磷石膏改性处理后的微观形貌和孔结构进行了分析。研究结果表明:矿渣和粉煤灰均能提高磷石膏的强度,且矿渣对磷石膏强度的增强作用更明显;但两者对磷石膏耐水性的增强作用并不明显,矿渣掺量过多时会由于延迟钙矾石的形成而导致石膏开裂。水泥和熟石灰作为激发剂时可以增强磷石膏的强度,熟石灰的增强作用更明显。水泥对磷石膏的耐水性能有一定的增强作用。磷石膏的水胶比、养护龄期和矿物掺合料可以改变其孔隙率,但不会改变其孔径分布;粉煤灰可以提高石膏的孔隙率,并且改变其孔径分布;水泥会降低石膏的孔隙率并改变其孔结构。     

Cement hydration: building bridges and dams at the microstructure level

The concurrent goals of cement hydration are to percolate (bridge) the original cement particles into a load-bearing network and to depercolate (dam) the original water-filled capillary porosity. The initial volume, particle size distribution, and flocculation/dispersion state of the cement particles have a large influence on both hydration rates and microstructure development. Likewise, the capillary porosity as characterized by its pore size distribution, percolation state, and saturation state also influences both hydration kinetics and microstructure. In this paper, experimental techniques and computer modeling are applied to further understanding several of the critical connections between these physical parameters and performance properties. First, the setting or bridging process is explored via a combination of needle penetration and rheological measurements, in concert with three-dimensional microstructural modeling. Second, low temperature calorimetry is shown to be a valuable indicator of the percolation state or damming of the water-filled pores with various size entryways in the three-dimensional microstructure. Porosity percolation (or depercolation) is shown to be strongly influenced by both curing conditions and the alkali content of the cement pastes. Finally, it is proposed that future efforts in this field be directed towards a greater understanding of the (nano)structures of cement hydration products, particularly the calcium silicate hydrate gel, and their influence on performance properties.