Porosity, pore size distribution and in situ strength of concrete

Porosity, pore size distribution and in situ strength of concreteRakesh Kumara, B. Bhattacharjeeb,*aBridges Division, Central Road Research Institute, New Delhi 110 020, IndiabDepartment of Civil Engineering, Indian Institute of Technology-Delhi, Hauz Khas, New Delhi 110 016, India

Received 23 July 1997; accepted 24 July 2002

AbstractIn this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. Theapparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed onsmall-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation intheir strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarseaggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With thedata so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-basedmaterials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics,cement content, aggregate type, exposure conditions, etc., is presented.

Keywords: Mercury porosimetry; Pore size distribution; Pore system; Concrete; Cement content

1. IntroductionConcrete prepared with hydraulic cement binder can beregarded as a chemically bonded ceramic. The hydrationreaction of cement results in a product consisting of solidand a pore system [1]. Pores are thus inherent to concrete.Pores in concrete can also result from inadequate compac-tion. This pore system governs the most important prop-erties of concrete, notably its strength [2,3]. Well-compactedconcrete prepared with hard low-porosity aggregates may beassumed to be a multiphase material consisting of coarseaggregates embedded in mortar matrix. The mortar matrixconsists of fine aggregates, the solid cement hydrates,unhydrated cement, etc., and the pore system [4]. The poresystem present in the mortar of concrete, however, ismarkedly different from the pores of well-compacted mortarprepared independently using identical proportions of therelevant ingredients. The above difference in the two poresystems is due to the transition zone pores present atmortar-aggregate interface [5-7]. Capillary porosity ofhardened cement paste depends on water-to-cement ratio.

Water-cement ratio also governs the transition zone por-osity in concrete [4]. Thus, there are a number of well-established strength versus water-cement ratio relation-ships, which indirectly relate the strength of concrete withits pore system characteristics [4,8,9]. These relationshipsserve their purpose very well in the design of concretemixes. A few of these relationships take into account aircontent and degree of hydration of concrete. However, suchindirect relationships do not take into account the porespresent in hardened concrete in structure due to inadequatecompaction, etc. Further, the pore system in concrete alsochanges with degree of hydration and chemical changes dueto aggressive environments, etc. A direct relationship, on theother hand, can facilitate the strength estimation of in situconcrete from the knowledge of its pore system character-istics. For the purpose of mix design, however, strength/water-cement ratio relationships are more useful. The mostimportant characteristics of pore system are porosity andpore size distribution, which can be determined throughmercury intrusion porosimetry (MIP). However, MIP resultsare affected by a number of factors and the same must besuitably accounted for in the experimental procedure adop-ted [10-14]. Secondly, the smallest size of pore, in whichmercury can intrude, depends upon the maximum intrusionpressure applied. Consequently, extent of porosity that can156R. Kumar, B. Bhattacharjee / Cement and Concrete Research 33 (2003) 155-164be determined by porosimetry test depends upon the natureof the pores, the size of the smallest pore likely to beencountered in the material and the maximum intrusionpressure applied.The pore system in cement-based materials consists offour types of pores. These are: (a) gel pores, which aremicropores of characteristic dimension 0.5-10 nm; (b)capillary pores, which are mesopores with average radiusranging from 5 to 5000 nm; (c) macropores due to deliber-ately entrained air; and (d) macropores due to inadequatecompaction. In concrete, in addition to the above pores,there can be cracks at aggregate-mortar interface due toshrinkage. The gel pores, which are mostly of 1.5-2.0 nmsize, do not influence the strength of concrete adverselythrough its porosity, although these pores are directly relatedto creep and shrinkage. Capillary pores and other largerpores, on the other hand, are responsible for reduction instrength and elasticity, etc. [4,7,15-17]. Thus, while dealingwith an empirical strength-porosity relationship of con-crete, contribution of the gel pores in the overall porosityand pore size distribution of concrete can be neglected,without introducing any significant error. Hence, to deter-mine the pore system characteristics influencing thestrength, the maximum pressure in the porosimetry testmust be sufficient to cause intrusion of mercury in thesmallest capillary pore. In mercury porosimetry, a majorpart of gel pores remains nonintruded. Further, the closedpores also remain nonintruded. One other limitation pointedout as regard to mercury porosimetry is that it measuresentry sizes rather than true pore size that is related to inkbottle effect [18]. Thus, the porosity determined, as above,is apparent porosity.A number of relationships relating strength of cement-based materials with their pore system characteristics areavailable in the literature. In this paper, firstly, the results ofan experimental investigation are presented, whereby dataon in situ cube compressive strength of concrete—estimatedthrough compression test of cores drilled out from labor-atory cast beams—are generated, together with MIP data forthe same concrete. This is followed by an appraisal wherebymost frequently referred relationships involving strengthand pore system characteristics of cement-based materialsare evaluated for their suitability in strength estimation of insitu concrete. Finally, a new empirical relationship for in situstrength of concrete is proposed, which takes apparentporosity, pore size characteristics (corresponding to 33,000psi intrusion pressure) and binder content of the concreteinto account.2. Experimental investigation2.1. Objectives and scopeThe main objectives of this experimental investigationwere to generate sufficient data on strength, apparentporosity and pore size distribution of concrete. It was alsodesired that the strength data so generated should adequatelycover the range of strength usually encountered for normal-strength concrete. High-strength concrete, produced usingwater-reducing agent and pozzolanic microfiller, was leftout of the scope of this work.