International Concrete Abstracts Portal

High-strength concrete shows particular characteristic behavior at ele- vated temperatures, such as explosive spalling, that is rarely observed in normal-strength concrete. This behavior has been attributed to the very dense concrete matrix usually associated with high-strength concrete. This paper presents the effect of fiber reinforcement on mitigation of explosive spalling and residual properties of high-strength concrete under elevated temperatures (600°C). The experimental work has been carried out on the influ- ence of three parameters associated with fire resistance. These three parameters are an addition of polypropylene fibers andor polymer beads, the moisture content of the concrete and the water to cement ratio. The experimental results showed that the internal temperature elevation was mitigated in the test specimens, that contained the fibers or beads, that were melted by heating, by low cement-water ratio and by high moisture content. The accompanying strains due to heating at these conditions were reduced in the test specimens.

This paper provides results of different types of curing in hot weather environment on the compressive strength of concrete made with portland cement and complementary cementitious materials (CCM) such as natural pozzolans, fly ash (FA), granulated blast furnace slag (GBFS), and silica fume (SF). In all concrete mixtures, a superplasticizing admixture (SPA) was used. Nine series of concrete mixture were made. In seven of them (1,2,3,4,5, 6 and 9) the normal portland cement (NPC) content was 200 kg/m3 and in two of them (7 and 8) the same amount of cement was used but it was a portland-natural pozzolan cement (PNPC). The CCM varies from 9.9 to 60.6% of the total cementitious material. The W/C in all series was 0.70 when using NPC or PNPC. The W/C+CM varied from 0.28 to 0.63. In all series the same amount of 1260 kg/m3 of coarse aggregate was used. Five different ways of curing were used. One was the initial and final ASTM curing at 23°C up till the age of testing as reference, and four different ways of curing in hot weather environment at 37°C for the first 24h were used. These final curings were: A) ASTM; F) three day and G) seven-day water spray for 15 minutes every 2h; and (E) covered by two layers of membrane. Adequate compressive strength development (CSD) can be obtained using CCM but very good curing is necessary. Generally, by casting specimens at 37°C and put them under ASTM curing next day at 231t2"C (A), the strength at 28 days was lowered by about 8% and at six months by about 8% lower than these casting at 23°C. Membrane curing was less effective at later ages mainly when fly ash was used. There exist an optimum amount of fly ash to obtain maximum compressive strength at later ages.

The study is aimed at obtaining basic data for evaluating the residual strength and deformability of damaged reinforced concrete members. The behavior of membqs that suffered damage during cyclic loading such as cracking that penetrate into the cross section, plastic deformation or partial failure was compared with that of member tested under simple torsion. Reversed cyclic torsion was applied for loading because few studies have been reported in open literature although it has been estimated to cause the most bride failure. Tests and analyses identified the effect of reversed cyclic torsion with varying numbers of cycles and with varying amplitudes had on the reduction of residual strength and torsional rigidity. It was .also confirmed that a closed joint that encircles longitudinal reinforcement at the ends of transverse reinforcement was stronger and more resistant to deformatioQ than a lapped joint.

The durability and perme&i&y of concrete are of special concern in the Arabian Gulf due to the prevailing aggressive weather conditions. Very few practical examples showing the ability of the local ready-mixed industry to adapt to extreqie hot weather, characteristic of this region of the world, have and analyzed. The following paper describes extreme concrete properties in two particular projects in Saudi Arabia demonstrating how the local concrete industry has used specific measutes to produce high-performance concrete in very hot weather. In the first project concrete was hauled for 8 hours in hot weather using high dose of standard retarders and superplasticisers and following rigorous measures to control the mix temperature. In the second project high-strength high-performance concrete was hauled for 45 minutes at an ambient temperatup of 40" C and achieved 24 hours strength in excess of 50 MPa. It is concluded that controlling the temperature of fresh concrete in hot weather should be primarily based on parameters related to cost, workability retention and long-term strength. Besides, the cited examples'demonstrated how the use of admixtures in hot weather could be very beneficial even if the dosage considered did not comply with ASTM C 494 requirements and the manufacturer standard recommendations.

Norida Department of Transportation (FDOT) requires contractors to submit proposed concrete mixture proportions prior to the production of any concrete. The contractor must use mixture proportions approved by FDOT. Substitutions of ingredients, other than coarse aggregate, must be justified through trial mixtures, and authorized in writing by FDOT Engineers. The substitutions of fly ash, slag, air-entraining admixtures, and 'I).pcs A, D and G water reducing admixtures were performed on two typical FDOT hot weather mixture proportions in this study. The concrete properties considered were slump, air content, and compressive strength. Test data for substitution mixture propohons were compared with the data for the original mixture proportions on the basis of statistical criteria. Results show that the substitutions cause significant change in concrete properties for the fly ash mixture proportions. The substitution of slag and air-entraining agent in the slag mixture proportions does not cause significant change in the properties of concrete.